Effect of Surface Hardness and Hydrogen Sulfide Partial Pressure on Sulfide Stress Cracking Behavior in Low Alloy Linepipe Steel

TMCP (thermo-mechanical controlled process) linepipes have long been used in severe sour environments, but recently sulfide stress cracking (SSC) caused by local hard zones has become a concern. In order to clarify the hardness threshold that leads SSC, four-point bend (4PB) SSC tests as specified NACE TM0316 were conducted under several H2S partial pressure conditions. At pressures of 1 bar and higher, surface (at 0.25 mm from surface) observed 4PB specimens without was approximately correlated maximum acceptable level 250 HV0.1. A stable low or less HV0.1 achieved suppressing lath bainite (LB) obtaining soft granular (GB) microstructure, resulting superior SSC-resistant property. It found crack propagated when increased due an increasing volume fraction LB microstructure. Under 16 condition, growth rate environment, hydrogen embrittlement promoted. However, test at bar, concentration transition suppressed because shape localized corrosion semicircular corrosivity. This can be reason why susceptibility similar especially using samples with lower