PSI - Issue 69

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

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3. Results and discussion As shown in Fig. 1(a, d), the lamellar spacing of pearlite formed at 540 °C is significantly smaller than that at 570 °C. As revealed by TEM in Fig. 1(b, e), the lamellar pearlite consists of Mn-depleted ferrite and Mn-enriched cementite. Forming at 540 °C, the widths of cementite and ferrite are determined to be 11.4±2.1 nm and 109.3±21.6 nm, respectively, considerably smaller than those at 570 °C (18.3±2.8 nm and 165.3±42.9 nm). An EDS line across the lamellae demonstrates Mn heterogeneity within pearlite (Fig. 1(c, f)), showing strong enrichment in cementite and depletion in ferrite (Table 1). Furthermore, at 540 °C and 570 °C, the Mn concentration in cementite ( U Mn ) is similar, which are 21.7 ± 3.4 % at 540 °C and 20.8 ± 3.9 % at 570 °C, respectively. The U Mn is calculated by Mn/(Mn+Fe) where Mn and Fe represent their atomic fractions [3, 14]. Consequently, the pearlite formation temperature predominantly controls the lamellar widths of cementite and ferrite, and the differences in cementite width will play a pivotal role in the subsequent austenitization process.

Fig. 1. The Mn-partitioned pearlite consisting of Mn-enriched cementite and Mn-depleted ferrite after holding at (a-c) 540 ℃ and (d-f) 570 ℃ for 12 h: (a, d) SEM; (b, e) TEM; (c, f) TEM-EDS.

Table 1 Pearlite characteristics after holding at 540 °C and 570 °C for 12 h.

Temperature, °C

Width of cementite, nm Width of ferrite, nm

U Mn of cementite, %

U Mn of ferrite, %

540 570

11.4±2.1 18.3±2.8

109.3±21.6 165.3±42.9

21.7±3.4 20.8±3.9

1.8±0.2 1.7±0.3

The Mn partitioned pearlite samples with two different cementite lamellar widths (formation at 540 °C and 570 °C) are austenitized at 750 °C for 50 s and water quenched to room temperature, resulting in the microstructures shown in Fig. 2(a-b). The microstructure can be divided into the ghost pearlite regions and the conventional martensite regions, where the ghost pearlite region has a volume fraction of 45.0±3.7 % for 540 °C and 61.0±3.0 % for 570 °C. The ghost pearlite fraction is estimated using at least five SEM images and is calculated in a manual way by Photoshop software. The so-called ghost pearlite consists of alternating lath martensite and film RA, exhibiting a pearlite morphology resembling structure (Fig. 1). Similar to Mn-partitioned pearlite, the significant Mn fluctuation is detected between RA film and martensite lath (Fig. 3), where RA exhibits Mn enrichment and the martensite presents Mn depletion.