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. Marine diesel engines can be classified into low speed (70-200rpm) two-cycle engines and middle speed (200-800rpm) four-cycle engines. The exhaust valves of low speed diesel engines are usually operated to the environments of high temperature(400-600℃), high pressure to enhance thermal efficiency and exposed to the corrosion atmosphere by the exhaust gas. Also, the exhaust valve is subjected to repeated thermal and mechanical loads.
The microstructure evolution during hot forging process is composed of dynamic recrystallization during deformation as well as grain growth during dwell time. The control of hot forging parameters such as strain, strain rate, temperature and holding time is important because the microstructure change in hot working affects the mechanical properties.
The dynamic recrystallization has been studied in the temperature range of 950-1250℃ and strain rate range of 0.05-5/sec using hot compression tests. The grain growth has been studied in the temperature range of 950-1250℃ and strain rates of 0.05, 5/sec, holding times of 5, 10, 100, 600sec using hot compression tests. Modeling equations are developed to represent the flow curve and recrystallized grain size, recrystallized volume 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.
Flow curve, dynamic recrystallized grain size and grain growth are expressed by the following equation.
The developed modeling equations were combined with thermo-viscoplastic finite element modeling to predict the microstructure change evolution during hot forging process. The predicted grain size in FE simulation is compared with results obtained in field product. In order to obtain a fine and homogeneous microstructure and good mechanical properties in forging, the FEM would become a useful tool in the simulation of the microstructure development.
To obtain the fine grain microstructure in forging, appropriate temperature, strain and strain rate and rapid cooling are required. The optimal forging temperature and effective strain range of Nimonic 80A for large exhaust valve are about 1080-1120℃ and 150-200%.