Sulfide Stress Cracking Susceptibility of Low Alloy Steels for Casing Application in Sour Environments

Sulfide stress cracking (SSC) resistance of casing steels with different alloying chemistries (i.e. Ti-B and Mn-Cr-Mo) was evaluated using constant load tensile test and slow strain rate test in NACE-TM0177 environment. SSC resistance in terms of RAH2S/RAair and time to failure was found to decrease with increase of material strength. For more susceptible steels, a mix mode of transgranular and intergranular fracture was observed. For less susceptible steels, intergranular fracture was less prevalent.;Inclusions were found to be the dominant factor contributing to SSC susceptibility. Mn-Cr-Mo steels were more susceptible to inclusions than Ti-B steels. Cracking was found to be initiated mainly from elongated inclusions such as MnS, Al-Si-O, or large globular Ca-enriched oxide inclusions. Clustering of inclusions were main SSC initiation sites. The distribution and morphology of carbides played an important role in SSC propagation. A uniform microstructure with fine globular carbides was found to improve SSC resistance.