6. Conclusions
Uniaxial stress-controlled static creep and cyclic creep tests of bainite 2.25Cr-1Mo steel have been performed at 455 °C. The tests are conducted under changed unloading conditions, with different unloading rates and durations under valley stress being applied. The principal observations and conclusions of this study are as follows:
  1. Compared with static creep, a longer endurance life is observed under cyclic creep. The fracture modes of tested specimens suffer ductile fracture under static creep and cyclic creep which involves a long-term duration of peak stress. The ductility of specimen failed after cyclic creep is slightly lower than that after static creep.
  2. The unloading elastic modulus varies with different unloading rate. The anelastic strain during the unloading period shows dependence on both unloading rate and valley stress duration. Based on strain classification, the evolution of the strain (\(\varepsilon^{r}\),\(\ \varepsilon^{c}\)) and the recovery strain (\(\varepsilon^{e}\),\(\ \varepsilon_{\text{un}}^{\text{an}}\), and\(\ \varepsilon_{v}^{\text{an}}\)) with cyclic life under different unloading conditions can be divided into distinct stages, which are found to enter the stable evolution stage after the first cycle.
  3. The actual damage created per cycle is systematically considered by the classified strain components during the stable stage. By introducing the difference between maximum strain and recovery strains as a damage parameter, a fatigue life prediction method is proposed. The model provides satisfactory estimated results for cyclic creep loading conditions.