Effect of Biaxial Stress Condition on Creep Damage of a Steam Turbine Rotor Material
Masao OKAZAKI, Kenichi TANAKA, Toshiyuki OSAWA and Takashi SATO
Abstract:A central hole of the steam turbine rotor can be degraded with creep damage during the operation. And it is important to measure the creep damage of this part to determine the replacement time. Usually the damage is measured with the hardness change, electric resistance change or the observation of the creep void (for example A parameter: damaged grain boundary parameter). However, the inspected results of the replaced rotor indicate the measured damage is overestimated and too conservative. This is caused by the change of hardness and electric resistance with heat history during the operation not only with the creep damage. And furthermore the master curves to evaluate the creep damage are obtained from the uniaxial creep test results. In this study biaxial creep tests to simulate the stress distribution of the central hole of the rotor under centrifugal force is conducted to clarify the effect of the biaxial stress on creep damage of a steam turbine rotor mterial. Micro creep cracks are proved to be initiated from a little inner location from the surface and the obtained A parameter is greater than that of uniaxial creep test under the equal creep damage ratio. Key Words:Creep, Biaxial creep test, Cr-Mo-V steel, Central hole of turbine rotor, A parameter