Effect of Annealing on Microstructure of Thermally Aged 308L Stainless Steel Weld Metal

Austenitic stainless steel weld metal has been widely used as nozzle/safe-end joint and inner surface cladding of reactor pressure vessel, due to its good mechanical property and corrosion resistance. However, long-term thermal ageing at the service temperature (280 similar to 330 degrees C) could induce hardening and embrittlement of the weld metal. To recover the thermal ageing embrittlement, the annealing treatment has been proposed since the annealing could affect the ageing-induced microstructurel changes such as spinodal decomposition and G-phase precipitation in ferrite. However, there is still an incomplete understanding as well as a lack of nanoscale investigation about the annealing effect on the microstructural change of the weld metal. In this work, 308L stainless steel weld metal was thermally aged at 410 degrees C for 7000 h, followed by an annealing treatment at 550 degrees C for 1 h. Since the weld metal has a dual-phase structure of austenite and delta-ferrite, the phase transformation of austenite and delta-ferrite as well as the ele ment segregation at the delta-ferrite/austenite phase boundary were investigated by TEM and atom probe tomography. The results revealed that austenite was unaffected by annealing while the ageing-induced spinodal decomposition of delta-ferrite was completely recovered. In addition, the number density of G phase in delta-ferrite was significantly reduced following annealing. This indicates that austenite has a higher stability compared with delta-ferrite. As for the delta-ferrite/austenite phase boundary, thermal ageing induced the segregation of Ni, Mn and C at the phase boundary, while the contents of Cr, Si and P remained almost unchanged. Following the annealing treatment, the segregation of all elements was eliminated. Further, only a small quantity of Ni and Mn was enriched in austenite near the phase boundary. The results suggested that the microstructure of the annealed specimen was similar to that of the unaged specimen, indicating a good recovery of the microstructure by annealing.