Evidence was obtained correlating the susceptibility of Type 304 (UNS 530400) stainless steel (SS) to irradiation-assisted stress corrosion cracking (IASCC) and grain boundary segregation. Susceptibility to IASCC was measured in constant extension rate tensile (CERT) tests, and grain boundary segregation was demonstrated by Auger electron spectroscopy and HNO3/Cr+6 corrosion tests.
A high degree of IASCC susceptibility was imparted to commercial purity Type 304 SS by solution annealing in the temperature range 2200 to 2400°F (1204 to 1316°C), and then irradiating it in the advanced test reactor to a fast neutron fluence of 2.22 to 3.08 × 1021 n/cm2 (E > 1 MeV). The high temperature solution anneal effectively purged the grain boundaries of trace elements such as sulfur, phosphorus, and silicon. In the solution annealed, unirradiated condition, the stainless steel was resistant to stress corrosion cracking. Following irradiation, the material was very susceptible to IASCC. Auger analysis and corrosion testing indicated that the irradiated material was no longer free of grain boundary segregation. Sulfur, phosphorus, and silicon were found on grain boundaries in addition to an enrichment in nickel and a depletion of chromium.