Low Cycle Fatigue Characterization of a HP-IP Steam Turbine Rotor

Results of an extensive low cycle fatigue characterization of HP and IP stages of a 1Cr-Mo-V steam turbine rotor are presented. Uniaxial strain controlled fatigue tests were carried out at 480 and 540°C, with hold times up to 24 h and strain rates down to 3 X 10 - 6 s - 1 . Moderate life reductions due to long hold periods were found only in tests with strain ranges less than 0.8%, though microstructural observations revealed evidence of creep damage morphologies in a few tests with 24 h hold times carried out at high strain ranges. Life reductions caused by the lowest strain rate were more significant. Generally, inferior fatigue resistance was provided by the rotor bore material, particularly in the more heavily segregated zones of the intermediate pressure stage. Application of the linear damage summation (LDS) rule to hold time data was not satisfactory. Better results were generally obtained by the strainrange partitioning (SRP) method, with a few unconservative life predictions mainly due to inclusions that dominated early failures. A life correlation was attempted by an energy-based criterion suitable to provide rationalization of creep and fatigue damage in terms of thermodynamic concepts.