SSC Resistant X65 UOE Linepipe for Severe Sour Environment

Linepipe steels need to have a sufficient resistance to sulfide stress cracking (SSC) in a wet sour environment containing H2S gas. In 2013, actual sour gas pipeline failure occurred due to SSC under high H2S partial pressure condition over 1bar. One of the possible root causes of SSC was assumed to be a formation of hard zone in the steel surface. By improving cooling homogeneity as well as surface cooling rate control, homogeneous granular bainite (GB) microstructure is obtained, resulting in stable low surface hardness in the pipe surface. Lowering the surface hardness less than 250 HV0.1 at 0.25 mm from the surface by suppressing the hard lath bainite (LB) microstructure led to no SSC even for severe sour environment over 1bar H2S in both four-point bend (4PB) test and full ring test. As for the SSC mechanism, the round bar tensile SSC test showed that the increase of H2S partial pressure and the increase of hard LB microstructure increased the contribution of hydrogen embrittlement (HE) mechanism. Grade X60 UOE linepipes with low surface hardness were mass produced and showed good SSC resistant properties. No hard-spots were detected by the full surface hardness inspection in each plate steel before UOE pipe forming.