Creep Void Growth Simulation for Reliable Maintenance of High Temperature Components

Abstract

To reduce maintenance cost of high temperature components in aged thermal power plants, it is necessary to improve accuracy of remaining life assessment methods. High temperature components such as steam turbine rotors are operated under creep loading where creep voids initiate and grow on grain boundaries. Development of a quantitative evaluation method of void growth is important for reliable maintenance of these components. Purposes of this study are to clarify void growth behavior of a turbine rotor material under creep loading condition, and to develop void growth simulation program that can predict damage extension process in the components under actual operating condition quantitatively. From the creep tests and observation of a steam turbine rotor material, spherical shape voids initiate and grow up to their length of 2mm on the grain boundary at initial stage of damage, and then these voids change their shape to crack-like to grow until their length reaches around 10mm. Finally, crack-like voids coalesce each other to form one micro crack along boundary. It can be concluded that void growth rates are controlled by diffusion and power law creep under constrained condition based on theoretical consideration of void growth mechanism. Through these discussions, a new void growth model was proposed by modifying conventional models. A void growth simulation program was developed by incorporating the void growth model in a personal computer. It was confirmed that void growth process under the creep condition in the experiment could be well predicted quantitatively by the simulation program. The simulation program was also applied to predict creep void growth behavior in an actual steam turbine rotor during operation.