Irradiation-Induced Deformation in Cold-Worked SUS 304 Stainless Steel

SUS 304 stainless steel has been used in the light-water reactors constructed in earlier days, in which irradiation-assisted stress corrosion cracking has drawn increasing attention and tensile residual stress is believed to be one of the major causes. It is, therefore, essential to assess its stress relaxation behavior under irradiation, which can be evaluated from the irradiation creep data, and the effect of cold work on it. Creep experiments under 17 MeV proton irradiation (2x10−7 dpa/s) at 288°C were conducted for SUS 304 with 5% and 25% cold work (CW). Irradiation creep rate of 5%CW was only slightly larger than that of 25%CW. Stress dependence was almost quadratic in both specimens, in contrast with the linear dependence in cold-worked SUS 316L reported earlier. Stress relaxation under irradiation was found to reflect this quadratic dependence. Martensite is induced by cold-working in SUS 304, not in SUS 316L, and marked difference in its amount was found between 5%CW and 25%CW, despite the small difference in irradiation creep behavior. Thus, the observed quadratic dependence appears to result not directly from the induced martensite itself but from a climb-enabled glide of the tangled dislocations densely formed in the vicinity of martensite phase boundaries.