Damage Evaluation for Hot Reheat Pipe Elbow Used at the Thermal Power Plant of Mod.9Cr-1Mo Steel
Akigo FUJITA, Masamichi MIYABE, Yukio KAGIYA, Yoshinori MURATA and Masahiko MORINAGA
Abstract:In order to confirm the validity of the model for creep damage mechanism constructed from a series of experiments on reheat pipe elbow of mod. 9Cr-1Mo steel with a practical size, microstructural observation and uniaxial creep rupture tests were carried out using the welded joint of the hot reheat pipe elbows (No.5 and No.25) taken from a practical plant after long term-service. The obtained results are as follows : (1) Nucleation and growth of creep voids were observed in the fine-grained HAZ region of the inside-elbow region, where the triaxiality factor of stress was high compared to the other regions in the HAZ. The phenomenon of creep damage in the long-term used elbows was consistent with that in the practical sized experimental elbow used for the model construction. (2) The degree of creep damage observed for No.5 and No.25 elbows corresponded to the early stage of creep damage. But the creep damage of No.5 elbow was more serious than that of No.25 elbow due to the difference in degree of circularity between the two elbows. (3) Considering that the long-term used elbows underwent creep damage to some extent, the creep rupture strength except for the repair-welded specimen exhibited high enough to exceed the demand strength estimated from the life assessment curve for the welded joint of high chromium ferritic steels presented by Nuclear and Industrial Safety Agency. Key Words:Modified 9Cr-1Mo steel, Elbow, Weld joint, Life assessment, Long-term serviced, Repair-welded, Laves phase.