Long-term thermal aging embrittlement and recovery behaviors of TIG-welded stainless-steel joint

Cast austenitic stainless steels (CASS) commonly used in nuclear power plants due to their strength and corrosion resistance. In this work, the thermal aging embrittlement mechanisms in narrow-gap TIG-welded CASS joints were investigated through integrated mechanical testing and multi-scale microstructural characterization. It was found that the prolonged aging at 400 °C for 30000 h reduced fracture toughness by higher than 50%, attributed to spinodal decomposition and G-phase precipitation in δ-ferrite, which transformed this phase into brittle domains. These microstructural changes induced premature ferrite fracture, constraining crack-tip plasticity and accelerating crack propagation. Moreover, a reversion heat treatment was found to fully recover the fracture toughness to pre-aged levels by dissolving embrittling precipitates. The re-aged samples exhibited re-embrittlement kinetics identical to virgin-aged welds during subsequent thermal exposure, confirming the feasibility of reversion treatment as a life-extension strategy for nuclear primary piping weldments. This work provides a foundation for mitigating aging degradation in reactor service environments.