typical weld in stainless steel made without backing gas

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PIPE FABRICATION

WALTER J. SPERKO

Fabricators turn

to backing flux.

as an economzc

alternati ve

to purge gas.

When heated by welding, the fluxmelts and floats over the weld-poolsurface; the solvent evaporates quicklyand leaves no residue. Another alter-native is use of a flux-coated weld-fillerwire that delivers a protective layer offlux over the weld.

To compare the results of weldingwith a backing flux and use of purgegas, Larry Peterson, Peterson Welding(Clemmons, N.C.), and I prepared gas-tungsten-arc-welded GTAW Type 304stainless-steel pipe coupons, NPS 3Schedule 40 (3.5-inch o.d., 0.216-inchwall thickness). Test weldments were !made with an inert backing shield;backing flux; a flux-coated filler metal;and without root-side protection. Re- ,fer to the photos to view effects on weld !quality of each procedure. t

And the winner is ...Welding without root-side protec-

tion, as Peterson welded he struggledto obtain good pipe-wall fusion. Root-pass welds made with backing gas,

may exhibit thin black deposits at theouter edges of the heat-affected zone(HAZ), but the weld metal should befree of black oxides. Slight discol-oration-a golden hue to the weld-in-dicates a bit of oxygen remained in thepipe during welding. This thin andtightly adherent oxide layer generallydoes not affect weld corrosion resis-tance, and is usually acceptable.

Fabricators turn to backing flux toprotect weld backsides as an economicalternative to purge gas. Fluxes comeas fine powder that the fabricatormixes with a solvent such as alcohol toform a thick mud. The welder appliesthe mud to the inside surfaces of thepipe at the edges of the joint bevel.

Mr. Sperko is president, Sperko Engi-neering Services, Inc., 4803 ArchwoodDr., Greensboro, NC 27406-9795;(910) 674-0600, fax (910) 674-0202.

" c an't you use backing flux in-stead of argon purging?" Fab-ricators of pipe frequently

hear this question from contractors in-stalling stainless-steel piping. Purg-ing to protect the root side of stainless-steel pipe weldments, or any pipeweldment of corrosion-resistant alloy,can be an expensive and time-consum-ing task. But welding the pipe re-quires the fabricator to use somemethod of preventing oxygen contami -nation of the weld root. If not, theweld surface will be coarse and rough,covered with a heavy black oxidelayer, and it may exhibit excessiveconcavity and incomplete penetration.Use of backing fluxes or flux-coatedwelding wire may prove an economicalalternative; however they leave aresidual flux and slag on the pipe in-ner surface that, when the piping is inservice, can contaminate the processstream or damage equipment. Theresidue may also cause pitting andcrevice corrosion of the pipe. ,

Purge gas, backing fluxesfight off oxygen

Purging stainless-steel pipe with ar-gon (or nitrogen for 300-series alloyswith less than 10 percent nickel) re-quires the purge gas to flow longenough to displace the air from thepipe and create an atmosphere of lessthan 1 percent oxygen. Use of purgedams, which reduce the volume to bepurged, minimizes the amount ofpurge gas needed and the purge time.With a gas flow of 50 ft3/min, purgetime can range from 30 seconds perfoot of pipe for 3-inch pipe and smallerto 90 minutes per foot of 48-inch pipe.The inside surface of a completed weldon properly purged stainless-steel pipewill be smooth, even, and uniform. It

26 WELDING DESIGN & FABRICATION MAY 95

PIPE FABRICATION

formed due to decomposition of ma-chining fluids remaining in the pipeafter bevelling, easily flaked off.While the root pass of all of the weldsreceiving some method of backing eas-ily fused, the second pass oxidized theroot-side surface when backing withflux and when welding with coatedwire. Only when depositing the sec-ond pass with purge gas protecting theroot-side surface did the inside surfaceremain unoxidized by the heat of sub-sequent passes.

As was expected, welding withoutbacking left rough-surfaced heavilyoxidized welds plagued with cavities.

Use of backing fluxes and flux-coated wire eases wetting of the weldmetal and base metal, enabling thewelder to easily achieve full penetra-tion. However, welding left behind aresidual slag fused to the inside sur-face of the pipe over the weld and HAZ.As the test coupons cooled, some of theresidue---flux and slag, a hard ceramicoxide---flaked off in chunks. The re-maining residue proved tough to re-move-water spray and hand-wirebrushing failed, and use of a slag ham-mer proved only somewhat successful.The flux-coated wire left less of a slagthan did use of backing fluxes.

Residual slag that adheres to the in-side of a pipe weld can lead to corro-sion under the slag, including pittingand crevice corrosion. Applicationswhere the slag may not cause prob-lems include elevated- temperaturesites where electrolytic corrosion is ofno concern, transportation of noncor-rosive products, and handling ofstrong acids and other aggressiveproducts. Strong acids may even dis-solve and dislodge the residue, leavinga clean corrosion-resistant surface.

These photos show the root side of gas-tungsten-arc welds deposited from one side onstainless-steel pipe. From top left {1), deposit made with inert-gas backing is smoothand even, without oxides; {2)weld back-shielded with flux exhibits residual flux and slagon and near the weld; {3) the root side of a two-pass weld using backing flux shows thatthe second pass has disrupted the residual slag, leaving a cleaner weld surface; {4)anduse of a flux-coated filler wire results in a small amount of slag covering the weld andheavy oxidation of the base metal next to the weld.

ConclusionsUse of backing fluxes and flux-

coated wire improves quality of root-side welds in stainless-steel pipe whencompared to welding without backing.The flux causes tack and root-passwelds to fuse easily and fully. How-ever, piping-system designers shouldnot approve use of these productswithout first determining the effectsof the residue left behind.

The only way to ensure that the in-side surfaces of welds made in stain-less steel or any other corrosion-resis-tant material is free of contaminationand excessive oxides that may causecorrosion of the pipe is to deliver ashield of an inert backing gas that suf-ficiently displaces the oxygen from theinside of the pipe. 8

backing flux, and flux-coated wirefused smoothly and easily, althoughthe flux on the coated wire would occa-sionally pop off of the wire before melt-ing into the weld pool, and some slagwould float on the weld pool. Bothtypes of backing fluxes used tended tobubble up through the puddle, dis-turbing the arc plasma. The weld de-

posited withinert-gas purge wassmooth and uniform, with slightgolden and thin blue discoloration onthe weld and HAZ, and brown discol-oration at the edge of the HAZ. Thegold and blue layers, caused by a bit ofoxygen present in the pipe duringwelding, tightly adhered to the weld-ment; the brown layer, which probably

WELDING DESIGN & FABRICATION 27MAY 95

Typical weld in stainless steel madewithout backing gas