PSI - Issue 71

Prasanna Dupare et al. / Procedia Structural Integrity 71 (2025) 118–125

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Fig. 3 SEM image of as received specimen showing presence of carbides and (b) EDS of as received specimen confirming presence of Cr and Mo rich carbides 3.2 Cyclic Stress Response Cyclic stress response (CSR) curve i.e. peak stress vs. cycle number was plotted to analyse cyclic behaviour of alloy 617M. Fig.4(a) depicts CSR curve of strain-controlled fatigue tests at a strain amplitude of ± 0.25% and temperature of 650 °C for without hold 0 minute (LCF) and hold time of 1 minute, 10 minutes (CFI). Alloy 617M showed a distinct difference in the cyclic stress response (CSR) of pure LCF tests and CFI tests. In the case of pure LCF conditions (without hold time), the CSR initially exhibited cyclic hardening, followed by a steady-state plateau, which is typical of LCF behaviour. This hardening was attributed to dislocation accumulation and interaction during cyclic loading (Shankar & Kumar, 2022). However, when hold times of 1 minute and 10 minutes were introduced, the CSR showed a noticeable decrease. The inclusion of hold periods during each cycle allowed time-dependent mechanisms, such as creep, to take effect, resulting in a reduction in CSR. The prolonged hold times led to accelerated softening, which was associated with stress relaxation during the hold period, thereby reducing the material's resistance to further cyclic loading. Consequently, the fatigue life of the material decreased significantly as the hold time increased, highlighting the detrimental effect of creep on the fatigue performance of alloy 617M. Fatigue life reduction for 1 minute and 10 minute hold test was 10% and 77.87%. respectively.

Fig.4 (a)CSR, (b) Hysteresis loop Hysteresis loop for hold time 0 minute (LCF), 1 minute and 10 minutes are compared in fig. 4(b). All hysteresis loop plotted for cycle at half-life. The hysteresis loop for the 0-minute LCF (continuous cycling) test indicated greater elastic strain recovery compared to the loop observed during the hold test. With the introduction of hold time, creep damage accumulated, leading to an increase in additional plastic strain. 3.3 Stress relaxation The variation of stress values during hold time was plotted to analyze stress relaxation behavior. Fig. 5 depicts the stress relaxation behavior of alloy 617M at a strain amplitude of ±0.25% and a temperature of 650 °C during 1 minute and 10 minutes hold. It can be seen that, the extent of stress relaxation increased with an increase in hold