The effect of back shielding gas types and contained oxygen level on the corrosion resistance of weld was investigated for gas tungsten arc welding (GTAW) of UNS S32750 super duplex stainless steel (SDSS) pipes. The SDSS pipes were welded with three types of back shielding gases of nominal 100% Ar, 100% N2 and a mixture of 90% N2+10% H2 along with controlled O2 contents at 100, 500 and 2,000 ppm as pre-mixed gas. It was found that 100% N2 back shielding gas improves the corrosion resistance compared to 100% Ar, however, a mixture of 90% N2+10% H2 had an adverse effect. In addition, the specimen employing 100% N2 back shielding gas with 2,000 ppmO2 was acceptable in corrosion test. It was observed that the corrosion resistance of SDSS weld joint is mainly affected by depletion of nitrogen in the surface of root bead and less affected by oxygen content levels. The nitrogen depletion behavior is affected by the type of back shielding gas.
Key words: Super duplex stainless steel, UNS S32750, single-sided welding, GTAW, oxidation, corrosion resistance