SULFUR IN AUSTENITIC STAINLESS STEEL ANDIRRADIATION-ASSISTED STRESS CORROSION CRACKING

摘要

Slow-strain-rate tensile (SSRT) tests in boiling-water-reactor-like (BWR-like) waterrnand fractographic analysis were conducted on 13 model austenitic stainless steelsrn(SSs) irradiated at 289℃ in helium to ≈2.0 x 1021 n·cm–2 (E > 1 MeV). Therncompositions of the model steels, similar to those of Types 304 and 316 SS, werernvaried systematically to investigate the effects of Ni, Si, P, S, Mn, C, N, Cr, and O onrnsusceptibility to irradiation-assisted stress corrosion cracking (IASCC). Susceptibilityrnto intergranular stress corrosion cracking (IGSCC), and hence to IASCC, could not berncorrelated well with bulk Ni, Si, P, Mn, C, N, Cr, or O content. However, bulk Srncontent provided good correlation with IASCC susceptibility. Good resistance tornIASCC was observed in Types 304 and 316 SS steels that contain very lowrnconcentrations of S of ≤0.002 wt.%. As fluence increased, the IASCC susceptibility ofrnTypes 304, 304L, 316, and 316L steels that contain >0.003 wt.% S increasedrndrastically, indicating that the deleterious effect of S is predominant in the failure ofrnin-core components at high fluence. In contrast to Types 304 and 316 SS, Srnconcentration of ≤0.002 wt.% does not necessarily render low-carbon Types 304L,rn316L, or high-purity-grade steels resistant to IASCC, indicating a synergism betweenrnC and S. A small amount of globular delta ferrite is beneficial in mitigating therndeleterious effect of S, probably because S atoms are trapped in the ferrite. AnrnIASCC model has been proposed which is based on oxidation characteristics of cracktiprngrain boundary and loss of cohesive strength in thin Ni- and S-rich film, producedrnbetween the grain-boundary oxide and metal, at high S concentration in the film.