Microstructure evolution of delta-processed IN718 during holding period after hot deformation

Two IN718 alloys with different initial δ phase contents have been used to study the microstructure evolution during the holding period after the hot deformation by the heat treatment at 950–1040 °C after deformation at temperature of 950 °C and strain rate of 0.005 s⁻¹. The results indicate that the grain growth is restrained due to the pinning effect of δ phase. The activation energy of grain growth for the alloys with initial δ phase content of 0 and 8.14 wt% in the heat treatment after hot working were 273.8 and 882.8 kJ/mol, respectively. In addition, the dissolution of δ phase occurred during the heat treatment after the hot deformation, and the δ phase dissolved largely as the temperature is higher than 980 °C. Therefore, the heating and deformation temperature should be in the range of 950–980 °C for the Delta Process and multi-stage hot working processes.