Fundamental Study On The Hot Working And Heat Treatment Technology Of GH169 And GH696 Superalloys

On the basis of the electrochemistry analysis of GH169 (Inconel718) superalloy and its precipitates, a respective extraction technique of (γ″+γ′), δ and NbC phases was employed, and their quantitative analysis was successfully fulfilled. The quantitative phase analysis results are in good agreement with those obtained through the X-ray diffraction (XRD) quantitative phase analysis method previously proposed. According to the distribution relationship of alloy elements in γ″ to γ′ phase, the ratio of weight fraction of γ″ to γ′ phase is calculated and is believed to be a constant equal to 3. This result is adopted for the correction of the XRD quantitative phase analysis method. Dissolution kinetics and microstructure evolution of δ phase in GH169 during holding at 980℃, 1000℃ and 1020℃ were analyzed using XRD and scanning electron microscopy (SEM). For the specimens with initial content of δ phase of 8.4wt%, a dynamic equilibrium state reaches after holding at 980℃ for more than 30 minutes and at 1000℃ for more than 2 hours, and the equilibrium contents are 3.0wt% and 0.6wt% respectively. Nearly all δ phase dissolves into the austenitic matrix at 1020℃ for more than 1 hours. Dissolution models of δ phase were established and the controlling step was revealed. Dissolution process of needle-like δ phase can be divided into two stages, the initial stage is mainly characterized as the fracture from long needle shape to short bars or particles and the following stage is the decrease in size for the short bars or particles. The whole dissolution process is governed by the interface reaction mechanism, but not by the long-distance diffusion of Nb or Ni atoms in the austenite matrix. The dissolution kinetics equations are established by applying the reaction shrinking core model and the third order reaction model for the initial and final stage of δ phase dissolution, respectively.Influences of δ phase dissolution treatment on the mechanical properties and notch sensitivity of GH169 were studied. Quantitative relationship between the notch sensitivity and the content of δ phase is established. GH169 alloy is not sensitive to notch when the content of δ phase is higher than 0.62wt%. By contrast, seriously notch sensitivity is observed when the content of δ phase is 0.33wt%. The influence of shot peening on the notch sensitivity of GH169 was studied. For the notch sensitive specimens having been heated at 1020℃ for 1h, stress rupture life can be improved for 10 times and failure occurs at the smooth region when shot peening is performed before double ageing. Based on the above results, an advice for the hot working technology of GH169 is proposed. Quantitative analysis of δ phase using the XRD method should be performed before the double ageing treatment and shot peenig should be performed, if the content of δ phase is lower than 0.62wt%. In this case, notch sensitivity can be effectively avoided.Hot deformation behavior of hot-rolled GH696 (ЭИ696) was investigated by the use of Gleeble 3500 simulator. Hot deformation equation is given to characterized the dependence of peak stress on the temperature and strain rate. The apparent activation energy is about 499kJ/mol. The strain rate sensitivity and efficiency of power dissipation is determined under different hot deformation conditions. Processing maps are established and interpreted on the basis of microstructure observation. The maps obtained at true strains range from 0.1 to 0.7 are essentially similar, and efficiency of power dissipation increases gradually with the increase of temperature and decrease of strain rate. A domain with its peak efficiency about 40-48% appears at about 1150℃ and 0.001/s-1, and is correspondent to dynamic recrystallization (DRX) for the true strain of 0.7.Influence of solid solution treatment temperature (range from 1000℃ to 1150℃) on the mechanical properties of GH696 was studied. Strength and plasticity decrease with the increase of solution temperature. Fractography and microstructure analys...