Considerations to the An-Isothermal Creep-Fatigue Assessment of Steam Turbine Rotor Steels

The high creep-fatigue loading of fast starting steam turbines is investigated. Based on real turbine operation, but assuming worse conditions like increased notch factors, typical stress-strain cycles of the rotor are calculated. These an-isothermal cycles consists of phases of different start-types (cold start, warm start and hot start), of steady-state operation, shut-down and stand-still. Experiments, performed with these cycles, are evaluated with different methods. For the LCF fraction of the lifetime consumption, several reference temperatures are compared, whereby the so-called ‘corner temperature’ is preferably incorporated, which is the temperature at the end of the compression phase. The creep fraction of the lifetime consumption is firstly assessed with the time fraction rule, either considering the midlife cycles only or considering each individual cycle, either considering all phases of the cycles or the steady state operation phase only. Then, the ductility exhaustion method for the creep damage is applied, after defining the ductility for several experiments.