Type IV damage was found at several ultra-supercritical (USC) plants that used creep-strength-enhanced ferritic (CSEF) steels in Japan, and the assessment of the remaining life of the CSEF steels is important for electric power companies. However, there has been little research on the remaining life of material that has actually served at a plant. In this study, the damage and remaining life of a Gr.91 welded elbow pipe that served for 54,000 h at a USC plant were investigated. First, microscopic observation and hardness testing were conducted on specimen cut from the welded joint; the results indicated that the damage to the elbow was more severe in the fine-grain heat-affected zone near the inner surface. Furthermore, creep rupture tests were performed using specimens cut from the welded joint of the elbow, and from these results, the remaining life was evaluated using the time fraction rule as almost 110,000 h. Finite-element analysis was also conducted to assess the damage and remaining life, and the results were compared with the experimental results.

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