PSI - Issue 69

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

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decreases from 8.4 % to 7.8 % in 570 °C sample. As demonstrated from previous research [4], the film RA in ghost pearlite remains stable during tempering, whereas unstable RA in conventional martensite decomposes, accounting for this reduction in RA fraction. Fig. 5(a) shows the tensile curves for samples transformed at different pearlite temperatures. As the pearlite transformation temperature increased from 540 °C to 570 °C, the yield strength (YS) increased from 1587±17 MPa to 1635±13 MPa, while the ultimate tensile strength (UTS) remained approximately constant at around 2000 MPa. Meanwhile, the 570 °C sample exhibits larger uniform elongation (UE) than the 540 °C sample, while the total elongation (TE) is similar. The increase in YS is primarily attributed to the elevated volume fraction of ghost pearlite. Compared to conventional martensite, the refined width of martensitic laths in ghost pearlite enhances grain refinement strengthening [5, 11]. Furthermore, the strain hardening exponent is significantly higher at 570 °C than at 540 °C. This is attributed to the increased volume fraction and higher Mn concentration of RA, which enhances the TRIP effect during deformation, thereby improving strain hardening capability and increasing UE (Table 2).

Fig. 5. (a) Engineering strain-engineering stress curves and (b) strain hardening exponent as a function of true strain after quenching and tempering.

Table 2 Tensile properties after tempering at 200 °C when holding at different pearlite transformation temperature.

Temperature, °C

YS,MPa

UTS, MPa

UE, %

TE,%

540 570

1587±17 1635±13

1973±11 1965±15

4.6±0.1 5.5±0.2

11.9±0.2 12.2±0.1

4. Conclusions The lamellar ghost pearlite, which consists of alternating film RA and lath martensite, is formed after fast and short austenitization from the Mn-partitioned pearlite. Effect of the lamellar width of cementite on microstructures and tensile properties in Mn-heterogeneous steels is investigated, leading to the following conclusions. (1) With the pearlite transformation temperature increases from 540 °C to 570 °C, the lamellar width of cementite increases from 11.4±2.1 nm to 18.3±2.8 nm. Notably, the cementite exhibits Mn enrichment, while the ferrite presents Mn depletion. (2) Increasing the pearlite transformation temperature raises the fractions of both ghost pearlite and RA. This results from the larger cementite lamellar width at higher temperatures (18.3 ± 2.8 nm at 570 °C vs . 11.4 ± 2.1 nm at 540 °C), which effectively enlarges Mn-enriched regions in high-temperature austenite, thereby facilitating ghost pearlite formation after quenching.