YIELD STRESS EVOLUTION IN INCONEL 718 SAMPLES UNDER STANDARD HEAT TREATMENT PROCESS CONDITIONS OF TURBINE DISKS

摘要

Finite element simulations of the evolution of residual stresses in Inconel 718 turbine disks during the standard heat treatment process require a quantitative understanding of the yield stress evolution during this process. The yield stress is strongly influenced by the precipitation of intermetallic N_3Al γ' and Ni3Nb γ" phases. The dissolution temperature of both precipitate types is around 900°C. After forging at around 1000°C the turbine disks are air-cooled before they are furnace-reheated to 720°C, aged at 720°C for 8 hours, followed by furnace-cooling to 620°C and final aging at this temperature for 8 hours. Mechanical tests are performed to study the yield strength evolution as function of temperature history. The temperature history for the specimens is extracted from the heat treatment simulation of the turbine disk. This work provides a deep understanding of the yield stress evolution and its dependence on the precipitation state. The found yield stress evolution can be used on one hand to validate the results of precipitation kinetics simulations, and on the other hand to discuss the relevant temperature and time ranges for the residual stress evolution in turbine disks.