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.