Change in mechanical properties of austenitic stainless steels due to very high cycle fatigue

Xiong, Z.; Naoe, Takashi   ; Futakawa, Masatoshi  ; Maekawa, Katsuhiro*

An austenitic stainless steel, SUS316L is currently being used in the liquid mercury pulsed spallation neutron source as the structural material of the mercury enclosure vessel, so called target vessel. The target vessel suffers cyclic loading in which the total number of cycles in the service life is higher than 210, with a high strain rate of 50 1/s at maximum under intensive proton and neutron irradiation environment. In present work, the very high cycle fatigue strength was investigated using the specimens that had the different dislocation density at room temperature. The tensile strength and hardness of the failed specimen were measured to understand the change in mechanical properties during fatigue test in addition to the fatigue strength evaluation. It was found that the fatigue failure still occurred in the very high cycle region. The fatigue strength was increased with the dislocation density. For the SA 316L, cyclic hardening, the hardness was increasing with the number of cycles, was observed. However, 10% CW316L showed obvious softening, the hardness decreased while the number of cycles was less than 106 and then increased while the number of cycles was beyond 10.