Embrittlement of ER309L stainless steel clad by σ-phase and neutron irradiation

The degradation of mechanical properties in RPV clad by the presence of σ-phase and neutron irradiation was investigated using small punch (SP) tests. The irradiation doses at 290 °C ranged from 5.01 × 1018 to 5.79 × 1019 n/cm2 (>1 MeV). After irradiation, the increase in SP ductile-to-brittle transition temperature (SP-ΔDBTT) was ≈14 K, independent of the fluence. However, it was more strongly affected by the amount of brittle σ-phase. The cracking appearances in the SP specimens gradually changed from circumferential to radial cracking as a function of test temperature, content of σ-phase and neutron irradiation. These results are discussed in terms of the different fracture stress of each phase as well as the irradiation-induced hardening combined with the changes of governing stress components for crack initiation during deformation by means of finite element stress analysis.