Effects of post-welding heat treatment on microstructure and mechanical properties of welding joint of new Ni-Fe based superalloy with Haynes 282 filler metal

Abstract

The gas tungsten arc welding process was employed to weld a new wrought Ni-Fe based superalloy with the Haynes 282 filler metal, which was designed for advanced ultra-supercritical boiler tubes operated beyond 700°C. In this study, the evolution of microstructure and mechanical properties of the welding joint prior to and following the post-welding heat treatment (such as annealing treatment at 1010°C for 2h and 788°C for 8h) were investigated. The microstructure stability and hardness were also studied following 500h, 1000h and 3000h at the service temperature (750°C). An epitaxial growth of the grains near the fusion line was observed, whereas the partition coefficients and solidification temperature range during the thermodynamic state of equilibrium were calculated in the weld metal zone. The welding heat affected zone of the as-welded welding joint was the weakest zone. In contrast, following the post-welding heat treatment, the segregation was significantly eliminated in the weld metal zone. Also, the mechanical properties of the welding heat affected zone were restored to the property levels of the base metal, with the weld strength reduction factor of 4%. Therefore, the effects of post-welding heat treatment on the microstructure and properties of welding joint were also investigated. Following 3000h thermal exposure at 750°C, the coarsening rates of the spherical γ′ precipitates of both the as-welded and post-welding heat treatment samples were consistent with the Lifshitz-Slyozov-Wagner (LSW) model. In addition, the microstructure stability of the welding joint was excellent, without any topologically closed packed phase precipitation. The Haynes 282 filler metal could be a potential candidate material for the welding of this new Ni-Fe based superalloy.