Effect of neutron irradiation on intergranular stress corrosion cracking susceptibility of thermally-sensitized type 304 stainless steels

Austenitic stainless steels have been extensively used as core component materials for light water reactor (LWR). During long time operation, however, the material tends to suffer from degradation in high temperature aqueous and radiation environment. Here, the effect of neutron irradiation on intergranular stress corrosion cracking (IGSCC) of thermally-sensitized Type 304 stainless steels was studied. Neutron irradiation was carried out to neutron fluences from 4 x 1022 to 1 x 1024 n/m2 (E > 1 MeV) at 563--613 K in helium gas at the Japan Material Test Reactor. The irradiated specimens were examined by slow strain rate (SSR) stress corrosion cracking tests in 563 K pure water of different dissolved oxygen concentrations, SSR tensile tests and microhardness measurements. The IGSCC susceptibility of the irradiated specimens increased with neutron fluence for the same irradiation temperature, although the IGSCC susceptibility at lower neutron fluences was lower than that for the unirradiated specimens. This trend was consistent with that for neutron fluence dependent grain boundary microhardness of the irradiated materials. Present materials irradiated in helium gas and low gamma ray field did not show any intergranular (IG) cracking in non-oxidizing water as well as in inert gas, despite the fact that previously another heat material irradiated to 3 x 1023 n/m2 at 563 K in boiling water showed IG cracking in the same environment