The exact life prediction is an important issue, because the working condition of rotors are severe and the breakdown of rotors can cause miserable accidents. Although the existing thermal power plants were designed and built for continuous working condition, the thermal power plants are operated in peak time usage because of the starting of the nuclear power plants. The turbine rotors in power plants are operated under condition in which the creep-fatigue interaction occurs. As such, the peak time usage of rotors has prompted the life prediction by hold time fatigue tests.
The hold time fatigue test conditions are as follows. For HIP rotors, testing temperatures are 700K and 811K, and total strain range is ±1.5~2.5%, and the tensile-hold time is 5min.,10min. and compressive hold time is 5min. For LP rotors, testing temperature is 616K, and total strain range is ±1.7~2.5% and the tensile-hold and compressive-hold time is 5min., respectively.
The introduction of hold time during low cycle fatigue test is known to decrease the fatigue life relative to that of the continuous cycling, and it is observed that the fatigue life decreases with increasing the hold time. But the fracture mode of the specimens tested with hold time is observed to be transgranular, regardless of hold modes. In case of the austenitic stainless steel, it is reported that the fracture mode is intergranular. It is known that the difference of microstructure causes difference of fracture mode. But the exact reason why the fatigue life of compressive-hold mode is shorter than that of tensile-hold mode was not found yet.
The present work investigates that reason. The shorter fatigue life at compressive-hold mode results from the larger plastic strain range. In compressive-hold mode, the amount of stress relaxation and peak stress are larger than those of tensile-hold mode. This means that the fatigue damage and creep damage at compressive-hold mode is much larger than those of tensile-hold mode.
