Effects of microstructural evolution on charpy impact properties of modified ferritic/martensitic steel after neutron irradiation

Yano, Yasuhide  ; Oka, Keiichiro*; Akasaka, Naoaki; Yoshitake, Tsunemitsu ; Abe, Yasuhiro; Onuki, Somei

The embrittlement behavior of the modified ferritic/martensitic 11Cr-0.5Mo-2W, V, Nb steel (PNC-FMS) after neutron irradiation in JOYO was investigated by Charpy impact tests and TEM and SEM observations. The impact properties of the specimens after irradiation to 4.4 dpa at 773 K were similar to the as-received PNC-FMS. The ductile-brittle transition temperature (DBTT) remarkably decreased due to irradiation to 2.8 dpa at 923 K. The precipitates formed in the martensitic lath were still stable under neutron irradiation at 773 K, however they were unstable under irradiation at 923 K. The martensitic lath structure was also stable at the former irradiation temperature, but it was significantly changed at the latter. The decrease in the upper shelf energy after irradiation was related to the precipitate distribution. The changes of DBTT due to irradiation were attributed to decreased the dislocation recovery and to increased broadening of the martensitic lath.