Thermomechanical fatigue behaviour and damage mechanisms in a 9% Cr steel: Effect of strain rate

Thermomechanical fatigue (TMF) loading is inevitable during the operation of ultra-supercritical power plants. In the present work, TMF tests are carried out at strain rates of 5 × 10⁻⁵/s to 4 × 10⁻⁴/s and in-phase (IP) and out-of-phase (OP) to investigate the cyclic deformation and damage mechanisms of P92 steel. The results reveal that the increase in strain rate leads to a significant improvement in fatigue life under both IP-TMF and OP-TMF, in which OP-TMF induces more severe damage to fatigue life than IP-TMF. It was found that the fatigue life reaches a maximum after a strain rate of 1 × 10⁻⁴ s⁻¹. It is important to note that the action of dynamic strain ageing (DSA) is strain rate-, fatigue cycle- and phase angle-dependent. It was also demonstrated that the effect of the strain rate on the fatigue life is a combined effect of DSA, fatigue cracks, creep voids and oxidation. Additionally, the predictive abilities of various traditional life prediction methods under various loading conditions are comparatively assessed.