PSI - Issue 69

Zeynab Aalipour et al. / Procedia Structural Integrity 69 (2025) 105–112

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As time progresses, Figures 4(b) and 4 (c) reveal a more pronounced formation of tempered martensite laths and the presence of fine carbides within the martensitic matrix, indicating that extended heat treatment promotes carbon redistribution and partial decomposition of primary martensite. By the final image, Figure 4 (d), substantial regions of tempered martensite are clearly visible, underscoring the ongoing transformation that occurs over longer dwell times in L-Si steels. Notably, when compared with H-Si steels, these L-Si samples exhibit a greater fraction of tempered martensite, reflecting the reduced capability of lower silicon content to stabilize retained austenite. While H-Si alloys tend to facilitate more austenite retention through carbon partitioning, the L-Si alloy is more prone to martensitic decomposition, resulting in higher amounts of tempered martensite. Consequently, these differences in silicon content and resulting phase fractions can significantly influence the balance of strength and toughness, as tempered martensite contributes different mechanical properties than RA.

Figure 4-scanning electron microscope (SEM) images for H-Si during the final quench at holding times of (a) 10 s, (c) 100 s, (e) 1000 s, and (g) 10000 s, and Electron Backscatter Diffraction (EBSD) images for L-Si during the final quench at holding times of (b) 10 s, (d) 100 s, (f) 1000 s, and (h) 10000 s.