Cyclic deformation and life time behaviour of NICR22CO12MO9 at isothermal and thermal-mechanical fatigue

Abstract

In the present study, the materials reaction and the microstructural changes during isothermal and thermal-mechanical fatigue are presented. In total strain controlled isothermal fatigue tests at temperatures between 1123 and 1473K and a frequency of 10 2 Hz the cyclic deformation behaviour is influenced by thermally activated recovery and a neutral cyclic deformation behaviour is found. At this condition the life time behaviour is determined by creep-fatigue interactions. In total strain controlled in-phase and out-of-phase thermal-mechanical fatigue tests the initial values of the induced stress amplitudes and plastic strain amplitudes are the higher and the cyclic hardening is the more pronounced, the higher the total mechanical strain amplitude is. The observed cyclic hardening is on the one hand caused by the development of high dislocation densities due to plastic deformation at lower temperatures, and on the other hand by the precipitation of small semi-coherent carbides at higher temperatures. At high total mechanical strain amplitudes with the same magnitude, in-phase tests yield smaller lifetimes than out-of-phase tests. At low total mechanical strain amplitudes the contrary is true. This is the result of competitive processes: creep damage favoured by high tensile stresses at high temperatures under in-phase loading and tensile mean stresses developing during out-of-phase loading.