OLAC (On-Line Accelerated Cooling)-Based X65 Linepipe with Low Surface Hardness for Severe Sour Environment

摘要

TMCP (Thermo-mechanical controlled process) large diameter linepipes have been used since a long time for high-strength and severe sour application, but recently Sulfide Stress Cracking (SSC) caused by local hard zones has become a concern. In this paper, material design concept to prevent SSC was firstly introduced. In the author's organization, Advanced-OLAC-based cooling rate control for surface and mid-thick area can achieve uniform and low hardness at the surface portion while maintaining high strength with lean chemistry. By improving cooling homogeneity as well as surface cooling rate control, homogeneous granular bainite microstructure is obtained, resulting in a stable low surface hardness at pipe inner surface. Developed OLAC-based Grade X65 LSAW Linepipe showed no SSC even for severe sour environment over than 1bar H_2S while tested as per both four-point bend test and full ring test. The surface hardness limit in full ring SSC tests gave good agreement with that of 4PB SSC tests in 1bar and 16bar H_2S partial pressure condition. The effect of H_2S partial pressure condition on surface corrosion behavior and SSC crack propagation behavior is also discussed.