PSI - Issue 69

R. Surki Aliabad et al. / Procedia Structural Integrity 69 (2025) 69–75

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Fig. 3. SEM micrographs of the heated samples step-quenched from different temperatures (denoted in Fig. 2b by black dotes); (a and d) 250°C; (b and e) 450°C; (c and f) 680°C.

The microstructural evolution of the studied steel at three different heating temperatures is shown in Fig. 3. The general microstructure, including tempered martensite, remains largely consistent with the initial structure across all temperatures. At 250°C, some degree of carbide coarsening becomes evident. As the temperature increases to 450°C, significant coarsening of carbide particles is observed. At higher temperature, 680°C, carbide transitions dominate, resulting in the formation of numerous nano-sized spherical carbides distributed both within and on the martensitic blocks. Figures 3d and 3e illustrate this coarsening behavior at 250°C and 450°C, respectively. HEXRD data reveals clear carbon enrichment in retained austenite (RA) between 165°C and 375°C. Following this enrichment, the estimated carbon content in the BCC matrix phase, attributed to cementite precipitation, can be estimated by mass balance calculation as shown in Equation 2: C γ .f γ +C α .f α =0.4 →1×0.1+C α ×0.9=0.4 → C α =0.33wt.% (2) Where C and f are carbon content in mass weight and phase fraction of each phase, respectively. Using Thermo-Calc software, the maximum amount of cementite that could precipitate, based on the nominal composition of the material with a carbon content of 0.33 wt.%, was calculated to be 4.8 vol% at 540°C. Although SEM observations suggest the potential formation of significant amounts of cementite, HEXRD failed to detect carbide peaks with high intensity. One key factor contributing to this is the size of the carbides. When the particle size falls below a critical threshold, the corresponding diffraction peaks become broadened, which prevents their detection with the current experimental setup [24]. Between 530°C and 640°C, the volume fraction of retained austenite (RA) decreased to 9.0%. At 680°C, SEM micrographs (Fig. 3f) revealed the formation of a pearlite-type microstructure, resulting from the decomposition of RA into colonies of large cementite lamellae. This transformation is consistent with previous studies. For instance,