Importance of Molybdenum on Irradiation-Assisted Stress Corrosion Cracking in Austenitic Stainless Steels

Abstract

Constant extension rate tensile (CERT) specimens were irradiated in the core of a commercial operating boiling-water reactor (BWR). Subsequent to irradiation, CERT testing was performed in a test loop attached to the reactor water cleanup system in the same BWR, using reactor water at a high flow rate. Testing was performed in BWR normal water chemistry (NWC) and hydrogen water chemistry (HWC). Type 316 (UNS S31600) stainless steel (SS) was much less susceptible to irradiation-assisted stress corrosion cracking (IASCC) than type 304 SS (UNS S30400) in NWC and HWC. High-purity (HP) type 304 SS was more susceptible to IASCC than commercial purity (CP) type 304 SS in NWC at intermediate fluence (∼ 1 × 1021 n/cm2, E > 1 MeV). Results suggested chromium depletion resulting from radiation-induced segregation (RIS) was an operating mechanism in NWC and may have been the primary radiation effect at fluences < 3 × 1021 n/cm2 (E > 1 MeV). At higher fluences, another mechanism supervenes. Detailed microchemical analysis was conducted on unirradiated archive material and irradiated material. Two complementary techniques were used: field emission gun scanning transmission electron microscopy (FEGSTEM) and Auger electron spectroscopy (AES). The unirradiated materials showed nonequilibrium grain-boundary segregation of chromium, molybdenum, and phosphorus. Irradiation to intermediate fluences (∼ 1 × 1021 n/cm2, E > 1 MeV) resulted in further enhancements of silicon and phosphorus at the grain boundaries. In the CP materials, molybdenum and chromium remained enhanced at the grain boundaries, with chromium depletion on either side of the grain boundary. Chromium was depleted significantly in the HP material. At higher fluences (∼ 1 × 1022 n/cm2, E > 1 MeV) chromium was depleted significantly in the CP type 304 SS. The presence of molybdenum correlated with retention of the chromium level at the grain boundary during irradiation up to a certain fluence. For the HP type 304 SS, which contained virtually no molybdenum, the chromium level was lower than for the CP heats.