Sulfur in Austenitic Stainless Steel and Irradiation-Assisted Stress Corrosion Cracking

Abstract Slow-strain-rate tensile (SSRT) tests in boiling-water-reactor-like (BWR-like) water and fractographic analysis were conducted on 13 model austenitic stainless steels (SSs) irradiated at 289°C in helium to ≈2.0 x 102 1 n·cm–2 (E > 1 MeV). The compositions of the model steels, similar to those of Types 304 and 316 SS, were varied systematically to investigate the effects of Ni, Si, P, S, Mn, C, N, Cr, and O on susceptibility to irradiation-assisted stress corrosion cracking (IASCC). Susceptibility to intergranular stress corrosion cracking (IGSCC), and hence to IASCC, could not be correlated well with bulk Ni, Si, P, Mn, C, N, Cr, or O content. However, bulk S content provided good correlation with IASCC susceptibility. Good resistance to IASCC was observed in Types 304 and 316 SS steels that contain very low concentrations of S of ≤0.002 wt.%. As fluence increased, the IASCC susceptibility of Types 304, 304L, 316, and 316L steels that contain >0.003 wt.% S increased drastically, indicating that the deleterious effect of S is predominant in the failure of in-core components at high fluence. In contrast to Types 304 and 316 SS, S concentration of ≤0.002 wt.% does not necessarily render low-carbon Types 304L, 316L, or high-purity-grade steels resistant to IASCC, indicating a synergism between C and S. A small amount of globular delta ferrite is beneficial in mitigating the deleterious effect of S, probably because S atoms are trapped in the ferrite. An IASCC model has been proposed which is based on oxidation characteristics of crack-tip grain boundary and loss of cohesive strength in thin Ni- and S-rich film, produced between the grain-boundary oxide and metal, at high S concentration in the film.