A comparative study of thermal aging behavior of CF8M cast austenitic stainless steel and ER316L austenitic stainless steel weld

Cast austenitic stainless steels (CASSs) and austenitic stainless steel welds (ASSWs) are susceptible to thermal aging embrittlement due to hardening of δ-ferrite during long-term exposure to high temperature of nuclear power plants. This study focused on comparison of thermal aging behavior of CF8M CASS and ER316L ASSW, with similar ferrite content. To simulate the thermal aging embrittlement observed during light water reactor operations, CF8M and ER316L weld were thermally aged at 343 °C, 375 °C and 400 °C for up to 20,000 h. The microstructural analysis revealed that thermal aging induced spinodal decomposition of δ-ferrite was observed in both CF8M and ER316L weld. In CF8M, G-phase and ω-phase precipitates appeared after 5000 h of aging at 400 °C while in ER316L weld, Ni-clusters appeared after 5000 h and G-phase precipitates formed after 10,000 h and 20,000 h of thermal aging. The mechanical properties such as nanohardness of δ-ferrite, tensile strength and fracture toughness were compared for unaged and aged CF8M and ER316L weld. The kinetics of thermal aging embrittlement was evaluated by determining the activation energy based on δ-ferrite hardening and fracture toughness degradation. While the hardening of δ-ferrite exhibited slightly faster kinetics for CF8M, the loss of fracture toughness exhibited comparable kinetics for both CF8M and ER316L weld.