Metal Joining Processes

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’s employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Welding For additional information on this subject, contactFile Reference: COE11401 A.A. Omar on 874-6127

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Metal Joining Processes Used In Saudi Aramco

Engineering Encyclopedia Welding


Saudi Aramco DeskTop Standards

Contents Pages

COURSE SCHEDULE......................................................................................................... 1

COURSE INTRODUCTION................................................................................................ 1

Course Goals............................................................................................................. 1

INTRODUCTION................................................................................................................ 2

FUNDAMENTALS OF WELDING ..................................................................................... 3

Importance of Welding .............................................................................................. 3

Basic Welding Circuit................................................................................................ 3

Principle of Electric Circuit ............................................................................ 4

Welding Power Sources ............................................................................................ 5

Welding Electrodes ................................................................................................... 5

Consumable Electrodes.................................................................................. 5

Nonconsumable Electrodes ............................................................................ 6

Weld Joint Description .............................................................................................. 8

Fusion Zone................................................................................................... 9

Fusion Line...................................................................................................10

Heat-Affected Zone (HAZ)...........................................................................10

Weld Joint Properties ...............................................................................................11

Tensile Strength............................................................................................11

Ductility........................................................................................................13

Hardness.......................................................................................................15

Impact Strength ............................................................................................16

IDENTIFY THE MAJOR JOINING PROCESSES USED IN SAUDI ARAMCO...............18

Shielded Metal Arc Welding .....................................................................................18

Process Description ......................................................................................18

Power Supply ...............................................................................................19

Electric Leads ...............................................................................................20

Electrode Holder ..........................................................................................20

Filler Metal Form..........................................................................................20




Purpose of Covering .....................................................................................21

Common Uses ..............................................................................................21

Advantages...................................................................................................21

Disadvantages...............................................................................................22

Gas Tungsten Arc Welding.......................................................................................22


Power Supply ...............................................................................................23

Electric Leads ...............................................................................................24

Torch............................................................................................................24


Purpose of Shielding Gas ..............................................................................25

Common Uses ..............................................................................................25

Advantages...................................................................................................26

Disadvantages...............................................................................................26

Gas Metal Arc Welding ............................................................................................27


Power Supply ...............................................................................................29

Electric Leads ...............................................................................................30

Wire Feed Gun .............................................................................................30


Purpose of Shielding Gas ..............................................................................30

Common Uses ..............................................................................................31

Advantages...................................................................................................31

Disadvantages...............................................................................................31

Flux-Cored Arc Welding ..........................................................................................32


Power Supply ...............................................................................................33

Electric Leads ...............................................................................................33

Wire Feed Gun .............................................................................................33


Purpose of Flux ............................................................................................34




Common Uses ..............................................................................................34

Advantages...................................................................................................34

Disadvantages...............................................................................................35

Submerged Arc Welding...........................................................................................35


Power Supply ...............................................................................................37

Electric Leads ...............................................................................................37

Wire Feed Gun .............................................................................................37

Flux Hopper .................................................................................................37


Purpose of Flux ............................................................................................38

Common Uses ..............................................................................................38

Advantages...................................................................................................38

Disadvantages...............................................................................................39

Stud Welding ...........................................................................................................39


Power Supply ...............................................................................................41

Electric Leads ...............................................................................................41

Stud Gun ......................................................................................................41

Stud Form ....................................................................................................42

Common Uses ..............................................................................................42

Advantages...................................................................................................42

Disadvantages...............................................................................................43

Oxyacetylene Welding ..............................................................................................43


Fuel Gas Combustion....................................................................................45

Gas Supplies .................................................................................................46

Torch............................................................................................................46


Common Uses ..............................................................................................46

Cutting .........................................................................................................47




Preheating.....................................................................................................47

Advantages...................................................................................................47

Disadvantages...............................................................................................47

Electric Resistance Welding......................................................................................48


Major Components .......................................................................................49

Common Uses ..............................................................................................50

Brazing ....................................................................................................................50

Completed Braze Joint..................................................................................50

Typical Interface Gaps ..................................................................................51

Soldering..................................................................................................................52

Completed Solder Joint.................................................................................52

GLOSSARY........................................................................................................................53

WORK AIDS ......................................................................................................................54

Work Aid 1. How To Describe The Fundementals Of Welding .................................54

Work Aid 2. How To Identify The Major Joining Processes Used In SaudiAramco ....................................................................................................................54

BIBLIOGRAPHY ...............................................................................................................62




Table of Figures Pages

Figure 1. Basic Welding Circuit ................................................................................. 4

Figure 2. Consumable Electrode ................................................................................ 6

Figure 3. Nonconsumable Electrode ......................................................................... 8

Figure 4. Five Basic Weld Joint Types ...................................................................... 9

Figure 5. Full Penetration Weld Joint........................................................................10

Figure 6. Tensile Specimen Removal Locations .......................................................12

Figure 7. Guided-Bend Test Jigs..............................................................................14

Figure 8. Hardness Testing of Weld Test Coupons ..................................................16

Figure 9. Impact Specimen Removal Locations........................................................17

Figure 10. SMAW Welding Circuit...........................................................................19

Figure 11. The SMAW Process ...............................................................................19

Figure 12. GTAW Welding Circuit ...........................................................................23

Figure 13. The GTAW Process................................................................................23

Figure 14. Purge Scheme for a Piping Weld.............................................................25

Figure 15. GMAW Welding Circuit ..........................................................................28

Figure 16. The GMAW Process...............................................................................28

Figure 17. FCAW Welding Circuit............................................................................32

Figure 18. The FCAW Process ................................................................................33

Figure 19. SAW Welding Circuit ..............................................................................36

Figure 20. The SAW Process...................................................................................37

Figure 21. SW Welding Circuit.................................................................................40

Figure 22. Sequence of the SW Process...................................................................41

Figure 23. Oxyacetylene Welding Equipment............................................................44

Figure 24. The OAW Process ..................................................................................45

Figure 25. HFRW Pipe Fabrication...........................................................................49

Figure 26. Completed Braze Joint.............................................................................51

Figure 27. Completed Solder Joint...........................................................................52



Saudi Aramco DeskTop Standards 1

COURSE SCHEDULE

COURSE INTRODUCTION

Course Goals

On completion of the Welding Course (COE 1114), the participant will be able to:

• Apply Saudi Aramco standards to situations in which welding is used.

• Perform basic quality control inspections to ensure proper welds.

• Determine the personnel qualifications for welders.

• Determine proper weld qualifications.

• Recognize improperly joined metals.




INTRODUCTION

This module provides information on the various welding processes that are commonly used atSaudi Aramco. The primary emphasis is placed on a basic understanding of the fundamentals ofarc welding and key welding processes, such as, shielded metal arc, gas tungsten arc, gas metalarc, flux cored arc, submerged arc, stud, and oxyacetylene.The material is presented in the following sections:

• Fundamentals of Welding

• Joining Processes

Note: All references to standards within this module are listed in the Bibliographywith the latest publication date. All references were correct and current at thepublication date of this module which is listed in the revision page of thismodule. For simplicity in reading, the titles of publications will only be giventhe first time they appear in the module. Refer to the Bibliography if titleinformation is required.




FUNDAMENTALS OF WELDING

This section provides background information on the fundamentals of welding and will include thefollowing topics:

• Importance of welding

• Basic welding circuits

• Welding power sources

• Welding electrodes

• Weld joint description

• Weld joint properties

Importance of Welding

Welding is used in all industries to fabricate, maintain and repair equipment and facilities. Weldingapplications are used in petrochemical facilities to accomplish the following tasks:

• Manufacture steel pipe at the steel mills

• Fabricate piping spools in the fabricating shops

• Prefabricate structural steel members in the fabricating shops

• Maintain corrosion-resistance cladding in pressure vessels

• Replace corroded sections of pipe or vessels

• Repair cracked welds caused by service fatigue

• Construct drilling platforms

• Construct structural steel frames in the field

• Construct oil tanks in the field

• Modify or alter existing piping systems

• Repair mechanical equipment that has failed or become worn

The process of welding produces localized coalescence of metals by heating the metals to asuitable temperature. Localized coalescence can occur with or without the application of pressureor filler metals. Coalescence means a “growing together” or a “growing into one body”.

Fusion is the complete blending of two pieces of base to form a weld. This can be accomplishedwith or without the use of filler metal. Filler metal is not used in all applications. When fillermetal is absent, the welding process merely melts both pieces of base metal together.




Basic Welding Circuit

Figure 1 shows a basic welding circuit. This circuit represents a shielded metal arc weldingcircuit in which the power supply is connected to the work piece through use of a ground cable.The current flows from the negative terminal of the power supply to the work piece (cathode).The electrode holder becomes the anode through connection of a welding cable between theelectrode holder and the positive terminal of the power supply. When the power supply isenergized and the electrode tip is touched to the work piece, and then withdrawn and held closeto the spot of contact, an arc is created across the gap. The arc produces a temperature of about4000°C (6,500oF) at the tip of the electrode. This temperature is more than adequate for meltingmost metals.

Figure 1. Basic Welding Circuit

The circuit shown in Figure 1 is a “reverse polarity” welding circuit because the negative groundprovides the power. In a straight polarity welding circuit, the current flows from the positiveterminal of the power supply to the work piece (anode). The electrode holder is connected to thenegative terminal of the power supply with a welding cable that becomes the cathode.

Principle of Electric Circuit

To understand the principle of a typical electric welding circuit, the nature of the current and thetransport medium must be examined. During arc welding, the space between the electrode andthe work piece is the point at which the arc is initiated and maintained. This point is referred to asthe “arc plasma”. The welding arc is characterized as a high current, low voltage arc that requiresa high concentration of electrons to carry the current. Negative electrons are emitted from thecathode (work piece) and flow along the negative ions of the plasma to the positive anode(electrode). Positive ions flow in the reverse direction.




The cathode, anode, and arc plasma are all areas of heat generation. Heat is mainly generated inthe work piece when the positive ions strike the surface of the work piece. Heat at the electrodeis mainly generated by the electrons. These electrons have been accelerated by the arc voltage asthey pass through the plasma. The electrons then give up their energy as heat when they strikethe electrode.

Welding Power Sources

All arc welding processes require a continuous energy source. This energy source, which is morecommonly referred to as a welding machine or power supply, must supply electrical current that iseither alternating (AC) or direct (DC) to the welding electrode through a device that enables theprecise control of the current.

Welding machines are classified by the type of current (AC or DC) and the voltage output(variable or constant). A further classification designates the method by which energy is suppliedto the welding machine, such as, directly from a power line, or from a gasoline or diesel engine.The main function of any type of welding machine is to supply the type of needed to weld.

Transformers, rectifiers, and generators are the three basic types of welding machines. Thetransformer welding machines are a voltage step-down transformer that changes high voltage, lowamperage AC input current to low voltage, high amperage AC welding current.

The transformer-rectifier welding machines are similar to the transformer machines. Thedifference between these machines is the addition of a rectifier that allows the transformer-rectifierwelding machine to also produce DC welding current.

Generators are either motor-driven or engine-driven. Motor-driven generators convert an ACinput current into a DC welding current. Engine-driven (either gasoline or diesel) generators canproduce both AC and DC welding current.

Welding Electrodes

Several types of welding electrodes are available for different welding processes and materials.The original welding electrode was a piece of bare metal wire. Bare wire electrodes are still usedtoday. Bare wire electrodes are manufactured in 36" straight lengths that range in sizes from1/16" to 1/8" in diameter. Bare wire electrodes are also manufactured in continuous lengths thatare wrapped on spools and that range in sizes from 0.035" to 0.045" in diameter.

Covered electrodes are very common and are readily adaptable to field welding applications.These electrodes have a bare metal rod as a core and are covered with baked-on flux that providessuch functions as shielding from the atmosphere, deoxidation, and arc stabilization. Flux can alsoserve as a source of metallic additions to the weld. Flux cored electrodes are similar to coatedelectrodes. Each of these electrodes consist of a tubular wire that is filled with a flux material.These electrodes are generally manufactured in sizes from 0.045" up to 0.063" in diameter.




Consumable Electrodes

A consumable electrode is one that is consumed in the heat of the welding arc and adds metal tothe weld. Consumable electrodes are considered to be filler metal. Figure 2 shows a consumableelectrode used for shielded metal arc welding to make a weld on base metal.

Figure 2. Consumable Electrode

As the solid metal electrode is drawn near the base metal, the electrical circuit is completed and anarc is created. The solid metal electrode is heated in the arc and melts. As the electrode melts,small metal droplets are transferred from the solid metal electrode to the molten weld metal. Asthe molten weld metal cools under the slag, solidified weld metal that is fused to the base metal isformed. When the solid metal electrode is heated and burned in the arc, the covering that is onthe solid metal electrode forms a protective gas over the molten weld metal.

Some of the electrode covering material is metallic and becomes molten weld metal. Theremainder of the electrode covering material melts and forms over the solidified weld metal aceramic cover called slag. COE 114.04 gives more detail of electrode coverings.

As the solid metal electrode melts and becomes shorter, the consumable electrode must becontinuously lowered towards the base metal to maintain the proper arc length. When the entireusable portion of the consumable electrode has melted, the arc can no longer be maintained, andmelting no longer occurs. The consumable electrode has then been fully consumed and anotherconsumable electrode must be used to continue the welding process.




Nonconsumable Electrodes

A nonconsumable electrode is one that is not consumed in the heat of the welding arc and thatdoes not provide any metal to the weld. Figure 3 shows a nonconsumable electrode that is usedwith the gas tungsten arc welding process.

Figure 3. Nonconsumable Electrode

As the nonconsumable electrode is drawn near the base metal, the electrical circuit is completedand an arc is created. Unlike the consumable electrode, nonconsumable electrodes can effectivelymaintain the welding arc without melting. In this case, the nonconsumable electrode is made fromtungsten, which melts at about 3400°C (6200°F). Although the molten weld metal may reach andexceed this temperature, the actual temperature of the nonconsumable electrode is well below themelting point of tungsten.

Because the nonconsumable electrode does not melt, it is very easy to maintain a constant arclength with the base metal. Welding processes that use nonconsumable electrodes do not have tobe interrupted to replace the nonconsumable electrode. To deposit filler metal when anonconsumable electrodes is being used, a filler metal must be added to the welding arc.

Weld Joint Description

Welds are made at the junction of at least two members. These weld junctions are called joints,which are defined as the location at which two or more members are to be joined. Parts that arejoined by welding may be in the form of rolled plate, sheet, shapes, pipe, or the parts may becastings, forgings, or billets. The physical placement of the members that are to be joined definesthe weld joint




Figure 4 shows the five basic types of weld joints used to join members. In some instances,several weld joints may be used in combination to complete a weld. A more detailed discussion ofweld joints and bevel designs will be presented in COE 114.04.




Figure 4. Five Basic Weld Joint TypesFusion Zone

Figure 5 shows a full penetration weld joint. The fusion zone identified in Figure 5 represents thearea of base metal that was melted during the welding process. The boundaries of the fusion zoneare between the weld metal and the fusion line. The actual fusion zone can only be determinedthrough removal of a cross-section of the weld to examine the metallurgical structure of the basemetal.




The depth of the fusion zone depends on the amount of heat that was input to the weld jointduring welding. As more heat is input to the weld joint during welding, the size of the fusionwidens. As less heat is input to the weld joint, the size of fusion zone narrows. The heat input tothe weld joint is mostly controlled by the welding voltage and the electrode travel speed.

Figure 5. Full Penetration Weld Joint

Fusion Line

The fusion line that is identified in Figure 5 represents the border of fusion during welding and theheat-affected zone. Beyond the fusion line, no melting of the base metal occurs.

Heat-Affected Zone (HAZ)

The heat-affected zone identified in Figure 5 represents that portion of the base metal that has notbeen melted, but whose mechanical properties or microstructure have been altered by the heat ofwelding. The boundaries of the HAZ are between the base metal and the fusion line.

When heat is input to a weldment from the electrode, the heat also transfers into the adjacent basemetal. As the heat input during welding travels through the base metal, the heat dissipates as itgets farther from the weld. Even though the temperature may not be great enough to melt thebase metal in the HAZ, the heat is sufficient to alter the microstructure and physical properties ofthe base metal in the HAZ next to the fusion zone.




Weld Joint Properties

The mechanical and physical properties of materials determine which materials are consideredapplicable in the design of a product. In the design of weldments, the properties of primaryconcern are those properties that indicate the behavior of metallic materials under variousconditions of loading. These properties are determined in testing laboratories, where standardizedprocedures and equipment are used to gather data.

The adequacy of a weld depends on whether the completed weld provides properties that areequal to or that exceed those of the base metals that are being joined. Properly executed weldsgenerally have mechanical properties that are superior to the mechanical properties of the basemetals that were joined. The following mechanical properties will be discussed in this section:

• Tensile Strength

• Ductility

• Hardness

• Impact strength

Tensile Strength

Tensile strength is the maximum strength that is developed in a metal tension test. The tensiontest is a method to determine the behavior of a metal under an axial stretch loading. To determinethe tensile strength of a weldment, two base metals are welded together, sectioned, and machinedto make a reduced-section tensile specimen. Under a tensile load, the tensile specimen will exhibitelastic elongation in proportion to the applied tensile load. At the yield point, the specimen willcontinue to exhibit plastic elongation without an increase in the load. Ultimately, the load isincreased until the tensile specimen is pulled apart and fails. The ultimate load divided by thecross sectional area of the tensile specimen determines the actual tensile strength of the weldedassembly.

Tensile Specimens - Figure 6 A and B show where to remove the reduced-section tensilespecimens in weld test coupons. Figure 6A illustrates the location of the two reduced-sectiontensile specimens in plate coupons. Figure 6 B illustrates the location of the two reduced-sectiontensile specimens in pipe coupons.

The reduced-section tensile specimens are located so as to provide representative tensile strengthdata for the entire weld test coupon. Figure 6 A and B also show the location of the followingspecimens for purposes of reference:

• Root-bend specimens

• Face-bend specimens




Figure 6. Tensile Specimen Removal Locations




Acceptance Criteria - To be acceptable, the tensile specimens generally have to meet or exceedthe tensile strength of the base metal. In cases in which two different base metals are weldedtogether, the tensile specimens must meet or exceed the tensile strength of the weaker of the twobase metals.

Ductility

The ductility of a metal is the property that allows the metal to be stretched or otherwise changedin shape without breaking and then be able to retain the changed shape after the load has beenremoved. To determine the ductility of a weldment, two base metals are welded together,sectioned, and machined to make a guided-bend specimen. The guided-bend specimen is thenbent in half to a specific radius that is based on the thickness of the specimen. Bending isaccomplished with either a roller jig or a wrap-around jig as illustrated in Figure 7 A and B. Theductility of a weldment is very important because a higher ductility indicates a weld that would beless likely to crack in service.




Figure 7. Guided-Bend Test Jigs

Bend Specimens - The types of guided-bend specimens that are used to test weld ductilityinclude face, root, and side. A root bend specimen is a specimen in which the root of the weldbecomes the convex surface of the bend specimen; a face bend specimen is a specimen in whichthe face of the weld becomes the convex surface of the bend specimen; a side bend specimen is aspecimen in which one of the side surfaces of the weld becomes the convex surface of the bendspecimen.




Figure 6 A and B showed where to remove transverse bend specimens in weld test coupons.Figure 6 A illustrated the location of the four bend specimens (2 face and 2 root) in platecoupons and Figure 6 B illustrated the location of the four bend specimens (2 face and 2 root) inpipe coupons. The root and face bend specimens are located to provide representative ductilitydata for the entire weld test coupon.

Acceptance Criteria - To be acceptable, the weld and heat-affected zone of the weld must becompletely located within the bent portion of the specimen and no open defects that exceed 1/8"can be visible on the convex surface of the specimen.

Hardness

The hardness of a metal is determined by the resistance of a metal to local indentation by a hardersubstance. Hardness testing is not a requirement of the fabrication codes, but it is often requiredby a job specification.

Hardness data from a weldment provides an indication of the following items:

• Metallurgical effects of the welding process on both the weld metal and the heat-affected zone

• An indication of the approximate tensile strength of a metal,

• The ductility of the weldment

• The ability of the metal to withstand impact loads.

The hardness of a weldment is important because very hard welds are more likely to crack inservice.

Testing - To determine the hardness of a weldment, a hardened steel ball or diamond is forcedinto the surface of the metal under a definite weight in a hardness testing machine. The amount ofindentation is converted into a numerical value used to compare the relative hardness of a specificmetallic surface.

Saudi Aramco procedures require that welding procedure qualifications and productionweldments be hardness tested.

For welding procedure qualification coupons, the hardness testing must be performed on a cross-section of the weldment that has been etched to clearly show the base metal, weld metal, andHAZ as illustrated in Figure 8.

For production weldments, the hardness testing must be performed on the ground surface of theweld near the middle of the deposited weld bead.




Additional hardness testing of the HAZ may be required by the applicable construction standard.The actual location, number, and acceptance criteria for hardness readings are identified in SaudiAramco Engineering Standards (SAES) -W-SERIES.

Figure 8. Hardness Testing of Weld Test Coupons

Acceptance Criteria - To be acceptable, the hardness of the base metal, the weld metal, and theheat-affected zone must be within the limits determined by the specifications of the particular job.

Impact Strength

Impact strength is the ability of a metal to absorb the energy of a load that is rapidly applied to themember. A metal may have good tensile strength and good ductility under static loading, yet itmay fracture from a high velocity impact. A material or weldment that does not have sufficientimpact strength may be too brittle for the intended service. Adequate ductility is an importantengineering consideration because it allows the material or weldment to redistribute concentratedstresses and prevent material failures. Even if no stress concentrations are present in a brittlematerial, fracture will still occur suddenly because the yield stress and tensile strength arepractically identical.

Knowledge of the impact properties of materials and weldments is very important because amaterial or weldment that is ductile at room temperature can become brittle in the presence ofstress concentrations, low temperature, high rates of loading, or embrittling agents such ashydrogen.

Impact strength testing is required by certain fabrication codes, such as ASME Section VIII, andit is most often determined by the Charpy V-notch test. To determine the impact strength of aweldment, two base metals are welded together, sectioned, and machined to make impactspecimens.




The impact strength of a material is determined through measurement of the energy that isabsorbed by the impact specimen while a weighted pendulum strikes and breaks the specimen.The absorbed energy is measured in foot-pounds. The temperature at which impact testing isperformed depends on the application of the weldment. Impact testing at temperatures as low as -423o F (temperature of liquid hydrogen) is not uncommon.

Impact Specimens - Dependent on the fabrication code, impact specimens that represent theweld metal, heat-affected zone, and base metal areas may have to be tested. A set of three impactspecimens is generally required from each area to adequately characterize the impact strength ofthe welded assembly.

Figure 9 shows where to remove impact specimens in weld test coupons used for pressure vesselconstruction. The impact specimens are removed so that the top of the specimens are 1/16"below the surface of the base metal. In relatively thick weld test coupons, additional specimenswould be removed somewhere between the root of the weld and the middle of the weld.

Figure 9. Impact Specimen Removal Locations

Acceptance Criteria - The average absorbed energy of a set of three impact specimens (from aspecific area) generally has to meet specified minimum absorbed energy values that are providedin a fabrication code. Dependent on the code or the type of base metal that is welded, impactspecimens may have to meet minimum lateral expansion values rather than absorbed energyvalues.




IDENTIFY THE MAJOR JOINING PROCESSES USED IN SAUDI ARAMCO

The following welding and joining processes are most commonly used in Saudi Aramco:

• Shielded Metal Arc Welding

• Gas Tungsten Arc Welding

• Gas Metal Arc Welding

• Flux Cored Arc Welding

• Submerged Arc Welding

• Stud Welding

• Oxyacetylene Welding

• Electric Resistance Welding

• Brazing

• Soldering

Shielded Metal Arc Welding

The shielded metal arc welding (SMAW) process, commonly called "stick" welding, is the mostwidely used arc welding process. SMAW is characterized by application versatility and flexibility,and relative simplicity of the equipment.

Process Description

Shielded metal arc welding is a manual joining process in which coalescence of metals is producedby heat from an electric arc that is maintained between the tip of a covered electrode and thesurface of the base metal in the weld joint. Figure 10 shows a simple schematic diagram of anSMAW welding circuit.




Figure 10. SMAW Welding Circuit

Figure 11 details the SMAW process. As the electric arc melts the base metal and electrode wire,metal droplets are transferred to the weld and become solidified metal. The electrode coveringalso partially melts into the weld and burns to form a protective gas.

Figure 11. The SMAW Process




Power Supply

Either AC or DC current can be used for shielded metal arc welding. To select a power supply,the following factors should be considered:

• The type of electrode to be used

• The required amperage range

• The welding positions

• The availability of a primary electrical power source

A transformer-type of power supply would be used for AC welding, and a transformer-rectifier ora motor-driven generator power supply would be used for DC welding. The motor-drivengenerator would have to be used in remote field applications in which primary electrical power isnot readily available.

Electric Leads

Electric leads are used to connect the electrode holder and the ground clamp to the power supply.These electric leads are generally copper cables that are constructed for maximum flexibility topermit easy manipulation of the electrode holder and to prevent wear and abrasion resistance.The electric leads are jacketed with a synthetic rubber that has high toughness, high electricalresistance, and good heat resistance.

Because of the rugged environment of most field welding operations, the durability requirementsof the electric leads cannot be overemphasized. The size of the electric leads that are required fora particular application depends on the maximum amperage that is to be used during welding andthe voltage drop between the electrode holder and the power supply.

As the length of the electric leads increases, the associated voltage drop through the cable alsoincreases. To compensate for this drop, larger diameter electric leads would be required.

Electrode Holder

The device that is used to hold and control the electrode in SMAW is known as an electrodeholder. The electrode holder has metal jaws that firmly hold the electrode and that conduct thewelding current from the electric lead to the electrode. These jaws are covered with insulation tokeep the jaws from grounding to the base metal. An insulated handle on the electrode holderseparates the welder's hand from the welding current. Electrode holders come in several differentsizes to accommodate specific ranges of welding current without overheating.




Filler Metal Form

The filler metal form for SMAW is a covered electrode. The covered electrode has a bare metalrod as a core and is covered with baked-on flux. Typically, this flux consists of either ironpowder-low hydrogen or cellulosic materials. A more detailed discussion of flux types will bepresented in COE 114.04.

The electrodes are readily available in sizes that range from 3/32" to 1/4" in diameter and are from9" to 18" in length. The size of the covered electrode is based on the diameter of the bare wirecore: It is not based on the overall diameter of the covered electrode.

Purpose of Covering

The purpose of the electrode covering (flux) is to perform one or more of the following functions:

• Provide a gas that prevents excessive oxygen contamination of the molten fillermetal during solidification.

• Provide scavengers, deoxidizers, and fluxing agents that cleanse the weld and thatprevent excessive grain growth in the weld metal.

• Establish the electrical characteristics of the electrode.

• Provide a slag blanket that protects the hot weld metal from the air and thatenhances the mechanical properties, bead shape, and surface cleanliness of the weldmetal.

• Provide a means to add alloying elements that change the mechanical properties ofthe weld metal.

Common Uses

SMAW is one of the most versatile welding processes that is available for use in thepetrochemical industry. At Saudi Aramco, SMAW is commonly used both in the shop and in thefield to perform the following tasks:

• Weld piping, structural steel members and pipe supports

• Construct oil tanks on site

• Perform maintenance welding operations to include the following tasks:

− Repair defective welds

− Addition of corrosion resistant material to pressure vessel internals

− Repair failed mechanical equipment.

SMAW is readily used on carbon steel, chrome-moly steel, stainless steel, and cast iron materialsin the form of plates, shapes, pipe, castings, and forgings.’




Advantages

The SMAW process has the following advantages:

• Uses relatively simple, inexpensive, and portable equipment.

• Has moderate filler metal deposition rates.

• Requires relatively low skill levels for welders.

• Can be used in all welding positions.

• Requires no auxiliary gas shielding or flux.

• Is less sensitive to wind and to drafts than are gas-shielded processes.

• Is suitable for most of the commonly used metals and alloys.

Disadvantages

The SMAW process has the following disadvantages:

• Requires significant interpass cleaning to remove slag.

• Has a low operating factor because of the interpass cleaning and constant addition ofnew electrodes.

• Has limited current capability due to the diameter and length of the electrodes.

• Is not applicable to low melting metals such as lead, tin, and zinc.

Gas Tungsten Arc Welding

Because of the high quality welds that are produced, the gas tungsten arc welding (GTAW)process, which is often called "TIG" (tungsten inert gas) welding, has become an indispensablewelding process for many industries.

Process Description

Gas tungsten arc welding can be a manual or an automatic joining process in which coalescence ofmetals is produced by heat from an electric arc that is maintained between the tip of a tungstenelectrode (nonconsumable) and the surface of the base metal in the weld joint. Figure 1 shows asimple schematic diagram of a GTAW welding circuit.




Figure 12. GTAW Welding Circuit

Figure 13 details the GTAW process. As the electric arc melts the base metal, the filler metal isintroduced to the arc area where the filler metal also melts. After the filler metal cools, it becomessolidified weld metal. GTAW requires the simultaneous use of both hands; one hand to controlthe torch and one hand to control the addition of filler metal.

Because of this two-handed technique, gas tungsten arc welding requires more skill of the welder.Care must be taken to ensure that the nonconsumable tungsten electrode does not come intocontact with the molten weld puddle. Such contact would cause the electrode tip to melt anddistort.

Figure 13. The GTAW Process




Power Supply

Either AC or DC current may be used for gas tungsten arc welding. The following factors shouldbe considered to select a power supply:

• Type of electrode to be used

• Amperage range that is required

• Welding positions

• Availability of a primary electrical power source

Typically, a transformer-rectifier or an engine-driven generator power supply is used for gastungsten arc welding.

For more specific applications such as thin sheet metal, the power supplies are equipped with apulsed DC welding current, which results in a lower overall heat input to reduce distortion andwarpage.

A high frequency feature is used on some power supplies to stabilize or "stiffen" the welding arcduring precision applications, at very low currents, and in outdoor areas.

Electric Leads

Electric leads are used to connect the torch and the ground clamp to the power supply. Theground lead and the torch lead are identical to the leads that are used for shielded metal arcwelding; however, the short torch lead also has an internal tube to convey shielding gas to thetorch.

Torch

The torch used in GTAW holds the nonconsumable tungsten electrode that conducts weldingcurrent to the arc and provides a means to convey the shielding gas to the arc zone.

Torches are rated in accordance with the maximum welding current that can be used withoutoverheating the torch. For high current (300 to 500 amps) welding applications, torches areavailable with a continuous flow of water through internal passageways to cool the torch.

Torches are available in several different head configurations to facilitate unique welding positionsand welder comfort.

GTAW torches often have auxiliary switches and valves to control current and gas flow.

Filler Metal Form

The filler metal form for GTAW is a bare electrode. The bare wire electrodes are readily availablein 36" length and in diameters that range from 1/16" to 3/16".




For automatic welding applications, the filler metal is a continuous wire, as small as 0.020", and itis wound on a spool.

Extra care must be exercised to keep the filler metal clean and free of all contaminants, such as oiland moisture. Clean, uncontaminated filler metal helps to ensure high quality welds.

Purpose of Shielding Gas

The purpose of the shielding gas is to provide an inert atmosphere that prevents excessive oxygencontamination of the molten filler metal during solidification. Shielding gas is usually supplied tothe torch from a local high pressure gas storage cylinder.

Typical inert gases that are used with GTAW are argon and helium; however, argon-hydrogenand argon-helium blends are sometimes used.

Argon is heavier than air, and it tends to cover the weld. Argon generally provides a smoothwelding arc with adequate penetration at a low cost.

Helium is lighter than air, and it does not provide adequate shielding unless the flow issignificantly increased. Helium provides greater penetration of the welding arc, and it is usuallypreferred on thick materials.

When root passes are made on certain materials such as stainless steel and nickel-based alloys, theair that is on the back side of the weld joint can actually corrode the weld. To avoid this problem,the air must be purged from the root pass to prevent the formation of oxide and scale that wouldresult in loss of passivity in stainless steel and nickel based alloys.

Figure 14 shows a purge gas scheme for a piping weld. Generally, this purge gas is the same asthe shielding gas that is used in the torch. The purge gas must be conveyed to the back side of theweld through a separate hose that can be regulated independently of the torch shielding gas. Byflowing the purge gas into the pipe, one can force the air out through the weld joint and the purgegas outlet.

Figure 14. Purge Scheme for a Piping Weld




Common Uses

Because GTAW produces high quality welds. GTAW is commonly used at Saudi Aramco to weldthe following items:

• Root pass of pipe butt welds for high alloy materials such as stainless steel andnickel base alloys.

• Open butt GTAW root pass welds are used on pipe to provide the following things:

− Excellent radiographic quality welds

− A smooth shallow bead contour that does not affect flow conditions

− A weld that can be made from only one side,

− A weld that can be made in all positions around the pipe.

• On thin wall materials, GTAW may be used to weld the complete joint without asignificant loss of productivity.

• When radiographic quality welds are required on carbon steel weld joints.

• To ensure complete fusion of the root pass.

• For small diameter (less than 2") piping welds that include butt, fillet, and socketwelds. For these items it is the process of choice.

• Maintenance welding operations on small intricate parts and on materials that arevery thin because of the excellent low current control of this process.

Advantages

The GTAW process has the following advantages:

• Provides high quality weld.

• Requires little interpass cleaning.


• Allows excellent control of root pass weld penetration.

• Can be used with or without filler metal.

• Suitable for most of the commonly used metal and alloys.

• Allows the heat source and filler metal additions to be independently controlled.

Disadvantages

The GTAW process has the following disadvantages:

• Requires relatively high skill levels for welders.




• Has a relatively low deposition rate compared to consumable electrode weldingprocesses.

• Is not cost effective to use on thick sections.

• Is not conducive to welding in windy or drafty areas.

Gas Metal Arc Welding

The gas metal arc welding (GMAW) process, which is often called "MIG" (metal inert gas)welding, became commercially available in 1948, and it has become one of the most popular arcwelding processes for efficient production welding.

Process Description

Gas metal arc welding is widely used as a semi-automatic joining process in which coalescence ofmetals is produced by heat from an electric arc that is maintained between the tip of a consumablebare wire electrode and the surface of the base metal in the weld joint.

The GMAW process uses a continuously fed consumable electrode that is shielded by anexternally supplied gas. After the initial process settings are made by the welder (voltage, wirefeed speed, and gas flow), the equipment provides for the automatic self-regulation of theelectrical characteristics of the arc. The only manual controls that are required by the welder forsemi-automatic operation are the travel speed, travel direction, and wire feed gun position.

Figure 15 shows a simple schematic diagram of a GMAW welding circuit.




Figure 15. GMAW Welding Circuit

Figure 16 details the GMAW process. As the electric arc melts the base metal, the consumableelectrode also melts and forms metal droplets. These metal droplets are transferred to the weldand become solidified metal. Because the electrode exits the wire feed gun at speeds of up to 500inches per minute, GMAW requires excellent hand-eye coordination to maintain a consistent arclength and travel speed. Because of the speed of this process, care must be taken to ensure thatcomplete fusion of the base metal occurs.

Figure 16. The GMAW Process

The GMAW process is capable of operating in three separate arc transfer modes. The followingthree arc transfer modes are slightly different in the way the consumable electrode melts and fillermetal is transferred to the weld puddle:




• "Short-circuiting" transfer mode

• "Globular" transfer mode

• "Spray" transfer mode

The "short-circuiting" transfer mode - is the most common mode, and it encompasses the lowestrange of welding currents and electrode diameters. Filler metal (electrode) is transferred in theform of tiny droplets from the electrode to the weld when the electrode is in contact with the weldpuddle. The electrode will contact the weld puddle at up to 200 times per second. The shortcircuiting transfer mode is limited to the use of relatively small diameter wires at current rangethat are below approximately 200 amperes.

The "globular" transfer mode - is similar to the short-circuiting transfer mode except that thedroplets of molten filler metal will grow two to three times the diameter of the electrode beforegravity causes transfer to the weld puddle. When the arc length is too short, considerable spatterwill be produced. However, when the arc length is too long, the weld can exhibit lack of fusion,insufficient penetration, and excessive reinforcement. These characteristics greatly limit the use ofthe globular transfer mode in production applications.

The "spray" transfer mode - occurs at higher currents when at least 80% argon shielding gas isprovided. The droplets of molten filler metal are smaller than the droplets in the short-circuitingmode and they spray across the arc. This mode provides higher deposition rates and greater basemetal penetration. The limitations of the spray transfer mode are that it can only be used in theflat and vertical positions, and only on relatively thick materials. Also, the spray transfer moderequires the use of expensive argon gas and high amperage welding power supplies.

Power Supply

DC current is used for the majority of GMAW applications. The following factors should beconsidered in selecting a power supply:

• Required amperage range

• Welding positions

• Duty cycle

• Availability of a primary electrical power source

Typically, a constant voltage transformer-rectifier or an engine-driven generator power supplythat is rated for a 100% duty cycle would be used for gas metal arc welding.




Electric Leads

Electric leads are used to connect the wire feed gun and the ground clamp to the power supply.The ground lead and the wire feed gun lead are identical to the leads that are used for gastungsten arc welding. The short wire feed gun lead also has an internal tube that is used to conveyshielding gas to the torch.

Wire Feed Gun

The hand-held wire feed gun that is used in GMAW guides the consumable electrode thatconducts welding current to the arc and provides a way to convey the shielding gas to the arczone. Wire feed guns are rated in accordance with the maximum welding current that can beused without overheating the torch and in accordance with the maximum size of filler metal thatcan be fed through the gun. For high current (300 to 500 amps) and continuous weldingapplications, wire feed guns are available with a continuous flow of water through passageways tocool the gun. For softer filler metals such as aluminum, small spools of filler that are metal-feddirectly from a hand-held gun are used to improve the uniform delivery of the filler metal.

Filler Metal Form

The filler metal form for GMAW is a continuous bare wire electrode. The filler metal is acontinuous wire that is wound on 4" to 30" diameter spools with wire sizes that range from0.030" to 0.125" in diameter. These wire spools can hold, depending on the nature of the weldingoperations, from 2 pounds to 60 pounds of filler metal. Care must be taken to avoid causingkinks and bends in the bare wire that can jam the wire feeder and halt welding operations.

Purpose of Shielding Gas

The purpose of shielding gas is to provide a gas that prevents excessive oxygen contamination ofthe molten filler metal during solidification.

Shielding gas is usually supplied to the torch from a local, high-pressure gas storage cylinder.Typical gases that are used with GMAW are argon, helium, argon-helium and argon-oxygenblends, oxygen, carbon dioxide, argon-carbon dioxide blends, and even argon-helium-carbondioxide blends.

The gas that is used depends on the application. Factors that can affect the choice of shieldinggas include the FOLLOWING

• Type of base metal

• Weld joint design

• Welding position

• Required penetration




• Control of heat-affected zone

• Required arc stability

As with GTAW, gas metal arc welding of root passes on certain materials, such as stainless steeland nickel based alloys, can contaminate the weld. Purging operations may be required on thesetypes of base metals to prevent contamination of the root pass.

Common Uses

In Saudi Aramco, GMAW is used primarily for pipeline construction using the short circuitingtransfer mode. To improve GMAW's productivity, the piping welds are placed in devices thatslowly turn the pipe so that the welder never has to change positions.

Many maintenance welding applications are readily adaptable to the highly efficient GMAWprocess.

GMAW is also used in the globular and spray transfer modes to weld structural members thatrequire a high deposition rate, and that can be positioned so that most of the welding is performedin the flat position.

Advantages

The GMAW process display the following advantages:

• A high deposition rate when compared to manual arc processes.

• A high operating factor because the filler metal is continuously fed.

• Reduces interpass cleaning because there is no slag.

• Used in all welding positions.

• Suitable for most of the commonly used metals and alloys.

Disadvantages

The GMAW process displays the following disadvantages:

• Requires relatively high skill levels for welders.

• Has a tendency to develop lack of fusion defects due to the high welding speed.

• Is more difficult to use in hard to reach places because of the size of the wire feedgun.

• Can produce welder fatigue due to the high operating factor.




Flux-Cored Arc Welding

The flux-cored arc welding (FCAW) process is very similar to GMAW. FCAW has the highestdeposition rates of any semi-automatic welding process.

Process Description

Flux-cored arc welding can be an automatic joining process, but it is used mostly as a semi-automatic joining process.

In the flux-cored arc welding process, coalescence of metals is produced by heat from an electricarc maintained between the tip of a consumable flux-filled wire electrode and the surface of thebase metal in the weld joint.

The FCAW process uses a continuously fed consumable electrode that is shielded by an internallysupplied flux.

A variation of the FCAW process, which is known as "dual-shield," includes the use of an externalshielding gas in addition to the internal flux for certain applications and base metals.

Like GMAW, after the initial process settings are made by the welder (voltage, wire feed speed,and gas flow), the equipment provides for the automatic self-regulation of the electricalcharacteristics of the arc. The only manual controls that are required by the welder for semi-automatic operation are the travel speed, travel direction, and wire feed gun position.

Figure 17 shows a simple schematic diagram of an FCAW welding circuit.

Figure 17. FCAW Welding Circuit




Figure 18 details the FCAW process. As the electric arc melts the base metal, the consumableelectrode also melts and forms metal droplets that are transferred to the weld and then becomesolidified metal.

Because the electrode exits the wire feed gun at speeds of up to 500 inches per minute, FCAWalso requires excellent hand-eye coordination to maintain a consistent arc length and travel speed.

Figure 18. The FCAW Process

Power Supply

In a manner that is similar to GMAW, FCAW uses a power supply that provides DC current froma constant voltage transformer-rectifier or an engine-driven generator power supply that is ratedfor a 100% duty cycle.

Electric Leads

Electric leads are used to connect the wire feed gun and the ground clamp to the power supply.The ground lead and the wire feed gun lead are identical to the leads that are used for gas metalarc welding; however, the short wire feed gun lead would generally not have an internal tube toconvey shielding gas to the torch.

Wire Feed Gun

The hand-held wire feed gun that is used in FCAW performs the same function as the wire feedgun in gas metal arc welding, and it is similarly rated.




Filler Metal Form

The filler metal form for FCAW is a continuous tubular wire electrode that is filled with flux. Thefiller metal is a continuous wire that is wound on 12" to 30" diameter spools of wire with sizesthat range from 0.045" to 5/32" in diameter.

These wire spools can hold 14 pounds to 60 pounds of filler metal. The nature of the weldingoperation determines the amount of filler metal that can be held on the wire spool. For highproduction applications, the filler metal may even be supplied in drums that weigh up to 600pounds.

Purpose of Flux

The purpose of the flux that is within the tubular wire is to provide one or more of the followingfunctions:

• A gas that prevents excessive oxygen contamination of the molten filler metal duringsolidification

• Scavengers, deoxidizers, and fluxing agents that cleanse the weld and that preventexcessive grain growth in the weld metal

• A slag blanket that protects the hot weld metal from the air and that enhances themechanical properties, bead shape, and surface cleanliness of the weld metal

• A means to add alloying elements that change the mechanical properties of the weldmetal

Common Uses

Semi-automatic FCAW is primarily used at Saudi Aramco to weld structure members that requirea significant amount of welding and that can be positioned so that most of the welding isperformed in the flat position.

Maintenance welding applications include the butt welding of sections of carbon steel piping thathave been replaced and the build-up of corroded material.

As with GMAW, FCAW's productivity can be improved through placement of the piping welds indevices that slowly turn the pipe so that the welder never has to change position.

Advantages

The FCAW process displays the following advantages:

• A high deposition rate when compared to manual arc processes

• A high operating factor because the filler metal is continuously fed




• Can be used in all welding positions

• Requires no auxiliary gas shielding or flux

• Is less sensitive to wind and drafts than are gas shielded processes

• Is suitable for most of the commonly used metal and alloys

Disadvantages

The FCAW process displays the following disadvantages:

• Requires relatively high skill levels for welders

• Requires significant interpass cleaning to remove slag

• Can produce welder fatigue due to the high operating factor

Submerged Arc Welding

The submerged arc welding (SAW) is one of the older automatic welding processes. The SAWprocess is referred to as "submerged" because the electric arc is actually submerged under agranular flux.

The SAW process is currently used in heavy steel fabrication that includes the welding ofstructural shapes, longitudinal seam welds in pipes, and the manufacture of machine components.

Process Description

Submerged arc welding can be a semi-automatic joining process, but it is mostly used as anautomatic joining process in which coalescence of metals is produced by heat from an electric arcthat is maintained between the tip of a consumable bare wire electrode and the surface of the basemetal in the weld joint.

After the initial process settings are made by the welder (voltage, wire feed speed, and flux flow),the equipment provides for the automatic self-regulation of the electrical characteristics of the arc.The only manual controls that are required by the welder for semi-automatic operation are thetravel speed, travel direction, and wire feed gun position.

Figure 19 shows a simple schematic diagram of an SAW welding circuit.




Figure 19. SAW Welding Circuit

Figure 20 details the SAW process. As the electric arc melts the base metal that is underneaththe granular flux, the consumable electrode also melts, and becomes solidified weld metal.




Figure 20. The SAW Process

Power Supply

Like FCAW and GMAW, SAW generally requires a power supply that provides DC current froma constant voltage transformer-rectifier or an engine-driven generator power supply that is ratedfor a 100% duty cycle.

Certain applications of SAW require the use of AC current from a transformer-type weldingmachine. SAW welding machines are often capable of providing up to 1500 amperes of current.

Electric Leads

Electric leads are used to connect the wire feed gun and the ground clamp to the power supply.The ground lead and the wire feed gun lead are identical to the leads that are used for other arcwelding processes.

Wire Feed Gun

The hand-held wire feed gun used in SAW performs the same function as did the wire feed gun inFCAW, and it is similarly rated. The only difference is that the SAW wire feed gun also has anattached flux feed tube. For small welding jobs, the flux hopper may be mounted directly on thewire feed gun.




Flux Hopper

The flux hopper provides a reservoir of welding flux that maintains a constant flow of flux to theweld joint during welding. The flux hopper is sized according to the following factors:

• Thickness and width of the weld bead

• Amount of arc time for the application

• Desired thickness of flux.

Filler Metal Form

The filler metal form for SAW is a continuous bare wire electrode. The filler metal is acontinuous wire that is wound on spools that have diameters of 12" or larger. The wire sizesrange from 1/16" to 1/4" in diameter.

The nature of the welding operation determines the amount of filler metal that the wire spools canhold. For high production applications, the filler metal may be supplied in drums.

Purpose of Flux

The purpose of the flux is to provide one or more of the following functions:

• A gas that prevents excessive oxygen contamination of the molten filler metal duringsolidification.

• Scavengers, deoxidizers, and fluxing agents that cleanse the weld metal.

• A slag blanket that protects the hot weld metal from the air and that enhances themechanical properties, bead shape, and surface cleanliness of the weld metal.

• A means to add alloying elements that change the mechanical properties of the weldmetal.

Common Uses

SAW is used at Saudi Aramco primarily to weld carbon steel materials such as structural steelmembers and heavy wall pressure vessel sections.

For smaller duration projects, the use of the FCAW process or even the SMAW process may bemore cost effective.

The most productive maintenance use for SAW would be the addition of weld overlay to pressurevessel interiors, although SAW is not conducive to vertical welding applications.




Advantages

The SAW process displays the following advantages:

• A high deposition rate as compared to other semi-automatic arc welding processes.

• A high operating factor.


• Is less sensitive than gas shield processes to winds and drafts.

• Provides a high quality of weld metal.

• Produces little or no smoke.

• Has no arc flash; therefore, minimal protective clothing is required.

Disadvantages

The SAW process displays the following disadvantages:

• Can only be used in the flat position.

• Is not cost effective for small welding jobs.

• Is not capable of welding thin materials.

Stud Welding

Stud welding is a general term for the joining of a metal stud or similar part to a work piece.Stud welding can be accomplished by a number of different welding processes that include arc,resistance, friction, and percussion. The process that is known as stud arc welding (SW) will bediscussed in this section.

Process Description

In stud welding, the end of a stud is joined to a base metal by heating the stud and the base metalwith an electric arc. When the two surfaces to be joined are properly heated, the surfaces arebrought together under low pressure. A stud welding gun is used to hold the studs during thewelding process.

Welding time and the plunging of the stud into the molten weld pool to complete the stud weldare controlled automatically. The stud that is held in the stud gun is positioned by the welder.The welder actuates the stud gun by pressing a trigger switch. The stud weld is quicklycompleted, usually in less than one second.

Figure 21 shows a simple schematic diagram of an SW welding circuit.




Figure 21. SW Welding Circuit

Figure 22 shows the sequence of the SW process. The stud gun holds the stud in contact withthe base metal until the welder depresses the gun trigger switch, which causes the welding currentto flow from the power supply through the stud (that acts as the electrode) to the base metal. Thewelding current flow actuates a solenoid within the stud gun to draw the stud away from the basemetal and establish the arc.

At the appropriate time, the welding current is shut off and the stud gun solenoid releases its pullon the stud. The spring-loaded action plunges the stud into the molten pool in the base metal.

The time duration for each phase of the operation is controlled by a timer in the control unit. Themolten metal solidifies and produces the weld and, in addition, a small reinforcing fillet weld.

The stud gun is released after the weld metal solidifies, and the ferrule is broken off of the stud.




Figure 22. Sequence of the SW Process

Power Supply

Stud welding requires a power supply that produces DC current from a transformer-rectifier orfrom a motor-generator that is similar to the power supplies that are used for shielded metal arcwelding. The DC current is directed through a control unit that is used specifically for studwelding.

The control unit controls the duration of the welding current and the timing of the stud gunplunger action. The time interval for the application of the welding current and the plunger actiontypically varies from 0.05 to 2.0 seconds.

Electric Leads

Electric leads are used to connect the control unit and the ground clamp to the power supply.The ground lead and the control unit lead are identical to the leads that are used for other arcwelding processes.

Stud Gun

The hand-held stud gun used in the SW process holds and positions the stud during the weldingprocess. The size of the stud gun depends on the diameter of the studs that are to be used.The stud gun consists of the following parts:

• Lifting mechanism which has the following parts:




− Solenoid

− Clutch

− Mainspring

• Chuck holder

• Adjustable support for the ferrule holder

• Connecting weld and control cables.

The lift mechanism controls the lift of the stud and is typically between 0.030" and 0.125".

Stud Form

SW is capable of welding many configurations of studs. The studs are commonly made fromcarbon steel, stainless steel or aluminum. Most stud bases are round; however, there also areapplications that use square- or rectangular-shaped studs. In addition to conventional straight-threaded studs, J-bolts, and punched, slotted, grooved, and pointed studs are used.

Ferrules are placed over the stud at the weld end for most stud welding applications. The ferrulesare designed to perform the following functions:

• Concentrate the heat of the arc in the weld area.

• Restrict the flow of air into the area to control oxidation of the molten weld metal.

• Confine the molten metal to the weld area.

• Prevent the charring of adjacent non-metallic materials.

• Protect the welder from the arc.

Common Uses

Stud welding is primarily used at Saudi Aramco in both shop and field applications for attachmentof the following:

• Insulation pins to pressure vessels and tanks

• Grating bolts to platforms

• Concrete anchors to structural members

Advantages

The SW process displays the following advantages:






• Can be used on curved and angled surfaces.

• Provides welds with a very shallow heat-affected zone.

Disadvantages

The SW process displays the following disadvantages:

• Can only weld one end of a stud to the work piece.

• Stud shape and design are limited by the capability of the stud gun chucking device.

Oxyacetylene Welding

Oxyacetylene welding (OAW) is a type of Oxyfuel Gas Welding (OFW) that uses acetylene as afuel gas and gaseous oxygen to support combustion to create a heating medium.

The wide field of applications, as well as the convenience and economy of oxyacetylene welding,are recognized in most metal working industries. OAW is universally used and accepted in thefield of maintenance and repair where the flexibility and mobility of this process results in reducedtime and labor expenses.

Process Description

The OAW process involves melting the base metal and usually a filler metal through use of aflame that is produced at the tip of a welding torch. Fuel gas and oxygen are combined to form amixture that is 2.5 parts oxygen to 1 part fuel gas (acetylene).

The mixing chamber may be part of the welding tip assembly. Molten metal from the edges of theplate and filler metal, if used, intermix in a common molten pool and coalesce when they arecooled.




Figure 23 shows a simple schematic diagram of oxyacetylene welding equipment.

Figure 23. Oxyacetylene Welding EquipmentFigure 24 details the oxyacetylene welding process. The combustion flame from the weldingtorch tip is brought into contact with the base metal, the base metal begins to absorb the heat andmelt.

When a pool of molten metal is established, filler metal can be introduced into the flame to meltthe filler metal and to add filler metal to the weld. The weld metal becomes solidified weld metalas the weld cools.




Figure 24. The OAW Process

Fuel Gas Combustion

All fuel gases require oxygen to support combustion. Fuel gases that are suitable for weldingoperations must have the following characteristics:

• High flame temperature

• High flame rate propagation

• Adequate heat content,

• Minimal chemical reaction between the flame and the base

• Filler metals

Acetylene most closely meets all of these requirements. When acetylene and oxygen are broughttogether and mixed, the fuel is readily ignited by an ignition source such as a striker. Bycontrolling the ratio of acetylene to oxygen, the nature of the combustion flame can be altered toprovide one of these three types of flames:

• A neutral flame (an equal ratio of the two gases)

• A carburizing flame (excess acetylene)

• An oxidizing flame (excess oxygen).

For most welding applications, the neutral flame is preferred.




Gas Supplies

Typical OAW applications use local gas cylinders to provide the necessary acetylene and oxygengases. Each cylinder is outfitted with a pressure regulator to reduce the gas pressure that issupplied to the welding torch and to adjust the gas flow rates.

Torch

A typical welding torch consists of these three parts:

• Torch handle

• Mixer

• Tip assembly

The torch can accomplish the following things;

• Independently control the flow of each gas

• Accept a variety of welding tips

• Control the movement and direction of the flame

The gases flow through control valves in separate passages in the torch handle to the torch head.The gases then flow into a mixer assembly in which the oxygen and acetylene are mixed. Thegases finally flow out through an orifice at the end of the tip.

Sealing rings or surfaces are provided in the torch head or on the mixer seats to facilitate a leak-tight assembly.

Torch tips come in a variety of sizes to provide various flame sizes that are required for differentmaterials and thicknesses.

Filler Metal Form

The filler metal form for OAW is straight bare rods. The filler metal is usually manufactured in24" and 36" lengths and is readily available in sizes from 1/16" to 3/8" in diameter.

To ensure high quality welds, extra care must be exercised to keep the filler metal clean and freeof all contaminants, such as oil and moisture.

Common Uses

Oxyacetylene welding is used at Saudi Aramco primarily to fabricate handrails for stairways andother non-pressure retaining equipment in both shop and field applications.




In addition, the oxyacetylene process is used to cut ferrous metals and to preheat welds asdescribed in the sections that follows.

Cutting

When the welding tip on the torch is replaced with a special cutting attachment, the oxyacetylenewelding process becomes the oxyacetylene cutting (OAC) process. This process can easily andeconomically cut ferrous metals that are up to 8" thick.

The cutting attachment has a nozzle with several perimeter preheat flame ports and a center portfor the oxygen. With this process, the oxyacetylene flame is used to preheat the area to be cut orsevered.

When the metal is sufficiently heated, a handle on the cutting torch is depressed to release a highpressure jet of oxygen directly at the metal surface. It is this jet of oxygen that actually makes thecut in the steel. The oxygen oxidizes the hot metal and also blows the molten metal from thejoint.

Preheating

With minor changes to the design of the welding torch tip, the same equipment that is used forwelding can also be used to provide a flame of combustion for localized preheat to weldments.This technique is usually used on piping that is less than 8" in diameter, and that has wallthicknesses that are less than 1/2". This technique is also used on structural members that are lessthan 1/2" thick.

A key to this type of preheating operation is the ability to cover the entire area to be heated sothat a uniform temperature can be achieved. An area that is too large, or a member that is toothick, will be difficult to uniformly heat.

Advantages

The OAW process has the following advantages:

• Requires no electric power supply

• Can be used in all welding positions

• Provides excellent control of heat input and temperature

Disadvantages

The OAW process has the following disadvantages:

• Has low deposition rate

• Requires relatively high skill levels for welders




• Is not efficient for large welding jobs

Electric Resistance Welding

Because filler metal and fluxes are not used in electric resistance welding (ERW), ERW is a groupof welding processes that differ from all of those processes that were previously mentioned. Atleast seven important resistance welding processes exist: They include the following types:

• Flash

• Percussion

• Projection

• Seam

• Spot

• Upset

• High frequency

This section will only address the high frequency resistance welding process because it is the mostcommonly used pipe manufacturing process.

Process Description

The high frequency resistance welding (HFRW) process involves the coalescence of metalsproduced by the heat that is generated from the electrical resistance of the work piece to aninduced high frequency current. The heat that is generated from both the HFRW process and theapplication of an upsetting force combine to produce a forged weld.

The joint upsetting process that occurs downstream of the weld not only forces most of themolten metal and the contaminants out of the joint, but it also hot-works the adjacent base metal.The additional metal that remains outside of the joint is typically ground flush with the contour ofthe adjacent base metal.

American Petroleum Institute (API) line pipe and some other tubular (pipe) specifications requirethat the ERW weld seam be normalized after welding. This heat treatment eliminates anyuntempered martensite (brittle metallurgical structure) in the weld's heat-affected zone that couldmake the weld susceptible to cracking.

Figure 25 shows a simple schematic diagram of HFRW pipe fabrication. As the rolled plate isformed and guided through the pressure rollers, the high frequency current is introduced into themetal with a pair of sliding contacts placed on either side of the seam to be welded, and ahead ofthe weld point.




The welding current travels directly from one sliding contact along one edge of the seam to theweld point, and back along the opposite edge to the other sliding contact. The edges are forcedtogether by the weld pressure rolls at the weld point where the two edges are fused.

Figure 25. HFRW Pipe Fabrication

Major Components

The following are the major components of the HFRW process and their functions:

• An electrical circuit that consists of a welding transformer and a secondary circuitwith sliding contacts that conduct the current into the work piece.




• A mechanical system that consists of a machine frame and associated weldingmechanisms to hold the work piece and to apply the welding force.

• The control equipment that is used to initiate and to time the duration of the current,to control the current magnitude, and to establish the sequence and the time of theother aspects of the welding cycle.

Common Uses

The ERW process is not used in either shop or field fabrication at Saudi Aramco, or elsewhere inthe oil and gas industry at Saudi Aramco.

High frequency resistance welding is primarily used by piping and tubing manufacturers aroundthe world to fabricate what is commonly referred to as ERW pipe and tube. ERW piping is usedin all facets of the petroleum extraction and refining industry, including cross country pipelinesand on-plot process piping..

Brazing

Brazing is a group of joining processes that produces coalescence of materials by heating the basemetal to a suitable temperature and by using a filler metal that has a liquidus that is above 840o Fand that is below the solidus of the base metal.

The many joining processes include the following:

• Torch brazing

• Furnace brazing

• Induction brazing

• Resistance brazing

• Dip brazing

• Infrared brazing.

The filler metal is distributed between the closely fitted surfaces of the joint by capillary attraction.Unlike welding, the base metal does not melt during brazing to create fusion between the basemetal and filler metal.

There is a process that is called braze welding, which is different from brazing because, in brazewelding, the filler metal is not distributed by capillary attraction.

Completed Braze Joint

Figure 26 shows a diagram of a completed braze joint. This braze joint is typical of a copperpiping connection between two pipe sections and a coupling. Notice that a slight gap existsbetween the two pipe sections to allow for thermal expansion when the joint is in service. Thejoint overlap distance must be one of the following:




• Equal to 4T, where T is the thickness of the brazed joint or,

• As specified by the design

Figure 26. Completed Braze Joint

The brazing process often requires that a flux be used to accomplish the these factors:

• Improve the adhesion of the filler metal to the base metal,

• To prevent the formation of oxides

• To clean the base metal surfaces.

Although brazing is seldom used at Saudi Aramco, common applications include the following:

• Joining copper tubing

• Materials in which little distortion due to heating can be tolerated like the followingmaterials:

− Electrical components

− Dissimilar base metals

− Precious metals

Typical Interface Gaps

In order to obtain the maximum strength from a brazed joint, the interface gap must be largeenough to permit both entry of the molten filler metal and the escape of molten flux and gasesduring heating. When the interface gap is too large, the filler metal will not be held in the joint bycapillary attraction. Ideal interface gaps for production brazing are 0.002" to 0.005." Gaps aslarge as 0.008" can be adequately brazed.




Soldering

Soldering is a group of joining processes that produces coalescence of materials by heating thebase metal to a suitable temperature and by using a filler metal that has a liquidus that does notexceed 840o F and that is below the solidus of the base metal. The many joining processesinclude the following:

• Torch soldering

• Dip soldering

• Furnace soldering

• Infrared soldering

• Iron soldering

• Resistance soldering

• Induction soldering

• Wave soldering

The filler metal is distributed between closely fitted surfaces of the joint by capillary attraction.

Completed Solder Joint

Figure 27shows a diagram of a completed solder joint. This solder joint is typical of a connectionbetween two electrical connectors. Notice that the solder joint is similar to the brazed joint.

Figure 27. Completed Solder Joint




GLOSSARY

Coalescence Coming together into one body

Fusion The complete blending of two pieces of base metal, with orwithout the addition of a filler metal, to form one weld.

Reverse polarity” A welding circuit where the negative pole provides the powerto the work piece.

Arc plasma The space between the electrode and the work piece at whichthe arc is initiated and maintained.

flux

consumable

slag

Charpy V-notch test




WORK AIDS

Work Aid 1. How To Describe The Fundementals Of Welding

This work aid consists of the first section of the information sheet and Figure 1 through 9, thelocation of which is listed in the Table of Contents, Figure List. Use this work aid to completeExercise 1.

Work Aid 2. How To Identify The Major Joining Processes Used In Saudi Aramco

This work aid consists of Figures 10 through 26, the location of which is listed in the figure listin Table of Contents, Figures List. It also consists of the tables of advantages, disadvantage,and common uses in this work aid. These tables are listed in the Table of Contents, List ofTables.

Table 1 is a list of types of joining processes and their abbreviations.

Table 1. Welding Procedures and their Abbreviations

JOINING PROCESS ABBREVIATION

Shielded Metal Arc Welding SMAW

Gas Tungsten Arc Welding GTAW

Gas Metal Arc Welding GMAW

Flux Cored Arc Welding FAW

Submerged Arc Welding SAW

Stud Welding SW

Oxyacetylene Welding OAW

Electric Resistance Welding

• High Frequency Resistant Welding.

ERW

• HFRW

Brazing NONE

Soldering NONE




Table 2. Advantages, Disadvantages, and Common uses of SMAW process.

ADVANTAGES DISADVANTAGES

Uses relatively simple, inexpensive, andportable equipment.

Requires significant interpass cleaning toremove slag.

Has moderate filler metal deposition rates. Has a low operating factor because of theinterpass cleaning and constant addition ofnew electrodes.

Requires relatively low skill levels forwelders.

Has limited current capability due to thediameter and length of the electrodes.

Can be used in all welding positions. Is not applicable to low melting metalssuch as lead, tin, and zinc.

Requires no auxiliary gas shielding or flux.

Is less sensitive to wind and to drafts thanare gas-shielded processes.

Is suitable for most of the commonly usedmetals and alloys.

COMMON USES

Weld piping, structural steel members and pipe supports

Construct oil tanks on site

Perform maintenance welding operations to include the following tasks:

• Repair defective welds

• Addition of corrosion resistant material to pressure vessel internals

• Repair failed mechanical equipment.




Table 3. Advantages, Disadvantages, and Common uses of GTAW process


Provides high quality weld. Requires relatively high skill levels forwelders.

Requires little interpass cleaning. Has a relatively low deposition ratecompared to consumable electrodewelding processes.

Can be used in all welding positions. Is not cost effective to use on thicksections.

Allows excellent control of root pass weldpenetration.

Is not conducive to welding in windy ordrafty areas.

Can be used with or without filler metal.

Suitable for most of the commonly usedmetal and alloys.

Allows the heat source and filler metaladditions to be independently controlled.

COMMON USES

Root pass of pipe butt welds for high alloy materials such as stainless steel and nickel basealloys.

Open butt GTAW root pass welds are used on pipe to provide the following things:

• Excellent radiographic quality welds

• A smooth shallow bead contour that does not affect flow conditions

• A weld that can be made from only one side,

• A weld that can be made in all positions around the pipe.

On thin wall materials, GTAW may be used to weld the complete joint without a significantloss of productivity.

TABLE 2, COMMON USES (Cont’d)

When radiographic quality welds are required on carbon steel weld joints.

To ensure complete fusion of the root pass.

For small diameter (less than 2") piping welds that include butt, fillet, and socket welds. Forthese items it is the process of choice.

Maintenance welding operations on small intricate parts and on materials that are very thinbecause of the excellent low current control of this process.




Table 4. Advantages, Disadvantages, and Common uses of GMAW process


A high deposition rate when compared tomanual arc processes.

Requires relatively high skill levels forwelders.

A high operating factor because the fillermetal is continuously fed.

Has a tendency to develop lack of fusiondefects due to the high welding speed.

Reduces interpass cleaning because there isno slag.

Is more difficult to use in hard to reachplaces because of the size of the wire feedgun.

Used in all welding positions. Can produce welder fatigue due to the highoperating factor.

Suitable for most of the commonly usedmetals and alloys.

COMMON USES

In Saudi Aramco, GMAW is used primarily for pipeline construction using the short circuitingtransfer mode

Many maintenance welding applications

Globular and spray transfer modes to weld structural members that require a high depositionrate




Table 5. Advantages, Disadvantages, and Common uses of FCAW process


A high deposition rate when compared tomanual arc processes

Requires relatively high skill levels for welders

A high operating factor because the fillermetal is continuously fed

Requires significant interpass cleaning toremove slag

Can be used in all welding positions Can produce welder fatigue due to the highoperating factor

Requires no auxiliary gas shielding or flux

Is less sensitive to wind and drafts than aregas shielded processes

Is suitable for most of the commonly usedmetal and alloys

COMMON USES

Weld structure members that require a significant amount of welding

Maintenance welding applications include the butt welding of sections of carbon steel pipingthat have been replaced and the build-up of corroded material




Table 6. Advantages, Disadvantages, and Common uses of SAW process


A high deposition rate as compared to othersemi-automatic arc welding processes.

Can only be used in the flat position.

A high operating factor. Is not cost effective for small welding jobs.

Requires no auxiliary gas shielding or flux. Is not capable of welding thin materials.

Is less sensitive than gas shield processes towinds and drafts.

Provides a high quality of weld metal.

Produces little or no smoke.

Has no arc flash; therefore, minimal protectiveclothing is required.

COMMON USES

To weld structural steel members and heavy wall pressure vessel sections

For smaller duration projects

For maintenance - use to add weld overlay to pressure vessel interiors not conducive to verticalwelding applications).




Table 7. Advantages, Disadvantages, and Common uses of SW process.


Can be used in all welding positions. Can only weld one end of a stud to thework piece.

Requires no auxiliary gas shielding or flux. Stud shape and design are limited by thecapability of the stud gun chucking device.

Can be used on curved and angled surfaces.

Provides welds with a very shallow heat-affected zone

COMMON USES

Attach insulation pins to pressure vessels and tanks

Attach grating bolts to platforms

Attach concrete anchors to structural members

Table 8. Advantages, Disadvantages, and Common uses of OAW process


Requires no electric power supply Has low deposition rate

Can be used in all welding positions Requires relatively high skill levels forwelders

Provides excellent control of heat input andtemperature

Is not efficient for large welding jobs

COMMON USES

Used at Saudi Aramco primarily to fabricate handrails for stairways and other non-pressureretaining equipment in both shop and field applications

Used to cut ferrous metals and to preheat metals prior to welds




Table 9. Advantages, Disadvantages, and Common uses of HFRW process


Not Used in Saudi Aramco Not Used in Saudi Aramco

COMMON USES

Not Used in Saudi Aramco

Otherwise, used to fabricate pipe and tubing around the world.

Table 10. Advantages, Disadvantages, and Common uses of the brazing process


Seldom Used in Saudi Aramco Seldom Used in Saudi Aramco

COMMON USES

Seldom used in Saudi AramcoJoining copper tubing

Joining materials in which little distortion due to heating can be tolerated like the followingmaterials:

• Electrical components

• Dissimilar base metals

• Precious metals




BIBLIOGRAPHY

SAES-W-010, Welding Requirements for Pressure Vessels, 15 January 1997ASME Section VIII, 1995

Documents

Metal Joining Processes