PSI - Issue 69

Dezhen Yang et al. / Procedia Structural Integrity 69 (2025) 97–104

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This is due to the fact that fast and short austenitization limits Mn diffusion, which in turn inherits the Mn heterogeneity from the initial Mn partitioned pearlite [3, 5, 8, 11]. Notably, after austenitization, the 570 °C sample exhibits exceptionally high Mn concentration in RA ( U Mn = 9.5 ± 1.4 %) and continuous lamellar morphology (Fig. 3(b)). In contrast, film RA in the 540 °C sample shows reduced Mn concentration ( U Mn = 5.7 ± 0.8 %) and chain-like morphology (Fig. 3(a)). The elevated Mn concentration significantly enhances RA stability in the 570 °C sample compared to the lower-temperature counterpart (540 °C). As pearlite transformation temperature increases from 540 °C to 570 °C, the volume fraction of ghost pearlite increases from 45.0±3.7 % to 61.0±3.0 %. This is attributed to the lamellar width of cementite increased from 11.4±2.1 nm at 540 °C to 18.3±2.8 nm at 570 °C. Specifically, the wider cementite lamellae (18.3±2.8 nm at 570°C vs . 11.4±2.1 nm at 540°C) extend the diffusion pathways for Mn atoms. Given the constrained austenitization time at 750 °C (50 s), this geometric factor imposes kinetic limitations on Mn homogenization. Consequently, wider cementite lamellae require longer diffusion times for complete Mn homogenization in austenite, effectively expanding Mn-enriched regions and facilitating ghost pearlite formation (Fig. 4). Additionally, as the pearlite transformation temperature increases from 540 °C to 570 °C, the RA fraction increases from 5.7 % to 8.4 %. This increase is primarily linked to the elevated volume fraction of ghost pearlite.

Fig. 2. The microstructures after (a-b) directly quenching to room temperature and (c-d) tempering at 200 °C for 30 min. The samples are held at (a, c) 540 °C and (b, d) 570 °C for pearlite formation, then austenitized at 750 °C for 60 s, and finally quenched to room temperature.