Results
3.1 Cyclic creep behaviour
Fig. 3 shows the variation of accumulated engineering strain with time under static creep and cyclic creep tests. The evolution of accumulated strain under all cyclic creep loadings in Fig. 3 (a) exhibit the similar three typical stages as static creep, which includes primary creep, secondary creep and tertiary creep stages. The primary creep stage of 2.25Cr-1Mo steel is quite short, and this feature has also been observed under the ratcheting-fatigue tests of 2.25Cr-1MoV steel.34 However, the strain accumulation rate quickly decreases afterwards and keeps constant after entering the secondary creep stage, of which the constancy result from the equilibrium condition between strain hardening and recovery softening. The secondary creep stage accounts for nearly 80% of the endurance life and is also referred to as the stable stage. The comparison of static creep and cyclic creep within the initial 10 hours enclosed in a box in Fig. 3 (a) is presented in Fig. 3 (b). It also appears that the stable strain accumulation rate under cyclic creep is much lower than that under static creep, and this difference eventually leads to the significant increase of endurance life under cyclic creep. The retardation in the accumulation of strain under cyclic creep has also been found in other Cr-Mo steels10, 35 and 316H austenitic stainless steel.20 The width of strain range of cyclic creep is varied by unloading conditions, which leads to a difference in accumulation of strain since the first cycle and the difference is increased gradually. On the other hand, it has to be admitted that the increase in endurance life is partly occupied by loading, unloading, and valley stress dwelling under cyclic creep as compared with the static creep, which cannot be ignored especially in cases involving long durations of valley stress.
To unify the evaluation of life, the endurance life which refers to the total duration under the peak stress is proposed, and corresponding results are listed and compared in Table 2. Since the peak duration of a cycle is set as 60 min, i.e. one hour, the endurance life is approximately equal in value to the cycles to failure. The variations of endurance life with unloading rate and duration under valley stress are shown in Fig. 4 (a) and (b) respectively. The endurance life under static creep is still below that under cyclic creep, which confirms that 2.25Cr-1Mo steel under cyclic creep shows the prolongation effect on life compared with the static creep. Besides, the influence of prolongation effect changes with different unloading conditions. As shown in Fig. 4 (a), the endurance life grows with the increase of unloading rate when the duration of valley stress is none. The endurance life also increases with the extension of duration of valley stress when the loading and unloading rate is the same, as shown in Fig. 4 (b). The reasons for these two trends are discussed in detail later.