PSI - Issue 69

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

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effectively eliminated. The contrasting trends in H-Si and L-Si underscore their distinct transformation kinetics. In the L-Si steel, RA exhibits lower stability, leading to the persistence of some secondary martensite over time. In contrast, the H-Si steel achieves rapid stabilization or elimination of secondary martensite. These differences are likely driven by variations in silicon content and temperature, which affect the stability of austenite and the carbon diffusion coefficient. Figure 2(c) shows the evolution of the bainite fraction over time for both H-Si and L-Si steels, capturing their transformation behavior up to 10000 s. The two steels exhibit distinct patterns in bainite formation and stabilization. In the L-Si steel, the bainite fraction begins at 0% and rises rapidly within the initial time frame, reaching approximately 5% within the first 1000 s. This sharp increase suggests a rapid transformation process, driven by favorable conditions for bainite formation. Beyond 1000 s, the rate of increase slows down, yet the bainite fraction continues to climb steadily, ultimately reaching about 7% by 10000 s. This gradual upward trend indicates ongoing transformation of bainite over time. In contrast, the H-Si steel demonstrates markedly different behavior. The bainite fraction remains near 0% throughout the entire duration, showing no discernible increase. Although higher partitioning temperature, this lack of bainite formation implies that the H-Si steel is resistant to bainitic transformation under the experimental conditions, possibly due to differences in chemical composition i.e. higher silicon content, which can suppress bainitic transformation [8]. The carbon content within RA plays a pivotal role in governing its thermal stability. Figure 2(d) presents the evolution of the total carbon content in RA over time for the H-Si and L-Si steels. Both steels display a general trend of increasing carbon content in RA, although subtle differences emerge during the initial time frame. In the L-Si steel, the carbon content in RA increases sharply during the first 1000 s, rising from approximately 0.4% to 0.6%. This rapid increase suggests accelerated carbon partitioning into retained austenite at early stages. Beyond 1000 s, the rate of increase moderates, but the carbon content continues to rise steadily, ultimately reaching about 0.8% by 10000 s. The absence of a clear plateau indicates that the L-Si steel supports ongoing carbon enrichment throughout the studied period. In contrast, the H-Si steel also exhibits a marked initial rise in RA carbon content (from approximately 0.4% to just above 0.6% in the first 1000 s), but this early-stage increase is slightly less steep than that observed in the L-Si steel. After the initial phase, the H-Si steel shows a similarly steady rise, culminating in a final value of roughly 0.8% by 10000 s. Overall, both steels display a common trend of progressive carbon enrichment in retained austenite, underscoring the active role of carbon partitioning.

Figure 2-(a) Retained austenite fraction for H-Si and L-Si, (b) Secondary martensite fraction for H-Si and L-Si, (c) Bainite fraction for H-Si and L-Si, (d) Carbon content of H-Si and L-Si.