| Inconel 718 superalloy is an important material used for high temperature turbine diskof aero engine and gas turbine. The improvement of high temperature strength and lowcycle fatigue properties of Inconel 718 becomes critical because of the rapiddevelopment in aerospace and industry. Therefore obtain an uniform and refinedmicrostructure is crucial for Inconel 718.Theδphase in Inconel 718 can control grainsize and it is an important and effective method to obtain an uniform and refinedmicrostructure via control ofδphase. In this study, the microstructure of ordinaryforging and the following typical heat treatment were analyzed, and theδphaseevolution and characteristics during isothermal aging were observed. The hotdeformation behavior of delta-processed Inconel 718, as well as the effect ofdeformation conditions on microstructure was investigated by means of hot compressivedeformation tests. And the hot working char; cteristics and evolution ofδphase in thedelta-processed Inconel 718 was studied. All of the microstructure were investigated byoptical microscopy (OM) and scanning electron microscopy (SEM) .The results show that the uniformity of grain Inconel 718 with inequable grain sizecan not be effectively improved after three typical heat treatments, and the uniform andrefined grain can be obtained by hot deformation.The microstructure observation results of Inconel 718 alloy with different agingtreatment process show that the peak temperature ofδphase precipitation in Inconel 718is 900℃. The morphology ofδphase changes from granular particles to platelets withthe aging time increasing. The time toδphase precipitate increases with the increase ofaging temperature.The results of the thermal simulation compression tests show that the peak stressfallows down with the deformation temperature increasing and the decreas of strain rate.The dynamic recrystallization is the main softening mechanism of the hot deformingdelta-processed Inconel 718.The activation energy (Q) of the delta-processed Inconel718 is 497KJ/mol. The hyperbolic sine-rype function that used for describing the relationship between the flow stress and the deformation condition for delta-processedInconel 718 is:Microstructure analysis of the hot deformed samples indicates that the degree of thedynamic recrystallization of the delta-processed Inconel 718 increases with the decreaseof strain rate, as well as a slightly larger of grain size. The grain size and uniformity arenot greatly affected by deformation temperature in the condition of the nominal strain70% and the strain rate 0.01s-1. The delta-processed Inconel 718 deformed at thetemperature 1050℃-980℃and the strain rate 0.01s-1 can obtain ASTM11 and uniformgrain.δphase under the total strain of 70% was spheroidized in different degree. Due tothe effect of deformation breakage and dissolution breakage, the spheroidization ofplate-likeδphase took place during the deformation, and the deformation promotesdissolution breakage. High temperature is advantageous to spheroidization ofδphase,and the effect of the strain rate on spheroidization ofδphase may depend on thedislocation for atom diffusing and the time for atom diffusion. Considering synthetically,the hot working parameters for the delta-processed Inconel 718 alloy are suggested to beat strain rates 0.1s-1 or 0.01s-1,as well as the starting forging temperature between1020℃and 1050℃, and the final forging temperature between 950℃and 980℃withlarger strains.
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