Outstanding ductility of flash-butt welded Inconel 718 joints after post-weld heat treatment

Abstract

Inconel 718 flash-butt welding is characterized by the gradient distribution of the welding temperature field and the deformation resistance, which affects the evolution of the microstructure after post-weld heat treatment (PWHT). In this paper, the evolution of the microstructure and mechanical properties of the joints are investigated after PWHT at different solution temperatures. The results show that the weld seam (WS) is formed through dynamic recrystallization at the welding interface under the action of the upsetting deformation process, and a large amount of distortion energy remains in the heat-affected zone (HAZ). The residual distortion energy promotes the formation of fine recrystallized grains and precipitation of a large amount of δ phase in the HAZ after treatment at 960 °C, especially on both sides of the WS. The recrystallized grains in the HAZ grew with the dissolution of a large amount of δ phase after treatment at 980 °C, resulting in the formation of gradient grains from the WS to the HAZ to the matrix. However, the grains in the HAZ and the matrix grew more quickly than that of the WS after treatment at 1000 °C, which resulted in the mutation of the grain size around the WS. The tensile test results show that the ductility of the welded samples treated at 960 °C and 1000 °C is much lower than that of the base material (BM). Nevertheless, the gradient grains obtained in the welded samples after treatment at 980 °C alleviate the strain concentration and facilitate the accumulation of more dislocations which generate additional strain, which ultimately results in a 16.57% higher ductility without any loss of strength compared with the base material (BM) with the same treatment.