JUNE 200758

Pipe made from highly alloyed metalsrequires a higher level of gas tungsten arcwelding (GTAW) proficiency to meet thestandards required in the power-genera-tion, nuclear, petrochemical, food, phar-maceutical, and processing industries.

While alloying elements such as nickel,chromium, molybdenum, copper, andcobalt add greater mechanical propertiesto carbon steel pipe, they also make thepipe more difficult to weld. This articlefocuses on the techniques required fromthe welders perspective, so that he or shecan make the transition from GTA weld-ing carbon steel to GTA welding alloy met-als such as stainless steel, duplex stainlesssteel, Inconel (Ni-Cr, Ni-Cr-Mo, andNi-Cr-Co-Mo alloys), Monel (Ni-Cuand Cu-Ni alloys, and pipe from relatedalloys. While each alloy has its own spe-cific considerations, similarities do exist.

Slow Down, You MoveToo Fast

Alloy metals have lower thermal con-ductivities than steel and the alloy weldpool has a lower viscosity. Because theweld pool is not as fluid, it does not wetas readily. The welder familiar with mildsteel must physically and psychologicallymake adjustments. Key among them is toslow down. Slow down for stainless steel,slow down more for duplex stainless steel,and slow down even more for nickel al-loys. Traveling too quickly can lead toproblems like incomplete fusion.

Do not attempt to compensate for asluggish pool by increasing weld current.Using more current than the weld proce-dure dictates will vaporize (burn out)the alloying elements, leading to corro-

NEAL BORCHERT is business development manager, Miller Electric Mfg. Co., Appleton, Wis. DANNY PHILLIPS is welding coordinator,UA Local 598, Pasco, Wash.

While alloying elements add greater mechanical properties to carbon steel pipe, they also make it more difficult to weld.

These tips can help you

make the transition from

gas tungsten arc welding

carbon steel to GTA

welding alloy metals

BY NEAL BORCHERT ANDDANNY PHILLIPS

Techniques forSuccessfully Welding

Alloy Pipe

Techniques forSuccessfully Welding

Alloy Pipe

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sion and reduced service life. For this rea-son, use a low-to-moderate heat input,and visually inspect all welds for indica-tions of excess heat input, typically a colorchange. This step is critical.

Keeping the slow down advice inmind, the following images and adviceshowcase the basic steps necessary toachieve good results when welding alloymetals. Be aware that in several instancesthe welder in the photos deliberatelymade mistakes. This was done to help thebeginning welder learn to identify mistakes, their causes, and how to correct them.

Weld Preparation

Weld backing gas. Protect the root sideof all complete penetration welds by usingthe proper tooling to close off the pipeand purging the atmosphere from the pipe.

Somethings wrong. This tack weldshows numerous defects: a sluggish weld

pool that couldnt be controlled, discol-oration that indicates too much heatinput, and a star crack (left) from improper gas coverage and the pool so-lidifying too quickly. The cause: no backing gas.

Carbide precipitation on the inside ofthe pipe, commonly called sugaring be-cause of the appearance, indicates poorbacking gas coverage.

Shielding gas. Argon may be used forall thicknesses, but for nickel alloysgreater than 18 in. thick, adding helium in-creases penetration and allows for fastertravel speeds (consult your local gas dis-tributor for a recommendation for yourapplication). For argon, set the flow rateat 10 to 20 ft3/h. For helium mixtures, increase flow rates by one and a half tothree times to compensate for heliums buoyancy.

Use a gas lens. To create a smoother,more stable flow of shielding gas and su-

perior shielding gas envelope, always usea gas lens. Use the largest cup practicalfor the application (here, a size 12 cup isused for 6-in.-diameter, Schedule 10 du-plex stainless pipe).

Tungsten extension. Keep the tungstenextension (stickout) as short as possible,ideally 316 in. or less on butt joints. Thishelps ensure that the welding arc stayswithin the shielding gas envelope. Thickermaterial or fillet welds may require exten-sions of 38 to 12 in. Use a 2% thoriated tung-sten electrode for all welds.

Welding parameters. Set the weldingmachine for DC electrode negative. Arcstarting methods include high frequency(HF), Lift-Arc, or scratch start, with thefirst two being preferred.

Clean filler rod. Use a chemical sol-vent to clean the filler rod before use(safety note: remove the solvent and any

Weld backing gas.

No backing gas: outside the pipe.

No backing gas: inside the pipe.

Set the gas flow.

Always use a gas lens.

Set tungsten extension.

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flammable material away from the weld-ing area before striking an arc). Ideally,wear gloves when cleaning the rod, as evenoil from your fingers can lead to contam-ination. Generally, the filler metal shouldmatch the base metal. When welding dis-similar materials, match the filler metalto the higher-alloy base material.

No contaminants. Prior to welding, re-move all oxide film from the bevel and 2in. back from the joint edge. Use a rotaryfile, hand file, or brush made from stain-less steel; dedicate these tools to this ap-plication. Do not use disc grinders, as thealumina oxide in the sanding medium andgum-based adhesive can contaminate thejoint. Next, use a chemical solvent to cleanthe joint surface. Grease, oil, paint, mark-ing crayons, ink, lubricants, cutting fluid,and processing chemicals can contain sul-

fur, phosphorus, lead, and other contam-inants can lead to embrittlement. Safetynote: remove the solvent and any flamma-ble material away from the welding areabefore striking an arc.

Joint preparation. To obtain good fu-sion on alloy metals, bevel the joint to awide angle (80 to 90 deg for a V-groove),create a narrow land, and set a wide rootwidth. This will enable the arc to breakdown the bevel and ensure complete fusion.

Mind the root opening. Pipe weldersuse their filler metal to set the root open-ing. This 18-in.-diameter filler rod shouldbe just on the verge of falling through theroot on this 6-in.-diameter, Schedule 10duplex stainless pipe.

Welding Advice

Awkward positions. Pipe welders needto be contortionists. Practice while kneel-ing on the remote amperage control(shown here), using your elbow, or put-ting the control between your thighs. Foradded convenience, consider using a gastungsten arc welding torch with a remotefingertip control.

Get tight. The proper arc length whenwelding alloy metals is as tight as possible on the verge of touching the metal. Atight arc produces a well-defined pool andbetter directional control, which is neces-

sary when dealing with a sluggish weldpool. Should the tungsten touch the weldpool, stop immediately and regrind thetungsten.

Too long, no control. Long arc lengths(shown here) preheat everything but theweld pool. If the heat of the arc gets to thebevel before the pool does, it can under-cut the edge of the bevel. A long arc mayalso cause the end of the tungsten elec-trode to ball up prematurely.

See the arc. Welders must put theirhead in a position so that they can see thearc to maintain good arc length dontguess. Generally, this involves being closeto the weld. Some welders may find ithelpful to use a magnifying cheater lens.Complicating matters, the pool on alloymetals is harder to read because, as alloy-ing elements like chromium and nickel va-porize, they create a film or haze of plasmagas over the top of the pool.

Set welding parameters.

Prepare the joint.

Set a wide root width.

Using knee control.

Example of proper arc length.

Clean filler rod.

Clean the metals surface.

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Bevel breakdown. The inside of thesame tack weld shows how the proper(slow) travel speed and tight arc brokedown the bevel and complete fusion wasobtained.

Root problem and solution. To illus-trate the effects of travel speed, the welder

intentionally moved too quickly whenwelding on the left side of this joint, thenslowed down to a proper speed. Noticehow quick travel failed to break down thebevel, as indicated by a weld that is nar-rower than the width of the root opening.

Angling for success. Keep the torch di-rected nearly straight at the joint, anglingit back just enough to see the arc. Main-tain a 90-deg angle (perpendicular) be-tween the torch and filler metal. Anglingthe torch too much (beyond 15 deg) di-rects the heat away from the base metal,preheats the welding wire, and may causeit to ball up prematurely. This may leadto incomplete fusion.

Slightly convex bead. This root passshows good sidewall fusion and a slightlyconvex appearance. A slightly convexbead (as opposed to a flat or concavebead) prevents centerline cracking whenthe weld cools.

Too convex. This root bead is too con-vex, which can lead to fusion problems onthe hot pass. Breaking down the ropey topof the bead and obtaining good fusion onboth sidewalls will be extremely difficult.

Cracking up. This star crack orcrater crack occurred because the edge ofthe weld pool solidified before the center.To prevent cracking, prepare the tack weldwhere youll end the weld by grinding itdown. Then, follow this welding routine:about 1 in. before the end of the weld,bring the pool down to half its size. This

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Example of too long arc length.

Proper angle of the torch and filler metal.

The inside of the same tack weld.

Root problem and solution.

A slightly convex bead prevents centerlinecracking when the weld cools.

A root bead that is too convex can lead tofusion problems.

Keep the torch directed nearly straight at thejoint.

This star or crater crack occurred becausethe edge of the weld pool solidified beforethe center.

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will allow the pool to solidify or dry upfrom the inside out. After bringing the sizeof the pool down, increase travel speed toslightly faster than normal while main-taining a tight arc right up to the momentthe arc is broken off. Many welders havea problem with star cracks because theyincrease arc length to help dry the poolup, which actually causes the pool to so-lidify too quickly. This causes star crackson the root bead and causes arc marks onthe surface of the pipe.

Tie-in and weld start. To start the rootpass, butt the filler metal against the tackweld and fuse it in place (shown here.Note that the tack is not ground or other-wise prepared). Then, step back 14 in. andestablish the weld pool on the tack weld;next, move forward and bring the poolthrough to where the filler metal is tacked,and carry on. When pipe welding, notethat the filler metal is not dabbed in andout of the weld pool. It remains in posi-tion and is almost wrapped into theweld. This technique keeps the rod insidethe protective shielding gas and minimizespool agitation, which prevents burning outthe deoxidizing elements.

Weld pass pause. When welding a weldpass freehand, move the torch from oneside, always pausing at the side of the weldto allow the pool behind the arc to solid-ify before moving on. Also, when movingacross, adjust travel speed so that abouthalf the width of the joint is molten andthe other half solid. Lastly, do not wigglethe torch as when cup walking; keep thearc pointed straight down at all times.

Stringer beads. After the first pass fol-lowing the root pass, make all subsequentpasses using the stringer bead technique.Alloy metals do not flow (wet out) read-ily, so the wide weave beads associatedwith carbon steel are difficult or impossi-ble to make.

Extended postflow. When weldingalloy metals, set the gas postflow to 5 to10 seconds longer than normal and keepthe torch in position until the weld cools.Prior to using the filler rod again, trim theend to prevent contamination.

Following the steps described abovewill help welders make the transition fromwelding mild carbon steel to a variety ofalloy metals. While this advice is broadlyapplicable, always obtain welding proce-dures for the specific metal on which you

are welding. Whenever possible, practiceuntil working with alloy metals becomessecond nature. Having confidence in yourability to make quality welds prepares youmentally for passing weld tests, which inturn opens up new job and career possi-bilities working with alloy metals.

Acknowledgments

All of the images depict the work ofFrank Carney, a first-year apprentice withUA Local 598.

In addition to the authors own knowl-edge, this article reflects more than 100years of pipe welding experience fromDave Stanley, journeyman welder, UALocal 598; Steve Davis, Joint ApprenticeTraining Committee instructor; and GaryCulvert, a 50-year Local 598 member,welding consultant, and retired collegeprofessor who is currently serving on theboard of directors at Columbia BasinCommunity College in Pasco, Wash.

For dedicating the resources to makethis article possible, the authors would liketo thank Rick Berglund, business man-ager, and Pete Nicacio, training coordina-tor, UA Local 598.

JUNE 200762

Starting the root pass.

When welding a pass freehand, pause at theside of the weld to allow the pool behind thearc to solidify.

Make subsequent passes using the stringerbead technique.

Set the gas postflow to 5 to 10 s longer thannormal.

Want to be a Welding Journal

Advertiser?For information, contact

Rob Saltzstein at (800) 443-9353, ext. 243,

or via e-mail atsalty@aws.org.

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