Effects of weld heat and neutron irradiation on microstructure of reactor pressure vessel steels

Katsuyama, Jinya  ; Onizawa, Kunio ; Kuramoto, Akira*; Nozawa, Yasuko*; Toyama, Takeshi*; Nagai, Yasuyoshi*

In order to characterize an inhomogeneity of heat affected zone (HAZ) in reactor pressure vessel (RPV) steels, effects of weld heat input and neutron irradiation have been investigated by SEM/EDX, 3D atom probe and positron annihilation methods. Coarse grain HAZ with/without post weld heat treatment (PWHT) showed higher toughness compared to base metal (BM) caused by mixed structures of lower-Bainite and Martensite. Fine grain HAZ before PWHT showed much higher strength and lower toughness due to Martensite-Austenite constituent (MA). After PWHT, FGHAZ toughness became similar to BM since MA disappeared. There are no clear differences among microstructures such as carbide, grain boundary for unirradiated HAZ materials and base metal, and Cu precipitation and irradiation damage for irradiated materials. Although HAZ materials of RPV steels might show equal or lower toughness compared to BM, irradiation embrittlement of HAZ materials would not become much larger than that of BM.