PSI - Issue 69

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

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Figure 3 (b, d, f) presents EBSD-IPF maps of L-Si steel samples at different partitioning time intervals. Although the overall trend of retained austenite (RA) growth over time is qualitatively similar to that observed in H-Si samples, the size and continuity of FCC regions are visibly smaller in the L-Si steel. Despite the reduced presence of RA in the EBSD maps, XRD analyses confirm that the L-Si samples do indeed contain measurable amounts of RA. This discrepancy arises because EBSD, especially at coarse pixel resolutions, may not fully capture very fine or thin RA regions that fall below the spatial resolution limit. Additionally, since the partitioning temperature is lower for L Si, the diffusion characteristics of carbon in the microstructure differ significantly. To determine the average diffusion distance of carbon from martensite to austenite, equation (2) was used, whereD=D 0 exp (-Q/RT) is the diffusion coefficient of carbon,D 0 is a constant (D 0 γ=1.5×10−5 m 2 /s),Qis the activation energy of carbon diffusion (Qγ=141.2 kJ/mol), R is the gas constant, T is the absolute temperature, and t is the time of isothermal holding [12]. X= √6 (2) The results, shown in Table 2, indicate that the average diffusion distance of carbon in the austenite is approximately 84.66 nm in L-Si steel, compared to 348.88 nm in H-Si steel. This significant difference in diffusion distances explains why detecting RA in low-silicon steels using EBSD is more challenging compared to high-silicon steels.

Table 2. Average diffusion distance of carbon in austenite in both L-Si and H-Si (nm).

Steel

Time (s)

Average Diffusion Distance (X) (nm)

L-Si L-Si L-Si L-Si H-Si H-Si H-Si H-Si

10

2.677388 8.466646 26.77388 84.66646 11.03271 34.88849 110.3271 348.8849

100

1000

10000

10

100

1000

10000

3.2.2. Scanning Electron Microscope Figure 4(a–d) shows a series of scanning electron microscope (SEM) images for a H-Si alloy at increasing partitioning times, illustrating how RA content evolves. In Figure 4(a), the microstructure is primarily composed of martensitic laths, with only small amounts of RA visible. The dashed lines highlight the boundaries between different regions, indicating that austenite is mostly confined to isolated areas or thin films at early stages. As time progresses, Figures 4(b) and 4(c) reveal an increase in the fraction of RA, which can be observed forming along prior austenite boundaries and within the martensitic matrix. This growth suggests that extended thermal exposure allows additional carbon partitioning from martensite into austenite, lowering the Ms in those regions and stabilizing the austenite phase. In Figure 4(d), RA is more prevalent and appears in larger, more continuous networks, reflecting further carbon enrichment and phase stability. The morphological changes observed in these sequential SEM images underscore the role of holding time and silicon content in promoting austenite stabilization, ultimately influencing the steel’s mechanical behavior by enhancing toughness and ductility through mechanisms such as the TRIP effect. Figure 4(a– d) shows sequential SEM micrographs of a L-Si steel at progressively longer partitioning times, highlighting how the microstructure evolves to contain increasing amounts of tempered martensite. In the earliest image, Figure 4 (a), the structure is mainly composed of primary martensite, with only subtle signs of early tempering near prior austenite boundaries.