Effects of low-temperature annealing on the microstructure and grain boundary chemistry of irradiated type 304SS and correlations with IASCC resistance

Abstract

Low-temperature annealing (LTA) is a known technique for mitigating irradiation-assisted stress corrosion cracking (IASCC) in laboratory stress corrosion cracking tests. A combined transmission electron microscope (TEM)/scanning transmission electron microscope (STEM) study was undertaken to determine the microstructural and/or grain boundary compositional differences between an IASCC-resistant material, which had been mill-annealed, then irradiated and subjected to a low-temperature anneal; and an IASCC-susceptible material, which had been mill-annealed and irradiated only. The material used was commercial-purity Type 304SS from a control blade sheath. Stress corrosion resistance was measured in constant extension rate tensile (CERT) and constant deflection tests in a hot cell. Attempts were made to correlate microhardness data obtained from tested constant deflection specimens with dislocation density measured in the TEM and with IASCC resistance. Phosphorus and silicon segregation and chromium depletion are reported as a function of heat treatment and IASCC resistance.