VOID GROWTH SIMULATION OF A STEAM TURBINE CASING MATERIAL UNDER CREEP AND CREEP-FATIGUE LOADING

摘要

High temperature components in thermal power plants are subjected to creep and creep-fatigue loading where creep voids initiate and grow on grain boundaries. Development of a quantitative evaluation method of the void growth is important for reliable maintenance of these components. In this study, creep and creep-fatigue tests were carried out at 600 °C on a lCr-Mo-V casting steel. Creep damaged materials were produced by interrupting the creep tests and microstructure of the damaged materials were observed carefully by a scanning microscope. The creep-fatigue tests were also conducted in a scanning electron microscope, and continuous observation of void growth behavior during the tests was made. From the observations, spherical shape voids initiate and grow up to their length of 2μm on grain boundaries at initial stage of damage, and then these voids change their shape to crack-like to grow until their length reaches around 10μm under both the creep and the creep-fatigue conditions. Under the same stress level, the void growth rate in the creep-fatigue condition was faster than that in the creep condition indicating acceleration of void growth rate by cyclic loading. Previously proposed void growth simulation model, in which the void growth was controlled by diffusion and power law creep, was modified to express acceleration of the void growth by the cyclic loading. Void growth behavior within a certain area under both the creep and the creep-fatigue condition were simulated by the modified program. Predicted void growth behaviors agreed with observed ones. The void growth behavior of an actual turbine casing was also simulated and void growth behavior was discussed based on the result.