Radiation Hardening and Radiation-Induced Chromium Depletion Effects on Intergranular Stress Corrosion Cracking in Austenitic Stainless Steels

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Abstract
Radiation hardening and radiation-induced chromium (Cr) depletion were related to intergranular stress corrosion cracking (IGSCC) response among various stainless steels (SS). Available data on neutron-irradiated materials were analyzed and correlations developed between fluence, yield strength, grain-boundary Cr concentration, and cracking susceptibility in high-temperature water environments. Large heat-to-heat differences in the critical fluence (0.2 neutrons/cm2 to 2.5 × 1021 neutrons/cm2) for IGSCC were documented. Variability often was consistent with yield strength differences among irradiated materials. IGSCC correlated better to yield strength than to fluence for most heats, suggesting a possible role for radiation-induced hardening (and microstructure) on cracking. However, isolated heats revealed a wide range of yield strengths (450 MPa to 800 MPa) necessary to promote IGSCC which could not be explained by strength effects alone. Grain-boundary Cr depletion qualitatively explained differences in IGSCC susceptibility for irradiated SS. Examination of measured Cr content vs SCC showed that all materials showing IG cracking had some grain-boundary depletion (≥ 2%). Grain-boundary Cr concentrations for cracking (< ∼ 16 wt%) were in good agreement with results from similar SCC tests on unirradiated type 304 SS (UNS S30400) with controlled depletion profiles. Heats that prompted variability in the yield strength correlation were accounted for by differences in their interfacial Cr contents. Thus, certain SS apparently were more resistant to cracking, even though they had significant radiation-induced Cr depletion. Cr depletion was believed to be required for SCC of irradiated SS, but susceptibility in this study was modified by other microchemical and microstructural components.

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