The purpose of this research is to eva‎luate the hot deformation behavior of Udimet 720 nickel base superalloy and to know the appropriate temperature range for applying hot deformation in processes such as rolling and radial forging. For this purpose, after determining the chemical composition of the alloy, the primary ingot with a diameter of 95mm was prepared using melting, alloying and vacuum casting. The homogenization operation was performed at different temperatures and times and the desired temperature and time were obtained. The dissolution temperature of the γ′ phase was determined by the dilatometric method. Continuous hot compression tests were performed in the temperature range of 1050-1150 oC and constant strain rates of 0.001 and 0.01 s-1 and the strain value was 0.7. Multipass hot compression tests at constant strain rates of 0.01 and 0.1 s-1 in four stages from 1125 to 1050 oC with interpass times of 30, 60, 120 and 240 seconds and 0.2 strain was applied in each step. True stress-strain curves were drawn at different temperatures and times, and the resulting microstructures were examined using optical and electron microscopes. The results showed that the desired homogenization is done before hot deformation for this alloy at a temperature of 1150 oC. The dissolution of γ′ precipitates also occurs at approximately 1150 oC. The results of continuous hot compression tests showed that this alloy has low workability and continuous deformation up to a strain of 0.7 on the casting sample causes cracks or destruction of the sample. The results of the multipass hot compression tests showed that the increase in stress in each stage is due to the decrease in the recrystallization rate due to the temperature drop and the increase in the precipitation rate of γ' precipitates. Deformation at a temperature of 1150 oC caused the destruction of the sample, so the beginning of the deformation should be done at lower temperatures. The results showed that the interaction of recrystallization and precipitation causes deformation at temperatures lower than 1075 oC to be accompanied by a significant increase in stress. In the short (30 seconds) and long (240 seconds) interpass times, the stress increased greatly, and the optimal time is obtained in the average times of 60 and 120 seconds. The microstructure of the alloy after multipass deformation was in the form of elongated grains in the direction of force application. This structure contained partially recrystallized grains at the strain rate of 0.01 s-1 and the fraction of recrystallized grains decreased at the strain rate of 0.1 s-1. The optimum temperature range for hot deformation of Udimet720 alloy was obtained in the temperature range of 1075 to 1125 oC for strain rates of 0.01 and 0.1 s-1.

محمدرضا طرقي نژاد , محمود منيرواقفي

محمدرضا طرقي نژاد , محمود منيرواقفي