STAINLESSQ&A BY DAMIAN J. KOTECKI

Q: Is it acceptable to use air carbon arcgouging to produce a weld joint prepara-tion or to remove defects found in 308L or316L stainless steel weld metal — eitherjoints or cladding? Concerns have beenexpressed about carbon pickup from thecarbon electrode causing damage to cor-rosion resistance of the stainless steelweld metal.

A: This question has been around formore than 50 years, and recent discussionscan still be found on the AWS Web siteforum. It was considered by Hard (Ref. 1)already in 1954, although he did not lookat low-carbon stainless steel. The concernusually expressed is that carbon contami-nation in the metal surface will lead tochromium carbide precipitation with re-sulting sensitization of the weld metaland/or heat-affected zone.

Actually, there is a second concern thatno one seems to mention. This concern isthe dross and molten metal that is notblown away by the air stream will also ex-perience nitrogen pickup from the air. Ifwelding is done on the dross and thin filmof metal melted by the air carbon arc butnot blown away, this nitrogen will enter

the weld metal. Since nitrogen is anaustenite-promoting element, ferrite con-tent will be reduced and could potentiallybe reduced to a level low enough that so-lidification cracking could occur in weldmetals like 308L and 316L, which are de-signed to contain some ferrite.

The manufacturers of air carbon arcgouging equipment provide lots of infor-

mation on how to correctly apply themethod to carbon steels, low-alloy steels,stainless steels, aluminum, etc. AWSC5.3:2000, Recommended Practices for AirCarbon Arc Gouging and Cutting, is an ex-cellent resource for noncommercial infor-mation. The correct technique generallyinvolves a pushing inclination to the car-bon electrode, with the air stream directed

Fig. 1 — Proper air carbon arc gouging technique.

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17WELDING JOURNAL

behind the advancing electrode. Thenthere are very specific recommendationsconcerning the correct current range for agiven carbon electrode, the correct elec-trode extension, and the correct air pres-sure. Figure 1, reproduced from AWSC5.3:2000, shows the correct technique.

Along with correct gouging technique,correct cleanup after gouging is essential.For stainless steels, this means grindingaway all traces of dross and oxidized sur-faces from the gouge area. Only a brightmetallic surface should remain beforewelding is initiated. If this is done, withproper technique in the application of aircarbon arc gouging, all traces of nitrogenpickup and carbon pickup will be removedand the ground surface will be quite suit-able for subsequent welding.

Low air pressure in air carbon arcgouging is a particular concern because itcan permit greater depth of carburizedand nitrided metal than normal grindingwould remove. Christensen (Ref. 2) delib-erately used low air pressure for air carbonarc gouging 304L stainless cladding ascompared to a gouge done with proper airpressure and to a machined groove. Chipswere removed by a superficial cut fromeach of the three surfaces then analyzedfor carbon content. The surface preparedentirely by machining was found to con-tain 0.03% C. The surface gouged withproper air pressure was found to contain

0.04% C, and that gouged with low airpressure was found to contain 0.10% C.Next, each prepared groove was weldedwith low-carbon stainless steel (grade notspecified), and the weld deposit was ana-lyzed for carbon content. The depositedweld metal in all three cases was found tocontain 0.03% C. Christensen furthernoted that nitric acid corrosion tests ac-cording to ASTM A262, on backgougedand welded root pass surfaces, showed noadverse corrosion results.

Christensen does not state whether ornot the gouged surfaces were subse-quently ground to bright metal before thechips were taken for chemical analysis orbefore welding. From the overall tone ofthe paper, I believe that no grinding wasdone. Christensen also did not considernitrogen pickup or ferrite loss. Even ifChristensen’s results are based upon notgrinding, I would not advocate weldingover the as-gouged surface without grind-ing to bright metal. It is not good practice,from the viewpoint of producing sound,defect-free welds, to weld over heavily ox-idized surfaces. And nitrogen contamina-tion of the oxidized surface can result inlower than expected ferrite content and apossibility for cracking. A competent in-spector can easily determine visuallywhether grinding to bright metal has beenproperly done before welding begins.

As long as grinding to bright metal fol-

lows air carbon arc gouging, I see no rea-son that air carbon arc gouging cannot bean acceptable method for joint prepara-tion of stainless steel or removal of defectsfrom stainless steel welds before repairwelding.♦

References

1. Hard, A. R. 1954. Exploratory tests of theair-carbon arc cutting process. Welding Journal33(6): 261-s to 264-s.

2. Christensen, L. J. 1973. Air carbon-arcgouging. Welding Journal 52(12): 782–791.

DAMIAN J. KOTECKI is president,Damian Kotecki Welding Consultants, Inc.He is a past president of the American Weld-ing Society, currently treasurer and a pastvice president of the International Instituteof Welding, and a member of the AWS A5DSubcommittee on Stainless Steel Filler Met-als, and the AWS D1K Subcommittee onStainless Steel Structural Welding. He is amember and past chair of the Welding Re-search Council Subcommittee on WeldingStainless Steels and Nickel-Base Alloys.Send your questions to Dr. Kotecki atdamian@damian kotecki.com, or toDamian Kotecki, c/o Welding Journal, 550NW LeJeune Rd., Miami, FL 33126.

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