Simple modelling of the constitutive behaviour of a 1%CrMoV rotor steel in service-like thermo-mechanical fatigue tests

Thermo-mechanical fatigue tests have been performed to determine the cyclic constitutive behaviour of a 1%CrMoV rotor steel. The mechanical strain and temperature control profiles adopted in these tests were selected to represent loading conditions which could occur at critical rotor locations in a high temperature turbine rotor in service. The cyclic evolution of the elastic-plastic and creep stress/strain states is analysed. Cyclic deformation influences creep behaviour as well as the yield characteristics of this steel, the resistance to time independent and time dependent plasticity being reduced by approximately 20% at midlife. A constitutive model based on monotonic data (with time dependent and time independent plasticity components) is developed to determine midlife cycle behaviour and predictions are compared with measured TMF stress-strain curves. Simple rules are proposed which are suitable for application in turbine component lifetime calculations.