Effects of alloying elements and microstructure on the susceptibility of the welded HSLA steel to hydrogen-induced cracking and sulfide stress cracking

Hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC) susceptibility of the submerged arc welded API 5L-X70 pipeline steel with different amounts of titanium at two levels of manganese (1.4% and 2%) were studied. The centerline segregation region (CSR) observed in the X70 pipe steel played an important role in the HIC susceptibility. Increased acicular ferrite content in the microstructure improved HIC resistance and SSC resistance, while bainite and martensite/austenite constituents deteriorated the workability of the welded specimens in sour environments. The 2% Mn-series welds showed higher SSC susceptibility than the 1.4% Mn-series welds due to the higher hardness values of the welds. The precipitated titanium carbonitrides in the welds can act as beneficial hydrogen traps and delay cracking in hydrogen sulfide environments. By further addition of titanium, the appearance of bainite and martensite/austenite in the microstructure outweigh any beneficial effect of titanium carbonitrides. The weld metals contained high percentage of acicular ferrite and good distribution of titanium carbonitrides yielded the best performance in sour environments. In two series of the welds, the best sour service properties were obtained at two compositions, 1.40% Mn-0.08% Ti and 1.92% Mn-0.02% Ti.