Microstructure Evolution of Superalloy for Large Exhaust Valve during Hot Forging

Abstract

The nickel‐based alloy Nimonic 80A possesses strength, and corrosion, creep and oxidation resistance at high temperature. These products are used for aerospace, marine engineering and power generation, etc. The control of forging parameters such as strain, strain rate, temperature and holding time is important because the microstructure change in hot working affects the mechanical properties. It is necessary to understand the microstructure variation evolution. The microstructure change evolution occurs by recovery, recrystallization and grain growth phenomena. The dynamic recrystallization evolution has been studied in the temperature range 950–1250°C and strain rate range 0.05–5s−1 using hot compression tests. The metadynamic recrystallization and grain growth evolution has been studied in the temperature range 950–1250°C and strain rate range 0.05, 5s−1, holding time range 5, 10, 100, 600 sec using hot compression tests. Modeling equations are developed to represent the flow curve, recrystallized grain size, recrystallized fraction and grain growth phenomena by various tests. Parameters of modeling equation are expressed as a function of the Zener‐Hollomon parameter. The modeling equation for grain growth is expressed as a function of initial grain size and holding time. The developed modeling equation was combined with thermo‐viscoplastic finite element modeling to predict various microstructure change evolution during thermo mechanical processing. The predicted grain size in developed FE simulation results is compared with results obtained in various tests. In order to obtain a final microstructure and good mechanical properties in forging, the FEM would become a useful tool in the simulation of the microstructure development.