Microstructural changes in 1Cr-0.5Mo steel after 20 years of service

Metallographie studies have been conducted on 1Cr-0.5Mo steel “taken from a pressure vessel which had been in service for 20 years in a hydrogenous environment at 524 °C. The original microstructure of the steel, reproduced by reheat treatment of the exposed material, consisted of proeutectoid ferrite and tempered bainite, the carbides being mainly cementite. The service exposure caused precipitation of needle-like M₂C carbides in the ferritic regions and M₇C₃ carbides in the vicinity of the original cementite particles. Chromium and molybdenum moved from solid solution to the carbides during the service exposure with 72 pct and 32 pct of the total chromium and molybdenum contents, respectively, remaining in solid solution after service for 20 years. Formerly with AMAX Materials Research Center (formerly Climax Molybdenum Company of Michigan)