Long-term thermal aging of austenitic stainless-steel weld: Microstructure evolution in δ-ferrite and δ / γ phase boundary and apparent recovery of mechanical properties

This study aimed at investigation of thermal aging effects on evolution of microstructure and mechanical properties of ER316L austenitic stainless-steel weld (ASSW). The ASSW was subjected to thermal aging at 400 degrees C for up to 30,000 h. Initially, thermal aging triggered spinodal decomposition of delta-ferrite and clustering of Ni. Thermal aging up to 20,000 h further enhanced spinodal decomposition and G-phase formation. After 30,000 h of aging, precipitation of Mo-rich chi-phase occurred adjacent to the G-phase. Concurrently, thermal aging induced segregation of different elements and formation of nickel depletion zone was observed at the ferrite/austenite (delta/gamma) phase boundary. The consequence of these microstructure changes was the hardening of delta-ferrite and loss of fracture toughness of ASSW. However, after 30,000 h of aging at 400 degrees C, degraded fracture toughness and tensile properties were partially recovered. This recovery in fracture toughness was attributed to the evolution of microchemistry and the formation of a softer shell in the delta/gamma phase boundary region.