Process Pipe and Tube Welding. A Guide to Welding Process Options, Techniques, Equipment, Ndt and Codes of Practice

Edited by W Lucas, DSc, PhD, CEng, FIM, Weld1

Process pipe and tube welding A guide to welding process options, techniques, equipment, NDT and codes of practice

A B I N G T O N P U B L I S H I N G Woodbead Publishing Ltd in association with The Welding Institute Cambridge England

Published by Abington Publishing. Woodhead Publishing Ltd. Abington Hall. Abington. Cambridge CB I 6 A H . England

First published 1991. Abington Publishing

0 Woodhead Publishing Ltd

Conditions of sale All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means. electronic or mechanical. including photocopy. recording. or any information storage and retrieval system. without permission in writing from the publisher.

British Library Cataloguing in Publication Data Process pipe and tube welding 1. Piping. Metal tubes welding. 1. Lucas. William 67 I .52 ISBN 1 85573 012 X

Designed by Geoff Green (text) and Chris Feely Cjacket). typeset by BookEns Limited, Baldock Herts and printed by Crampton & Sons Ltd. Sawston. Cambridge

Introduction

The welding of tubes is an essential requirement in the fabrication of components for chemical plant, power generation and aeroengine industries. The original idea for this book arose from a seminar, organised by The Welding Institute in 1984, which attracted over 100 engineers concerned with design, fabrication, production and quality assurance. The speakers were specially invited from major UK companies as acknowledged experts in the fabrication of tubular products. This book contains contributions from those speakers together with additional chapters to provide unique, comprehensive and up-to-date coverage of all aspects of tube welding from initial design considerations through production to final inspection.

In the first three chapters the authors have outlined the process and equipment options available for both manual and mechanised welding. This type of information is essential for design and production planning when engineers are often faced with difficult choices, for example, between competing processes (MMG MIG, TIG, plasma) and indeed process variants such as pulsing. The information contained here will help engineers make the right choice for particular applications, ensuring that the most cost effective welding techniques are employed.

Five chapters are devoted to the application of tube welding in the aeroengine, ship building, power generation, petrochemical and chemical plant industries. Here numerous applications are described with detailed information on the choice of process, materials, techniques and equipment. The welding parameters and production data provided by the authors will be invaluable to engineers seeking to apply tube welding techniques in a practical situation. In addition, the experiences of the authors will be a valuable source of information and will help engineers to avoid making what could be costly mistakes or to overcome problems when they occur in production.

For completeness, specialised information is also provided on the production management aspects of tube welding. For example, in site welding

V

vi Introduction

the need to integrate all activities from material procurement through fabrication to final inspection is shown to be an essential requirement. More generally, when high quality applications are considered, the pipework must be fabricated to meet construction codes and here guid- ance is given on interpreting the requirements of the major codes for pressurised pipework. In the final chapter, NDT techniques are described for inspecting tubes to ensure that weld quality meets the exacting requirements of steam generation plant.

The information contained in the book represents without doubt the most comprehensive study of tube welding in industry. The data presented here will become a source of reference for designers, metallurgists. welding engineers and production engineers who are concerned with pipework fabrication. The authors are not only acknowledged experts in this technology but have also built up a reputation for providing sound practical advice in their particular industrial sectors. For these reasons this book, containing technical information and practical experience, is essential for those engineers wishing to manufacture tube components in the most productive manner.

W Lucas

Chapter 1

K R S P I L L E R

Process options and manual techniques for welding pipework fabrications

Of the methods available for joining pipe, a principal technique is manual welding using the manual metal arc (MMA). tungsten inert gas (TIG), and metal inert gas (MIG, MAG or CO,) processes.

Joining pipework with any of these processes is regarded as a specialised operation, with the most important requirement relating to the quality and profile of the penetration bead. The successful achievement of this critical application is one of the most exacting tasks the welder encounters, particularly when root runs are made on unbacked, unrotated butt joints. Where high quality root runs are required, TIG welding, with or without the use of fusible inserts, is usually preferred. Where the quality becomes less stringent, MMA with basic, rutile and cellulosic electrodes, or the MIG process find acceptance.

Whichever process is used, the production of controlled penetration beads on an unrotated pipe is possible, provided the correct application of specific techniques is carried out. This chapter discusses, in turn, the relative merits of the manual techniques of each process.

Physical and geometrical aspects of pipewelding

Welding a pipe butt joint, especially when its axis is in the fixed position, i.e. vertical, horizontal, or 45" inclined (classified as 2G, 5G, and 6G respectively, Fig. l.l), is one of the most difficult challenges to the welder, particularly when making the root run in the overhead quadrant, which will henceforth be referred to as the 'critical area'. It will be appreciated that on larger pipe diameters, for example, 300mm OD, a greater degree of tolerance will be present than when pipes of smaller diameters, i.e. 100mm. are welded.

1

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(a) (b) (C)

1 . I Classification of non-rotatable pipewelding positions: a) 26: pipe vertical; b) 56: pipe horizontal; c) 66: pipe 45' inclined.

The techniques used to make the all-important root run in pipe butt joints are given in Table 1.1.

The limitations imposed on the maximum permissible protrusion of the penetration bead into the bore of the pipe are given in Table 1.2. While these acceptance limits are laid down, any concavity should merge smoothly into the adjacent surfaces and should be of a continuous and uniform profile around the internal circumference of the welded joint. The mismatch of the internal bore will also affect the penetration bead formation and profile. Many codes give allowable tolerances on the amount of bore mismatch which can be permitted, but this should never be taken as a licence to tolerate the limits and every effort should be exercised to keep the bore mismatch to a minimum.

Manual metal arc (MMA) welding The MMA process may be used on a wide range of materials and pipe sizes and is one of the most commonly practised for welding steel pipe. Although it is capable of being used to produce high integrity joints, consistent control of the penetration bead profile is difficult to achieve and can vary from being concave to a typical 3mm convex protrusion. The design of the joint preparation and the fit-up obtained make a significant contribution towards the achievement of controlled penetration beads. A representative sample of joint designs as given in various piping codes is reproduced in Fig. 1.2. Two techniques are available for making the root run: vertical-up and vertical-down welding. The choice of method is not affected by the diameter of the pipe, but depends primarily upon the wall thickness and type of material being welded.

Process options and manual techniques 3

Table 1.1 Techniques for making root runs on unbacked pipe butt joints

Welding process Manual welding technique

MMA 1 2 3 4

TIC 1 L 3

2 MIG or CO, 1

Vertical-up: rutile electrodes Vertical-up: basic electrodes Vertical-down: cellulosic electrodes Vertical-down: basic electrodes Vertical-up: filler rod additions Vertical-up: fusible inserts Vertical-up; fusing the root. No fusible inserts or filler rod Vertical-down: solid wire dip transfer Vertical-up: solid wire dip transfer

Table 1.2 Limits of bore protrusion and root concavity

Nominal bore of pipe

Less than 12mm 0.8 1.5

50mm up to but not including lOOmm 2.5 lOOmm and larger 3.0

Maximum penetration in bore, mm -

l2mm up to but not including 50mm

Root concavity: 1 The bore surface of the joint is of smooth contour 2 The depth of root concavity is no greater than 10% of the pipe wall

thickness or 1.2mm. whichever is the smaller 3 The thickness of the weld is not less than the pipe wall thickness

When a vertical-up procedure is used it is customary to start welding at the 6 o’clock position and weld upwards until the 12 o’clock position is reached. This method is preferred for thick-walled pipe where the walls act as a ‘heat sink by dissipating the heat more rapidly from the weld area. The rapid cooling rates can cause metallurgical changes within the heat affected zone (HAZ) which could prove detrimental. To overcome this the cooling rate must be reduced, and this is accomplished by decreasing the rate of welding and depositing a heavier weld bead.

When vertical-down welding is used, the weld is started at the top side of the pipe (12 o’clock position) and is then continued downwards to the 6 o’clock position. Vertical-down welding is primarily used to weld relatively thin-walled steel pipe since it allows fast travel speeds. On heavier walled pipe it is possible to use the vertical-down procedure with larger diameter electrodes and current values, but in doing this the welder will experience considerable difficulty in maintaining control over the weld pool. The weld pool increases in size which results in the pool over-running the arc

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30-40" r-7 1 1 , 5 + 0 . 5 m m i t 1.5-4.0 mm

(a )

3&37.5"

i 1.5" -0.5 mm

(C )

I / \

1.2 Typical joint preparatlons for MMA welding of process pipework. Pipe walls: a) Up to and including 20mm; b) Over 20mm; c) Up to and including 12.7mm, suitable for vertical-down welding.


and so flooding the joint. Moreover, because of gravitational forces, the increased weld pool and associated slag coverage cushions the arc and can cause defects such as pinholes. lack of root fusion and cold lapping.

Formation of root penetration around the pipe

When making the root run on a fixed 5 G position pipe using the conventional vertical-up welding, the welder has to maintain correct electrode angles and execute small changes in welding technique to produce the desired penetration bead.

In the critical area, the welder will rely on the position of the electrode tip and combined arc force to give the weld pool some support. As the weld is progressed to the vertical quadrant, the weld pool tends to sag downwards under the arc and tries to flow into the bore of the pipe at a much faster rate than when welding in the critical area, and the penetration bead becomes markedly convex. In the inclined and flat quadrant, the weld pool becomes even more fluid and the penetration bead can become excessively convex and, in more severe cases, burnthrough can occur. Irrespective of which type of electrode is used, achieving a controlled penetration bead will be highly dependent upon the formation of a keyhole.

The keyhole or pear-shaped enlargement is formed by the root faces burning away, which causes a localised enlargement of the root gap just ahead of the arc. Ideally penetration is obtained when the keyhole is between one and one and one-third times the electrode diameter (core) being used. If the keyhole becomes enlarged, excess penetration, burnthrough or internal undercut will develop. Conversely, if the keyhole is too small, lack of penetration may occur. The keyhole size is controlled by changing the electrode angle and the manipulation of the electrode tip and arc.

When vertical-down welding, relatively flat to slightly convex penetration bead profiles are more readily achieved around the pipe bore than with vertical-up welding. With vertical-down welding, the weld pool, because it readily flows downwards, is aided by gravity and so requires high welding speeds to prevent it from over-running the arc.

When a keyhole is correctly formed, the fluid weld metal flows in behind the hole to form the weld bead and automatically produces the required penetration bead profile.

€fleet of electrode type on welding technique

Verzical-up welding with basic electrodes On pipe joints which require the root run to be of high quality it is customary to use basic cover electrodes with diameters of either 2.5 or

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3.25mm. The 2.5mm diameter electrode is preferred as it results in the formation of a smaller weld pool which can be more easily controlled.

Once the arc has been established. the tip of the electrode is directed into the root gap and pointed at the weld pool. Extreme care is exercised at this stage to ensure that the arc does not 'blow' into the root gap. A short arc length is maintained throughout welding. since porosity will occur if the arc is too long. The welder can, if so desired. execute a V- shaped weave of the electrode tip. Fig. 1.3. the weave being made in a continuous smooth movement. In doing this the arc is brought out of the weld pool and up along the bevel face with a quick movement. The return movement is slowed sufficiently to allow just enough time for the weld pool to lose its fluidity, with the arc being returned to the weld pool and held for a short pause. This combined movement is then repeated up and along the other bevel face.

The practice of 'whipping' the tip of the electrode as used with rutile covered electrodes must be avoided to eliminate the occurrence of porosity and loss of control over the formation of the penetration bead profile. Another common practice - that of 'hopping' the electrode tip from one bevel to the other at the root of the joint - is also detrimental.

When stopping a weld bead which will be continued, care must be taken to ensure that the arc is not 'broken off abruptly. otherwise a shrinkage cavity forms in the weld crater. Unless these are dressed and tapered by grinding it is unlikely that they will be completely remelted out when a restart is made. The recommended practice to avoid forming a crater is to 'tail out' the arc to the sidewall of the joint. then break the arc. The restarting technique requires a preheat to allow fusion and penetration to take place at the leading edge and underside of the crater. This is achieved by initiating the arc on the bevel face adjacent to the crater and

1.3 Pattern of V weave when making root run using basic electrodes.


the electrode angle of 80" to the vertical being altered to 110" for a few seconds so that 'heat' is passed over and under the crater.

brtical-up welding with rutile electrodes For root runs in pipe in which the acceptance criteria are not so stringent, either 2.5 or 3.25mm diameter mile electrodes may be used. When the arc is established. the tip of the electrode is gently pushed into the root gap so that the arc initially burns inside the pipe until the root gap is bridged. Welding is then progressed across the critical area. and upwards with a linear movement along the joint in the direction of welding. The relatively softer arc does not give as clear an indication of the degree of penetration achieved as that of a basic electrode. and the welder has to rely more on the formation of the keyhole and associated electrode tip manipulation.

When the vertical-up quadrant of the pipe is reached i t is often necessary to use a whipping action of the electrode tip to prevent overheating the weld pool and burnthrough. With this whipping action. which may be classified as a 'whip and pause' movement, the whip width should not exceed one and one-half times the electrode diameter, and is applied until the 12 o'clock position on the pipe has been reached.

The primary objective of the whipping action is to allow the weld pool to cool sufficiently so that i t loses its initial fluidity and so prevents the formation of excessive penetration anti overflow of the weld pool. The pronounced and deliberate, but controlled. movement of the electrode tip causes the arc length to fluctuate. Rutile electrodes are far more tolerant to arc length variations (within practical limits) which result in fewer problems when the arc is broken. The electrode is held at 90-100" to the pipe tangent so that the slag is encouraged to run out of the weld pool.

brtical-down welding with cellulosic electrodes With this type of electrode the arc is struck at the 12 o'clock position on the pipe and, once initiated, the electrode angle is held at 10-15" and a long arc held until it has stabilised with the formation of the gaseous shield which envelops the arc area. At this point the cup of the electrode tip is pushed into the root of the joint thus promoting the formation of a small crescent-shaped keyhole. Fig. 1.4. No electrode weaving is necessary. and a drag action is used to ensure that the arc is allowed to burn inside the pipe. To achieve an acceptable penetration bead it is desirable that a small, visible keyhole is maintained.

The size of the keyhole is significant because. if it is allowed to become too large, internal undercut and/or burnthrough (windows) will occur. In practice a keyhole approximately 3mm in length gives the right amount of penetration inside the pipe.

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1.4 Example of keyhole when welding vertical-down.

Variations in root gap are undesirable, but where they do occur the following corrective action should be taken. With wider root gaps the weld travel speed is increased slightly, and the electrode angle is increased so that the arc force is directed upwards towards the leading edge of the solidifying weld pool. When the root gap becomes too narrow. the weld travel speed is decreased and the electrode angle is also reduced. I t is. however. essential to produce and maintain a keyhole since. frequently. good internal penetration beads cannot be obtained without its formation. This is particularly applicable where small root gaps are encountered and the keyhole disappears with the arc burning 'inside' the pipe. Of the two alternatives, the small visible keyhole technique is much preferred since it is easier to work with. coupled with the fact that a far greater degree of control over the welding operation is possible.

Tungsten inert gas (TIG) welding

This process allows precise control over the formation of the penetration bead to produce a smooth internal bead profile. I t is particularly suitable for positional welding of pipe, because the weld travel speed is slow enough to allow the welder to follow the curvature of the pipe. Moreover the welding current. being independent of the filler rod feed rate, permits close control over the formation of the penetration bead and compen- sates for slight variations in the fit-up of the pipe joint.

The use of fusible inserts also finds a wide application in process pipework where high quality root runs with smooth internal profiles are mandatory. The TIC process invariably is used to fuse such inserts in these situations. A representative sample of the types of joint preparation which are applicable to the TIC process with and without the use of filler rod and fusible inserts is presented in Fig. 1 .5


Gus purging

To achieve high quality penetration bead profiles, the use of a back purge of a non-active gas to protect the root bead from atmospheric contamination is desirable. and for many applications is mandatory. Gas purging is particularly recommended for stainless steels, low alloy steels and non-ferrous pipework, but is not considered to have any technical benefit when copper and aluminium alloy pipes are welded. When fusible inserts are incorporated into the joint, it then becomes a mandatory requirement to gas purge the pipe.

Welding grade argon is the most commonly used gas for the purging operation. The gas purge may be contained within an area inside the pipe enclosed at suitable distances from either side of the joint by blanking the pipe diameter with dams.

1 I 1

Nil gap (a)

0-3mm

(b)

3&45" BJ 1.5 f0.5 mm

0.8-2 mm Nil gap

( C )

1.5 Typical joint preparatlons for TIG welding process pipework. Pipe walls: a) Up to and including 2.5mm; b) From 2.5mm upwards; c) Greater than 6mm.

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Prior to starting the root run, the purge gas pressure within the pipe should not be allowed to become excessive, otherwise it may force the penetration bead to become concave or even cause metal expulsion at the point of the final tie-in when the root run is overlapped. Where open butt joints are encountered, it is normal practice to tape the joint to permit the pre-purging operation and prevent escape of the purge gas. A higher purge flow rate can be tolerated than that used on close butt joints or when fusible inserts are fitted. The higher flow rate helps to compensate for the loss of gas as the tape is peeled back in stages, by the welder, as welding progresses round the pipe.

Wlding techniques

Three variations in welding technique can be used when making the root run on pipe joints which are fixed in the 5G and 6G positions:

Starting the root run 30mm off centre from the 6 o’clock position, welding is continued across the critical area and proceeds up to the crown on one side. On reaching the 12 o’clock position, the root run on the opposite side of the pipe is then made in the same manner, starting about lOmm off centre from the 6 o’clock position, Fig. 1.6a. The root run is started at the 8 o’clock position and continued across the critical area until the 4 o’clock position is reached. A restart is made at 5 o’clock, terminating at 12 o’clock. The root run is then completed by restarting at 7 o’clock and progressing upwards until a smooth tie-in takes place at the 12 o’clock position. Fig. 1.6b. This technique is much preferred to counteract the occurrence of root gap closures, particularly on pipe diameters in excess of 200mm. The root run is accomplished using an identical weld progression to that described in (b), with the exception that the vertical and flat quadrants on each half of the pipe are sequenced, Fig. 1.6c.

1.6 TIG welding techniques for production of fixed position process pipework.

(c)

penetration bead on


TlG torch and filler rod angles and feeding techniques

The angle at which the TIG torch and filler rod are held in relation to the pipe wall curvature, when making the root run, must be carefully maintained as welding progresses through one quadrant to another. The feeding of the filler rod has to accomplish two aims:

0 Minimal disturbance of the stability of the arc and weld pool; 0 Ensure complete freedom from root concavity across the critical

area.

For the completion of the root run, the following filler rod feeding techniques may be used, the choice depending upon whether the joint preparation is a close or open butt. The dip feed method: Usually preferred on U preparations where the lands at the root of the joint simulate a thin sheet butt weld. The welder is then able to maintain a small cross-sectional weld pool into which the tiller rod is introduced with a rhythmical forward and backward movement. The dip push method: The rod is pushed hard into the weld pool each time a portion of filler rod is melted as welding progresses. This may be used to advantage particularly on aluminium alloys where the rod is literally pushed into the weld pool to disrupt the surface oxide skin on the inside of the pipe. The continuous feed method: The end of the rod is held in contact with the weld pool and melted off at a steady rate as welding progresses. On open butt joints the root gap is set slightly below the diameter of the filler rod to act as a channel in which to rest the rod. The rod is then continuously fed into the weld pool as the arc. with a slight side-to-side oscillation, passes over it. The reversed continuous feed method: The rod is passed through the joint root gap and fed to the weld pool on the inside of the pipe as welding progresses, Fig. 1.7. This technique is restricted to welding across the critical area only.

Failure to use the various filler rod feeding techniques properly will contribute to the following defects: lack of root fusion areas, excessive penetration, burnthrough, or fully fused but inconsistent penetration bead profiles.

Welding without fusible inserts

Close butt joints Once the initial weld pool has been established, irrespective of whether or not filler rod has been added, the molten pool will take the form of a pear

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\ \

I , \ -

\ 5"

Weld progression

2

1.7 Filler rod and TIG torch angles for root run across critical area using reversed continuous feed method.

shape, Fig. 1.8a. This pear shape is significant, because it indicates that the correct amount of penetration is being obtained. If this weld pool shape is maintained throughout the root run, consistent and complete penetration will result. Should welding conditions vary, i.e. if travel speed becomes too slow or if weld current is too high, the pool will tend to become elongated with the formation of a long sharp point at the leading edge of the pool. Fig. 1.8b. This pool shape indicates that excessive penetration is being formed.

If the operating conditions are reversed. with travel speed too high or current too low. the pool will revert to a round shape. with the formation


of a re-entrant angle at the leading edge of the weld pool. Fig. 1 .8~ . This re-entrant angle signifies the loss of penetration inside the pipe.

The weld pool shapes discussed will, in general, be applicable to the carbon, low alloy and stainless steels, nickel, nickel-copper and cupro- nickel alloys, but in aluminium and copper the weld pool will tend to develop into an oval shape. and exhibit a convex appearance until sinkage occurs. This sinkage will be more evident in aluminium than copper in that the copper weld pool will become distinctly convex in appearance.

During the welding of the root run it will be inevitable that a restart will need to be made. Ideally these restart junctions should be planned to occur when tack welds are reached. In practice this does not always occur and it is necessary to ensure that potentially defective areas are eliminated by attention to detail when making a restart. In close butt joints the arc is slowly moved away from the weld pool to the face of the joint preparation, with the travel speed being steadily increased to tail off the weld pool subsequent to extinguishing the arc.

If cracking occurs in the root run craters. the restart is made by initiating the arc on the already deposited weld bead about 6mm back from the crater. The weld pool. once formed, is then carried forward until refusion of the crater takes place.

Open butt joints Where open butt joints are welded, the use of a filler rod is necessary to bridge the root gap and thus form a penetration bead without sacrificing full fusion ofthe root faces. The technique used to produce the root run in open butt joints is termed the ‘keyhole’ technique. The keyhole is formed by melting the root faces until a definite hole is formed. O n formation of ---

1.8 Examples of weld pool shape: a) Complete penetration; b) Excessive penetration; c) Incomplete penetration.

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the keyhole, a filler rod is introduced into the arc to bridge the molten root gap. Once the bridge has been established between the root faces the TIG torch is moved forward just sufficiently to re-form the keyhole. Filler rod is again added to fill the hole, and is subsequently added in incre- ments each time the keyhole is formed, as welding progresses around the joint.

With open butt joints the arc, once it is extinguished, leaves an existing keyhole. The restart in these situations is similar to that described for close butt joints. Upon reaching the keyhole, care must be taken to avoid remelting the leading edge and thus causing it to become enlarged. A small increment of filler rod is deposited to fill the existing keyhole which is re-formed and filled accordingly to complete the root run. For the continuous filler rod method, however. re-forming the keyhole is unnecessary.

Welding with fusible inserts

Fusible inserts work on the principle that, once a weld pool has been formed, there are certain forces which control the molten pool. These forces which are associated with the surface tension of the material, are present on the underside of the pool as well as on the surface and tend to pull the pool upwards to minimise surface width.

Fusible inserts do not require the addition of a filler rod on the initial root run because their design is such that sufficient weld metal is formed during the completion of this weld run. They can also serve to avoid cracking in crack-sensitive material and porosity by providing a greater cross-section at the root and by the incorporation of vital metallurgical constituents and deoxidants.

‘EB’ insert This fusible insert was developed by the Electric Boat Company of America (hence the name ‘EB’) and is perhaps the most commonly used. It has. in cross section, the profile of a short shanked rivet head and is fitted and tack welded into the joint preparation, Fig. 1.9a. It is generally necessary to bore match the pipes to ensure an accurate tit-up.

Once the pipe bore has been adequately purged. welding is started within the critical area of the pipe joint, welding up first one side then the other. The TIG torch is held at right angles to the pipe tangent. The arc length, should be kept as ‘short’ as possible and never longer than 3mm.

Once the arc has been established and the weld pool formed. the TIG torch is weaved very slightly from side to side with a pause at each side wall. This motion is essential to distribute the heat across the shank of the insert to both halves of the joint.


Y-rype insert As its name implies, this insert has the shape of a Y and may be used only on V-type joint preparations, Fig. 1.9b. Good tit is essential for successful welding. To counteract any movement of the insert during welding, equally spaced tack welds of 6-12mm in length are made between the arms of the insert and the side walls of the joint preparation.

Once the pipe is purged, welding progresses from the critical area towards the top of the pipe, first on one half and then the other, with the TIG torch positioned at 90" to the pipe tangent. Excessive weaving of the TIG torch from side to side should be avoided, although some side-to- side movement of the torch is permissible provided it is within the extremities of the insert arms. Persistently wide weaving and any erratic movement of the torch will have a marked effect on the weld pool and associated penetration bead formation, and both will be of an irregular appearance.

'Grinnel' ring insert This insert has a rectangular cross-section with the overall appearance of a thin-walled washer. The ring is first tack welded to one side of the joint, compressed to eliminate gaps between the lateral face of the insert and the root face of the prepared joint, and then tack welded to its other prepared pipe joint, Fig. 1.9~.

The penetration bead can, to a certain extent, be regulated by the placement of the insert between the pipes. By positioning the insert eccentric to the centre line of the pipe, the upward portion of the insert

1.9 Insetis a) Conventional EB; b) Y-type; c) Grinnel ring.

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will counteract the gravitational pull of the weld pool across the critical area and so avoid the formation of a concave bead on the inside of the pipe.

The weld progression and TIC torch angle are essentially the same as those used for the EB and Y-type inserts. A short arc length is required with a slight side-to-side torch movement to ensure that the tongue of the insert is fused into the root of the joint preparation.

Influence of material on choice of technique

Carbon. low alloy and stainless steel On these materials the root run may be produced on closed and open V and closed U butt joints, but, because of the formation of high fluidity weld pools, closed V and U preparations are preferable. To facilitate ease of control over the root run without the addition of filler rod and to obtain the correct penetration bead shape, fusible inserts are technically desirable, Fig. 1.10.

Nickel, nickel alloys, and cupro-nickel When making the root run on these materials it is essential that copious additions of filler rod are added, the fused root containing at least 50% by volume of added filler rod. Failure to do this will inevitably result in porosity in the root run and. in some instances, weld cracking. As an alternative. however, root runs may be produced with fusible inserts instead of one of the conventional filler rod feeding techniques.

Aluminium alloys For the accomplishment of the root run AC argon TIG is the accepted method, but DC helium TIG may also be used. Of the two systems, the latter finds less acceptance on unrotated pipe, since the absence of any

1 2 3 Without fusible insert

QZuDam 1 2 3

1 With fusible insert

1.10 Penetration bead shapes obtained on: 1 - critical area; 2 - vertical quadrant; 3 -flat quadrant; and their positions in the pipe wall.


arc cleaning action. increased penetration characteristics, and difficulties with filler rod feeding makes the process more suited for welding rotated pipe.

Adequate access to the root of the joint is important because it allows the use of modified V and conventional U preparations, with no root gaps, to maintain small controllable weld pools. Flat lands and thin root faces, together with the addition of filler rod, are recommended to promote ‘rapid’ solidification of the weld pool as welding progresses so that the desired penetration bead profile can be obtained. The tiller rod has to be pushed into the weld pool to break the oxide skin on the inside of the pipe wall and thus promote the formation of a convex penetration bead when welding across the critical area.

Copper Because of the high heat sink produced. wide angle V preparations must be used. Close butt joints with U preparations, thin root faces and generous lands are also essential. This means that welding currents can be selected to control the heat input when making the root run, thus maintaining a controllable weld pool without too great a heat loss. Helium is widely used on wall thicknesses in excess of 6mm. because it enhances the arc cleaning action and. since a hotter arc is obtained, there is less need for overall auxiliary preheating. Filler rod is generally added in small incre- ments. For the V preparation, especially on heavier walled pipe (12mm). a root opening is often advantageous with the keyhole technique being applied.

Metal inert gas (MIG, MAG or CO,) welding

The MIG process provides a convenient and rapid means of producing root runs in process pipework. It has a considerably higher deposition rate than TIG with filler and does not suffer to the same extent from deslagging as necessary with MMA.

Two modes of metal transfer can be used with the all-positional MIG systems, the short-circuiting arc or ‘dip transfer’ condition and pulsed MIG welding. The former arc type is favoured for pipewelding in preference to pulsed MIG which. it is argued, does not afford the same weld pool control as that achievable with the short circuiting condition.

The direction in which the root run is made is an important factor for the achievement of an acceptable penetration bead. The root run is generally welded vertically-down from 12 to 6 o’clock. Vertical-up welding may also be used, but tends to produce a weld bead with a pronounced convex profile which is difficult to fuse adequately with filler runs. Weaving the gun produces a flatter bead surface, but also reduces the travel speed and

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increases heat input, which can result in burnthrough. However, it is feasible, depending upon material type and wall thickness, to weld in both vertical-down and vertical-up directions either singularly or in combination, Fig. 1.1 1.

The joint preparations which may be used are similar to those for MMA welding. A single V preparation is generally used on wall thicknesses up to 19mm; above this thickness compound bevels are preferred. Root gaps in each case are set slightly wider than those for MMA welding, particularly for vertical-down root runs. Typical joint preparations are shown in Fig. 1.12.

The diameter of the filler wire used will affect the root run. Welding with too thin a wire and too high a current/voltage combination causes defects. Conversely, too thick a wire with too low a current/voltage can result in a broad heat source and hence insufficient fusion takes place at the root of the joint.

For carbon steel an Ar + CO, mixture or CO, is normally used but in stainless steels a He + Ar + CO, mixture is to be preferred. For non-ferrous pipe, Ar or mixtures of Ar + He find a wide application.

%/ding technique

When the root run is started at the 12 o'clock position, the welder should ensure that the MIG gun is not held at too acute an angle to the pipe tangent, otherwise the gas shield will be deflected causing porosity.

To avoid excess penetration at the start of the weld, the gun is held at a trailing angle of 30". A trailing angle of 10-20" is then held until the 6 o'clock position is reached. The nozzle-to-work distance is held at approximately 9- 12mm throughout the welding operation. During welding the gun is weaved to bridge the root gap.

To avoid defective root runs the travel speed should always be such that the tip of the wire is arcing on to the leading edge of the weld pool.

1.1 I MIG/CO, welding techniques to produce penetration bead on fixed position process pipework.


The weld pool should not be allowed to flood the arc otherwise lack of root penetration/root fusion can occur. If the arc is allowed to impinge just ahead of the weld pool, usually caused by an increase in travel speed, a series of small lengths of wire known as 'whiskers' will protrude from the penetration bead inside the pipe.

Perhaps one of the principal difficulties to overcome is the lack-of- fusion defect which can easily occur at restart positions. To eliminate this defect the welder has to ensure that the restart position is feather ground. This also applies to tack welds, but the elimination of the incidence of crater and lack-of-fusion defects is extremely difficult.

I /

1.12 Typical joint preparations for MIG/CO, welding of process pipework. Pipe walls: a) Up to and including 19mm; b) Over 19mm.

20 K R Spiller

Conclusion

The most important factors concerning welding process options and manual techniques for producing penetration beads in process pipework have been discussed.

All three processes considered are capable of producing controlled penetration beads on unbacked. unrotated pipe butt joints provided the relevant techniques are correctly applied.

Where the highest quality and smooth penetration bead profiles are required, the TIG process with filler rod or alternatively with fusible inserts is best for meeting these criteria on both ferrous and non-ferrous pipe.

Where the quality becomes less stringent, the MMA process with basic. rutile and cellulosic electrodes using vertical-up and vertical-down techniques or the MIG/C02 process with solid wire are both capable of satisfying most code requirements depending on the material to be welded.

Further reading BS 2633: 1987:

BS 4677: 1984:

BS 2971: 1977:

BS 4515: 1984:

ANWASME B31.3: 1987:

Class I arc welding of ferritic steel pipework for carrying fluids.

Class I arc welding of austenitic stainless steel pipework for car-

Specification for class 11 arc welding of carbon steel pipework for

Process of welding steel pipelines on land and offshore. Pub1

Chemical plant and petroleum refinery piping. Pub1

Pub1 British Standards Institution. 1987.

rying fluids. Pub1 British Standards Institution. 1984.

carrying fluids. Pub1 British Standards Institution. 1977.

British Standards Institution. 1984.

American Society of Mechanical Engineers. 1987.

Chapter 2

M G M U R C H

Mechanised arc welding process options for pipework fabrications

Mechanised arc welding processes are increasingly being exploited for pipework fabrications in austenitic and femtic steels. The use of mechanised gas shielded arc welding stems from a requirement to obtain a more consistent weld quality in terms of both joint integrity and uniformity of weld surface profile. Further benefits to be expected are improved reproducibility with a reduced rejection level and, where possible, higher joint completion rates than are normally obtainable from manual welding.

Considerable development has occurred in power source technology and orbital welding equipment design based on the two basic gas shielded arc processes: tungsten inert gas (TIG) and metal inert gas (MIG) welding. Derivatives of the TIG system include the development of 'hot wire' and plasma arc welding which may be considered as separate processes. In MIG welding, pulsed operation with feedback control such as the recently developed synergic system' has greatly enhanced the suit- ability of the process for pipework applications. The advent of solid state switching applied to thyristor- and, in particular, transistor-regulated power supplies has led to precision setting of the pulse current level and duration in pulsed MIG welding.

The mechanised welding process options available for the fabrication of butt joints include:

0

0

0 Hot wireTIG: 0 Pulsed MIG.

Undoubtedly the root run is the most critical area in process pipework and, to achieve satisfactory results. special attention must be given not

Steady state and pulsed current TIG; Steady state and pulsed keyhole plasma:

21

22 M G Murch

only to the choice of welding process but also to the pipe end preparation. The position of welding, i.e. rotated or fixed pipe (orbital welding), will place constraints on the selection of available process options. Further limitations may occur through restricted access around fixed pipes, necessitating a welding system which is capable of performing within a small operating envelope.

This chapter reviews the application of these mechanised gas shielding arc welding process options and details some of the more recently developed beneficial features of equipments suited to the production of high integrity butt welds in pipe.

Pipe joint geometry

It is often satisfactory for manually welded pipe of wall thickness above 3mm to use a single V preparation of 60-70" inclusive angle. A root face of 2-3mm with a root gap of up to 3mm is conventional practice in manual metal-arc (MMA) welding. However, in T IC welded pipe butt joints it is usually necessary to provide a purging gas to the underside of the joint to prevent oxidation of the penetration bead. This precludes the use of root gaps and, in orbital welding, necessitates the use of a J type preparation for the control of root fusion.

In mechanised orbital welding the degree of latitude on joint geometry variation is very limited: typically less than 0.2mm on root face and preparation land dimensions. Joint geometry within these tolerances can be obtained using commercially available pipe end preparation machines.

A square edge preparation is normally used for thin wall pipe <2.5mm when TIC welding; however, exceptions exist, such as unkilled or semi- killed steels and copper-nickel alloys, where a filler is added to prevent porosity or weld metal cracking. In these materials a small bevel preparation is often used to accommodate the addition of filler wire and to increase the dilution in the weld pool.

TIG we Id i ng

The local intense TIC welding arc provides a process ideally suited to the control of weld pool formation in pipe butt joints.2 Weld pool control can be further enhanced by low frequency or thermal pulsing of the current sequentially between high and low levels. Filler wire may be added independently to the molten pool and can be used to improve the formation of the penetration bead in the pipe root. The advantageous features of TIC welding have been applied over many years and the process has become the logical choice and most favoured system for root run control.

Mechanised arc welding process options 23

A range of manufacturers' equipment especially designed for TIG orbital welding is available. An example of a track-mounted open head type orbital welding system with a full range of facilities such as arc voltage control. lateral weave, end dwells, synchronised pulsed current, wire feed and rotation capabilities is shown in Fig. 2.1.

Root run technique

In mechanised TIG welding, several techniques are available for the control of root fusion, and the procedure selected will be influenced by the requirements of joint integrity, penetration protile and joint completion rate. Butt welds in rotated tube with the torch in the fixed vertical (ASME 1G) position can be fabricated using autogenous welding as square edge preparations on tubes with <3mm wall thickness material. For tube walls above 3mm a simple 70-90" V preparation may be used and welded with the addition of filler wire. To improve the uniformity of root penetration, weld inserts can be preplaced and fused without adding tiller wire: this method is often employed for the most critical applications.

For orbital welding with the pipe fixed in the horizontal plane (ASME 5G). more complex joint preparations are required for root fusion control. Typical defects which may be experienced are:

2.1 Example of a sophlsticated track-mounted orbital welding system with comprehensive welding parameter control.

24 M G Murch

0 Incomplete root fusion: 0 Uneven root penetration profile: 0 Root penetration concavity (suckback).

Susceptibility to the formation of these defects can be minimised by giving attention to joint geometry and welding procedure details. In particular. the most satisfactory joint preparation for wall thicknesses above 3mm is the modified J type with a root face of <2mm and a total land width of 5mm between sidewalls to enable root fusion to be obtained in all positions of welding. Uniformity of root penetration is promoted through current pulsing at around 2Hz frequency; fusion takes place during the higher current level pulses followed by partial solidification of the weld pool on sequential switching to a lower current or pilot arc. Surface tension and weld pool fluidity have a marked effect on the formation of root bead concavity which is most likely to occur in the overhead position in orbital pipe welds. To minimise this effect i t is necessary to limit the weld pool size and maintain it within the land of the butt joint preparation geometry.

Conventional orbital TIG welding procedures using pulsed current and continuous wire feed can provide a positive penetration bead in all welding positions in carbon steel joints. However. when welding austenitic steel tubes. the fluidity of the weld pool increases the risk of root suckback and additional control measures e.g. incorporating a pulsed wire feed synchronous with the background current have been found beneficial for the following reasons:

0 When no filler wire is added during the high current pulse cycle. the minimum arc current level and hence smallest weld pool size can be used to obtain penetration: The addition of filler wire in the background cycle increases the amount of weld pool solidification between high current pulses: The weld penetration bead is supported after its formation through the pressure applied to the pool surface by the addition of filler wire in the background period.

0

Hot pass control The hot pass refers to the second run of a multipass weld. where sufficient arc current is required to achieve good sidewall fusion while at the same time avoiding repenetration through the root run. To satisfy these criteria, synchronised pulsing combined with end dwells on cross seam oscillation can be used to control the fusion characteristics. During the oscillation periods a low current level is selected to provide controlled fusion into the root pass. To ensure adequate sidewall fusion. an increased current or pulse is applied at the extremity of the oscillation width and synchronised with


the end dwells. This procedure is advantageous for fusion control of the hot pass in both ferritic and austenitic steels.

Weld joint completion

The technique and welding procedure required for fill and capping runs after the second pass in orbital welding are relatively simple. In the 5G position, weaving may be carried out at up to 15mm lateral width and end dwells used to control deposit shape. Complex pulsing of current and wire feed, in combination with oscillation end dwells, is not required; indeed. very satisfactory results are obtained with continuous wire feed and steady state current. To allow for pipe ovality and weld bead contour irregularities. the use of arc voltage control assumes increasing importance in maintaining the correct arc gap by controlling the torch to weld pool surface distance.

Examples of completed orbital pipe welds in carbon steel of lOOmm nominal bore (NB) X 6mm wall and austenitic steel of 150mm NB X 8mm wall are shown in Fig. 2.2. Transverse weld sections from the overhead position are shown in Fig. 2.3a and b for the carbon and stainless steel joints respectively.

2.2 Appearance of completed orbital TIG welds in 100mm NB carbon steel and 150mm NB stainless pipe.

26 M G Murch

2.3 Typical pulsed TIG weld cross sections from the overhead position of 6 and 8mm wall thickness pipe: a) Carbon steel; b) Stainless steel (x5).

Plasma welding

In the plasma welding system for fabricating butt joints in steel. a DC arc is formed between the tungsten electrode positioned in the body of the torch and the ~ o r k p i e c e . ~ A copper nozzle with a small bore orifice con- stricts the plasma-forming gas and separates it from the shielding gas envelope. The significance of plasma constriction is that the arc becomes very directional with deep penetration capabilities. Argon is normally used as the plasma gas, but argon, helium, or a mixture of argon-hydrogen may be used in the shielding envelope.

The plasma welding process has three distinct operating modes resulting from varying the plasma gas flow and current levels:


0 Microplasma welding: 0.1-15A at 0.2-0.5 litre/min plasma gas flow: 0 Medium current plasma welding: 15-70A at 0.5-1.5 litre/min plasma

gas flow: 0 Keyhole plasma welding: >70A at 1.5-3.5 litre/min plasma gas flow.

For pipework fabrications the plasma keyhole operating mode offers several distinct advantages over TIC welding. As shown in Fig. 2.4. the unique characteristic of the arc which passes completely through the material guarantees full penetration.

In this operating mode the keyhole is maintained continuously and the molten pool is formed behind the arc. The arc force is sufficiently power- ful to enable pipe up to 8mm wall thickness to be welded in a single pass. Filler wire is added to prevent undercut along the edges of the top bead and also to provide positive reinforcement. The welding speed on wall thicknesses above 4mm is typically over three times that achieved in TIC welding. Further benefits are gained by savings in joint preparation time where. in this operating mode. a simple square edge geometry is used.

Continuous current ke*o/e plasma

As the continuous steady state current ihe keyhole plasma process can only be applied to rotated pipe. it is limited to joints welded in the flat

2.4 Keyhole plasma and efflux plasma formatlon in welding 5.0mm stainless steel.

28 M G Murch

( 1G) or horizontal-vertical (2G) positions. However. joint completion rates are high and in this operating mode the bead surface profile is both smooth and uniform. A characteristic of keyhole plasma is the fusion contour which is broad at the top surface and narrow at the root. typically likened in shape to the bowl and stem of a wine glass. The surface appearance of a butt weld in stainless steel of 50mm NB X 4mm wall. with filler wire addition, fabricated completely in an arc time of 64 seconds. is shown in Fig. 2.5a. and a transverse weld section demonstrating the fusion shape is shown in Fig. 2%.

Pulsed keytrole plasma

In pulsed current operation the capabilities of keyhole plasma are extended to all positions of welding including vertical and overhead. However. the process is restricted to welding in a fixed position unless a programmable controller is used, as the welding parameters (plasma gas flow and current level) are required to accommodate the positional changes which occur in orbital welding. The advantage of pulsed operation is that a wider keyhole is formed with the beneficial effect of increasing the tolerance to joint fit-up variations and seamhorch alignment.

The travel speed in the pulsed mode is generally lower than in continuous current operation as each successive pulse must overlap the preceding pulse by 60-70% to guarantee underbead continuity. Thus a butt joint in stainless steel of 50mm NB X 4mm wall with filler wire added requires 108 seconds arc time to complete compared with 64 seconds for continuous current plasma. The characteristic weld surface ripple obtained in pulsed operation can be seen in the completed butt joint. Fig. 2.6a. and the transverse weld section shown in Fig. 2.6b demonstrates an increase in root penetration width.

TIG hot wire welding

The TIG hot wire process was developed in the mid 1960s4 as a method of achieving high metal deposition rates together with a weld integrity normally associated with conventional TIC welding. In its original concep- tion the system used two power sources. a DC supply for arc current and an AC supply for resistance heating of the wire. The system produced minimal interaction between the arc current and the wire. The filler wire which is heated to near its melting temperature enters the weld pool behind the TIC arc.

More recently. a commercial hot wire system has become available employing a single DC power supply which shares its output between the arc and the resistance heated wire, as represented schematically in


2.5 Constant current keyhole plasma weld in 50mm NB, 4mm wall thickness stainless steel pipe: a) Completed weld; b) Cross section (x10). Typical welding parameters: plasma gas - Ar; plasma gas flow rate - 1.8 litre/min; plasma current - 13OA; welding speed - 0.20 m/min.

30 M G Much

2.6 Pulse current keyhole plasma weld in 50mm NB, 4mm wall thickness stainless steel pipe: a) Completed weld; b) Cross section (XIO). Typical welding parameters: plasma gas - Ar; plasma gas flow rate - 1.8 litre/min; plasma pulse current - 1451% plasma background current - 25% pulse frequency - 1.3Hz; welding speed = 0.12 m/min.


High speed wire feeder

Travel , 7 Afc~y;,;wife Weld

Weld pool

2.7 Electrical system and torch arrangement for a single DC supply TIG hot wire process.

Fig. 2.7. The main advantages of this system are reduced capital equipment costs, small physical size (comparable with conventional TIG equipment) and ease of operation.

For circumferential pipe joints, the hot wire variant which operates in the continuous current mode is restricted to welding in the flat position on rotated pipe. It is used principally on pipe wall thicknesses greater than 6mm where a deposition level similar to that of MIG welding is beneficial. A typical example of a transverse section through a weld in 8mm thick wall Inconel 625* pipe, which was welded in three passes, is shown in Fig. 2.8. In comparison, welding pipe of the same wall thickness with conventional TIG would require six passes.

Pulsed MIG welding

The short circuiting MIG arc welding technique in mechanised operations has been applied to orbital welding of line-pipe over a number of years. However. because of a susceptibility to lack-of-fusion defects, this operating mode has not found general acceptance for process pipework. For rotated pipe fabricated in the flat position, the conventional spray and pulsed

* Trade name

32 M G Murch

open arc metal transfer modes can be used for high deposition filling and capping weld runs.

Recent improvements in the control of pulsed welding through transistor- regulated power supplies has substantially increased the potential of this operating mode to include a positional welding capability in process pipework. The technology of transistor control represents a significant advance, allowing precision setting of welding parameters together with fast switching between high and low arc current values. Systems incorporating feedback control to regulate welding current and arc voltage levels for any given wire feed speed have greatly enhanced the arc and metal transfer stability in pulsed MIG welding.

Pulsed MIG welding systems based on a transistor-controlled power supply and specifically designed for orbital welding are commercially available. Equipment comprises a MIG welding gun and orbital head mounted on a fixed ring positioned around the pipe. The MIG welding gun is supplied from a constant current transistor power source. In operation, the facility of rapid response from the transistor power supply is used to provide a pulsed frequency up to 999Hz. As the pulsed frequency is increased, the shape of the arc column changes from a broad soft fan at <450Hz to an increasingly narrow and directional stiff arc above 450Hz. This feature is used to provide accurate placement of the weld deposit together with a controlled fusion width at low average welding current levels. Low frequency or thermal pulsing from 0.5 to 20Hz (similar to that practised in TIG welding) can be superimposed on the higher frequency pulsed current waveform to further improve both fusion characteristics

2.8 TIG hot wire weld cross section from a 42mm OD, 8mm wall thickness lnconel 625 pipe (x5).


and weld pool control. Independent adjustment of pulsed parameters, together with weaving and travel operating variables, are all programmable on a remote pendant.

In orbital welding on a pipe fixed in the 5G position, root runs fabricated with this system are welded vertically down both sides of the butt joint with the weld start in the flat position. The fill and capping weld runs are started in the overhead position and traversed vertically up to finish in the flat position. High deposition rates and good joint integrity are claimed features of pulsed MIG operating systems.

Future trends Although it is unlikely that a radical process development will occur in gas shielded arc welding, the present mechanised systems described for welding process pipework can be improved in detail. Mechanised T I C arc welding systems will increasingly be applied to process pipework where high integrity joints are demanded. Exploitation of the small physical size of the weld torch and comparatively simple process parameter control will ensure the dominant position of TIC in orbital welding, particularly in situations of difficult access and where space between pipes is limited. Improvements in orbital equipment reliability, especially wire feeding, are most likely to occur through extended use. The correct positioning of filler wire in the molten pool can be difficult as most orbital TIC systems do not have sufficient adjustment to compensate for the wire spiralling as delivered from the spool, and here the use of wire-straightening rolls would be beneficial.

Increasing application of the plasma keyhole welding process is probable, where single pass welds on simple square edge preparations in butt joints of up to 8mm wall thickness can be produced in rotated pipe. The use of this process has been investigated at The Welding Institute for orbital welding of pipe fixed in the horizontal plane. Here the benefits of microprocessor control to facilitate ad.justment of the major welding parameters (welding current and plasma gas flow) for all positions around the pipe will provide, in the near future, a usable process option for the fabrication of fixed pipe.

The TIC hot wire system for welding rotated pipe of thick wall section merits further application. With a process capability for giving welded joints of high integrity. similar to that obtainable with cold wire TIC welding but at approximately three times the joint completion rate, the use of this system is likely to increase. In particular, through process development, simplification in operation, and a reduction of both equipment cost and physical size will enhance the attractions of the process.

Pulsed MIG welding. with the benefit of improved process parameter

34 M G Murch

control arising from the use of transistor-regulated power supplies, has an all-positional capability in pipework fabrications. The features of high deposition rates together with superior fusion characteristics and weld pool control over other MIG operating modes have greatly increased the potential of this process. However, further development in the control of root fusion and in improving the weld start/stop overlap is required for it to gain increasing acceptance for the fabrication of process pipework.

References 1 Allum C J: 'MIG welding - time for a re-assessment'. Merd Cor?srnrcrior? 1983

2 Lucas W: 'TIG and plasma welding'. Abington Publishing, 1990. 3 Halko K H: 'Plasma arc welding 2ViCr-IMo tubing'. WeldingJ 1978 57 (5) 23-

4 Manz A F: 'Hot wire welding and surfacing techniques.. WRC Bull January

15 (6) 347-350.

31.

1977 (223).

Chapter 3

W L U C A S

Process techniques and equipment for mechanised TIG welding of tubes

In recent years there has been a trend to replace manual techniques by mechanised welding systems. Whilst a welder can accommodate variations in joint fit-up and tube end preparation, mechanised techniques offer the potential for a more consistent level of weld quality. However, it must be emphasised that mechanised welding procedures are more inflexible than manual procedures. For this reason, process techniques and control systems have been developed to improve the performance and tolerance of the TIG process to material and component variations. The range of welding equipments available commercially today reflects the advances that have been made in recent years to improve equipment performance.

Simple mechanised welding equipment merely has the capability of rotating the electrode around the tube and the operator is required to make periodic adjustments to the welding current, electrode position and the arc length. The more advanced microelectronic controlled welding heads and power sources employ techniques such as arc oscillation and current pulsing to improve the control of the arc and weld pool behaviour. However, whilst such systems can achieve a higher level of quality and can accommodate automatically greater variations in the components. they are substantially more expensive and considerable operator training is required to derive the benefits.

In this chapter. the process techniques which can be used in mechanised TIG welding of tube to improve process performance will be described. The range of welding equipments available commercially will be categorised in terms of their operating features. Typical welding procedures and applications will be used to show the type of applications best suited for the various categories of equipment.

35

36 W Lucas

Process techniques

General techniques

Whenever possible mechanised TIC welding of tube is carried out in the 1G welding position. i.e. with the tube rotated beneath a fixed welding torch. When it is not possible to rotate the component. specialised equipment is available to rotate the torch around the tube. but this not only imposes restrictions on the range of welding parameters used to control the welding operation, but also requires greater care in the setting up and the alignment of the welding system.

Because of the reduced flexibility of mechanised welding, it is necessary to select the joint preparation and to employ process techniques which will increase the tolerance to material and component variations which inevitably occur in production.

Joint preparation

The joint preparations which can be used in tube welding are shown in Fig. 3.1. Below 3mm wall thickness, the tubes can be welded with the simple square edge. close butt joint configuration (Fig. 3.la). A simple V preparation (Fig. 3.lb) is used for welding tubes of wall thickness >3mm in the IG position. The U preparation (Fig. 3.lc and d), although more expensive to machine will facilitate control of the weld pool e.g. when welding in the 5G position. The U and narrow gap preparation (Fig. 3.10 will reduce the number of weld passes, particularly in thicker wall thickness material. Irrespective of the joint preparation employed. the tube end should be prepared by machining and not manually prepared; typical tolerances for the joint preparation are shown in the figure.

A consumable insert (Fig. 3.le) can be used to enable the tolerance on component fit-up to be relaxed although additional care will be required in fitting the insert before welding: the additional material from the insert will help to prevent suck-back when orbital welding especially in the 6 o'clock position. There are different types of inserts but the EB (Electric Boat) insert is probably the most popular for tube welding as i t facilitates positive location of the tube ends before tacking and welding.

Current pulsing

The pulsed current technique has several advantages in welding tubes especially with regard to improving tolerance to material, component and production variations.' The basic principle of the technique is that a high

Process techniques and equipment 37

current pulse is applied which causes rapid penetration of the material. When the desired degree of penetration has been achieved (for example, full penetration in the root pass or fusion of the sidewall and the melting of the tiller wire in the filler passes) the current is switched to a low background level to allow the weld pool to solidify. Thus welding proceeds in a series of overlapping spot welds, Fig. 3.2, which have an overlap of approximately 60%. Typical pulse current levels and times for various thicknesses of stainless steel are shown in Fig. 3.3.

Specific advantages of the pulsed operation are as follows:

It aids control of weld pool penetration which is beneficial when welding to the 5G position in ensuring a positive penetration profile: It increases the tolerance to material and production variations (especially tube end dimensions and joint fit-up):

0

70 to 90' Y-----7

I I f 3 m m {

l a )

30 to 40' r-----v

( b ) 0 tol.5mm

30 to 60' rn

10' It-

(e l 3mm 2.0 to 2.5mm

--l+ ( d ) 2 t o 3 m m

I P5"

3.1 Typical joint preparations used in welding tubes.

38 W Lucas

3.2 Pulsed TIG weld In stalnless steel (note, the pulses have been separated to demonstrate the principle of overlapping spot welds).

Pulse tirne,sec 0.1 0.2 0.3 0.L 0.5 0.6 0.7 0.8 0.9

Traverse rnm / sec

speed,

50 160 150 200

0

\ '

Penetration depth,rnrn 1

Pulse current, A

3.3 Nomograph as an aid to the selection of pulse parameters in TIG welding austenltic stainless steel.


0

0

0

It reduces the sensitivity to a heat sink disparity, for example, when welding tubes with different wall thicknesses: In specific instances, it will reduce the sensitivity to cast to cast. material variations’; In certain materials e.g. nickel alloys. it can reduce the sensitivity to surface oxides.

It should be noted. however, that the pulsing technique will inevitably reduce the welding speed as the weld pool is allowed to solidify between pulses. However. in mechanised welding this may not be significant as the setting up of the equipment, assembly and pre-tacking of the tubes will account for a large proportion of the total welding time.

Pulsed wire

Pulsing the wire feed in synchronism with the pulsed welding current can be an effective means of improving control of the welding operation. The wire can be fed either during the pulse or during the background period. If the wire is fed during the background period only, rapid freezing of the weld pool will occur which has the following advantages:

0 The pulse welding current can be set at a sufficient level to give penetration and no additional current is required to melt the tiller wire;

0 The weld poot is pushed through and supported whilst it freezes.

The disadvantages of the pulsed wire technique, especially feeding during the background period, are that the welding procedure is more difficult to set up and greater operator training is required.

Electrode oscillation

The simplest welding heads provide only for rotation of the torch around the joint. Whilst this is more than adequate for welding thin wall tubing which requires a single pass, it is somewhat inflexible for welding thicker wall tubes requiring several passes. The advantages of oscillating the welding head compared with a stringer bead technique are as follows:

0 The welding procedure is more tolerant to variations in the joint preparation and tube fit-up:

0 The procedure is easily set-up and operated in production; a substantial degree of operator skill is required in multi-run stringer welding and the operator must adjust the position of the welding head to follow the contours of the previously deposited weld bead; Fewer welding runs are required to fill the joint; for example. in 0

40 W Lucas

25"

2 rnm radius

1

i face

. .

3.4 Sections through tube welds in 60mm OD and 5.7mm wall (2in NB Schedule 80) tvpe 304 stainless steel pipe in the 5 6 position: a) Joint preparation; b) Weld with stringer bead; c) Weld with electrode oscillation.


welding a 5.7mm wall, type 309 stainless steel tube, by weaving the electrode, the number of filler passes could be halved from eight to four passes, Fig. 3.4.

Oscillation of the arc is not normally applied in the root run but weaving in tiller passes is strongly recommended to improve sidewall fusion. etc. Oscillation frequency is typically between 0.5 to 2Hz which is matched to the welding speed to give a smooth weld bead appearance.

Electrode oscillation wifh end dwells

In addition to electrode oscillation. the momentary dwelling of the head at the extremes of oscillation promotes fusion into the sidewall. The duration of the end dwell period is typically between 0.5 and 1.0 sec. In multi-run welds of thicker wall section tube. this technique helps to reduce the risk of lack-of-sidewall-fusion defects.

In the most sophisticated systems, the control system enables the welding current to be pulsed in synchronism with the weld head weaving. By applying a high current pulse at the end of the weave or during the end dwell period. the risk of sidewall-fusion defects can be further reduced.

Welding equipment A wide range of welding systems is available commercially which differs in the process features and operating characteristics available. The systems can be conveniently grouped into one of the following three categories:

Basic function; Intermediate function;

0 Full function.

The principal operating features of the three categories are described with examples of where these features have been exploited to improve weld quality or performance.

Basic function system

A simple, basic function system is shown in Fig. 3.5, and comprises a thyristor controlled power source. The control system has a single level programme, but has facilities for pulsed welding current, pulsed wire feed and stepped head rotation. The most important limitation of the basic function system is that the operator is restricted to a stringer bead welding technique. The advantages of this type of system for welding tubes are:

42 W Lucas

0 When welding thin wall tubes which require a single pass without filler wire. a completely sealed head can be used: gas shielding will he the most efficient with this type of head. As the welding head is very compact, i t is ideal for welding a nest of tubes which will offer only restricted access for the welding head.

Thus. the basic function systems are ideally suited to welding thin wall section tubes which require a single pass and no additional filler. As shown in Fig. 3.6a. the welding head is very compact and the gas cup, which completely surrounds the tube, provides excellent protection of the

0

3.5 A typlcal b a s k tunctlon tube welding head.


weld pool. For this reason this type of system is preferred for welding reactive metals such as titanium.

Basic function systems are also widely applied for simple mechanised welding of thicker wall tube, but as described above, the operator will be required to set the position of the stringer runs and to make periodic adjustments during welding. A filler wire must be used to fill the joint, the head must have the capacity to feed filler wire. A typical head for

3.6 Welding heads for a basic function tube welding system: a) Enclosed head (courtesy of Huntingdon Fusion Techniques Ltd); b) Non-weaved head but with wire feed (courtesy of ESAB Ltd); c) Typical application in weidtng boiler tubes (courtesy of Foster Wheeler Power Products Ltd).

44 W Lucas

multi-pass welding is shown in Fig. 3.6b and a typical application in Fig. 3 .6~ ; the welding procedure for this application is shown in Table 3.1 and here it can be seen that a simple V preparation was used but current pulsing was required to control the weld pool.

When selecting a welding system for welding thick wall tubes. it should be noted that this type of system (non-weaved) will necessitate more welding runs. reduced tolerance to variation to joint dimensions and fit-up and increased weld contraction may cause excessive bore constriction. To improve process tolerance, especially when welding in the 5G position, a power source with current pulsing is strongly recommended.

lnfermediufe function system

The intermediate function system provides the means for electrode oscillation and may contain other features such as multi-level programming of the welding current around the tube and automatic arc length control.


Table 3.1 Typical joint preparation and welding parameters for welding SOmm (2111) OD. 4mm (0.16in) wall thickness, carbon steel pipe in the flat (SG) position (courtesy of Foster Wheeler Power Products Ltd)

0.8 mm (0.03 in) max -I& 1 st pass 2nd pass

Current . peak

Pulse . peak

Slope , in

Rotational speed Wire feed speed Wire size. diameter

, background

. background

. out

A A S S

S

S

slrev mm/min mm

I10 60 0.2 0.5 2 8

I so 26-30 0.8

160 15 0.2 0.5 2 8

1 50 26-30 0.8

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A sophisticated intermediate function system is shown in Fig. 3.7 which has an extensive range of functions including arc voltage control. cross seam oscillation with end dwells together with synchronisation of pulsed welding current. pulsed wire feed and stepped welding head rotation. As shown in Fig. 3.8a, the disadvantage of this type of welding head is that it is bulkier compared to the basic function head (Fig. 3.6b).

In an attempt to keep the head size small. a small electromagnet is available commercially which bolts on to a basic function welding head:

3.7 Intermediate function welding system (courtesy of ESAB Ltd).


3.8 Wlding heads for the intermediate function welding system: a) Mechanical weaved system; b) Magnetic oscillation attached to non-weaved head.

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Comparison of costings

manual weave*

the mounting of the electromagnet adjacent to the electrode is shown in Fig. 3.8.’ The electromagnet is used to provide arc oscillation and end dwells, similar to mechanical oscillation. but at a substantially lower cost. Electromagnetic oscillation also allows a narrow gap joint preparation to be used without the risk of the electrode striking the sidewall.

Intermediate welding systems (with either mechanical or electromagnetic oscillation) can be used to reduce the number of passes which would otherwise be required with a stringer bead procedure. Typical welding procedures are shown in Tables 3.2-3.5 for 60mm OD, 5.7mm wall, type 304 stainless steel tube.

Because of the reduction in the number of filler passes. intermediate function systems have economic benefits over the basic function systems. As shown in Fig. 3.9. despite taking into consideration the increased cost of the arc weaving equipment, the cost in terms of depositing a metre of weld metal is considerably less for a weaved procedure compared with a stringer bead procedure; the savings are derived almost exclusively from the reduction in the labour cost. It should also be noted that the manual (weaved) procedure cost is, also slightly greater than the mechanised procedure because of the frequent stophtarts as the welder re-positions around the tube, picks up a new length of wire, etc.

f

WELDCOST Gas cost 0 Labour cost

The Welding Institute 0 Wire cost Plant cost

117.52

95.30

83.67

7 1.92

J prep

Narrow-gap preparation

3.9 Summary of cost per metre of weld for the five orbital IG process variants, taking into account the purchase of equipment (from The Welding Institute’s WELDCOST program).


Table 3.2 Optimised root pass parameters for 60mm OD X 5.7mm wall (2in NB Schedule 80), Type 304 stainless steel pipe in the 5G position

Welding parameter Unit Value

Pulsed peak current Pulse time Background current Background time Motor delay Slope up Slope down Rotation speed Insert size Shielding gas flow rate Purge gas flow rate

Electrode type Electrode diameter Electrode angle Electrode polarity Shielding gas Purge gas

2% thoria 2.4mm 60" DC- Argon Argon

A sec A sec sec sec sec sedrev mm I/min l/min

88 0.8 26 0.4 7 1 6 160 2.4 7 3

~~ ~

Table 3.3 Optimised parameters for the stringer bead filler passes, J preparation

Welding parameter Unit

Wire diameter mm Wire feed rate m/min Pulsed peak current A Pulse time sec Background current A Background time sec Motor delay sec Slope up sec Slope down sec Rotation speed seclrev Shielding gas flow rate I/min Purge gas flow rate I/min

Electrode type 2% thoria Electrode diameter 2.4mm Electrode angle 6 0 O

Electrode polarity DC- Shielding gas Ar-I%H, Purge gas Argon

Pass number

1

0.8 0.26 72 0.8 29 0.4 5 1 6 160 7 3

2

0.8 0.26 76 0.8 30 0.4 5 I 6 160 7 3

- 3 4 5 6 7

0.8 0.8 0.8 0.8 0.8 0.31 0.31 0.25 0.21 0.13 98 100 100 100 100 0.8 0.8 0.8 0.8 0.8 4 0 4 0 4 0 4 0 4 0 0.4 0.4 0.4 0.4 0.4 5 5 5 5 5 1 1 1 1 1 6 6 6 6 6 160 160 160 160 160 7 7 7 7 7 3 3 3 3 3

----- 8

70 0.8 28 0.4 5 1 6 160 7 3

50 W Lucas

Table 3.4 Optimised parameters for the filler passes. using magnetically controlled arc oscillation, J preparation

Pass number

Parameter Unit

Welding Wire diameter mm Wire feed rate mlmin Welding current A Motor delay sec Slope up sec Slope down sec Rotation speed sechev Shielding gas flow rate llmin Purge gas flow rate Ilmin Magnetic Frequency Hz End dwell SeC

Field strength. electrode tip gauss Field strength, magnetic Probe tip gauss

Electrode type 2% thoria Electrode diameter 2.4mm Electrode angle 28" Electrode polarity DC- Shielding gas Argon Purge gas Argon

0.8 0.8 0.80 0.85 89 92 5 5 1 1 6 6 140 140 1.5 1.5 3 3

1.35 1.35 0.36 0.36 26 22

130 110

3 4

0.8 0.85 96 5 1 6 140 1.5 3

1.4 0.35 15

76

0.8 0.1 91 5 1 6 140 1.5 3

1.5 0.33 100

519

Table 3.5 Optimised parameters for the filler passes. using magnetically controlled arc oscillation, narrow gap preparation

Pass number

Parameter Unit 1

Welding Wire diameter Wire feed rate Welding current Motor delay Slope up Slope down Rotation speed Shielding gas flow rate Purge gas flow rate

mm mlmin A sec sec sec seclrev Ilmin I h i n

0.8 0.11 96 5 1 6 145 1.5 3

2 3 4 --

0.8 0.8 0.8 0.11 0.17 0.44 96 96 68 5 5 5 1 1 6 6 6 I45 145 145 1.5 1.5 1.5 3 3 3


Table3.5cont’d

Magnaic

End dwell sec 0.35 0.35 0.35 0.33 Field strength, electrode tip gauss 20 20 20 123 Field strength. magnetic probe tip gauss 103 103 103 648

Electrode type 2% thoria Electrode diameter 2.4mm Electrode angle 28O Electrode polarity DC- Shielding gas Argon Purge gas Argon

Frequency Hz 1.4 1.4 1.4 1.5

Full function system

Full function welding systems contain additional features to electrode weaving such as:

0 Multi-level programming; Automatic arc length control;

Pulsed wire feed; 0 Pulsed travel.

Thus, the welding head and the power source are substantially more sophisticated than the previous two. An example of the type of system is shown in Fig. 3.10a which has a track mounted welding head, Fig. 3.10b. The transistor controlled power source has the facility for pulsing the welding current in synchronism with either the peak or background level, pulsed wire feed, end dwells and stepped head rotation.

In addition to the economic benefits of electrode weaving, the full function system allows the following welding techniques to be exploited to improve control over the behaviour of the weld pool:

In the root pass, pulsing the wire feed with the wire added during the background period promotes rapid freezing and control of the weld pool; in larger diameter pipes, e.g. >60mm, pulsed wire feed can be used to prevent root penetration concavity particularly in the overhead segment of the pipe: In the hot pass, synchronised pulsing of the current so that a high current level is applied during the end dwell period improves sidewall fusion: the synchronisation of the low current period on the centre of the joint reduces the risk of re-penetration through the root pass;

Pulsed current synchronised to the electrode weave:

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3.10a) Full function welding system; b) Welding head.


Table 3.6 Optimised TIG welding parameters for 168mm diameter pipe, using the full function welding system

Run No. 1

Wire diameter, mm Wire feed rate. m/min Electrode type Electrode diameter. mm Electrode polarity Pulsed peak current. A Background current. A Peak current duration. sec Background current duration. sec Mean welding current. A Weave width. mm Weave frequency, Hz End dwell, sec Shielding gas type Shielding gas flow rate. I/min Backing gas type Backing gas flow rate, I/min

0.9 0.8 1 2% thoria 3.2 d.c.- 160 80 0.2 0.5 103

Argon 7.0 Argon 9.0

2

0.9 0.89 2% thoria 3.2 d.c.- 140 90 0.2 0.5 104 4.54 0.63 0.40 Argon 7.0 Argon 9.0

3 4

0.9 0.76 2% thoria 3.2 d.c.-

130 6.30 0.53 0.55 Argon 7.0 Argon 9.0

0.9 1.06 2% thoria 3.2 d.c.-

120 8.82 0.43 0.75 Argon 7.0 Argon 9.0

0 When filling the joint the use of arc voltage control is especially useful as the arc length can be maintained despite the pipe ovality and variations in the weld bead contour.

The special techniques afforded by these additional control features will no doubt facilitate the production of sound welds especially on larger diameter, thick wall pipes: the welding parameters for 168mm diameter C-Mn pipes are given in Table 3.6. Here it should be noted that to avoid root run penetration by the second weld run, it was necessary to pulse the welding current in synchronism with the end dwells on the cross seam oscillation. It should be noted that, in addition to the higher price which can be as much as three times the price of an intermediate function system. the heads are larger and greater operator training is required to make the best use of the system.

References 1 Lucas W: 'TIG and plasma welding', Abington Publishing, 1990. 2 Lucas W and Rodwell M H: 'Process techniques to improve control of weld

3 Lucas W: 'Advances in TIG and plasma processes for welding and surfacing'. penetration in TIG welding', Welding Review August, 1987.

Welding and Metal Fabrication 1990 58 10.

Chapter 4

J W D A I N E S

Welding pipes for aero engines

Pipes for aero engines are typically stainless steel with two or more butt welded fittings made from bar, casting or forging. Pipe wall thickness is in the range 0.6-1.2mm. with diameters of 5-40mm. Lengths may be from 50mm to over 1000mm. Pipes may have several bends and they are attached to the engine exterior with bolted or clamped end fittings and if necessary intermediate supporting clips. There may be over I 0 0 different pipes per engine, each with two or more welds. A view of an engine with some of its pipes is shown in Fig. 4.1. One of the most complex assemblies, an annular fuel manifold, has over 30 welds. The failure or leakage of pipes carrying fuel or oil could jeopardise the safety of the aircraft. Welding and quality assurance (QA) procedures have been established to ensure that failures do not occur in service.

Stainless steel pipes are TIC welded but there are external casings of large diameter where plasma welding is the preferred process. An example of this is a titanium by pass duct with over 40 butt welded bosses.

Welding techniques

The preferred method of attaching end fittings to pipes is mechanised TIC welding. This is carried out on plain butt ended tube and fittings, which may or may not have autogenous lips. Acceptable procedures have been developed for many pipe and fitting combinations although on smaller pipes (5mm OD and 0.6mm wall thickness) it has been shown that the deletion of the autogenous lip has advantages, for example:

0 Cheaper end fittings; 0

0

0

0

However, on large diameter, thin wall pipes. i.e. above about 20mm

N o risk of damage to the autogenous lip; Mass of metal to be fused is reduced and is consistent; N o clearance problems between lip and pipe: No tacking required and no risk of oxidation.

Welding pipes for aero engines 55

diameter, ovality can cause alignment problems and the autogenous lip becomes a desirable location feature.

Experience has been gained over recent years of solid state power sources. They have performed well. are reliable and can be set to give the desired welding parameters with precision. This means that there are no parameter changes required if, for example, a particular job is moved from one power source to another.

Wlding with autogenous lips

There is a general requirement that each end fitting is within 0.65mm of its nominal position and that the finished pipe assembly must be within an envelope defined by ‘pipe diameter + 1.Smm’. To achieve these tolerances it is necessary to use an NC pipe bender and precision jigs.

Nominal dimensions of autogenous lips have been standardised as shown in Fig. 4.2 for pipes with a thickness in the range 0.6mm-1.00mm.

4.1 Plpes on the exterior of an englne.

56 J W Daines

Figure 4.3 shows a group of typical pipes and Fig. 4.4 a jig on which the pipes are cut to length. tack welded and checked after welding. Welding is then carried out using a welding head located directly on the pipe or jig.

An examination of welding parameters shows that welding time is being reduced on larger diameter pipes by the introduction of equipment which can control current at three or four levels for prescribed times. Where previously up to 3 orbits were necessary to achieve consistent and acceptable profiles, only 1% orbits are now required. An example is a pipe 18mm diameter and 1.2mm wall thickness. Pulsing with 0.15 sec ON and 0.15 sec OFF time is used. and at initiation there is a dwell o f 4 sec to establish penetration. Background current is 24A and welding speed is 150 mm/min. Other parameters are:

Wall thickness, mm TubeOD,mm A B

I 4-6 1 .o 0.5

8-50 1.4 0.7

4.2 Dimensions of autogenous lips.

4.3 Group of welded pipes and fifflngs.


Level 1 2 3 4 Current, A 71 64 60 Decay Time, sec 17 8 3 6

Where there is relatively low volume but considerable variety of work, it has been found cost effective to link one power source to three different welding heads via a switching arrangement.

Argon is used to protect top and bottom beads from oxidation, and underbead shape is controlled by having a positive internal pressure. This pressure is ensured by fitting a 1 mm bore restrictor in the end of the pipe and by adjusting the gas flow to give the required pressure. Gas pressure switches are incorporated to inhibit welding in the event of failure in the argon supply.

Welding without autogenous /ips

As previously noted it can be advantageous to delete autogenous lips on the smaller pipes. of say 5-10mm diameter. In collaboration with Startrite Designs Ltd, a tooling system' has been devised to locate the pipe and its end fittings accurately. The sequence of operations on this equipment includes grinding the pipe end square and to length, and the alignment of pipe and end fitting using a ball, Fig. 4.5.

4.4 Pipe and end fittings mounted on assembly Jig.

58 J W Daines

4.5 Alignment of pipe and end fiiihg using Startrite equipment.

Floor to floor time with this equipment averages under 5 min per weld. of which 35 sec is welding time. For these small pipes it has been shown that consistent welding around the whole circumference is achieved using 2-3 revolutions and a peak current in the range 15-18A.

Several sets of this equipment are being used successfully: including welding complex manifolds with over 30 welds per assembly.

The by pass duct

The by pass duct is a short, large diameter pipe carrying air from the low pressure compressor to the rear of the engine. The duct is made from 2mm thick titanium sheet wrapped and welded with one axial weld. Flanges are welded to the ends of the duct and more than 40 bosses, mostly circular but some racetrack shaped, are also butt welded to the duct.

With so many bosses to weld, procedures must be used which minimise distortion. A manufacturing sequence has therefore been devised in which the bosses are divided into three sets. This allows a step-by-step sequence where only a third of the holes for bosses are cut and the appro- priate bosses welded in each step. Both TIG and plasma welding have been used to attach bosses. Experience has shown that keyhole plasma


welding consistently gives fewer welds with quality problems such as unacceptable porosity. Equipment for carrying out this welding is shown in Fig. 4.6 and some typical bosses in Fig. 4.7. Local argon shielding in the form of a clear plastic dome surrounded by a flexible cuff is used to protect the top of the weld. Inside the duct, segmented tooling is expanded into position to limit distortion. This tooling also has features which direct argon to the underside of each boss. Torch position is controlled by optical line-following equipment and torch height is maintained by a simple cam system.

Argon arc welding is used for tack welding each boss in position. Keyhole

4.6 Tltanlum by pass duct wllh plasma welded bosses.

60 J W Daines

plasma welding is used for the first pass followed by a second non-penetrating weld to smooth the top bead. In the first pass a peak current of 5OA pulsed at 2Y2Hz allows welding at 220 mm/min. For the second pass the current is reduced to 40G speed is approximately halved and plasma gas flow reduced to ensure that no keyhole is produced. With this carefully controlled two pass process. satisfactory weld geometry has been achieved without the complexities of wire feed.

This process has been used successfully in production for several years.

4.7 By pass duct showing Wicai bosses.


Weld quality

Quality is engineered into components by careful consideration of the design and all facets of the manufacturing operation. Practices and mandatory procedures covering each topic are documented. Design philosophy is covered in a general document and from this specific documents are derived covering such items as design procedures, materials. end fittings, welding, X-ray, dye penetrant, pressure testing, quality, recovery. protection. handling and installation. For the welding process itself, all details of parameters and tooling are recorded on a standard data sheet which is approved by the controlling laboratory on the basis of representative test welds.

Requirements for satisfactory welds are contained in a company quality acceptance standard. These requirements may be summarised as follows:

0 Weld crown and weld underbead must blend smoothly with the parent material;

0 The weld must be fully penetrating over its entire periphery; 0 Undercutting and sinkage must be less than 10%: 0 Misalignment must be less than 15% of material thickness or0.13mm

whichever is the greater.

Overall thickness of the weld may be up to 2% times that of the parent material provided that the height of the crown on the underbead is less than that of the parent material.

Faults identified on visual. liquid penetrant or X-ray inspection such as cracks, lack-of-sidewall fusion, lack of penetration, unfused tacks, oxidation and ballooning are unacceptable. The target in manufacturing is zero defects but there are occasions when a weld fault causes an assembly to be rejected. The alternative is to scrap it or initiate recovery by applying procedures instructed in a standard recovery technique document. This laboratory approved document defines procedures for local removal of the defect and local manual rewelding with filler wire followed by mechanised welding. If problems with the weld are more extensive. T rings or make-up pieces may be used to recover the assembly. All welds that are carried out in this way must meet the normal quality requirements.

Any recovery procedure is an additional cost. For this reason. regular analysis is carried out to ensure that causes of rejection are identified and corrective action taken.

Future developments Joining of pipes has been developed progressively from brazed lap joints through manually welded joints to today's mechanised welding of plain

62 J W Daines

butt joints. Both power sources and tooling have been improved steadily and the scrap rate due to welding problems is low. However. there is a need for a low cost system that will guarantee consistent weld penetration and underbead dimensions.

From earlier comments it is clear that each aero engine requires a number of different pipes, so future developments in welding must cater for variety and a relatively low production rate. This is likely to make the cost of full automation unattractive but there is scope for improvements in tooling to minimise setting time for 'batch-of-one' production and to reduce floor to floor time for each joint.

There will continue to be a reduction in the range of pipe sizes and in the number of different types of fittings. Attention will continue to be paid to all items that assist in consistent achievement of the design intent to the satisfaction of MOD and the air worthiness authorities.

Reference 1 Anon: 'Orbital pulsed TIG welding of pipe assemblies'. Welding and Meral

Fabrication 1976 44 (4) 278-280.

Chapter 5

W W A T S O N

TIG welding of pipework for ships

Since the introduction of welding into the shipbuilding industry, there has generally been little progress in techniques used for pipework fabrication in commercial vessels. The norm is manual metal arc for ferrous materials using back and front welded flanges, forged T pieces and elbows. although. to some degree, the latter have been phased out by the use of bending equipment capable of producing tight radius bends, i.e. down to 2 X O/D centreline radius. For non-ferrous materials, brazing has been used except where clients have insisted on welded systems. Welding and brazing have almost totally been confined to workshops, with on-board joints being mechanical couplings.

These methods also applied to naval vessels well into the 1950s. Only high pressure steam systems were fabricated using forged tail flanges; these were welded with backing rings fitted, which, on completion of welding, were removed by machining before radiographic and magnetic particle examinations were carried out.

The start of the naval nuclear programme heralded the TIG welding age in shipbuilding. The butt weld, performed by TIC or combined TIG- MMA processes, is now the preferred method of achieving a permanent line joint, both in the workshops and on-board, not only in ferrous but also non-ferrous materials. Attachment welds are also frequently made using the TIG process.

Reasons for the change to TIG welding The reason for the rapid change to TIG welding in pipework will be readily understood when it is realised that the majority of pipework being fabricated is in the range 6-63mm diameter. There are. of course. larger sizes in use, up to 368mm diameter, but the quantities are relatively small when compared with total tubing quantities.

Further, whereas tube materials at one time comprised mild steel, 1% chrome, Yi% molybdenum and copper, the nuclear age brought with it

63

64 W Watson

various types of stainless steels, nickel alloys, copper alloys. carbon manganese steels and aluminium, in addition to the three basic materials previously used. As well as direct welds in these materials, there are several transition joints between them. In addition to butt welds, there are ‘special’ welds such as set-on branches. pintle bosses. vicolets. canopy seals and toroid seal welds.

Quality requirements of pipe welds, particularly the bore condition, also encouraged TIG welding to be adopted. The consumable insert was specified by the shipbuilder’s main customer as the preferred method of producing butt weld roots. The root contour was of importance to minimise erosion in high flow-rate systems and to minimise noise trans- missions through piping systems to the hull structure. The EB type insert was chosen and has continued to be used since its introduction. These inserts were also used on liner steam systems and in gas system pipework on the first liquid gas carriers built, Fig. 5.1.

A further reason for the use of TIG welding is the extremely restricted access conditions on board naval vessels. It is literally impossible to perform many of the welds by any other process. At least one mirror is standard

5.1 Sectlons through pipe with (top) tack welded and (bottom) fully fused EB Inserts.

TIG welding of pipework for ships 65

equipment in a pipe welder's bag and, in some cases, two mirrors are required to enable a satisfactory view of the joint.

Rarely is a pipe welder seen with a standard face mask. Most masks are cut down to protect face and forehead only. to enable the welder to get between pipe runs, etc, to see the joint to be welded.

The early days of TIG welding

Manual TIG welding was introduced when EB fusible inserts became available for high integrity systems in all materials other than copper alloys. Pipe sizes were such that it was economical to use TIG, since the extra welding time taken, when compared with MMA. was offset by reduced dressing times. Fabrications were made up using butt welded elbows, T pieces and reducers in most systems but socket welding was also permitted. Tight radius bending, when introduced, replaced elbows but the welding development effort had to be increased as copper alloy systems, previously brazed, were required to be welded. This led to the use of torch gas mixtures in place of argon which had previously been standard. Current practice is for minimum brazing, with none at all being performed on board vessels. Socket and sleeve welds, although acceptable, are only used where butt welds are too difficult to achieve.

In addition to the types of welds already mentioned, other welds carried out using the manual TIG process are:

0 Ferrous pipework set-on branches: 0 Pintle bosses in all materials: 0 Vicolets in all materials: 0 Canopy seal welds on valves and other equipment; 0 Toroid seal welds on valves and other equipment: 0 Transition butt welds: stainless steel/monel, stainless steel/copper

nickel, 1% chrome 95% moly/rnonel.

All of the above are classed as 'special' welds and must be carried out in the workshop and inspected to the highest standard. In the case of branch welds. pintle boss welds, vicolets and transition welds, the root must be removed by either machining or dressing. thus ensuring that any defects existing at this vulnerable location are removed, Fig. 5.2.

Weld procedures

Weld procedures are required for all welds to be carried out in production. Procedures are initially drafted and a limited issue made within the welding and quality control departments. The procedure test is carried out and any minor amendment made to the draft: the test piece, procedure and

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5.2 Section through small bore branch connection showing root area removed by final boring: a) Reinforced; b) Non-reinforced.


weld record sheet are then forwarded to the laboratory for destructive tests to be carried out. Any non-destructive testing (NDT) is performed in the department before submission to the laboratory.

During all operations, the customer’s representatives have full access to witness tests and NDT results. Finally, all results are forwarded to the Design Authority for the official report. Throughout the period of the nuclear programme, almost two hundred weld procedures have been raised and qualified.

Welder training Welder training and qualifications are a major part of the welding department’s problems, since some welds are only carried out once per vessel and build programmes are spaced such that welders’ qualifications expire. For nuclear systems, a welder’s qualifications expire thirteen weeks after performance of the last weld on a particular material. This period is twenty-six weeks on non-nuclear systems. Both apprentices and craftsmen undergo training in an area supervised by a foreman instructor.

Training commences in an open access situation, fusing consumable inserts in various materials. This operation is the most critical since the majority of defects found in small bore butt welds are at the root stage. Filling by manual TIG on small bore, and manual metal arc on the larger sizes, is undertaken when the welder is proficient at the root stage. Fur- ther training in ‘special’ welds is only given when i t is anticipated that the welder will be employed on these welds.

All welders, for whatever class of weld, must qualify to the satisfaction of the customer’s representatives before being employed on any production welding. The customer can withdraw qualifications. For those welders who will be employed on board vessels. qualification tests must be carried out in restricted access situations. The restriction is applied by setting-up the test weld in a cubicle with the weld being located with a maximum clearance of 480mm from the roof, back and side of the cubicle. This restriction has been found to be insufficient for many welds encountered on board and added training is given in a much more restricted cubicle, where management consider this would be beneficial.

Types of preparations For manual welding butt joints, three standard preparations have been used over the years, Fig. 5.3:

A square butt, with slight chamfer to guide the welder, for steel pipes with thin wall - up to 2mm;

68 W Watson

0

0

A 70" included angle V for steel pipes up to and including 57mm and 90/10 copper nickel of all sizes; A 50" included angle J for steel pipes up to 60mm and above and for all sizes of Monel and 70/30 copper nickel. Fig. 5.3.

Branch welds are only permitted in ferrous pipework and all must be of the full penetration, set-on type with the branch length being short enough to allow dressing of the bore. The adjoining pipe or fitting is then butt welded on using a consumable insert.

1.0 mm

30-40"

6 35 f 2.5"

/

1 .O mm max

V to enable welder / t o sight joint

0.5 mm max I 4 I I

0.5+0.1 mm 0.5 mm max

25 -0.5 mm max 0.5 mm max

Detail

1.05O.l mm 0.5 mm max

5.3 Standard joint preparations.


5.4 Copper nickel swaged and extruded Amng for butt weldlng In piping system: a) 70/30 b) 90/10.

Small branches in steel piping may be achieved by the use of a pintle boss or vicolet, both of which have the root removed by either machining or hand grinding on completion of welding. These small branch connectors may have a butt weld end, socket weld end or screwed end for connection to the next pipe.

Branch welds are not permitted in copper and copper alloy pipes, neither are back and front welded flanges. These joints are formed using the extrusion process and butt welds for branch connections. Fig. 5.4, either butt welded tail flanges or brazed flanges of the solid gun metal or nickel aluminium bronze type, or composite flange of gun metal ferrule and mild steel backing flange.

Gas for purging and shielding

When carrying out welding within a workshop, the extraction of fumes and gases is relatively simple by installed extraction plant. On board vessels, with fairly small compartments, the problem has much greater significance. argon, being heavier than air, tends to fall to lower levels and, unless

70 W Watson

stringent safety precautions are adhered to. there is a danger of an unacceptably high level of concentration and a hazard to personnel.

Because mechanical joints are kept to a minimum, it is frequently necessary to purge long runs of pipework and, on occasion. pressure vessels. Every effort is made to lead exhaust pipes clear of the vessel but careful monitoring must be maintained.

The confined space control department now authorises entry into compartments and an automatic safety air monitor, with visual and audible alarms, has replaced safety lamps which were carried by welders and fitters into areas considered as 'confined spaces'. A 'certificate of entry' is displayed at the entrance into confined spaces and a 'continuation certificate' will be appended following routine checks. 'If at any time a compartment becomes unsafe, then a 'NO ACCESS' notice is posted. such that it obscures the 'certificate of entry'.

Control of filler materials

Most of the filler wire used for manual TIG welding is purchased in cut lengths. On receipt into the works, all batches are quarantined until material certificates have been checked by the quality assurance materials section and have also had check analysis tests performed.

For materials which may be taken into the primary plant fabrication areas. the stores department advises the welding section of the quantity of wire released for production. The welding section then arranges for special heat shrinkable sleeves to be typed with identification information, i.e. material type and batch number. The sleeves also comply with a colour coding system which is part of the control system. The cut wire is then drawn from the stores and the identification sleeves applied at each end of the length. This procedure is carried out in a strictly controlled area. After application, the sleeves are subjected to infrared heat which permatises the typed identification by 'burning' it into the sleeve, following which the sleeves are shrunk on by application of heat in a special small oven. All wire is then returned to the main stores where it remains until required for production.

Wire for secondary systems only is subject to the same quality assurance checks on receipt but is then identified by an adhesive 'flag' which has the abbreviated coding printed on it at regular intervals. The batch number is maintained by tie-on labels. A new procedure will be introduced in the near future. Although the same type of adhesive tape will be used, it will be coloured in accordance with the colour coding scheme in addition to the printed lettering.

EB type consumable inserts are identified by the manufacturer with a coding number being impressed at 40mm intervals along the length. The


batch number is retained on the wire by application of a heat shrinkable sleeve at each end, so that one sleeve should remain even when short pieces are cut off one end.

Weld records For all welds that require non-destructive examination above the level of visual only, weld record cards are maintained. These cards record the base material's unique identity numbers, filler material identities. mechanical inspector's name, welding inspector's name, welder's name, in addition to the dates of various operations. On the reverse side of the card are details of the examinations carried out and the operators' names.

For primary plant welds. the card also states that an electro-chemical check has been carried out at the weld surface to ensure no contamination has occurred by the use of incorrect filler wire. Finally the card is signed by senior members of both the company and the customer, who are members of a weld acceptance team.

Codes of practice Although the vast majority of welds carried out at Barrow Shipyard are subject to Ministry of Defence (Navy) standards, other standards are also applicable when commercial work is undertaken. For offshore gas and oil pipework, either ASME code or ANSI specifications are generally applicable, although certain customers have their own standards based on these documents. On commercial ship contracts, BSI Specifications have. in the past, mainly been the codes of practice.

Automatic orbital equipment Those reading this chapter may have concluded that manual TIG welding has been considered the ultimate over the period covered. Nothing could be further from the truth. For over twenty years the pipe welding section at Barrow Shipyard has been endeavouring to obtain a suitable and reliable set of automatic orbital pipe welding equipment. Many suppliers have offered equipment which they believed was suitable for shipbuilding purposes, but most have failed to meet the particular requirements of pipework fabrication for ships.

The reasons for seeking automatic welding equipment are that it:

0 Requires less operator skill:

Produces more consistent welds.

Simplifies working in areas of restricted access:

Is faster than manual welding;

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€quipmenf trials and development

The early work on orbital welding equipment was carried out in the pipe welding development area at Vickers Shipbuilding Works in the early 1970s. Unfortunately, equipment design and/or performance was shown to be largely inadequate for production use. particularly where severely restricted access conditions were encountered, Fig. 5.5 and 5.6.

More recently Vickers purchased three machines of the latest design having the most compact dimensions available at the time of purchase and covering the range 3-220mm diameter. The equipment may be used with or without filler wire and provides stepless adjustment of current from 8 to 250G remote control facility, automatic flow control and current pulsing. Torch oscillation is not provided which may cause some problems with the larger pipes.

When the machine was purchased it was known that the French naval yards were using this type of equipment for small bore stainless steel pipe welding. Further, whereas we had previously been seeking an automatic pipe welding set for use on butt welds in the mid-size range, 76-200mm. the emphasis began to change as we were faced with a demand to butt

5.5 Astromatic AM1 1 orbital welding head showing excessive clearance required and out-of-balance weight distribution.

I 'adial


weld pipes in the range 20-38mm. which had previously been joined using mechanical couplings or socket welds. Further, the introduction of valves, in the same size range, which could be refurbished in-situ led to more butt welding and less mechanical couplings. These two changes were both in copper alloy material which is not the easiest material to weld in the fixed position - certainly in a different league to the stainless steel being welded by the French.

Initial procedural work was carried out on 33.4 diameter, 4.5mm wall for root fusion, both autogenous welds and with EB inserts. This was because many of the fittings already in stock had already been prepared with the standard V for manual welding and previous work had shown that a simplified weld preparation, with a 2.5mm thick nose, gave acceptable results without the case of a consumable insert. Work on 21.3 diameter, 3.7mm wall followed and customer’s approval of procedures was obtained.

Trouble was then experienced which set the procedure programme back to square one. When machines two and three were set to values used

5.6 Orbital welding head showing radial clearance required.

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on machine one which had been used for canying out procedure development, neither produced an acceptable weld. In fact, there was no comparison in the settings of the three machines to achieve successful welds. After several days were spent at Barrow endeavouring the calibrate the machines within acceptable limits, all machines were returned to the supplier.

Following this initial problem, procedure tests were again performed and all machines approved with similar settings; following procedure approval, two machines have worked regularly on production with a 99% success rate in the production shop. It is, of course, relevant to point out that only 44% of the welds carried out since introducing the machines have been performed due to accessibility.’The weld head used requires lateral straight length of 55-60mm and a minimum radial clearance of 57mm. This gives some idea of weld accessibility even for shop sub-assembly work. From surveys carried out it is anticipated that approximately 20% of welds will be accessible on board vessels of the class currently under construction.

Further development work has resulted in approved procedures in mild steel small bore pipework and it has been shown that Monel and 70/ 30 copper nickel pipes of similar scantlings can be welded using the same settings.

Although filler wire units were purchased with the equipment, early usage indicated that consistency could not be guaranteed, hence the decision to accept root fusions by automatic machines initially and to continue with manual TIG filling.

Recent modifications carried out by the development section have resulted in much improved performance with the feed wire units and procedural work has been undertaken in relation to stainless steel and mild steel.

Narrow gap welds For large wall thickness pipes the narrow gap process is currently being assessed: a typical section through a welded joint is shown in Fig. 5.7.

Suitable weld preparation machines Having succeeded in finding satisfactory automatic welding equipment, attention has now turned to either purchasing or designing and manufacturing a weld preparation machine for small bore pipes. Such a piece of equipment is considered essential. since configuration and accuracy of preparation are of paramount importance when automatic welding is con-


sidered. The machine cannot adjust itself during the welding cycle as the manual welder may do. Since complicated sub-assemblies often have excess length purposely left on during fabrication, it is difficult to machine final weld preparations successfully on a boring machine or similar fixed equipment. Portable machines, either electric. hydraulic or air driven, become necessary.

Most machines currently on the market require excessive straight lengths for either external or expanding mandrel type clamping. Alter- natively, machines using short clamping lengths require excessive radial clearance. This is precisely the problem experienced in the search for the welding heads.

Most small bore pipe preparations in the production shops are cut using Vickers designed and manufactured equipment but these are not ideal. For closing welds on board, the majority of preparations are produced using hand tools. Such preparations will be totally unacceptable when automatic machines are introduced for on-board welds.

It should not be beyond the ingenuity of a tool-making establishment to produce a piece of equipment suitable to meet fabricators' requirements within the next few years.

5.7 Section through plpe wall showing narrow gap weld carried out using orbital TIG process.

Chapter 6

K W R I G H T

Automatic tube welding in boiler fabrication

Since the decline of flash butt welding some 10-15 years ago joining of boiler tube has been primarily carried out by TIG welding. Initially, most welds were completed manually but the emergence of pulsed TIG power sources, together with more robust, easier to use orbital welding equipment, led to mechanised welding finding an increasing number of applications.

Previously, automatic orbital welding had been used for special applications and repairs where access was difficult, but had found little application on the shop floor requiring high productivity.

The advent of pulsed TIG provided much of the impetus to establish automatic orbital welding as a routine production process; the greater control over heat input resulted in greater consistency in weld quality and hence productivity was increased. Foster Wheeler Power Products Ltd also use automatic TIG welding equipment for welding tubes to tube plates where pulsed TIC and related developments have found increasing application.

Orbital welding Foster Wheeler Power Products Ltd is engaged in heavy fabrication work, part of which is the manufacture of works assembled and site erected boilers. The former are built and shipped as separate units while the latter are fabricated as sub-assemblies which are subsequently shipped and erected on site.

Boilers come in various sizes, the final design being dependent on the ultimate application. Nevertheless, even the smallest unit contains a large number of tube to tube butt welds, in possibly a wide range of materials and material combinations. This application is ideal, in terms of effrciency and effectiveness, for automatic welding.

In the mid 1970s the decision was made to evaluate the use of automatic orbital welding for tube butt welds as an alternative to manual TIC

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Automatic tube welding in boiler fabrication 77

welding in the fabrication of both loose and boiler tube panel assemblies. At that time a number of factors were imposed on any system considered, primarily that the system should be capable of producing welds of the necessary quality and consistency and at a rate equal to or greater than that of manual welding.

To achieve this it was considered essential that any welding equipment be simple to operate, easy to assemble and perform consistently. In addition, the equipment should be robust enough to withstand the rigours of a tube fabrication shop where, due to the varying workload, the welding location was constantly changing. A further requirement was that the equipment should consistently produce the correct weld parameters and these should be easily transferable from one unit to another. Finally, the system should be capable of welding over a range of tube sizes.

The initial evaluation of both in-house and commercially available equipment for automatic orbital welding under the fabrication conditions prevailing indicated that it would be very difficult to meet the basic requirements. Much of the equipment available was primarily designed for high technology applications where the quality of weld was the prime requirement and productivity, although important, only a secondary consideration. Other specialised equipment was intended for limited access. in-service special repair applications.

Almost all the equipment evaluated at that time was based on a bracelet concept, with the welding head attached to a stationary bracelet clamped around the tube. While this system was capable of producing satisfactory welds it had two major disadvantages: it was difficult to position and clamp on the tube, and the drive mechanism was usually an external motor connected to the welding head by a flexible drive cable or a connecting rod.

One system differed from the others in that instead of a bracelet locating system it utilised a horseshoe caliper welding head. This system seemed to meet most of the requirements of adaptability, handleability and quality. The welding head could weld a number of tube sizes; for example, one welding head could weld tubes between 36 and 80mm outside diameter, while smaller and greater tube sizes could be welded by using other, different sized welding heads. Attachment of the head to the tube was relatively easy; two shoes were clamped by a lever mechanism. The distance between the shoes could be adjusted by two small knobs.

A ring gear cames the welding head. wire feed nozzle and cable support. During welding the ring gear rotates, driven from a drive motor located in the stationary handle of the unit. To allow attachment and removal from the tube, the ring gear is disengaged from the drive system by a small spring-loaded clutch lever.

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The power source accompanying the welding head is a thyrister controlled, mains voltage stabilised rectifier capable of pulsed current operation. In addition. the whole system includes a control box to control both torch rotation and wire feed. a remote control for welder operation and a wire feed unit.

After exploratory work, a decision was made that this equipment was the most suitable and a single complete package was subsequently purchased for in-house, pre-production evaluation.

Pre-production development

The possible introduction of automatic orbital welding poses a number of questions. The adoption of such equipment cannot be considered as simply replacing an operator with a machine. for the effects on pre- and post-welding operations also require careful consideration. Thus a programme of work was undertaken to establish the basic weld parameters and the dimensional tolerances required for various aspects of the production operation. for example:

Weld preparation: Tube fit-up:

0 Cleanness levels: 0 Weld parameters, particularly the pulse details: 0 NDE techniques. particularly the interpretation of radiographs: 0 Welder attitude.

The various trials confirmed the previously held view that although mechanisation removes a number of the manual skills it introduces other factors. For example. a skilled welder, presented with a poor fit-up and unsatisfactory weld preparation. will almost always overcome such difficulties by adjusting technique. The automatic machine cannot do this, since all the parameters are established and the welding operation consists simply of set up, switch on and switch offwith only minor adjustments during the welding operation.

This factor is of considerable importance when rejections are analysed; it is all too easy to blame rhe machine for not performing efficiently when other factors are the real cause. For this reason all personnel involved need to be fully acquainted with the new requirements and critical areas. The welders operating the equipment must undergo thorough training and retraining. Not all welders have the aptitude or self-motivation for new processes so welder selection is of the utmost importance. In our case many young welders. just out of their apprenticeship. were found to be most adaptable: subsequent training and practice have produced a


nucleus of good operators. I t has also been noted that these welders are some of our best manual TIG welders.

Nondestructive testing is usually dependent on customer/code requirements and invariably includes radiography. The weld profiles from the automatic process are more uniform and consistent than those of equivalent manual welds. Radiographs can show innocuous linear indications which can be incorrectly diagnosed as cracks or lack of fusion. For this reason it is essential that weld and radiograph are examined carefully throughout the development work.

Welding

The orbital welding process is quite tolerant of variation in technique. Multipass welds can be produced continuously or in discrete passes; the root pass can be completed with or without filler additions: pulse characteristics can be changed to accommodate individual pass requirements. In some cases techniques have been developed that allow lack of fusion in the root run by correcting it in the second pass. In other cases, full root fusion on the first pass is essential, the remainder being considered strictly as filler passes.

All weldable boiler tube materials can be welded using orbital techniques. Carbon steel. low alloy steels (including 6 and 9% chromium), 300 and 400 series stainless steels have all been satisfactorily welded and in many cases to each other. Preheat can be accommodated without problem. providing the service cables are kept away from the heated tube.

Many forms of panel can be welded provided there is sufficient access for positioning the weld head and for restriction-free rotation. The location of any bend in relation to the weld is a major factor. Figures 6.1 and 6.2 are typical examples of applications where access is limited by the close proximity of a bend and finning on the tube.

The most common welding position used within the tube assembly area is with the tube in the fixed horizontal (5G) position. One major factor during pre-production evaluation was to determine weld parameters for the total weld cycle. so that once preset, no operator adjustment would be necessary. Typical weld parameters are shown in Table 6.1. together with a suitable weld preparation.

Before welding, the machine-prepared tube ends are cleaned and tack welded together, ensuring that the tubes are correctly aligned and that a constant root gap is maintained around the joint. The tack welds need to be kept to a minimum to avoid possible lack of fusion during the subsequent automatic root run, yet they must also be large enough not to break during subsequent handling or welding.

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6.1 Limited access simulatlon mock-up.

6.2 Production orbital welding with limited access.

Tubes are normally assembled into panels before welding. Welding may then be completed using one or more orbital welding systems.

In addition to normal tube butt welds. orbital welding has been used for fillet welding small end plate attachment tubes. This is achieved by


Table 6.1 Typical weld preparation and weld parameters for a 50mm OD X 4mm wall carbon steel orbital tube butt weld

1st pass 2nd pass

Peak current. A Base current. A Pulse on. s Pulse off. s Slope in. s Slope out, s Rotational speed. shev Wire feed speed. mm/min Wire size, mm

110 60 0.2 0.5 2 8

150 26-30

0.8

160 15 0.2 0.5 2 8

I50 26-30

0.8

modifying the electrode holder and wire feed guide system by the addition of a small angled base to the rotating ring gear.

Other orbital welding techniques have been developed by the company, using their own designed and manufactured horseshoe caliper type welding head. One example is the welding of Incoloy 800 pigtail to socket welds for oil refineries both in the UK and abroad. which is one of the few site applications of orbital welding.

Tube to tube plate welding

Mechanised welding has for many years found acceptance in the production of tube to tube plate welds in heat exchangers and power generation equipment. Tube to tube plate welds fall into two categories, those welded to the back and those welded to the front of the tube plate. The former are called internal bore welds and the latter back face welds.

lnfernal bore welding

The company developed the internal bore welding process for joining tubes to a castellated preparation machined onto the back face of a tube plate or the outside of a header. The process is now widely accepted for tube to tube plate welding.

The technique is an automatic TIG process with the weld being produced by an arc which is rotated through 360" on or near the inside

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interface between the component parts. The tungsten electrode is fitted into a water cooled holder connected to a rotating stem. The dimensions of this stem are determined by the inside diameter of the tube and the thickness of the tube plate. Figure 6.3 shows a typical internal bore welding torch.

To complete the weld the tube is inserted into the socket in the castellated preparation and the welding torch is inserted from the rear of the tube plate to a known position adjacent to the joint line. The arc is then struck and. following a short delay to establish the weld pool, the electrode is rotated at a predetermined speed around the inside circumference of the tube. The outer surface of the weld is protected from the atmosphere by a submerged-arc welding flux or by an inert gas, usually argon. The submerged-arc welding flux. besides giving a protective cover, also provides some support to the molten weld pool. Today, with the use of pulse current giving greater penetration and profile control, argon backing is often preferred, which eliminates the need for flux. A recent development is a system of heat sensing diodes which controls penetration by adjusting the power

6.3 Internal bore welding torch.


source output according to the heat emitted by the outer surface of the weld pool. This technique would of course be impossible if flux backing is used.

The internal bore welding process was developed to produce a consistent quality weld with a minimum of operator involvement during the welding cycle. To achieve this the welding cycle requires consistent control. The original control equipment contained a series of mechanical timers and a motorised variable resistance to slope clown the welding current at the end of the cycle. Today with the utilisation of solid state electronics the type of control system is dependent on the extent of the control required and the money available to finance its development. It is now possible to control weld parameters to a very fine limit. Pulse current ensures a much greater control of heat input than could be achieved with constant current: this control can be further refined by synchronising the torch rotation with the pulsed current. Instructing the controller can be as simple as a series of manually operated switches and timers or a microcomputer pro- grammed to reproduce the required weld parameters.

The original internal bore welding torch was rotated using a large motor placed adjacent to the welding station and connected by a flexible cable. This system has now been superseded by a motor small enough to be connected directly on to the torch itself, which gives increased positive driving.

The basic weld sequence is similar whether pulsed or constant current is adopted. The major differences are those which influence the heat input, i.e. welding current and travel speed. Following a short prepurge the arc is struck and the electrode is allowed to remain stationary for a preset time to establish the weld pool. The torch then rotates until the arc is finally extinguished at the end of the current rundown at the end of the weld cycle. Rundown is initiated when the electrode has completed one revolution plus a small overlap to ensure complete fusion. Typical weld parameters are shown in Table 6.2, together with a suitable weld preparation.

The overall object of the weld cycle is to produce a weld which exhibits a smooth profile across the change of diameter from the tube to the castellation. Figure 6.4 shows the internal and external profile of a typical weld.

A wide range of materials can be welded by this process: i t can be used for almost all those materials normally welded by the TIG process. Being an autogenous process, the behaviour of the weld pool depends on the composition of the materials being joined, but some variation in weld parameters may be required to accommodate cast to cast variations within the same material specification. It is sometimes possible to control

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Table 6.2 Typical weld preparation and weld parameters for the internal bore welding of 25mm OD X 2.4rnm wall. type 347 stainless steel tube to a type 347 stainless steel tube plate

Torch speed. sfrev Weld current. A Voltage Delay, s Rundown. s Arc gap. mm Electrode position (below joint line). mm Weld position

d - 25.0 mm I I t 2 5 . 0 5 mrn

60 50 13

5 12 1.2

0.w Tube vertical

the materials being joined by overlay cladding the tube plate or by adding a consumable insert to the weld joint. Two alternative weld preparations incorporating consumable inserts are shown in Fig. 6.5a and b.

Internal bore welding has also been successfully used to weld tubes directly into tube holes either with or without a recess.

6.4 Typical weld profile: a) inside; b) Outside.


(a)

6.5 Weld preparations with: a) Insert ring; b) Flat washer insert.

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Front face welding

Mechanised welding can also be used to weld tubes to the front face of the tube plate. The TIC process has been adapted to produce welds both with and without filler wire, yielding both seal and strength welds.

The equipment consists of a welding head which rotates around the tube end. the whole process being controlled by a sequence controller. This controller is similar to that used for internal bore welding but with an additional system for the wire feed. The welding head, Fig. 6.6. consists of a central mandrel which locks into the tube to be welded. The welding head is positioned at the correct height and angular distance in relation to the joint line: it can be adjusted to accommodate a number of tube sizes. Both the torch drive and the wire feed motor form an integral part of the unit. The wire drive motor and feed system can be removed if they are not required as in the case of non-reinforced simple fusion fillet welds.

Welding is usually carried out with the tube plate vertical, the welding head being supported by a ‘sky hook’, Fig. 6.7. Setting the equipment in this position is aided by a level device built into the torch.

Various types of weld can be produced with this equipment, the most common being either a single pass weld where the tube end is fused into the tube plate or a strength weld requiring two passes. Typical weld parameters for a stainless steel, two pass strength weld are shown in Table 6.3. Alternative weld preparations are shown in Fig. 6.8.

c

6.6 Front face welding torch.


Table 6.3 Typical weld preparation and weld parameters for a two pass strength type stainless steel automatic tube to tube plate weld

4R

Welding current. A Arc voltage Electrode angle. O

Delay. s Torch speed. drev Rundown. s Wire feed speed. mm/min Wire size. mm

1st pass 2nd pass

130 135 10 10 17 17 2% 2%

120 120 10 10

660 760 0.8 0.8

6.7 Application of the front face welding torch.

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c 6.8 Alternative tube to tube plate weld preparations.

Chapter 7

A R W A R D

TIG and MIG welding developments for fabrication of plant for the chemical, petrochemical and offshore oil and gas industries

Weld fabrication for the chemical and petrochemical industries involves extensive use of gas shielded welding methods. Root runs of butt welds are generally made using the TIG process for the wide range of materials involved, which include ferritic steels, duplex and austenitic corrosion- resisting steels, heat-resisting Fe-Ni-Cr alloys and also nickel and copper base alloys. For completion of butt joints with wall thicknesses up to 8mm, the manual TIG method is commonly preferred, but when shapes and repetitive quantities suit the special facilities, automatic TIG welding is used.

For repetitive filling work of butt joints over 8mm thickness the automatic MlG method is also well established, after completion of TIG root runs, because of the relatively high deposition rates it provides (3-5 kg/hr). These are substantially higher than those achieved both with MMA welding and with most of the TIG processes used for filling. However, Paralloy has established the use of automatic TIG (both cold and hot wire) for filling in a number of applications where metallurgical considerations favour TIG.' Improved deposition rates have been achieved to ensure that fabrication remains economic.

Automatic TIG methods have been employed alongside automatic MIG welding in one shop- used for the same product, namely the reformer catalyst tubes made by Paralloy Ltd. These tubes, Fig. 7.1, are used in the chemical industry for producing hydrogen in the manufacture of ammonia base fertiliser and methanol, and also for other refinery processes which employ hydrogen. The centrifugally cast tubes of typically 15m

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fabricated length are made of austenitic alloys. such as 2SCr/3SNi/lNb with 0.4%C. suitable for service at temperatures of 850-1OOO"C. and subject to high operating pressure. A typical butt weld is expected to last at least 10 years under creep conditions in service. This shop situation enables a comparison to be made between the respective characteristics of TIG and MIG methods for weld fabrication of high integrity items requiring 100% radiographic examination of welds.

In recent years. through the development of alloys with substantially improved creep rupture strengths. the reduction of tube wall thicknesses from typically 20mm to 9-12mm has lessened the relative importance of weld filling at high deposition rates. Moreover, the tubular cast product is extensively employed with only 5-9.5mm wall thickness in the coils used for high temperature steam cracking of hydrocarbon gases to yield ethylene. Elsewhere. the use of automatic plasma for root runs of say 6mm thickness. compared with 2.Smm and 3.Smm for manual and automatic TIG roots respectively in conventional processes. has permitted the completion of weld joints with only one subsequent automatic TIG filler run.

Semi-positional welds for complex coils made in the fabrication shop (generally in the 5G position) and similar field welds (mostly in the 2G position) have increasingly relied upon orbital TIG welding methods. The choice of this process has been influenced by the need for consistent control of root penetration weld beads to. for example. Imm maximum

7.1 Catalyst tubes In Paralloy N625 and Paralloy H3W.

TIG and MIG welding 91

penetration. When the beads of equivalent manual TIC welded roots are less uniform they have process disadvantages in service, in particular. for 50mm diameter reaction coils operating at over 1OOO"C. Pulsed TIG, with low heat input. and intermittent welding head progression. synchronised with arc pulsing, has enabled fusion of 2.2mm thick close butt roots in exotic alloys that can exhibit significant crater cracking in manual procedures, especially if thinner root thicknesses are attempted to suit manual TIG welding.

In the shop. filling is commonly completed by manual TIG. after estab- lishing the root profile by the orbital technique. Welding wires of 0.8mm have been introduced for a range of proprietary alloys to sustain the increasing use of orbital TIG for site welds in new capital plant where steam cracking coils are subject to increasingly onerous conditions. Since cast products. tubular and associated fittings are machined generally to very accurate diametric tolerances, they therefore provide ideal fit-up for the orbital TIG welding of close butt joints.

Manual gas shielded welds

Manual TIG welds

Especially with relatively small diameters (up to 150mm) and thin wall sections of up to 8mm. manual welding of pipes and associated fittings usually employs the TIC process for root and filling runs, in all welding positions. When weld profiles are machined the close butt technique is preferred using a J profile and matching bore diameters. An open gap V preparation of 75" (nominal) included angle to ANSI B16.25 applies when acceptable diametric fit-up cannot be achieved. The root run is produced using the TIG process, with or without filler wire addition, depending on the joint configuration and also the crack propensity of the alloys involved. Pulsed TIG welding is generally used, with conventional current decay to avoid cracking in the weld crater.

Dissimilar metal joints are common. for example: (a) wrought stainless steel flanges in 304 or 321 material to cast heat-resisting alloys; (b) Incoloy 800H reducers to cast or wrought alloy tubes; (c) Cr-Mo ferritic steel flanges to austenitic alloy tubes, involving an open gap root run (2.4mm). with Inconel wire addition. Generally. the joints are subject to 100% radiography.

For substantial quantities of pipework in the petrochemical and oil industries. involving type 3 I6L. duplex stainless steel, cupro-nickel and nickel base alloys, for example Incoloy 825, manual TIG welding is often specified exclusively. These applications generally involve wrought

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materials with many sources of supply for pipes and fittings, and the resulting dimensional variations affect the fit-up of butt welds. Conse- quently, open gap welds usually result, with close attention to tacking and back purging. Manual TIG welding of a stainless steel pigtail to a manifold is illustrated in Fig. 7.2.

I t is important to preserve metallurgical structure. for example on the process side of root runs of cast and wrought duplex stainless steels, or austenitic alloys with controlled ferrite, to resist potential stress corrosion, and to control the microsegregation of critical elements affecting the corrosion resistance of stainless steels. This has necessitated development of welding procedures that balance joint design.heat input and composition of filler metals, to satisfy strict corrosion test criteria.

Pipework for tubular reaction vessels and certain other items. such as collection manifolds and general vessels, involves internal conditions of service where gases. fluids and less frequently solid particles are carried.

7.2 Manual TIG weld of stalnless steel pigtail to manifold.


sometimes at very high speeds. The internal surfaces of welds are usually subject to the effects of the processes and reactions, often with superimposed high operating pressures and temperatures. Moreover, the welds may be subject to corrosion by solutions at, for example, ambient temperature. or gas at elevated temperatures (e.g. through oxidation/carbur- isation). As a result, the quality of the TIC process root runs of welds joining the respective component parts are usually highly significant in terms of adverse influences of any abrupt changes in contour. Excessive local convexity (from overpenetration). concavity, lack of root fusion. or cracks, providing stress concentrations. or crevices for corrosion, must be avoided. Generally TIC root runs are produced manually with extensive use of pulsing.

Manual MlG welding

Manual Synergic MIG welding is now applied quite extensively for filling of thick section welds, for example, for joining heavy section branches, involving up to l00mm ID forged weldolers, welded to Incoloy 800H and cast 20/32Nb manifolds. The MIG process achieves high deposition rates and good general weld soundness. Before the availability of Synergic MIG, pulsed spray MIG welding had achieved good general weld soundness, and had been commonly applied for X-radiographic work in some nickel and copper base alloys, such as Monel400 and aluminium bronze, and for manual as well as automatic welds. This enabled high deposition rates with control of heat input. Good sidewall fusion was demonstrated to satisfy users who had doubts arising from previous experience with conventional MIG welding.

The Synergic MIG developments have included argon shielded Inconel 625 cladding of high strength steel components? and for the associated structural attachments in ferritic steels, where conventional and former pulsed spray MIG welding had often been unacceptable due to poor side wall or root fusion, especially in narrow grooves or fillets.

Automatic TIG and MIG welding for filling

Automatic MIG welding for filling

For repetitive work involving filling with the MIG process the root runs are invariably completed using TIC and are subject to dye penetrant inspection before transfer to the MIG welding machine. Fig. 7.3.

MIG welds are produced in austenitic Fe-Ni-Cr and high nickel alloys with excellent contour and cleanness of the capping runs, so that modest

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wire brushing is sufficient to provide an inspectable high quality weld surface finish. Argon/helium/l%oxygen shielding gas has been introduced for optimum cleanness of more reactive alloys.

For automatic welding the machined weld preparation profiles are held within close tolerances, so that an automatic cycle can be used to produce consistent results in terms of weld geometry. Using a modern MIG welding machine for tubes in the diameter range 75-200mm. and with wall thicknesses varying from 7.4 to 25mm. and a J type profile with a 40" included angle, Fig. 7.4, the equipment enables the principal parameters to be established for each run. and a continuous MIG weld is made.

There is scope for the operator to adjust the set parameters during welding if circumstances demand. These parameters are:

(a) rotational speed of the pipe. which is driven from one end using a pipe-rotator;

(b) oscillation width and speed. and with scope to vary the dwell at the respective side walls; and

(c) wire feed speed related directly to the current.

7.3 Automatic MIG welding machine.


Wire may be within the range 0.8-1.2mm diameter. The arc voltage can be varied to suit the penetration and heat input requirements for each type of weld by adjusting the tapping of the power source. Stable arc conditions make it possible to produce welds that are radiographically sound as regards the incidence of spherical porosity and with good fusion between the weld and the parent metal. Argon with up to 20% CO, is used as the shielding gas and the mode of transfer is globular dip, for austenitic heat resistant alloys which derive creep-rupture strength from their 0.35/0.S% carbon content.

The main attraction of MIG weld filling is high deposition rate and compliance with specifications that require prior dye penetrant crack detection of weld TIC roots in a production flow line for the tubular product .

Dissimilar welds by the automatic Synergic MIG process

For dissimilar metal welds - for example of a ferritic low alloy steel to an austenitic heat-resisting alloy ( I .25%Cr/O.S%Mo to 25Cr/35Ni/lNb) - made using Inconel 82 filler wire (1.2mm diameter) with Synergic MIG provides spray transfer conditions (fine droplets) with an argon shielded stable arc. Formerly, when conventional MIG was used, these Inconel welds required a small addition of an oxidising gas to produce a relatively smooth arc. In addition, the different melting points and magnetic characteristics of the two alloys necessitate optimisation of the respective side dwell times to avoid undercut and excessively convex profiles.

40"

:I 7.4 J type weld preparation.

96 A R Ward

Comparison of automatic TlG welding for filling with automatic MlG for heat resisting alloy tubes

Why use TIC for thicknesses over 7.5mm when the MIG process is available and established? At first sight, the idea of feeding the wire separately into the weld pool formed by the arc between a tungsten electrode and the workpiece and with sufficient arc energy to melt the cold wire does seem comparatively unattractive for repetitive work. For example. wire of 2.4mm diameter fed at 250mm per minute will produce a deposition rate of perhaps 25% of that achieved using the MIG process. Nonetheless, there are cases where the TIG process is preferable.

While conventional MIG welding of austenitic alloys uses shielding gas mixtures based on argon o r argodhelium with CO, or oxygen stabilisers, TIG welding is based on 95-1000? argon, with up to 5% hydrogen. An inert shielding gas is particularly important when welding alloys which cannot tolerate oxidation. For example. Inconel 625 (65Ni/20Cr/ 9Mo/4Nb) is very reactive. Any oxides which form during welding d o not readily separate from the weld pool because their melting point is above that of the base metal. Should oxide inclusions occur, therefore, they tend to persist and appear like gas porosity in radiographic films. Thus. with highly reactive alloys, the TIG process with its inert shielding can contribute significantly to the quality of welds produced. The same is true.of fabrication with other reactive materials, for example titanium. However. more recently Synergic MIG has enabled good arc stability with 99.995% argon shielding gas for Inconel weld deposits though not eliminating spatter.

A further example of the superiority of TIG welds is found in service with tubular catalyst tubes of approximately 15m length. and l00mm internal diameter. Service experience of Chiyodaq3 Japan. indicates that weld macrostructures can be critical in resisting thermal stresses in thicker wall sections (of, for example, 17-25mm thickness).

The fabricated tubes, Fig. 7.5 and 7.6, involve two butt joints between the three cast tube segments. In service the tubes are heated from the outside (furnace side) to support an internal endothermic catalytic reaction at 800-900°C and 30bar. Process gas transfer inside the tube leads to a sufiiciently steep temperature gradient on the machined inside bore surface to generate significant thermal stresses. which are then superimposed upon the designed hoop stress (from the high operating pressure). It has been shown that a weld joint's susceptibility to thermal stress cracking. and slow propagation from the extreme inside surface, radially through the weld, is far less likely when the weld joints consist of an equiaxed or rounded grain structure. Cracking may not become apparent until after about five years in service, that is half-way through the normal life of the


welds, which are expected to last for at least ten years in most plants. Through the control of TIG welding parameters it is possible to obtain this far more resistant structure to hot thermal stress deterioration. Fig. 7.7a. (By comparison, sound high creep strength welds produced by the MIG process tend to have a directional (columnar) grain structure. which grows directionally from the inside to the outside surface. Fig. 7.8.

From experience. it is generally known which types of design are susceptible to thermal stress cracking. and the TIG process is preferred for these applications. In particular. when the design wall thickness is less than some 16mm. grain structure is not usually significant.

Using automatic TIG welding for pipe butt welds it is possible consistently to produce welds which are 100% radiographically sound. Equivalent MIG welds in austenitic alloys are more inclined to contain a few isolated

7.5 Manual TIG welds between lncoloy 800H weldolets and manifold using stringer beads (26 position) and to 25%Cr/20%Ni HK40 tubes in lG/SG semi-fixed position with lnconel 82 filler wire.

98 A R Ward

spherical gas pores, though very comfortably satisfying acceptance specifications for radiographic soundness, such as ASME VIII , UW-51.

One user standard limits any isolated gas pores less than 2.5mm diameter to a total of five in circumferential butt welds with over 600mm weld length made by automatic welding. This requirement in 200mm ID, lOmm thick reaction tubes, produced in substantial numbers for each furnace producing hydrogen rich gas, is at the limit for MIG welds, but is comfortably attained by automatic TIG welding.

Both gas shielded processes. especially when used for welding austenitic heat resisting alloys, produce welds with a high weld strength. In high temperature service, the creep-rupture strength of gas shielded welds is 80-1 10% of that of the parent metal. This is quite adequate for most circumferential welds. By contrast, the equivalent welds made by the MMA process, producing deposits of equal nominal composition, tend to be no more than 60% efficient. The reasons for this are not entirely clear, but the relative cleanness of gas shielded welds is important.

7.6 TIG welds In catah/st tubes.


7.7 a) Microstructure of a TIG narrow gap weid (equiaxed grain); b) Contrasting MMA profile.

100 A R Ward

7.8 Coarse columnar macrostructure of automatlc MIG weld. The manual TIG root has been pamy removed by internal grinding and the parent tube also has a coarse grain structure.

The strength at high temperatures of austenitic alloy welds relates to the form taken by stable carbides of niobium and chromium. Through weld joint designs (for example, the small weld volume made possible by the narrow gap TIG process) and control of thermal conditions; it is possible to produce a suitable microstructure in the welded condition. This is a specialised subject. Suffice it to say that multipass TIG and MIG welding is necessary to avoid the coarsening, and hence weakening, of carbide precipitates due to excessive heat build up during the respective weld runs.

The TIG process has an inherent flexibility to enable independent adjustment of the major welding parameters (wire feed speed, welding current. arc voltage and run out lengthhpeed).

Automatic narrow gap TIG welding of piping

For more than twenty five years tubes and associated fittings, including reducers in 'as cast' and wrought materials, have been welded using an automatic argon gas shielded technique, and with a narrow weld preparation or gap. For wall thicknesses between 7 and 25mm, this has an


essentially 6.5mm wide weld profile with parallel side walls. The electrode is held in a central position, and multipass welding proceeds immediately on completion of the automatic TIG root, without stopping. The limited weld volume relative to the cross section thickness of the parent metal ensures that heat input into the weld pool and cooling rate within the weld runs favours attainment of microstructures giving optimum creep strength, is illustrated in Fig. 7.8. One welding machine is shown in Fig. 7.9. This has computerised control of the continuous weld to ensure consistency between batches.

By contrast, some other automatic TI(; welding methods, which have employed a 40" included angle and a very substantial deposit for the filler run, develop a comparatively low hot strength as a consequence of thermal damage to the essential strengthening microstructural carbides.

7.9 Automatic TIG welding machine which has computerised control of continuous welding cycles.

102 A R Ward

The equipment

In narrow gap TIG welding. the thoriated tungsten electrode, positioned at the centre line of the weld, is not weaved as it is in manual TIG, MMA and MIG welding methods. Control of side wall fusion and penetration of the narrow gap TIG weld benefits from electromagnetic weaving of the arc only. This allows sensitive control of the oscillation and weave of the arc. In particular. achievement of the relatively flat, smooth and uniform cover passes is assisted by this weaving facility.

The weaving device consists of a low frequency oscillator (control unit) generating the current waveforms and an electromagnetic probe, which is mounted directly on to the welding torch, to develop a magnetic field. The unit enables precise control of the parallel forces applied to and interacting with the magnetic forces of the welding arc, so that the arc is swept across the weld joint under set controls. For semi-continuous processes and high currents the probe is water cooled.

The electromagnetic stirring action imparted by the device lessens the scope for metallurgical segregation during cooling of the liquid weld pool, and assists in elimination of 'hot shortness' and susceptibility to microfissuring.

The narrow gap TIG welding equipment involves boom mounting of the water cooled torch and an automatic arc length control unit. The weld commences with an arc length of nominally 3mm and the arc length control then maintains a consistent arc length (voltage). The roller bed for pipe rotation involves 'hold down' units to avoid longitudinal creep of the tubes, hence to assist the essential centralisation of the tungsten electrode in the narrow weld joint.

The pipes are pre-tacked or clamped together before rotation for welding commences. For instance. after pre-argon purging the root is completed at 9V and 180A. After this the welding parameters are automatically adjusted to provide the short root run overlap and to reduce simul- taneously the welding current (heat) and hence control the weld pool volume while the 2.4mm diameter wire is introduced. The wire enters the rear of the weld pool (behind the 12 o'clock position). The pipe rotation towards the operator enables viewing of the resultant weld.

Current delay is achieved automatically to eliminate crater cracks at the conclusion of the welds, and the sequence control speeds pipe rotation at this stage. Otherwise speed of rotation is directly related to the welding current via a tachogenerator.


lncreased deposition rates of TlG process filler

With cold-wire TIG welds, the heat capacity of the weld pool limits the consumption of 2.4mm wire to approximately 250 to 275mm feed speed per minute. unless metallurgical characteristics suit increased weld pool size and lower pipe rotational speeds. Clearly, economic considerations demand use of features which permit increased rates of deposition. but without causing problems with lack of fusion and excessive oxidation, or porosity to develop in the welds.

The normal shielding gas is argon. The introduction of up to 5% hydrogen produces a considerably hotter arc allowing increased wire feed at a given current, for example, by up to a factor of 2. The benefits of hydrogen addition have been applied extensively to TIG welding of austenitic steels, which have a comparatively good tolerance for hydrogen. This is in contrast with ferritic, low alloy and martensitic 12% chromium type steels, which have well known sensitivity to hydrogen as regards cracking.

Unfortunately, with austenitic alloys with higher alloy contents than 18%Cr/lO%Ni - for example, 25%Cr/2O%Ni (HK40) - weld metal gas porosity becomes a problem at 2% and particularly with a 5% hydrogen/ argon mixture. One per cent hydrogen is satisfactory in this respect, but this requires very careful gas mixing to achieve the thermal benefits without causing weld porosity to occur.

Automatic narrow gap hot wire TlG welding

In recent years the hot wire automatic TIG process has become significant as an effective means of increasing weld deposition rates. The filler wire, usually 1.2mm diameter, is preheated by resistance heating with a power supply separate from that for the welding arc. This wire heater consists of a constant potential, alternating current supply to control 'arc-blow' effects, related to the magnetic characteristics of the materials being welded. The equipment for hot wire involves a relatively high speed wire feeder, the current for wire preheating, with the contact tube and holder. Fig. 7.10.

The wire speed and preheating current for the wire may be adjusted in combination for optimum results, with the latter being not more than 50- 60% of the arc current to minimise interference through arc oscillation. This would arise due to interaction of the wire current. and the arc current. With the correct ratio of the respective currents, the interference produces arc oscillation of about 30" around the arc only.

The hot wire, with its own inert gas shield, is fed into the back edge of the weld pool at a steep angle approximately 45" to the arc, which is near

104 A R Ward

to the 12 o'clock position in the case of pipe welding, to ensure a consistent point of contact for the filler wire in the weld pool.

The process enables substantial increases in weld metal deposition rates without the need for the unacceptably large liquid pool of weld metal, which would be necessary with cold wire TIG. These would prove difficult to shield from oxidation with inert gas, and would also reduce general quality.

It has been reported that the hot wire process is applied in producing narrow gap welds in thick section plates in austenitic stainless steels for nuclear service. including use of the vertical technique.

Plasma welding

Plasma welding is used for producing longitudinal seam welds in about 6-8mm thick, rolled and seam welded piping and vessels (for example, in austenitic steel and copper base alloy welds). The plasma arc penetrates 6mm approximately, and is then followed by a separate conventional TIG arc. to complete the longitudinal seam of pipes up to 5m long. in stainless steel. nickel base alloys and cupro-nickel. Large diameter vessels for the food and beverage industries have also utilised this method.

Plasma welding has not been employed extensively in the UK for pipe welding involving circumferential butt weld joints. It is, however, applied elsewhere to root runs of some 5.0-6.0mm in 9-25mm thickness tubing. versus, say, 2.4mm for conventional TIG. Apparently, 25%Cr/20%Ni joints ( H K N ) , particularly in Japan, are being made entirely using the keyhole plasma process. The chemical and petrochemical industries use

~ Filler wire

High speed wire drive

Travel +

7.10 Schematic arrangement for hot wire TIG welding.


butt welded tubing for processes that involve substantial investments, which inevitably dictate a relatively conservative approach to acceptance of new welding methods. When alloy creep-rupture properties in such welds are determined by their solidification and cooling rates and take several years’ testing to prove in the laboratory, before being accepted for service, this prompts caution when the thermal considerations of a new welding process deviate from established data with more conventional welding methods. Short term tests are notoriously inclined to provide misleading confidence.

Summary From the contents of this chapter, it will be appreciated that effective automatic TIG welding processes have been developed, offering increased rates of weld deposition, to compare favourably with MIG procedures. In turn, piping requirements have involved smaller diameters and thicknesses for reaction tubes and coils, so that any cost differences between welding processes have been reduced relative to the total product value.

For fixed position welds in shop and on site (field) orbital pulsed TIG, automatic welding has become established especially for good control of root runs, and uniformity of the penetration beads.

Development of the Synergic MIG process4 with a stable pulsed arc and low spatter propensity has increased the scope for the MIG high deposition rate technique now used for relatively heavy section welds, such as vessel branch connections, and for surface overlaying high strength steels with corrosion resistant weld metal.

Changes in material selection for applications at the same time as new welding processes emerge, prompts a need for a balance between welding engineering for optimisation of production times and also quality revealed by non-destructive testing, and metallurgical features affecting the properties of welds in service. This observation applies to the more critical applications in the petrochemical and oil industries and where jobbing fabricators tend to be remote from the service conditions. In this context, for many years. major chemical and petrochemical industry users have observed that welds seldom fail prematurely. However. during the 1980s. the same organisations noted that their welds had a shorter normal operating life at high temperature than the expensive parent material. It often takes many years for this information to materialise, and then to prompt changes in specifications relating to welding parameters and metallurgical features. No doubt this type of evolution of technology will continue.

106 A R Ward

Acknowledgements

The author would like to thank the following companies for their help in preparing this chapter.

Para//o.v Ltd. of Billingham. Teesside. a Division of Lake and Elliot Industries. who are the successors to A P V Paramount Ltd. concerning cast tubular products. Paralloy is also the alloy designation for their range of corrosion and heat resisting alloys.

Scomark Eng. Ltd. of Burton on Trent. concerning fabrications for Paralloy and for other photographs.

Richards and Ross Ltd. of Wednesfield. Wolverhampton. who are also fabricators of tubular cast and wrought items using hot wire TIC automatic welding.

fnco Group. concerning proprietary Incoloy and lnconel trade designations.

References

1 D W 0 Dawson, R J Fivash and A R Ward: 'Automatic TIC welding'. Metal Construction, March. 1976.

2 S Smillie: 'Corrosion resistant synergic MIG overlaying'. Joining and Materials, August, 1989.

3 T Kawai. K Takemura, T Shibasaki, T Mohri: 'The effect of macrostructure on catalystic tube damage'. AIChE symposium on safety in ammonia plants and related facilities. San Francisco. November. 1979.

4 Symposium. 'Using synergic MIG successfully'. Cranfield Institute. October, 1984.

Chapter 8

R A W I L S O N

Fabrication of aluminium process pipework

Aluminium process pipework covers a wide spectrum of materials. tube diameters, and wall thicknesses depending upon specific requirements. These can range from the pure Al 1100 series through the Al-Mn 300 series up to the hardenable 6OOO series. However, application is generally limited to the non heat treatable high strength AI-Mg 5000 series. Physical pipe sizes can range from 6.3mm nominal bore (NB) up to 500mm OD over a wide range of wall thicknesses; diameters above 500mm OD are not commercially available and, if required, must be fabricated from rolled plate. Welding techniques for aluminium are currently limited to conventional MIG and TIG processes, which, although effective, have seen significant changes over the previous two decades.

This chapter seeks to clarify some of the problems associated with aluminium pipewelding, and, by examination of these, to offer solutions in terms of joint configuration, filler wire, cleanness, and welding process. Advanced welding processes are also discussed with particular regard for automatic orbital pipewelding techniques.

Parent materials Material choice is influenced by joint strength, ability to form, corrosion resistance and material availability, the most common alloys falling in the Al-Mg range, BS 1471, 5154A and 5083,' and to a lesser extent pure aluminium, BS 1471, 1200, and the hardenable alloys, BS 1471, 6063.

In general, most alloys can be successfully welded. However, in pure aluminium there is a tendency towards excessive porosity. This requires additional precleaning before welding and may be associated with higher gas solubility in the pure metal. Solubility of hydrogen in pure aluminium at 800°C is 1.68 cm3/100g against 0.04 cm3/100g at 660°C. From these figures the severity of the gas entrapment problem can be appreciated. Alloying

107

108 R A Wilson

additions, on the other hand, can drastically reduce gas solubility. For example, the addition of 16% Si can reduce the previous figures by 40%.

In heat treatable o r hardenable alloys involving precipitation mechanisms, heat affected zone (HAZ) cracking can occur. Figure 8.1 shows a macrosection, taken from 6061 tubing welded with 5356 filler wire, suffering from HAZ cracking along the grain boundaries. This probably stems from excess precipitate formed during the weld thermal cycle. Welding procedures employed on hardenable alloys should be designed to limit shrinkage stress in terms of weld pool volume, with the use of stringer beads instead of weave techniques. Levels of shrinkage stress can vary depending upon the alloy and weld pool volume. However, liquid-to- solid shrinkage of aluminium is approximately 20% and, when added to a thermal contraction of 15%. results in a high level of weld stress.

Choice of filler

Having chosen the type of material, choice of filler metal is generally dependent upon the following criteria:

Strength: Ductility;

. -

I .

5

8.1 Macrosectlon of 6061 pipe joint illustrating HA2 cracking. Filler wire 2.4mm, ASTM ER5356; welding current 160A AC, 17-21V

Fabrication of aluminium process pipework 109

500

375 E E 5- 0)

250 Y

!!

r-” 125

- m -

0

0 Corrosion resistance; 0 Elevated temperature properties; 0 Weldability.

Filler materials for aluminium have been commercially developed to suit most applications and are widely available. In normal circumstances filler metal selection is straightforward with the use of matching alloy compositions. Problems arise with the use of transition joints between differing alloys where dilution can result in hot short crack sensitivity.

Hot short crack sensitivity increases with increasing amounts of eutectic- forming elements, up to a maximum, then decreases.2 Figure 8.2 illustrates weld crack sensitivity of fillet welds on a T section testpiece over a range of magnesium contents, with maximum sensitivity at 1.5% Mg; curves for other eutectic-forming elements would be similar. It can be seen from the graph that, for example, a joint between aluminium and a 3.5% Mg alloy by dilution could be a problem. Selection of fillers should be such that compositions are held above or below crack-sensitive levels. In the example quoted a 5356 filler at 5.0% Mg would probably be chosen.

Other reasons may influence the choice of filler wires. For instance 4043 Si wire, because of its improved fluidity, can be used in areas where leak tightness is important, and it can be used on 6OOO series alloys. How- ever, it would be unwise to weld alloys in excess of 3% Mg with Si because of the formation of excess magnesium silicide. This would result in a brittle

* Magnesium in weld bead, %

8.2 Effect of Mg content on weld crack sensitivity (after Dowdz).

110 RAWilson

weld zone. Table 8.1 shows a filler metal selection chart listing the various options available for aluminium alloys.

In terms of filler metal quality. fabricators have generally formed their own preference for a particular supplier, this preference being based on cleanness, repeatability of analysis, and consistent quality. Of these points, cleanness is probably the most important. However, having established a cleanness standard, consistent quality requires regular monitoring. Figure 8.3 shows a 5556 1.6mm electrode with an internal oxide flaw which resulted in weld metal oxide inclusions.

Weld joint configuration

Types of joint depend upon a number of variables, each of these often being interrelated and contributing to the final choice. Typical considerations can be listed as follows, not necessarily in order of importance:

0

0 Shop or site weld: 0 Fixed position or manipulated; 0 Bore restrictions imposed by code authority, customer requirements,

or process considerations; 0 Ease of joint fit-up:

Weld process: 0 Number of joints.

As a general statement it is probably accurate to say that penetration welds are preferred rather than the use of permanent backing strips. Figure

Tube diameter and wall thickness:

8.3 AWS ER5556 1.6mm diameter TIG rod exhibiting an internal oxide flaw.

Fabrication of aluminium process plpework 11 1

Table 8.1 Filler metal selection chart for Al alloys: to AWS classifications

5356 5183 5556

5356 5183 5556

5183 5356 5556

5356 5183 5556

5183 5356 5556

Parent metal

4043 5356 5356 5183 5183 5556

5356 5356 5183 5183 5556 5556

5356 5556 5183 5356 5556 5183

5356 5183 5556

1060

1 100 3003

3004

~

5083

5154 5254

5456

6061 6062 6063

6061 6062 6063

4043 4047

4043 5356 4047

4043 5183 5356

5356 5183 5556

5356 5183 4043

5356 5183 5556

5356 4043 5154

5154 5456 5254 5083 ’

4043 5183 1 1 5356

8.4 shows a typical penetration weld joint for a 75mm NB schedule 10 tube. The exceptions to this statement are generally associated with small diameter tubes or difficulties with fit-up. Small bore tubes generally suffer from overpenetration with unacceptable bore restrictions which necessitate the use of backing rings. Figure 8.5a illustrates a small bore backing ring which has been used for normal applications and Fig. 8.5b a ring which is used in cases of difficult access. The type of ring shown in Fig. 8.5b does not require filler wire additions and is simply fused into the joint. How- ever, it is used only on welds whose main criterion is leak tightness. Other exceptions to full penetration joints involve site or shop welds where close fit tolerances cannot be achieved. The main problem in these situations is root gap. Gaps above approximately 2mm on penetration joints can result in unacceptable overpenetration, irregular protrusions. weld metal globules, and wire fingers, particularly on welds in the 6G position.

112 RAWilson

Backing rings can be machined from bar. cut from tube, o r rolled from strip. the last two generally when tubes are above 50mm NB. Some manufactured rings have indentations pressed in the ring to facilitate weld set- ups (gaps). However care should be taken with these designs as cracking of indentations can occur.

Wall thickness may influence the use of backing rings by process limitations. For example, at thicknesses greater than 40mm where J preparations are not feasible, sufficient heat input for adequate fusion may not be available from the TIC process, in which case MIG may be employed. Although MIG penetration welds have been investigated in the past3 there is a high risk in terms of penetration profile, and single-sided welds invariably utilise back-up systems. A further type of joint which can be

1 I

8.4 Typical penetration weld preparation for 75mm NB schedule 10 Al tube.

11.5 mm

8.5 Backing rings for use on: a) Conventional butt welded joints; b) Difficult access joints where fusible ring removes wire feeding requirement (dimensions in mm).


utilised in the pipe range 12-50mm NB is the socket joint. This provides ease of welding within wide fit-up tolerances with no bore restrictions. However crevices will exist and fatigue may be a problem in dynamic situations. To summarise:

0

0

0

6-25mm NB: use special backing rings; 25-50mm NB: use socket joints; 50mm NB and above: either weld backed or unbacked, depending upon tit-up.

Methods of edge preparation Edge preparation techniques on aluminium can vary considerably depending upon size, thickness, number, complexity of joint, and the ability to measure accurately the finished length required. Because of its softness, aluminium can encompass a wide range of tools which are normally employed in the woodworking trades. With repetitive weld joints, automatic preparation machines can be utilised. These are typically high speed fixed rotary routers, or portable pneumatic or electric end preparation tools.

In applications where the tube must be fitted in situ, manual techniques are generally employed. There are usually rotary rip saw or recip- rocating blade machines; rotary burrshouten on hand rasps are also used. Abrasive grinding discs can be used for rough edge preparation. However, they should always be followed by a metal-removal technique to avoid problems with abrasive particles embedded in the soft surface. Obviously, wherever possible, maximum advantage is taken from the use of end preparation machines.

Preweld cleaning and sources of weld defects The primary consideration in pipe fabrication is to produce a satisfactory welded joint economically and free from the three most frequent defects: porosity, oxide inclusions and cracks. Having previously considered the intricacies of material filler metal combinations and their effect on cracking, it can be assumed that the optimum choice of combinations has been made and the weld should be crack-free. This leaves porosity. and oxide or non-metallic inclusions.

Porosity is invariably caused by poor weld precleaning or incorrect welding procedure. Porosity originating from the parent material is unlikely and can usually be ignored. Precleaning the weld joint is essential and should consist of chemical cleaning, degreasing. wire brushing (stainless steel bristles), and scraping. Precleaning the wire should not be

114 RAWilson

necessary. However TIG rods can be scraped if the surface condition is suspect, i.e. dulled, contaminated, or heavily oxidised. On the other hand MIG wire must be satisfactorily cleaned by the manufacturer and carefully stored before use.

Weld procedure variables which can result in porosity apart from filler wire are:

0 Quality of shielding gas. which should have impurity levels below 0.01%: Ability to achieve and maintain lamellar shielding gas flows with maximum stability. This can generally be improved by the use of gas l e n s e ~ . ~

Oxide or non-metallic inclusions are unlikely to originate from the parent material and, like porosity, are usually a result of weld procedure. It is worth commenting at this point upon the considerable difference in melting points between aluminium at 660°C and aluminium oxide at 2O5O0C, which highlights the problems of oxide inclusions. The most effective solution to the oxide problem is one of prevention. This takes the form of ensuring scrupulous cleaning before welding, both the weld joint and the wire. by means of wire brushing, scraping, and degreasing.

A further problem with TIG tiller wire can be an incorrect 'dipping' technique, i.e. the rod is dipped into the weld pool and when removed should be held within the shroud gas, otherwise the molten tip is oxidised, which in turn is plunged beneath the pool surface and hence becomes a source of oxide inclusions. Figure 8.6 illustrates typical oxide inclusions in aluminium TIG welds.

Welding processes Established welding processes in general use for welding aluminium consist of AC TIG, DC TIG (helium shielded), and MIG. Each process has its own method of removing the tenacious oxide film which exists on the surface of aluminium; this film must be removed before wetting and fusion can occur. In the MIG process with the filler wire acting as the anode (positive), oxide removal is by fragmentation of the oxide at the surface, i.e. a cathodic stripping action. This results in maximum cleaning. In AC TIG welding a similar action is used with cleaning occurring only with the electrode in the positive half cycle.

Use of DC TIG with helium shielding relies predominantly on the workpiece being cleaned before welding, any areas of remaining oxide being floated out of the weld pool. However, it is very difficult to reach the high level of welder skill and cleanness required to produce satisfactory


welds. particularly when delays occur between cleaning, tit-up, and welding. This limits the DC TIG process to special applications where deep penetration of high input welds are required, e.g. bulky aluminium structures such as heat exchangers.

With conventional AC TIG welding and MIG techniques, for oxide stripping to be effective, the arc must have access to all critical surface areas. Figure 8.6 shows an oxide inclusion occurring at the root which was originally the internal surface and vertical sides of the weld preparation root face, i.e. areas inaccessible to the arc. Similar problems can occur with backing strip joints, where oxide is sandwiched between the internal wall and backing strip leading to oxide fingers on either side of the root. Fig. 8.7. It is therefore essential to design the joint with maximum arc accessibility and to use effective cleaning methods in areas of susceptibility: J type preparations are most effective in avoiding this problem.

However, with the advent of solid state square wave AC power sources, the cleaning action can be increased by varying the positive and negative cycle time. These machines can overcome the problems of tungsten spitting and weld craters, both of which arise from 'crash' starts and stops, i.e. all-on, all-off. Weld starts can now be controlled by upslope techniques to avoid tungsten transfer and downsloping by current decay for crater elimination. A further advantage appears to be the effect of the

8.6 Macrosection of two pass weld In 5154A 75mm NB, schedule 10 pipe showing oxide inclusion in the root and capping passes. Filler wire AWS ER5556 2.4mm diameter; welding current 170-1WA AC, 21-22v

UOSl!MV d 9CC

Fabrication of aluminium process pipework 11 7

c -

8.9 MIG capping pass on 750mm, 5mm wall, tube In 5083 material. Filler wire: TIG (root pass) 2.4mm diameter ER5556; MIG (capping) pass 1.6mm diameter ER5556. Welding current: TIG 160-180A AC, 18-24V; MIG 220-240A DC, 2628V

square waveform on the arc itself. This appears to be a more efficient con- centrated arc with a reduction in arc spread, smaller weld pools, and the ability to operate with smaller electrode diameters. The result is more control over the weld pool with subsequent improvements in weld profile.

118 RAWilson

particularly the penetration bead. Figure 8.8 shows a weld produced with a neat penetration profile on a square wave AC machine.

The MIG process is generally used in conjunction with TIG for pipewelding, T IC providing penetration control and MIG a rapid filling technique. Like TIC, MIG can be used readily in all positions, including vertical down and overhead, as a result of rapid weld pool freezing. Because of its relatively low heat input it is also used in areas where distortion can be a problem. Figure 8.9 shows a MIG capping pass on 750mm diameter, 5mm wall, tube.

Inter-run cleaning with TIC is not necessary; however, with MIG, wire brushing between runs to remove soot deposits would be required. With aluminium, preheating is generally not carried out except when porosity is a problem or at extreme thicknesses, but even then, preheat temperatures of approximately 150°C will suffice.

Future developments Welding processes currently in use for aluminium pipewelding are invariably manually operated. Automatic processes such as fixed station (stationary welding head, rotating tube) or orbital welding, which are commonplace in steam generation industries, are virtually nonexistent in aluminium fabrication. To maintain competitive effectiveness it is essential to auto- mate the welding operation, particularly with large volume repetitive pipework.

Whether pulsed or otherwise, MIG welding, because of the high risk of overpenetration coupled with equipment bulk, is unlikely to be utilised. On the other hand with consistent tit-ups TIC would appear to be feasible. However, because of the low melting point and high thermal conductivity of aluminium, transient heat fields (preheating effect) and the effect of filler wire entrant angle and force on the degree of penetration may prove difficult to control. Plasma welding in the keyhole mode would seem to be an ideal choice of process having the potential of flexibility in terms of weld preparation and misalignment. With autogenous plasma welding slight undercut occurred; this could be eliminated completely in the vertical position with filler wire additions. Weld cleaning is comparable to AC TIG.

Plasma keyhole welding is also possible in the electrode positive DC mode" with maximum oxide cleaning. Although some development work would be necessary, high travel speeds in the keyhole mode, coupled with rapid weld pool freezing, should make all-positional plasma welding of aluminium a distinct possibility.


Other high density processes examined on thin aluminium sheet, i.e. electron beam, CO,, and Nd-YAG lasers, would be impractical for true orbital welding. and difficult to apply in the fixed station mode. It would appear that, to date, plasma welding, whether AC or electrode positive DC polarity, is the most likely to succeed with automatic orbital welding.

References 1 BS 1471: 1972: Wrought aluminium and aluminium alloys for general engineer-

ing purposes - drawn tube. Pub1 British Standards Institution. 1972. 2 Dowd J D: Weld cracking of aluminium alloys. Welding J 1952 31 (10). pp

4485-4565. 3 Salter G R and Spiller K R: 'Alternative systems for the manual gas-shielded

arc welding of aluminium and its alloys'. Welding Institute Members Report 57/ 1978lPE.

4 Gorman E F: 'New developments in gas shielding'. Linde Publication 52-524. 5 Messer Griesheim. private demonstration.

Chapter 9

L C A R R I C K

A fabrication system for site mechanical construction

The production of any fabricated system should be coming to an end, not starting, when the welding arc is applied to the weld joint. In far too many situations, the fabrication system is the welder who carries complete responsibility for productivity and quality of the plant being welded. I t is normal practice for many organisations to use welds/welder/day as the criteria of productivity and to fall into the temptation to increase the number of welders to recover programme dates. irrespective of progress in other areas. These statements may be a little emotive but there is no doubt that. in general terms, there is a need to improve the application of technical and management control to fabrication on site mechanical construction.

The concept of fabrication must be revealed at the design stage when the necessary requirements must be fully understood and included in the design and project programmes. One approach, that followed by British Nuclear Fuels (BNF). is to produce a total fabrication philosophy.

Site environment

The fabrication environment on the Sellafield Site Construction, BNF, is mainly that of cell construction. A cell is a structure surrounded by concrete which provides a biological shield. Vessels and wall box pene- trations are positioned in the cells and joined together by pipework resulting in restricted working conditions, Fig. 9.1.

Fabrication philosophy

Several years ago it was decided within BNF to evaluate the stainless steel pipework fabrication technology of the Engineering Group and to make changes where necessary so that benefits could be achieved in future construction projects. In setting this objective it was considered necessary to

120

A fabrication system for site mechanical construction 121

develop a total integrated fabrication system bringing together a set of connected operations to form a whole. It was accepted that the introduction of any single new aspect of technology and expecting this to reflect independently a total improvement in fabrication productivity was not possible. For example. there are many fine welding developments on the market. any one of which can help the fabricator to produce good welds; but none of them, unless properly used, will give the increased productivity hoped for. Specifically with respect to pipe and tube welding, the substitution of automatic orbital welding for manual welding will not bring more than a limited success unless many other factors in the fabrication sequence have been changed to allow full use of the automatic technique.

It was therefore decided to evaluate both the technical aspects and a number of management functions which would also have to be changed if the full benefits were to be achieved. The result of this evaluation can be summarised under the two disciplines.

Technical aspects

hfUf8rkJh Of COnSffUCfiOn

The materials selected are readily weldable and not susceptible to defects caused by fabrication techniques. Any exception to this principle because

9.1 Site fabrication environment.

122 L Carrick

of especially severe environments will be limited as far as possible. Pur- chasing specifications for these raw materials were written with specific attention given to the standardisation of sizes and dimensional tolerances; this factor is very important with respect to fittings and forgings. A materials management system was introduced to control the supply of all material product forms.

Fabrication design detail

Computer-aided design (CAD) systems were in operation in the design offices and were of help when introducing the following steps to:

Standardise weld detail design using approved procedures: Standardise pipe-support structures using an approved manual; Introduce standard radial clearances around pipework to ensure that welding and inspection equipment can be used correctly; Ensure access for personnel, machines, and pipework into the buildings and within the buildings where fabrication is taking place. Design staging into the building as an integral part of the cell to replace scaffolding: Arrange pipe layout details to ensure the best use of grouping welds together in specific areas: Apply pipe-bending techniques wherever possible. Fittings should be used only for very tight bend radii and T connections: Introduce an improved surveying system to help with the accuracy of pipework isometrics: Improve the accuracy of the conventional civil and mechanical surveying techniques used on site: Design an argon supply line into the building as an integral part of the cell. to remove the need for the use of argon cylinders.

Fabrication techniques

A number of evaluation and development programmes resulted in the introduction of the following specialised equipment:

Pipe-rounding tools to ensure improved accuracy of the pipe end diameters. Fig. 9.2: Pipe preparation equipment to produce standardised weld preparations: Pipe alignment jigs to help set up the weld joint. Fig. 9.3: Purging systems to reduce the overall time and cost of purging. One of these systems is shown in Fig. 9.4:


Automatic orbital welding equipment to increase weld productivity and provide more reliable weld quality, Fig. 9.5; Computer numerically controlled (CNC) bending machines to produce accurate pipe assemblies with high productivity.

Fabrication specifications

The existing fabrication specifications were carefully rewritten for the fabrication and erection of stainless steel plant. In all cases where modifications to the specifications were considered, the safety and possible failure mechanisms of the plant were fully evaluated and considered before any changes were initiated.

The application of these new specifications has been a step-by-step approach. applying them to allocated areas of plant, monitoring the application, and feeding back results so that a complete study could be made before a formal change was approved.

9.2 Pipe-rounding tool.

124 L Carrick

9.3 Manual pipe alignment jig.

9.4 inflatable bladder system used for local purging In pipework.


9.5 Orbital weldlng equipment.

126 L Carrick

Non-destructive testing techniques

New non-destructive testing (NDT) techniques were developed, evaluated, and applied to the fabrication of vessels at fabricators' works and to the erection of pipework at the site. In all development work, specific emphasis was given to assessing the defect sensitivity of the various techniques to ensure a better understanding of the limits of acceptance written into the specifications.

Management functions The following management functions were considered and any changes needed to the existing procedures recorded so that the management requirements of future projects could be established:

Logistics of operating new technologies and their effect on site management (client and contractor); Contractor's management structure with emphasis given to the accommodation of the high level of technology being introduced; Increased site planning requirements and various control systems which will have to be introduced to improve the discipline necessary to ensure success; Effect on the established bonus scheme following the application of new technologies; Special training requirements for both engineers and industrial staff. Consideration should be given to the establishment of a test for pipefitters similar to the approval required by welders; The establishment of an inspection philosophy. It is said that quality cannot be inspected into a job but, by ensuring that each step of the fabrication system is controlled and monitored to a detailed specification, the discipline which is necessary to achieve success can be applied; The establishment of a philosophy for the use of modular fabrication to help site construction. This will include a policy on the use of 'on-site' and 'off-site' fabrication shops.

A fabrication system Interfacing the above developments into a management structure designed to accept and apply new technology, but still retaining the benefits gained from substantial experience, can allow a fabrication system to emerge. The fabrication concept will start in the early stages of design and thread its way through the engineering operations until the plant is


ready for commissioning. In large companies there will be the need for liaison procedures between engineers of each section and discipline so that actions do not fall between the responsibilities of two line manage- ments.

Having established the correct management structures for the operation of a fabrication system the connected operations which need to be brought together to form the whole can be represented schematically, Fig. 9.6.

Stages of Q fabrication system

Planning is shown to be an operation introduced at the design stage and continued through to detail site planning of the fabrication/welding operations. It is also part of the feedback operation to enable changes to be made during the various operational stages to rectify any problems which have developed. In the longer term the feedback to design will enable the experience gained in fabrication to be recorded and used to improve the detail design of future fabrication projects.

Inspection is also shown to be initiated at an early stage and continued through to final pipework acceptance. Inspection operations need to be integrated into the fabrication process, not just applied at the end of a work package like some alien function which is often blamed for delays in programmes. The other boxes show a sequence of events which. if

t Materials selection and purchasing specifications of a11 product forms required

Material management

I I Design CAD Application of surveying input fabrication systems techniques to increase

producing accurecv of isometrics detail Establish fabrication - pipework

- specifications isometrics

Fabrication planning j , Planning for i Inspection considered : ' : fabrication i

i j detail j

- ............................... .................... 1

. . ..

welding

Inspection

9.6 Stages of fabrication system.

128 L Carrick

carried out logically, will produce a fabrication approach that will enable a correctly structured management team to achieve reasonable productivity targets and a successful conclusion to a pipework project. There is no specific box showing quality assurance, the reason being that i t is considered to be an integral part of the fabrication system and is associated with every operation. Internal auditing ensures that procedures are written. approved. and correctly applied.

Application of a fabrication system

I t is not possible to apply a completely new system based on the changes outlined above in one major step. especially where fabrication projects are already in existence and newer ones are in various stages of imple- mentation. A transition period when new techniques are introduced and evaluated is necessary: such a period has been in existence at Sellafield for several years with the intention of applying a complete fabrication system to new projects.

One specific new technology - automatic orbital welding - was introduced in 1982 into two major projects. This step provided the know-how to organise the application of orbital systems and included:

0 Choice of equipment: 0 Training operators;

Maintenance of equipment: 0

Feedback to design and project management ..as also enabled some of the design and management aspects listed above to be resolved.

As a result of this work the application of orbital welding in future projects will now be able to go ahead with confidence in the knowledge that its full capability can be realised.

Other new techniques have been introduced in allocated areas of existing projects and fully monitored to ensure that the experience gained was fed back for evaluation and the formulation of approved operating procedures. One example of such an exercise was the application of a new purging technique to a specified area on a large project. This example has been chosen for inclusion in this chapter, because it demonstrates some of the points already made on how the management aspects have to be right if the technique’s full potential is to be realised.

Technical support in the field.

Introduction of a new technique

Approximately 650m of pipework requiring 110 welds in 80. 100. and


150mm nominal bore (NB), schedule 10,304L pipe had to be welded in a specific area of an existing project. The purging operation on pipework of this size is often extremely difficult and time consuming in a site environment. Vacuum purging. a technique developed at Sellafield to purge long lengths of pipework in an efficient manner, was used in this case. The technique was applied in the following sequence:

1 The total length of pipework was erected using a team of two fitters and one welder. Each weld joint was locally purged using dams so that the joint could be tack welded. At this stage the weld joint was sealed either by a complete circumferential seal tack or by applying a heat shrink sleeve (a special sealing material which was removed before welding).

2 The remaining outlets on the pipework were blanked using suitable sealing materials.

3 An automatic pressure control system. Fig. 9.7. was used to pump the

9.7 Automatic pressure control system.

130 L Carrick

oxygen from the pipework by reducing the pressure to one-tenth of the atmosphere. The pipework system was then isolated and pressure monitored for leakage. Having ensured that the system was leak tight the pipework was backfilled with argon. Once atmospheric pressure was attained the flow rate of purging gas was adjusted to the normal value. All the joints were welded at this stage. If a negative purge pressure was required for all or any of the welds to remove concavity at the 6 o'clock position. it was easily accommodated using the automatic pressure control system. The pipework could be isolated at any time and the purge gas held. This enabled NDT to be carried out and any subsequent repairs exe- cuted before releasing the purge.

Automatic pressure control system

Originally. an automatic negative purge system was developed to apply a small negative purge pressure to stainless steel pipe butt welds over 80mm NB so that concavity of the weld bead would not be a problem. This original system was modified so that the present model (automatic pressure control system) is a much improved piece of equipment capable of carrying out other functions such as vacuum purging. Briefly, the system will pump down a known volume to a specified pressure and maintain it for any period of time. Once the pressure and time cycle requirements are set, the operation will run automatically. A number of fail-safe and warning systems are built into the equipment; furthermore the controls have been designed to help the operators (welders/pipefitters) use the equipment.

Results

Of the 110 welds. two required repair for isolated porosity which was carried out successfully using the existing purge. The weld productivity figures were more than double those normally achieved on the rest of the project.

bedback from the exercise

The vacuum purging technique demonstrated an efficient method for purging long lengths of 80 to 150mm NB pipe. It also demonstrated that changing the fabrication sequence (erection of all pipe. purging all pipe. welding all joints) can give improved productivity.

The operation highlights the need for a different ratio of pipefitterl


welder (2/1) in a team if the welder is to be used more efficiently. It can be postulated that. if eight fitters and one welder were used to erect the pipework. a reduced time scale would result with better utilisation of the welder and much improved overall productivity.

The planning of work packages must be in such detail that personnel. machines, materials. and any specialised equipment with trained operators are available for the duration of the work package. A number of shifts were lost during this particular work package through qualified welders not being available.

This example of the application of a new technique to pipework erection has demonstrated that. although the technique was beneficial. its potential is limited unless traditional work practices are changed. It can also be observed that the improved purging technique also enhances the use of an improved welding technique: automatic orbital welding.

Conclusion In the pipework fabrication field many new techniques are available to the fabricator to improve production. These new techniques are often interfaced with traditional working practices resulting in disappointment at their lack of success in the improvement of overall productivity. Experi- ence has shown that the cost of applying these techniques successfully is very high in terms of resources and time and must start at the design stage and continue through to commissioning. Also, any organisation which aims to achieve success must be ready to change its management structure. manpower ratios. training programmes and planning effort.

Chapter 10

R S E T A Y L O R

Qualification of welding procedures for the chemical process industry

The welding of pressure pipework is controlled by construction codes or standards. These in turn specify the requirements for written weld procedures qualified by test to demonstrate their acceptability. Procedures are therefore mandatory to meet the design code and/or statutory requirements. In addition. the use of such procedures contributes to the achievement of the necessary weld quality to supplement the inspection procedure. This chapter studies those weld procedures required for pressurised piping systems. For non-pressurised systems, the usual code requirements are not applicable but the intention of demonstrating an acceptable procedure is valid, as outlined in BS 4872.'

Construction codes A construction code may be defined as a set of rules of procedure and standards of materials designed to secure uniformity and protect public interest, usually established by a public agency. Codes used in the chemical process industry include the American ASME VIII* for pressure vessels and ANSI B.31.33 for piping. Corresponding British codes are BS 55004 and BS 5135.5 A summary of relevant codes and standards is shown in Table 10.1. Construction codes specify requirements for written weld procedures and procedure qualification tests as well as qualification tests for welders and welding operators. Codes covering qualification of weld procedures and welders are. typically. ASME IX,6 DIN 8560,7 BS 48708 and BS 4871.9

Weld procedures

A welding procedure may be defined as 'The blueprint for the welder'.

132

Qualification of welding procedures 133

%able 10.1 Codes and standards used in chemical plant construction

Construction codes Weld pressure codes

American ASME W1I2 ANSI B.31.1' British BS 5500' BS 513S

ASME 1x6

BS 48708 BS 48719

ASME IX defines a Welding Procedure Specification (WPS) as a written document prepared to provide direction to the welder or welding operator for making production welds to code requirements. Qualification of the weld procedure is carried out by welding testpieces. testing specimens. and recording the welding data in a document known as a Procedure Qualification Record (PQR) in ASME IX and Record of Approval Test of Weld Procedure in BS 4870.

Mlding procedure specification (WPS)

ASME IX and BS 4870 cover the majority of welding processes used in pipework fabrication. including shielded metal-arc. gas tungsten-arc. gas metal-arc. submerged-arc. and gas welding. In addition. ASME IX includes electron beam welding.

Welding variables are grouped as essential or non-essential. Changes in essential variables entail requalification of the procedure: changes in those which are non-essential just require a rewritten procedure without requalification. Codes differ in the definition of variables but. basically, changes which affect the mechanical properties require requalification. A typical WPS is shown in Table 10.2.

Materials with similar metallurgical and welding characteristics are grouped for the purposes of welding procedure approval. Approval on a material within a particular group shall include approval for welding on any other alloy within the same group. Filler materials are similarly grouped by covering classification or usability characteristic (F number in ASME IX) and by chemical composition (A number). A change in either grouping requires requalification of the procedure.

Procedure qualification

A typical Procedure Qualification Record (PQR) format is shown in

134 R S ETaylor

Table 10.2 Welding procedure specification

(Company name)

Welding procedure specification no. - Revision - - -

Welding process (ES)

Joints (QW-402)

Groove design Backing Other

Filler metals (QW-404)

Rocess GTAW

F no. A no. SFA no. AWS no.

SMAW

Size of electrode Size of filler Flux composition Particle size Electrode flux composition Consumable insert Size of tungsten Electrode trade name GTAW

Date __ Supporting PQR No.(s)- - -

__ -

Base metals (QW 403) Material specification:

P no. to P no. Thickness range: Diameter range: Actual welded:

Position (QW 405)

to

Position of groove: Welding progression: Other:

Preheat (QW-406) Preheat temp. Interpass temp. Preheat maintenance Other

Post-weld heat treatment (QW-407)

Temperature Time range Other SMAW

5 G

Originator: Date:


Table 10.2 cont'd

G8s (QW-408)

Shielding gas(es)

Percent composition (mixtures)

Gas backing

Trailing shielding gas composition

Electricd chnractenstks

Current AC/DC

SMAW GTAW

Electrode polarity

Amps

Volts Other

Technique (QW-410)

Weave or bead

Orifice or gas cup size

Initial and interpass cleaning (brushing. grinding. etc.)

Method of backing gouging-

Oscillation

Contact tube-to-work distance

Multipass or single pass

Single or multiple electrodes

Travel speed

(per side)

(range)

Pass Electrode Shielding Volts Amps Speed Manual Bead no. wiredia. gas auto. weave ---

136 R S E Taylor

Table 10.3. The first part references the relevant weld procedure specifications and records the welding data used to weld a testpiece. The second part records the test results of the tested specimens and shows the type and number of tests made.

Classifications of weld positions are shown in Figs. 10.1 and 10.2. In general, qualification in one position qualifies the procedure for other positions. Butt welded test joints qualify fillet welds but qualification by the fillet weld test covers non-pressure-retaining fillet welds only. The procedure is tested by machining and mechanical testing of test specimens. Reduced section tensile, face, and root bend testpieces are machined as indicated in Fig. 10.3.

It is the responsibility of the manufacturer or contractor to carry out the approval tests and prepare the PQR. It is permissible to subcontract the preparation of the weld procedures and the subsequent mechanical testing. but the manufacturer or contractor must accept responsibility for such work.

Performance qualification

The performance qualification test has to be welded in accordance with the relevant WPS. The purpose of this requirement is to ensure that the manufacturer or contractor has determined that welders and welding operators using the procedures are capable of developing the minimum requirements specified for an acceptable weldment. The welder or welding operator who prepares the procedure qualification testpieces which meet the code requirements is also qualified within the limits of performance requirements. A typical format for performance qualification is shown in Table 10.4.

Changes in process variables which would require requalification include weld position. Such variables apply to welder qualification; welding operators qualify by process only to ASME IX Performance weld testpieces may be tested by bend tests or radiography. Alternatively, qualification may be by radiography of the first production weld.

Special code requirements

The system of weld procedure qualification has to cover certain special cases which affect the requirements for specification and test.

Impact tests A particular requirement for certain designs and classes of parent material is to determine the toughness of welded joints to ensure resistance to brittle


Table 10.3 Roecdure Q d i h t i o n Record (PQR)

(See QW-201.2)

COMPANY NAME

PROCtDURE QUALIFICATION RECORD NO. ___DATE

WELDING PROCESS(ES) TYPES

JOINTS (OW-4OZl

Cruarc Deuen Used

I ILLLR MrTALS (OW-404)

Weld Metal Anrlynr A No

suc 01 r i m r o d e

I illcr Metal I No

S I A Specilratlon

AWSC lnssilic~tion

Other

POSTWI LD HCAT TREATMENT (OW-401)

Tempcrdure

~

Other

ELECTHICAL CHARACTERISTICS iOW-409)

rurrcnt

Pahrlly

Amps Volts

Other

(Manual. Aulomitw. Semi-Awl - BAS1 MI TALS tOW403)

Materid S p c .

Type or Crrdc

P. NO ____ 10 r NO.

T h r k n e u

numeter

Othw

POSITION lOW-405)

Poririon 01 Crmw

Wild Progression

(Uphill. Downhill)

Olhci ~~~ ~~~

YRkIIEAT iOW406)

Preheat Temp.

Interpass Temp.

Other

GAS (OW408)

T y p of Gar or Csicr

Composition ofGar Mixture

TECHNIOUF (OW410) StrinK of Weave Bead Owillition Mullipass 01 Single Pass

(per ride)

Singla! or Multiple Electrodes

Travel S p c d

fracture. ASME IX defines the supplementary essential variables which are to be included in the WPS when impact tests are required, e.g. further limitations on parent metal specification and thickness. The welding procedure codes specify Charpy V notch testing to be in accordance with the relevant codes which specify impact test requirements.

138 R S E Taylor

Table 10.3 cont'd

TYPE AND TYPE AND FIGURE NO. RESULT FIGURE NO,

Rocsdu~e Qufl iat ion Record (PQR) (Cont'd) (See QW-201.2

RESULT

TENSILE 'IEST (QW-150)

ULTIMATE ULTIMATE SPECIMEN TOTAL LOAD UNIT STRESS

CHARACTER OF FAILURE C LOCATION

I I I

TOUGHNESS TESTS (OW-170)

T y p of Test

Deposit Analysts

Other

FILLET WELD TEST (QW-180)

R ~ s u l t - S.tisfaetory

T y p and Chamctcr of F.ilurc

PcncV.tlon into Rmnt MeUl Yes. No Yea. No

Macro-Rcrultr

Clock No. Stamp No. ~ Welderr Name Tests conducted by. Laboratory Test No.

We certify that the itatcmcnts in this record PIC correct and that the test welds wcrc prepared. welded and tested in accordance with the rmuilements of Section IX of the ASME Code.

S i n c d (Manufacturer)

Date BY (Detail of record of tests PIC iUuitrstivr only and may he modified to conform to the type and number of twts required by the Code.)


Table 10.4 Welder performance qualification test

Welder’s name: Stencil no: Welding process: Position (ASME code): Weld progression: In accordance with welding procedure no: Material spec. To: P. no.-To P no.- Pipe diameter: Wall t h i c k n e s s : P l a t e thickness: Thickness range this qualifies: Diameter qualified:

Filler metal Trade name &diameter: Specification no: “F no. ‘ A no. Describe filler metal:

Is backing strip used: Flux for submerged arc: Gas for GTAW Rate of flow-torch: Backing purge:

~ ~ ~ ~~ ~~

Type and figure no. Result Type and figure no. Result

Root 1 Face 1

Radiography: % - Results: Test conducted by:

Fillet weld test results Fracture test:

(Describe the location. nature and size of any crack or tearing of the specimen)

Length and percent of defects inches % Macro test-fusion Appearance-fillet size- in.- n, convexity or concavity

in. Test conducted by:

We certify. that the statements in this record are correct and that the test welds were prepared, welded and tested in accordance with the requirements of ASME: Boiler & Pressure Vessel Code. Section 9 BS DIN

Date: Signed (Manufacturer) Lloyd‘s

Signed (Inspection company)

140 RSETaylor

Dissimilar joints For dissimilar parent metal joints a separate PQR is generally required, even though the two metals have been independently qualified using the same procedure. Exceptions are permitted depending upon the parent metal classification.

Combination procedures For welded joints employing more than one process each welding process or procedure shall be qualified either separately or in combination. Like- wise the welder performance qualification may be made on separate testpieces for each procedure or in combination in a single testpiece.

Extension of code requirements

As noted above. the codes permit multiple process procedures to be qualified by qualifying each process separately without ever testing the compatibil- ity of the procedures. In some cases it is preferred that each combination of welding process or procedure or both require qualification as a combination, unless specifically approved by the purchaser.

Care is required in interpreting the rules of the codes with regard to parent metal classification (P number). An arbitrary substitution of materials within one grouping can cause serious problems and unsatisfactory end products. Potential problems are the use of a filler metal which is weaker than the parent metal or use of a filler metal with non- matching chemistry. Care should be taken to ensure that. when joining similar metals. the deposited weld metal matches the chemistry and mechanical properties of the parent metal as closely as possible.

m B m (b) (C)

(a)

10.1 QW-461.4 groove welds in pipe; test positions: a) 1 G (rotated); b) 2 6 ; c) 56; d) 66.


Application to plant modifications/repair Codes and specifications are invariably written in terms of new construc- tions and seldom take account of all requirements for onstream or shutdown maintenance and repair. Joint restraint may lead to cracking problems, particularly when high strength filler metal is used. Product integrity can be ensured if all welds suspected of high restraint are subjected to intermittent stress relieving heat treatments and/or additional non-destructive test methods to detect cracks. Limited accessibility has been the cause of many unsatisfactory welds. Additional procedure and performance qualifications should be performed whenever the environment of the code qualifications fails to simulate production conditions.

Conclusions As chemical process plant has to be constructed and maintained to recognised codes and standards, the qualification of written weld pro-

I / r@" '111

/45"

(a)

H

' I , , \\' @

(d) (e)

10.2 QW-461.6 fillet welds in pipe; test positions: a) IF (rotated); b) 2F; c) 2FR (rotated); d) 4F; e) 5F.

142 R S E Taylor

10.3 QW463.l(d) procedure qualification: pipes 1.5-20mm thickness.

cedures and welder performance may be considered to be a statutory requirement. The approach should be seen, though, as a n important contribution to quality control: the amount of final inspection required is reduced and costly rework is minimised.

References

BS 4872: 1982 Approval testing of welders when welding procedure approval is not required. Pt 1: Fusion welding of steel. Pub1 British Standards Institution. 1982.

ASME Boiler and pressure vessel code. Pt VII1: Pressure vessels. Publ American Society of Mechanical Engineers. 1989.

ANSI/ASME B.31.3: 1987 Chemical plant and petroleum refinery piping. Publ American Society of Mechanical Engineers. 1987.

BS 5500: 1988 Unfired fusion welded pressure vessels. Publ British Standards Institution. 1988.

BS 5135: 1984 Specification of the process of arc welding of carbon and carbon manganese steels. Publ British Standards Institution, 1984.

ASME Boiler and pressure vessel code. Pt 1X: Welding and brazing qualifications. Pub1 American Society of Mechanical Engineers. 1989.

DIN 8560: 1982: Qualification testing of welders for welding steel. Deutsches Institut fur Normung. 1982.

BS 4870: 1981, 1982 Approval testing of welding procedures, Pts 1 and 2. Pub1 British Standards Institution, 1981, 1982.

BS 4871: 1982 Approval testing of welders working to approved welding procedures, Pts 1 and 2. Publ British Standards Institution, 1982.

Chapter 11

J M W O O D

Non-destructive examination of welds in small diameter pipes in the nuclear industry

In the nuclear industry there are many components which require stringent and reliable non-destructive examination. Traditionally, inspection of welds for the detection of body defects has been done either by ultrasonics or radiography and there has been much debate over the years as to the relative merits of each technique. Certainly it can be stated that some defects are more readily detectable by ultrasonics and others by radiography. Techniques in both these areas are continually improving and making inspection more reliable.

Within the author’s company both these approaches are extensively used to examine large numbers of tubular butt welds in steam generator plant. Ultrasonic inspection has been carried out using assisted hand scanning as well as, more recently, automatic inspection equipment, which has the benefits of producing permanent records of the inspection as well as improved repeatability. Ultrasonic inspection of austenitic materials previously caused many problems, but these have now largely been overcome using modern techniques, and austenitic welds are now routinely inspected.

Conventional X-ray and gamma-ray equipments are also frequently used. Recently. microfocus X-ray equipment has been installed which allows very high definition radiography of small components. This is the result of the small focal spot size (0.04mm) which gives excellent geometric sharpness. Sensitivities of 1% can be achieved using this equipment.

Use of low energy isotopes Several years ago a study was carried out of low energy isotopes which were suitable for the inspection of small diameter tubular butt welds. The

143

144 J M Wood

wall thickness of the tubes was typically 3 to 5mm, i.e. below that recommended for inspection by Iridium 192.

To be useful for the radiographic inspection of small diameter tubular butt welds, an isotope should have the following characteristics:

0 Sufficiently small physical size (generally equal to or less than IXlmm or Imm sphere) to achieve an acceptable value of geometric unsharpness:

0 Radiation energy low enough to produce good radiographic contrast; 0 Sufficiently high specific emission to allow acceptable exposure

times; 0 A half-life that is long enough to enable it to be used economically; 0 An initial cost which should not be prohibitive.

This study resulted in the selection of four isotopes with which experi- mental trials were carried out, including the more commonly used Iridium 192 for comparative purposes. The other isotopes were Thulium 170, Ytterbium 169, and Europium 152/154.

The main characteristics of these isotopes are summarised in Table 11.1. It should be noted from these data that, although Ytterbium has a half-life of approximately one-quarter that of Thulium, its specific emission is 50 times greater and the activities of the sources are similar. This results in very short exposure times for Ytterbium compared with a Thulium source of equivalent activity, and also allows Ytterbium to be used over a period of several half-lives while still maintaining acceptable lengths of exposure. It can also be seen from Table 11.1 that Europium has an extremely long half-life (13 years) and, although it is available only in very low activities (20mCi), it has an exceptionally high specific emission (0.6rh/Ci at 1 m).

To obtain information on the exposures required for each source, test radiographs were made using a circular density step wedge, Fig. 11.1. This consisted of twelve strips of steel of thicknesses varying from 1 to 12mm arranged to form a cylinder. This allowed an isotope to be positioned cen- trally at a distance of 50mm from each strip and an exposure made with a film wrapped around the outside. From this an exposure curve for each isotope was plotted, Fig. 11.2.

An important factor in radiographic work is the safe working distance applicable to the technique used. For a classified worker, this is lmm for an unshielded 1Ci Thulium source, whereas for a 1.4Ci Ytterbium source this becomes SSm, and for a 2OmCi Europium source 2.2m. These figures may be compared with a safe working distance of 14m for a 1Ci Iridium source. Because of the short safe working distance for Thulium, it can readily be manipulated by hand on the end of a standard l m rod. We

Non-destructive examination of welds 145

Table 11.1 Characteristics of isotopes studied

Isotope Half-life Specific Principal Source size Maximum emission, energies, keV activity rh/Ci at Im

Thulium 170 128 days 0.0025 84 0.5XOSmm 1Ci l x l m m 5Ci

Ytterbium 169 30.7 days 0.125 177 198 0.3mmsphere 0.15Ci 0.6mm sphere 1.4Ci

Imm sphere 8.OCi

l x l m m 6.7Ci Iridium 192 74 days 0.48 316 468 0.5X0.5m 1Ci

Europium 152/154 I3 years 0.6 122 344 1408 0.5 X 0.5mm 2OmCi

12 mm to 6 mm tapered hole

mm ~

over __ strips

11.1 Circular density step wedge.

146 J M Wood

have used it in this way for the central source inspection of welds since 1974.

Isotope delivery system

The survey of low energy isotopes, however, had identified Ytterbium as being very useful, but it required a remote delivery system to enable its potential to be fully exploited. Initially, to enable test radiographs to be made, it was remotely handled on a pneumatically operated lm rod which removed it from the storage container and positioned it inside a small length of tube containing a weld. The need was therefore identified for the development of a delivery system which was capable of positioning an isotope inside a tube at a weld up to 12m from the open end. I t would need to be capable of negotiating bends in the tube and positioning the isotope radially and axially to within +5% of the tube bore.

A delivery system was therefore designed and manufactured which was capable of inspecting tubes from 18 to IlOmm bore. The system, the gamma-ray isotope projection system (GRIPS), Fig. 11.3. is based on an inflatable rubber head attached to the end of a guide tube, the other end

C .- E ._ V

P VI 0

W

a

10

1

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 Steel thickness, rnrn

11.2 Isotope exposure curve.


of which is connected to the isotope container, Fig. 11.4. The inflatable head contains a low activity isotope in the nose and a scintillation probe connected to a meter is mounted on the outside of the tube near the weld. The guide tube and head are then manually inserted into the tube to be inspected and pushed along it until the scintillation probe and meter detect the reference isotope. The guide tube is then further adjusted to maximise the meter reading, which indicates that the head is accurately

11.3 Gamma-ray Isotope pro)ectlon sysbm (GRIPS).

11.4 GRIPS inflatable head and guide tube.

148 J M Wood

positioned with respect to the weld. The head is then inflated and secured in this position. The film is now wrapped around the weld together with the weld identification, and the main isotope automatically driven out of its container to position itself in the middle of the inflated head. Thus, the isotope is positioned central to the weld and exactly in line with it. After the preset exposure time the isotope retracts into the container.

A panoramic radiograph of the entire weld is thus produced which allows ease of interpretation as well as a single radiograph rather than the multiple exposures required when using an X-ray head. Also, exposures can be made where limited access would not allow an X-ray head to be positioned. and better quality radiographs are attainable.

The equipment developed has been used extensively within the company, both within factories and on nuclear sites, and is now being marketed commercially. In its standard form, the automatic remote control unit which drives out the main isotope is permanently mounted on a trolley which can be manoeuvred to the locality of the inspection. Where additional manoeuvrability is required, it can be supplied in other versions including a hand wind-out version.

Applications of GRIPS The GRIPS system has been used extensively within the company both in factory-based applications and on site. One major factory-based application was the examination of welds in the replacement tube bundles manufactured for the Prototype Fast Reactor (PFR) at Dounreay, Fig. 11.5. The welds were in tubes 24mm OD X 1.8mm and also 25mm OD X 3mm wall.

The system has been used at both the Advanced Gas Cooled (AGR) nuclear sites at Heysham and Torness, where it has been used to inspect welds in the economiser tailpipes and also those connecting the main feed tubes to the boiler. The tailpipes were inspected after welding to the boiler unit while it was in the boiler store before its installation in the reactor annulus, Fig. 11.6. After installation of the boiler, the final weld connecting the feed tube to the boiler unit was inspected by passing the guide tube through the concrete reactor wall via the feed penetration.

The feed tubes in this AGR application were 22mm O D X 3mm wall, increasing to the tailpipe size of 25.4mm OD X 4mm wall. This was below the normal minimum limit of 18mm bore and therefore required the design of special miniature heads and guide tubes. Using the equipment, over 9ooo welds have been inspected at the two AGR sites.

These and other applications have demonstrated that GRIPS has greatly increased the scope and capability of radiographic inspection within the nuclear industry.


11.5 InspecRon of PFR replacement tube bundles using GRIPS.

150 J M Wood

11.6 inspection of AGR boiler tail pipes using GRIPS.

Index

Further reading

This list contains a selection of relevant literature puhlished during the last tenyears.

Anon: 'Remotely controlled repair at Douglas Poine. Nuclear Eng Int 198 I 26 (309)35-40.

Blake MAW, Carrick L and Paton A: "Automatic TIG welding in site fabrication'.Met Con 1983 15 (5) 260-263.

Blanchard B E: 'Process plant piping fabrication: the dream and the reality'. Pipesand Pipelines International 1985 30 (5) 19-28.

British Welded Steel Tube Association: 'Quality assured welded steel tube'. In'The Institution of Engineering Designers Official Reference Book and Buyer'sGuide 1986/7'. Publ Sterling Publications Ltd, 1986,477-479.

Canda F W: ·Choosing the right tube welding process' Fabricator 1987 17 (2) 14-17.Carrick L and Paton A: "Materials and welding of small hore pipework for

nuclear fuel reprocessing plant'. Proc conf ·Welding technology for energyapplications', Oak Ridge National Laboratory, 1982, 201-21l

Cook et al: 'Robotic arc welding of pipe and piping components'. In 'Exploitingrobots in arc welded fabrication' ed J Weston, Publ The Welding Institute, UK.1989.

Ebert H W: 'Repair welding of refinery and chemical plant piping systems.' WeldJ 1984 63 (2) 18-23.

Hahn H K: "Retractable backing system permits GMAW (MAG) welding onopen root pipe joints'. Proc conf 'Offshore mechanics and arctic engineering',The Hague, Mar 1989. publ American Society of Mechanical Engineers, Vol 5315-320.

Hex B and Parkin K: 'The development and applications in production of arange of mechanised tube butt welding equipment'. Proc conf "Welding and theengineer - the challenge of the 80s'. South Africa, March 1983. Vol I, Paper 4. 1-13.

Hirata H et al: 'Development of automatic boiler tube welding system'. MitsubishiHeavy Industries Technical Review 1983 20 (1) 29-33.

Iceland W F and Viri D P: 'Automatic machine welds 2.25%Cr-l%Mo (steel)pipe'. Weld Des & Fab 1982 55 (7) 57-9.

Kapranos P A: 'Compression crack closure effect (CCCE): a basis for an ultrasonic NDE (non-destructive evaluation) technique'. Mat Eval 1984 42 (4) 458-462.

Kiefer J H: 'Welding equipment and process development for inaccessible tubeto-tubesheet welding on a sodium-heated steam generator'. Proc conr.

151

152 Further reading

Gatlinburg~ Tennessee~ USA Oak Ridge National Laboratory~ 16-19 May 1982~

573-593.Klueh R L, King J F and Griffith J L: \A simple test for dissimilar metal welds·.

Weld J 1983 62 (6) 154-159.Lucas Wand Males B 0: \Recent advances in the TIG welding process and its

application to the welding of nuclear components'. Proc conf 'Welding innuclear engineering·~ Aachen~ 22-24 November 1982~ Deutscher Verband furSchweisstechnik. DVS Berichte 75. 1982, 119-123.

Lucas W: 'TIG and plasma welding'. Abington Publishing, 1990.Masubuchi K Hardt D E~ Paynter H M and Untel W: 'Improvement of reliability

of welding by in-process sensing and contror, Proc ASM Confirends in weldingresearch in the United States·, New Orleans. Louisiana~ USA American Societyfor Metals~ 16-18 November 1981. 667-88.

Nadeau F et a]: 'Computerised system automates GMA pipe welding'. WeldJ 199069 (6) 53-59.

Pollard F: 'Fabrication experience with high chromium ferritic steels'. Conf'Production~ fabrication, properties and application of ferritic steels for hightemperature applications~. Warren, PA USA 6-8 October 1981. AmericanSociety for Metals~ 44-47.

Reynolds S D et al: 'Successful welding of tubes-to-tubesheets'. Proc symp 'Shelland tube heat exchangers'~ American Society for Metals~ 1982, 141-89.

Schwartzbart H: 'In-bore gas-tungsten-arc welding of steam generator tube-totubesheet joints'. Weld J 1981 60 (3) 25-36.

Sewell R A: 'Gas purging for pipe welding'. Weld & Met Fab 1989 57 (1) 20-22.Sewell R A: 'Orbital pipeline welding techniques'. Weld & Met Fab 1989 57 (9) 456

460.Smith D S: 'Practical and process problems in heavy pipework fabrication~. Weld

& Met Fab 1989 57 (I) 7-12.Stout R: \When to use insert rings for root pass pipe welds'. Canadian Welder and

Fabricator 1981 72 (4) 14-16.Taylor T and Henderson L: 'Tube·to-tubeplate welding developed for steam gen

erators~. Weld & Met Fab 1982 50 (9) 421-430.Varley J: 'Feed water pipework replaced at Ringhals'. Nuclear Eng lnt 1982 27 (334)

21-26.Yoshida Y et al: 'Development of internal bore welding process for heat

exchangers'. Mitsubishi Heavy Industries Technical Review 1982 19 (2) 122-128.

Index

Illustrations are referenced in italics

aluminium. 107 alloys. 16; Al-Mg, 107

arc oscillation, 48 automatic MIG welding, 93

carbon, low alloy and stainless steels, 16 classification of pipewelding positions. 2 code requirements, 136-40 codes of practice, 71 construction codes. 132 copper, 17

current pulsing. 36-9

'EB insert. 14 edge preparation.

methods of. 113 electrode oscillation. 39-40 electromagnet, 48

filler material control, 70 filler metal selection for A1 alloys. 11 1 front face welding. 84-8 fusible inserts. 14

nickel. 69

gas.

'Grinnel' ring insert. 15

hot pass control. 24

Inconel. 96 internal bore welding. 81-4 isotopes, 144

joint preparation. 36. 67

keyhole plasma. 27-8. 58

management functions, 126

purging. 9-10, 69 shielding. 69

manual, metal arc welding, 2 MIG welding. 93 TIC welds. 91-3 welding, 1

mechanised arc welding, 21-2 metal inert gas, 17-8 MIG. see metal inert gas MMA, see manual metal arc welding

narrow gap hot wire TIG. see TIG. narrow

narrow gap TIC welding. sm TIG. narrow

negative, purge system. 130 nickel, nickel alloys and cupro-nickel, 16

gap hot wire

gap welding

alloys, 93

orbital. equipment, 71 welding. 76

performance qualification, 136 pipe alignment jig. 124 pipe joint geometry, 22 plasma welding. 26. 104 preweld cleaning, 113 procedure qualification, 133-6 pulsed MIG welding, 31-3 pulsed wire. 39 purging, 129

in pipework 124 vacuum. 129

radiography, 144 root penetration, 5 root run technique. 23-4

stainless steel, 48, 54 Synergic MIG process. 95-8

153

154 Index

TIG. see also tungsten inert gas hot wire welding. 28-9 narrow gap hot wire welding. 103-4 narrow gap welding. 100-3

tube to tube plate welding. 81 tungsten inert gas. 8

Ultrasonic inspection. 143

vacuum purging. see purging. vacuum

weld. joint configuration. 110

preparation machines. 74 procedures. 132: specification. 133 quality. 61 records. 71

welder training. 67 welding.

equipment. 40-4. 57 processes. 114-9 techniques. 10-14. 18-20

WPS. see welding procedures. specification

Y-type insert. 15

Documents

Process Pipe and Tube Welding. A Guide to Welding Process Options, Techniques, Equipment, Ndt and Codes of Practice