CAVITY GROWTH SIMULATION OF 2.25CR-1MO STEEL UNDER CREEP-FATIGUE LOADING

摘要

High temperature components in thermal power plants are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of a quantitative evaluation method of cavity growth is important for reliable maintenance of these components. In this study, a creep-fatigue test was carried out at 600°C on 2.25Cr-1Mo steel in a scanning electron microscope, and continuous observation of cavity growth behavior on the surface during the test was made. Based on the cavity growth observation, existing cavity growth models were modified and a simulation result by the modified model was discussed by comparing with observed cavity growth behavior. From the observation, spherical shape cavities initiate and grow up to their length of 2μm on the grain boundaries at initial stage of damage, and then these cavities change their shape to crack-like to grow until their length reaches around 10μm. Finally, crack-like cavities coalesce each other to form one micro crack along a grain boundary. It can be concluded that cavity growth rates of these cavities are controlled by diffusion and power law creep under constrained condition based on theoretical consideration of cavity growth mechanism. Through these discussions, a new cavity growth model was proposed by modifying conventional models. Both spherical and crack-like cavity growth rate equations were derived from the modified cavity growth model. It was indicated that measured cavity growth rate was well predicted by the growth rate equations derived from the modified model, and a cavity growth simulation result corresponds to the change in the maximum cavity size with cycles under the creep-fatigue loading.