RADIATION HARDENING AND RADIATION-INDUCED CHROMIUM DEPLETION EFFECTS ON INTERGRANULAR STRESS-CORROSION CRACKING IN AUSTENITIC STAINLESS-STEELS

Radiation hardening and radiation-induced chromium (Cr) depletion were related to intergranular stress corrosion cracking (IGSCC) response a mong various stainless steels (SS). Available data on neutron-irradiat ed materials were analyzed and correlations developed between fluence, yield strength, grain-boundary Cr concentration, and cracking suscept ibility in high-temperature water environments. Large heat-to-heat dif ferences in the critical fluence (0.2 neutrons/cm(2) to 2.5 x 10(21) n eutrons/cm(2)) for IGSCC were documented. Variability often was consis tent with yield strength differences among irradiated materials. IGSCC correlated better to yield strength than to fluence for most heats, s uggesting a possible role for radiation-induced hardening (and microst ructure) 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 d epletion qualitatively explained differences in IGSCC susceptibility f or irradiated SS. Examination of measured Cr content vs SCC showed tha t all materials showing IG cracking had some grain-boundary depletion (greater than or equal to 2%). Grain-boundary Cr concentrations for cr acking (< similar to 16 wt%) were in good agreement with results from similar SCC tests on unirradiated type 304 SS (UNS S30400) with contro lled depletion profiles. Heats that prompted variability in the yield strength correlation were accounted for by differences in their interf acial Cr contents. Thus, certain SS apparently were more resistant to cracking, even though they had significant radiation-induced Cr deplet ion. 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.