Effect of shielding gas composition on weld formation and H2S stress corrosion susceptibility of X65 pipeline steel

The effect of shielding-gas composition on weld formation, microstructure, and H 2 S stress corrosion cracking (SSCC) in cold metal transfer (CMT) welds was examined. Pure argon shielding led to incomplete penetration and carbide precipitation. Moderate addition of CO 2 (10–20 %) improved arc stability and weld quality, yielding a bainite-dominated coarse-grained heat-affected zone (CGHAZ) microstructure. Excessive CO 2 (50 %) increased the cooling rate and induced martensite formation in the CGHAZ. Severe pitting occurred in both the weld metal and CGHAZ. Strong Si segregation in the weld metal produced Volta potential differences > 50 mV, driving microgalvanic dissolution. Although the CGHAZ showed minimal elemental segregation, its microstructural heterogeneity, characterized by coarse grains, high dislocation density, and hydrogen trapping, enhanced local anodic activity even with a potential difference below 35 mV. Acicular ferrite impeded microcrack propagation, whereas martensite promoted hydrogen accumulation and accelerated crack growth.