PSI - Issue 71

Prasanna Dupare et al. / Procedia Structural Integrity 71 (2025) 118–125 119 strengthening (Dongke. Zhang, 2013). Alloy 617 characterized by the presence of Ti(C,N), M 6 C, M 23 C 6 , and γ ′ particles (Mehdizadeh & Farhangi, 2022) . At 700 °C, close to the TTT diagram's nose, these precipitates form in less than an hour (Shankar & Kumar, 2022) . Components operating at higher temperatures undergo strain cycling due to thermal gradient. Generally, start up and shut down of power plant cause low cycle fatigue damage in the component. Steady state operation of component at such elevated temperature causes creep damage (Rodriguez et al., 1993). However, effect of these damages is not independently accountable for failure and synergetic effect of these two damage modes should be considered while designing component for such application. Additionally, materials response in this temperature range exhibits a complex nature due to phenomena such as dynamic strain aging (DSA), precipitation, and oxidation. Alloy 617 shows dynamic strain aging (DSA) between 300 °C and 700 °C, causing matrix hardening (Ekaputra et al., 2016). Serrations in the plastic region in stress-strain curve is the characteristic of occurrence of DSA. These result from the interaction of mobile dislocations with solute particles. Occurrence of DSA can be influenced by temperature, strain amplitude, and strain rate (Rao et al., 2019; Rodriguez & Mannan, 1995) . Earlier studies have reported a reduction in fatigue life with the introduction of a hold period, with tensile hold exhibiting a more detrimental effect than compressive hold (Li et al., 2023; Sandhya et al., 2005; Srinivasan et al., 1991; X. Zhang et al., 2014). Goyal et.al. investigated the LCF behavior of Alloy 617M at 700 °C and 0.4% strain amplitude with tensile and compressive hold times of 1, 10, and 30 minutes. Fatigue life showed a saturating trend with longer holds. Fractography revealed intergranular creep cavitation under tensile holds, while compressive holds exhibited transgranular striations with intergranular cavitation. Kazuao et.al studied CFI of alloy 617 at 700 °C with tensile and compressive hold times. They observed intergranular cracking for CFI test (Kazuo KOBAYASHI, 2011). Caline cabet et.al. studied LCF and CFI of alloy 617 at 0.3%, 0.6% total strain amplitude and 950 °C temperature. LCF showed cyclic hardening, saturation stage, and final fracture whereas CFI test showed CSR curve with initial cyclic hardening and saturation followed by softening. Fracture surface showed oxidized cracks in CFI specimen (Cabet et al., 2013). In the present work, creep fatigue interaction study of alloy 617M was carried out at strain amplitude of ±0.25%, temperature of 650 °C and tensile hold time of 1 minute and 10 minute. Effect of tensile hold time on cyclic stress response, hysteresis loop, and fractography is studied. 2. Materials & Methodology Alloy 617M used in the present study was procured from the Indira Gandhi Centre for Atomic Research (IGCAR). The chemical composition of alloy 617M obtained by wet chemical analysis is given in Table 1. Cylindrical blanks were taken out from an 800 mm forged block to machine threaded M16 specimens with a gauge length of 28 mm and a gauge diameter of 8 mm. Low-cycle fatigue (LCF) tests were conducted using a 100 kN electromechanical testing (BiSS make) system in accordance with ASTM E606 standard. A calibrated extensometer with a gauge length of 25 mm was employed for strain control, involving a fully reversed triangular waveform (R = -1) for testing. To investigate the effect of hold time, tests were performed at a strain amplitude of 0.25% and a temperature of 650 °C, without hold time i.e. 0 minute (LCF/continuous cycling) and with hold time of 1 minute and 10 minutes at peak tensile strain. All tests were carried out at a strain rate of 3 × 10 -3 s -1 . The samples were heated in a three-zone furnace and constant temperature was monitored using a dedicated K-type thermocouple, maintaining temperature variation within ±2 °C. Testing was continued till a 50% reduction in peak tensile stress or fracture occurred. After the completion of the tests, the fracture surfaces were transversely sectioned using wire EDM. The fracture surfaces were then cleaned ultrasonically and examined under a scanning electron microscope (JEOL 6380A make). Table. 1 Chemical composition of Alloy 617M Elements C Mo Fe Co Ti Cr Si Al Nb B Ni Wt% 0.057 10.30 0.503 11.34 0.478 24.31 0.074 0.761 0.078 0.003 Bal