17-Residual Stress and Distortion

8/12/2019 17-Residual Stress and Distortion

1/20

world centre for materials

joining technology

17.0 RESIDUAL STRESS AND DISTORTION

17.1 WHAT CAUSES DISTORTION?

Because welding involves highly localised heating of joint edges to fuse thematerial, non-uniform stresses are set up in the component because of expansion andcontraction of the heated material.

Initially, compressive stresses are created in the surrounding cold parent metalwhen the weld pool is formed due to the thermal expansion of the hot metal (heataffected zone adjacent to the weld pool. !owever, tensile stresses occur on coolingwhen the contraction of the weld metal and immediate heat affected zone is resistedby the bul" of the cold parent metal.

#he magnitude of thermal stresses induced into the material can be seen bythe volume change in the weld area on solidification and subse$uent cooling to roomtemperature. %or example, when welding &-'n steel, the molten weld metal volumewill be reduced by approximately ) on solidification and the volume of the solidifiedweld metal*heat affected zone will be reduced by a further +) as its temperature fallsfrom the melting point of steel to room temperature.

If the stresses generated from thermal expansion*contraction exceed the yieldstrength of the parent metal, localised plastic deformation of the metal occurs. lasticdeformation causes a permanent reduction in the component dimensions and distortsthe structure.

17.2 WHAT ARE THE MAIN TYPES OF DISTORTION?

istortion occurs in several ways

/ongitudinal shrin"age

#ransverse shrin"age

0ngular distortion

Bowing and dishing

Buc"ling

&ontraction of the weld area on cooling results in both transvrs and!"n#$t%&$na!shrin"age.

1on-uniform contraction (through thic"ness produces an#%!ar distortion aswell as longitudinal and transverse shrin"ing.

2elding Inspection 3+.34ev 5 6un 574esidual 8tress 9 istortion

&opyright :557, #2I /td


2/20


joining technology

%or example, in a single ; butt weld, the first weld run produces longitudinaland transverse shrin"age and rotation. #he second run causes the plates to rotateusing the first weld deposit as a fulcrum. #herefore balanced welding in a double side; butt joint can be used to produce uniform contraction and prevent angular distortion.8imilarly, in a single sided fillet weld, non-uniform contraction will produce angular

distortion of the upstanding leg. ouble-sided fillet welds can therefore be used tocontrol distortion in the upstanding fillet but because the weld is only deposited on oneside of the base plate, angular distortion will now be produced in the plate.

/ongitudinal '"($n# in welded plates happens when the weld centre is notcoincident with the neutral axis of the section so that longitudinal shrin"age in thewelds bends the section into a curved shape. &lad plate tends to bow in two directionsdue to longitudinal and transverse shrin"age of the cladding. #his produces a dishedshape.

D$s)$n# is also produced in stiffened plating. lates usually dish inwards

between the stiffeners, because of angular distortion at the stiffener attachment welds.In plating, long range compressive stresses can cause elastic buc"ling in thin plates,resulting in dishing, bowing or rippling. (see Figure below

Examples of distortion

Increasing the leg length of fillet welds, in particular, increases shrin"age.

17.* WHAT ARE THE FACTORS AFFECTIN+ DISTORTION?

If a metal is uniformly heated and cooled there would be almost no distortion.!owever, because the material is locally heated and restrained by the surroundingcold metal, stresses are generated higher than the material yield stress causingpermanent distortion. #he principal factors affecting the type and degree of distortionare

2elding Inspection 3+.:4ev 5 6un 574esidual 8tress 9 istortion



3/20


joining technology

arent material properties

0mount of restraint

6oint design

art fit-up

2elding procedure

17.*.1 PARENT MATERIAL PROPERTIES

arent material properties, which influence distortion, are coefficient of thermalexpansion and specific heat per unit volume. 0s distortion is determined by expansionand contraction of the material, the coefficient of thermal expansion of the material

plays a significant role in determining the stresses generated during welding and,hence, the degree of distortion. %or example, as stainless steel has a highercoefficient of expansion than plain carbon steel, it is more li"ely to suffer fromdistortion.

17.*.2 RESTRAINT

If a component is welded without any external restraint, it distorts to relieve thewelding stresses. 8o, methods of restraint, such as xcessive joint gap can also increase the degree of distortion by increasing theamount of weld metal needed to fill the joint. #he joints should be ade$uately tac"edto prevent relative movement between the parts during welding.




4/20


joining technology

17.*./ WELDIN+ PROCEDURE

#his influences the degree of distortion mainly through its effect on the heatinput. 0s welding procedures are usually selected for reasons of $uality andproductivity, the welder has limited scope for reducing distortion. 0s a general rule,

weld volume should be "ept to a minimum. 0lso, the welding se$uence and techni$ueshould aim to balance the thermally induced stresses around the neutral axis of thecomponent.

17.- DISTORTION PREENTION Y PRESETTIN+ PREENDIN+OR USE OF RESTRAINT

istortion could often be prevented at the design stage, for example, by placingthe welds about the neutral axis, reducing the amount of welding and depositing the

weld metal using a balanced welding techni$ue. In designs where this is not possible,distortion may be prevented by one of the following methods

pre-setting of parts

pre-bending of parts

use of restraint

#he techni$ue chosen will be influenced by the size and complexity of thecomponent or assembly, the cost of any restraining e$uipment and the need to limit

residual stresses.

Pre-setting of parts to produce correct alignment after welding

a) Pre-setting of fillet joint to prevent angular distortionb) Pre-setting of butt joint to prevent angular distortion

17.-.1 PRESETTIN+ OF PARTS

#he parts are pre-set and left free to move during welding (see figure above. Inpractice, the parts are pre-set by a pre-determined amount so that distortion occurringduring welding is used to achieve overall alignment and dimensional control.

2elding Inspection 3+.?4ev 5 6un 574esidual 8tress 9 istortion



5/20


joining technology

#he main advantages compared with the use of restraint are that there is noexpensive e$uipment needed and there will be lower residual stress in the structure.

@nfortunately, as it is difficult to predict the amount of pre-setting needed toaccommodate shrin"age, a number of trial welds will be re$uired. %or example, when

''0 or 'IA*'0A welding butt joints, the joint gap will normally close ahead ofwelding when submerged arc welding the joint may open up during welding. 2hencarrying out trial welds, it is also essential that the test structure is reasonablyrepresentative of the full size structure in order to generate the level of distortion li"elyto occur in practice. %or these reasons, pre-setting is a techni$ue more suitable forsimple components or assemblies.

Pre-bending, using strongbacks and wedges, toaccommodate angular distortion in thin plates.

17.-.2 PREENDIN+ OF PARTS

re-bending, or pre-springing the parts before welding is a techni$ue used topre-stress the assembly to counteract shrin"age during welding. 0s shown in above

figure,pre-bending by means of strongbac"s and wedges can be used to pre-set aseam before welding to compensate for angular distortion. 4eleasing the wedges afterwelding will allow the parts to move bac" into alignment.

#he main photograph shows the diagonal bracings and centre jac" used to pre-bend the fixture, not the component. #his counteracts the distortion introduced thoughout-of-balance welding.

17.-.* USE OF RESTRAINT

Because of the difficulty in applying pre-setting and pre-bending, restraint is the

more widely practised techni$ue. #he basic principle is that the parts are placed inposition and held under restraint to minimise any movement during welding. 2henremoving the component from the restraining e$uipment, a relatively small amount ofmovement will occur due to loc"ed-in stresses. #his can be cured by either applying asmall amount of pre-set or stress relieving before removing the restraint.

2hen welding assemblies, all the component parts should be held in thecorrect position until completion of welding and a suitably balanced fabricationse$uence used to minimise distortion.

2elding Inspection 3+.C4ev 5 6un 574esidual 8tress 9 istortion



6/20


joining technology

2elding with restraint will generate additional residual stresses in the weld,which may cause crac"ing. 2hen welding susceptible materials, a suitable weldingse$uence and the use of preheating will reduce this ris".

4estraint is relatively simple to apply using clamps, jigs and fixtures to hold the

parts during welding.

17.-.*.1 WELDIN+ ,I+S AND FI3TURES

6igs and fixtures are used to locate the parts and to ensure that dimensionalaccuracy is maintained whilst welding. #hey can be of a relatively simple construction,as shown in figure a below but the welding engineer will need to ensure that thefinished fabrication can be removed easily after welding.

17.-.*.2 FLE3ILE CLAMPS

0 flexible clamp (Fig b below can be effective not only in applying restraint butalso in setting up and maintaining the joint gap (it can also be used to close a gap thatis too wide.

0 disadvantage is that as the restraining forces in the clamp will be transferredinto the joint when the clamps are removed, the level of residual stress across the jointcan be $uite high.

a4 W!&$n# 5$# '4 F!6$'! !a89s

4 Str"n#'a:s ($t) (s &4 F%!!; (!&& str"n#'a:s

Restraint techniues to prevent distortion




7/20


joining technology

17.-.*.* STRON+AC


8/20


joining technology

17./.1 ELIMINATION OF WELDIN+

0s distortion and shrin"age are an inevitable result of welding, good designre$uires that not only the amount of welding is "ept to a minimum, but also thesmallest amount of weld metal is deposited. 2elding can often be eliminated at the

design stage by forming the plate or using a standard rolled section, as shown in thisfigure.

Elimination of welds b!" a) forming the plate# b) use of rolled or extruded section

If possible, the design should use intermittent welds rather than a continuousrun, to reduce the amount of welding. %or example, in attaching stiffening plates, asubstantial reduction in the amount of welding can often be achieved whilstmaintaining ade$uate strength.

17./.2 WELD PLACEMENT

lacing and balancing of welds are important in designing for minimum

distortion. #he closer a weld is positioned to the neutral axis of a fabrication, the lowerthe leverage effect of the shrin"age forces and the final distortion. >xamples of poorand good designs are shown in the figure below.

$istortion ma! be reduced b! placing the welds around the neutral axis

0s most welds are deposited away from the neutral axis, distortion can beminimised by designing the fabrication so the shrin"age forces of an individual weldare balanced by placing another weld on the opposite side of the neutral axis.2henever possible, welding should be carried out alternately on opposite sides,instead of completing one side first. In large structures, if distortion is occurringpreferentially on one side, it may be possible to ta"e corrective actions, for example,by increasing welding on the other side to control the overall distortion.

2elding Inspection 3+.D4ev 5 6un 574esidual 8tress 9 istortion



9/20


joining technology

17./.* REDUCIN+ THE OLUME OF WELD METAL

#o minimise distortion, as well as for economic reasons, the volume of weldmetal should be limited to the design re$uirements. %or a single-sided joint, the cross-section of the weld should be "ept as small as possible to reduce the level of angular

distortion, as illustrated in below figure.

Reducing the amount of angular distortion and lateral shrinkage

2ays of reducing angular distortion and lateral shrin"age

a reducing the volume of weld metalb using single pass weld

6oint preparation angle and root gap should be minimised providing the weldcan be made satisfactorily. #o facilitate access, it may be possible to specify a largerroot gap and smaller preparation angle. By cutting down the difference in the amountof weld metal at the root and the face of the weld, the degree of angular distortion will

be correspondingly reduced. Butt joints made in a single pass using deep penetrationhave little angular distortion, especially if a closed butt joint can be welded (%igureabove. %or example, thin section material can be welded using plasma and laserwelding processes and thic" section can be welded, in the vertical position, usingelectrogas and electroslag processes. 0lthough angular distortion can be eliminated,there will still be longitudinal and transverse shrin"age.

In thic" section material, as the cross sectional area of a double-; jointpreparation is often only half that of a single-; preparation, the volume of weld metalto be deposited can be substantially reduced. #he double-; joint preparation alsopermits balanced welding about the middle of the joint to eliminate angular distortion.

0s weld shrin"age is proportional to the amount of weld metal both poor joint fit-up and over-welding will increase the amount of distortion. 0ngular distortion in filletwelds is particularly affected by over-welding. 0s design strength is based on throatthic"ness, over-welding to produce a convex weld bead does not increase theallowable design strength but it will increase the shrin"age and distortion.

2elding Inspection 3+.E4ev 5 6un 574esidual 8tress 9 istortion



10/20


joining technology

17./.- REDUCIN+ THE NUMER OF RUNS

#here are conflicting opinions on whether it is better to deposit a given volumeof weld metal using a small number of large weld passes or a large number of smallpasses. >xperience shows that for a single-sided butt joint, or a single-side fillet weld,

a large single weld deposit gives less angular distortion than if the weld is made with anumber of small runs. Aenerally, in an unrestrained joint, the degree of angulardistortion is approximately proportional to the number of passes.

&ompleting the joint with a small number of large weld deposits results in morelongitudinal and transverse shrin"age than a weld completed in a larger number ofsmall passes. In a multi-pass weld, previously deposited weld metal provides restraint,so the angular distortion per pass decreases as the weld is built up. /arge depositsalso increase the ris" of elastic buc"ling particularly in thin section plate.

17././ USE OF ALANCED WELDIN+Balanced welding is an effective means of controlling angular distortion in a

multi-pass butt weld by arranging the welding se$uence to ensure that angulardistortion is continually being corrected and not allowed to accumulate during welding.&omparative amounts of angular distortion from balanced welding and welding oneside of the joint first are shown schematically in this figure. #he balanced weldingtechni$ue can also be applied to fillet joints.

%alanced welding to reduce the amount of angular distortion

If welding alternately on either side of the joint is not possible, or if one side hasto be completed first, an asymmetrical joint preparation may be used with more weldmetal being deposited on the second side. #he greater contraction resulting fromdepositing the weld metal on the second side will help counteract the distortion on thefirst side.




11/20


joining technology

17./.> EST PRACTICE

#he following design principles can control distortion

eliminate welding by forming the plate and using rolled or extruded sections

minimise the amount of weld metal

do not over weld

use intermittent welding in preference to a continuous weld pass

place welds about the neutral axis

balance the welding about the middle of the joint by using a double-; joint inpreference to a single-; joint

0dopting best practice principles can have surprising cost benefits. %orexample, for a design fillet leg length of 7mm, depositing an Dmm-leg length will resultin the deposition of C+) additional weld metal. Besides the extra cost of depositingweld metal and the increase ris" of distortion, it is costly to remove this extra weldmetal later. !owever, designing for distortion control may incur additional fabricationcosts. %or example, the use of a double-; joint preparation is an excellent way toreduce weld volume and control distortion, but extra costs may be incurred inproduction through manipulation of the wor"piece for the welder to access the reverseside.

17.> DISTORTION PREENTION Y FARICATION TECHNIUES

17.>.1 ASSEMLY TECHNIUES

In general, the welder has little influence on the choice of welding procedurebut assembly techni$ues can often be crucial in minimising distortion. #he principalassembly techni$ues are

tac" welding

bac"-to-bac" assembly

stiffening

17.>.1.1 TAC< WELDIN+

#ac" welds are ideal for setting and maintaining the joint gap but can also beused to resist transverse shrin"age. #o be effective, thought should be given to thenumber of tac" welds, their length and the distance between them. 2ith too few, there

is the ris" of the joint progressively closing up as welding proceeds. In a long seam,2elding Inspection 3+.334ev 5 6un 574esidual 8tress 9 istortion



12/20


joining technology

using ''0 or 'IA*'0A, the joint edges may even overlap. It should be noted thatwhen using the submerged arc process, the joint might open up if not ade$uatelytac"ed.

#he tac" welding se$uence is important to maintain a uniform root gap along

the length of the joint. #hree alternative tac"-welding se$uences are shown in thefigure below

tac" weld straight through to the end of the joint (figure a. It is necessary to

clamp the plates or to use wedges to maintain the joint gap during tac"ing

tac" weld one end and then use a bac" stepping techni$ue for tac"ing the rest

of the joint (figure b

tac" weld the centre and complete the tac" welding by bac" stepping (figure c.

&lternative procedures used for tack welding to prevent transverse shrinkage

irectional tac"ing is a useful techni$ue for controlling the joint gap, forexample closing a joint gap which is (or has become too wide.

2hen tac" welding, it is important that tac"s which are to be fused into the mainweld, are produced to an approved procedure using appropriately $ualified welders.#he procedure may re$uire preheat and an approved consumable as specified for themain weld. 4emoval of the tac"s also needs careful control to avoid causing defects inthe component surface.

17.>.1.2 AC


13/20


joining technology

a assemblies tac"ed together before welding

b use of wedges for components that distort onseparation after welding

%ack-to-back assembl! to control distortion when welding two identical components

17.>.1.* STIFFENIN+

'ongitudinal stiffeners prevent bowing in butt welded thin plate joints

/ongitudinal shrin"age in butt-welded seams often results in bowing, especiallywhen fabricating thin plate structures. /ongitudinal stiffeners in the form of flats orangles, welded along each side of the seam (%ig just above are effective inpreventing longitudinal bowing. 8tiffener location is important they must be placed ata sufficient distance from the joint so they do not interfere with welding, unless locatedon the reverse side of a joint welded from one side.

17.>.2 WELDIN+ PROCEDURE

0 suitable welding procedure is usually determined by productivity and $ualityre$uirements rather than the need to control distortion. 1evertheless, the weldingprocess, techni$ue and se$uence do influence the distortion level.

W!&$n# Pr"ssAeneral rules for selecting a welding process to prevent angular distortion are

deposit the weld metal as $uic"ly as possible

use the least number of runs to fill the joint




14/20


joining technology

@nfortunately, selecting a suitable welding process based on these rules mayincrease longitudinal shrin"age resulting in bowing and buc"ling.

In manual welding, 'IA*'0A, a high deposition rate process, is preferred to''0. 2eld metal should be deposited using the largest diameter electrode (''0, or

the highest current level ('IA*'0A, without causing lac"-of-fusion imperfections. 0sheating is much slower and more diffuse, gas welding normally produces moreangular distortion than the arc processes.

'echanised techni$ues combining high deposition rates and high weldingspeeds have the greatest potential for preventing distortion. 0s the distortion is moreconsistent, simple techni$ues such as pre-setting are more effective in controllingangular distortion.

W!&$n# T)n$@%Aeneral rules for preventing distortion are

"eep the weld (fillet to the minimum specified size

use balanced welding about the neutral axis

"eep the time between runs to a minimum

&ngular distortion of the joint as determined b! the number of runs in the fillet weld

In the absence of restraint, angular distortion in both fillet and butt joints will bea function of the joint geometry, weld size and the number of runs for a given crosssection. 0ngular distortion (measured in degrees as a function of the number of runsfor a 35mm leg length fillet weld is shown in the figure above.

If possible, balanced welding around the neutral axis should be done, forexample on double-sided fillet joints, by two people welding simultaneously. In buttjoints, the run order may be crucial in that balanced welding can be used to correctangular distortion as it develops.

2elding Inspection 3+.3?4ev 5 6un 574esidual 8tress 9 istortion



15/20


joining technology

a Bac"-step welding

b 8"ip welding

(se of welding direction to control distortion

W!&$n# S@%n#he welding se$uence, or direction, of welding is important and should be

towards the free end of the joint. %or long welds, the whole of the weld is notcompleted in one direction. 8hort runs, for example using the bac"-step or s"ipwelding techni$ue, are very effective in distortion control (figure just above

Bac"-step welding involves depositing short adjacent weld lengths in theopposite direction to the general progression (a in above figure

8"ip welding is laying short weld lengths in a predetermined, evenly spaced,se$uence along the seam (b in above figure. 2eld lengths and the spacesbetween them are generally e$ual to the natural run-out length of oneelectrode. #he direction of deposit for each electrode is the same, but it is not

necessary for the welding direction to be opposite to the direction of generalprogression.

17.>.* EST PRACTICE

#he following fabrication techni$ues are used to control distortion

using tac" welds to set up and maintain the joint gap

identical components welded bac" to bac" so welding can be balanced about

the neutral axis

attachment of longitudinal stiffeners to prevent longitudinal bowing in butt welds

of thin plate structures

where there is choice of welding procedure, process and techni$ue should aim

to deposit the weld metal as $uic"ly as possible 'IA*'0A in preference to''0 or gas welding and mechanised rather than manual welding

in long runs, the whole weld should not be completed in one direction bac"-step or s"ip welding techni$ues should be used.

2elding Inspection 3+.3C4ev 5 6un 574esidual 8tress 9 istortion



16/20


joining technology

17.7 DISTORTION CORRECTIE TECHNIUES

>very effort should be made to avoid distortion at the design stage and byusing suitable fabrication procedures. 0s it is not always possible to avoid distortionduring fabrication, several well-established corrective techni$ues can be employed.

!owever, rewor"ing to correct distortion should not be underta"en lightly as it is costlyand needs considerable s"ill to avoid damaging the component.

In this issue, general guidelines are provided on


17/20


joining technology

17.7.1.1 EST PRACTICE FOR MECHANICAL STRAI+HTENIN+

#he following should be adopted when using pressing techni$ues to removedistortion

@se pac"ing pieces which will over correct the distortion so that spring-bac" willreturn the component to the correct shape

&hec" that the component is ade$uately supported during pressing to prevent

buc"ling

@se a former (or rolling to achieve a straight component or produce a curvature

0s unsecured pac"ing pieces may fly out from the press, the following safe

practice must be adopted- bolt the pac"ing pieces to the platen- place a metal plate of ade$uate thic"ness to intercept the


18/20


joining technology

17.7.2.1 SPOT HEATIN+

*pot heating for correcting buckling

8pot heating (figure above, is used to remove buc"ling, for example when arelatively thin sheet has been welded to a stiff frame. istortion is corrected by spotheating on the convex side. If the buc"ling is regular, the spots can be arrangedsymmetrically, starting at the centre of the buc"le and wor"ing outwards.

17.7.2.2 LINE HEATIN+

'ine heating to correct angular distortion in a fillet weld

!eating in straight lines is often used to correct angular distortion, for example,in fillet welds (above figure. #he component is heated along the line of the weldedjoint but on the opposite side to the weld so the induced stresses will pull the flangeflat.

17.7.2.* WED+ESHAPED HEATIN+

#o correct distortion in larger complex fabrications it may be necessary to heatwhole areas in addition to employing line heating. #he pattern aims at shrin"ing onepart of the fabrication to pull the material bac" into shape.

2elding Inspection 3+.3D4ev 5 6un 574esidual 8tress 9 istortion



19/20


joining technology

(se of wedge shaped heating to straighten plate

0part from spot heating of thin panels, a wedge-shaped heating zone should beused, (figure above from base to apex and the temperature profile should be

uniform through the plate thic"ness. %or thic"er section material, it may be necessaryto use two torches, one on each side of the plate.

0s a general guideline, to straighten a curved plate (%ig. 3D wedge dimensionsshould be

3. /ength of wedge - two-thirds of the plate width

:. 2idth of wedge (base - one sixth of its length (base to apex

#he degree of straightening will typically be Cmm in a m length of plate.

2edge-shaped heating can be used to correct distortion in a variety of situations,(8ee following figure

3. 8tandard rolled section, which needs correction in two planes, (%ig. a

:. Buc"le at edge of plate as an alternative to rolling (%ig b

. Box section fabrication, which is distorted out of plane (%ig. c

a4 Stan&ar& r"!!&st! st$"n

'4 %:!& " 9!at 4 "6 a'r$at$"n

+edge shaped heating to correct distortion

2elding Inspection 3+.3E4ev 5 6un 574esidual 8tress 9 istortion



20/20


joining technology

17.7.2.- +ENERAL PRECAUTIONS

#he dangers of using thermal straightening techni$ues are the ris" of over-shrin"ing too large an area or causing metallurgical changes by heating to too high atemperature. 0s a general rule, when correcting distortion in steels the temperature of

the area should be restricted to approximately to 75G - 7C5G& - dull red heat.If the heating is interrupted, or the heat lost, the operator must allow the metal to cooland then begin again.

17.7.2./ EST PRACTICE FOR DISTORTION CORRECTION YTHERMAL HEATIN+

#he following should be adopted when using thermal techni$ues to remove distortion

use spot heating to remove buc"ling in thin sheet structures

other than in spot heating of thin panels, use a wedge-shaped heating

techni$ue

use line heating to correct angular distortion in plate

restrict the area of heating to avoid over-shrin"ing the component

limit the temperature to 75G to 7C5G& (dull red heat in steels to prevent

metallurgical damage

in wedge heating, heat from the base to the apex of the wedge, penetrateevenly through the plate thic"ness and maintain an even temperature

.

2elding Inspection 3+.:54ev 5 6un 574esidual 8tress 9 istortion

&opyright :557 #2I /td

Documents

17-Residual Stress and Distortion