STAINLESS Q&A included in the study, but I think the results offer a rationale with which to approach your problem with porosity. Stützer et al. indicate that increasing the quantity of CO2 in the shielding gas tends to increase the amount of porosity in the weld metal. In particular, the shielding gases in Table 2 containing 0.5% CO2 (gas G3) and 2.0% CO2 (gas G4) are most prone to producing weld metal with large amounts of porosity. The authors propose to restrict CO2 in the shielding gas to very low levels. Gas G1 (no CO2) and gas G2 (0.05% CO2) seem to have produced the lowest levels of porosity. They further propose that there is some sort of interaction between nitrogen and CO2 (or oxygen) that causes the porosity. If this is correct, then the same sort of porosity issues could be expected with argon plus 1 or 2% oxygen. Stützer et al. do not indicate whether wetting was acceptable with 30 WELDING JOURNAL / NOVEMBER 2016 the gases free of carbon dioxide. I expect that the use of 30% helium in the shielding gas was an attempt to obtain better wetting and bead shape than would be obtained without the helium, especially when there was no oxygen or carbon dioxide in the shielding gas. Since no data was included for argon oxygen gas mixes, no definitive statement can be made concerning use of these gas mixes. However, if the proposal of Stützer et al. that oxidizing gas additions interact with nitrogen to promote porosity, then it would seem that the elimination or reduction of oxygen in the shielding gas would have the same effect as elimination or reduction of carbon dioxide. It is noteworthy Stützer et al. indicate their study also considered other factors that might affect porosity, and reported heat input per se with a given gas did not seem to have any effect on the observed porosity. However, pulsed GMAW seemed to produce more porosity than spray transfer GMAW. No mechanism for this effect is proposed. In conclusion, I would be very interested in feedback from fabricators trying this approach of low oxygen potential to prevent or limit porosity. If I receive additional results, I will include them in a future column. Reference 1. Stützer, J., Zinke, M., and Jüttner, S. 2016. Studies on the pore formation in super duplex stainless steel welds. IIW Document II-1967-16. Paris, France: International Institute of Welding. DAMIAN J. KOTECKI is president, Damian Kotecki Welding Consultants, Inc. He is a past treasurer of the IIW and a member of the A5D Subcommittee on Stainless Steel Filler Metals, D1K Subcommittee on Stainless Steel Structural Welding; and WRC Subcommittee on Welding Stainless Steels and NickelBase Alloys. He is a past chair of the A5 Committee on Filler Metals and Allied Materials, and served as AWS president (2005–2006). Questions may be sent to Damian J. Kotecki c/o Welding Journal, 8669 NW 36 St., # 130, Miami, FL 33166, or via email at damian@damiankotecki.com. — continued from page 28 For info, go to aws.org/adindex WJ
Welding Journal | November 2016
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