PSI - Issue 69

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

98

1. Introduction The retained austenite (RA), as a crucial phase in martensitic steels, induces the TRIP (Transformation Induced Plasticity) effect through martensitic transformation during deformation, which enhances strain hardening capability and improves the strength-ductility balance [1]. In recent years, there has been considerable attention from researchers on controlling the fraction, morphology, and stability of RA through the manipulation of chemical heterogeneity in high-temperature austenite [2-6]. It is possible to increase the volume fraction and stability of RA through heat treatment, thereby improving the strength-ductility balance and the corrosion resistance. The concept of chemical heterogeneity in high-temperature austenite involves using alloy-rich phases as precursor microstructures, followed by fast and short austenitization treatments, which preserve the heterogeneous distribution of alloying elements in high-temperature austenite [3, 5, 7-9]. This heterogeneity subsequently influences martensitic transformation kinetics during cooling, providing significant flexibility to tailor RA and create novel microstructures in steels. The Mn-partitioned pearlite, as the initial microstructure, plays a crucial role in forming Mn-heterogeneous distribution in high-temperature austenite. During fast austenitizing of Mn-partitioned pearlite consisting of Mn enriched cementite and Mn-depleted ferrite, this Mn patterning can be inherited upon cooling to room temperature. This process leads to the formation of Mn-depleted martensite and Mn-enriched austenite [3, 5, 10]. For example, through fast heating and short austenitization from Mn-partitioned pearlite consisting of lamellar Mn-enriched cementite and lamellar Mn-depleted ferrite, Mn-enriched film RA and Mn-depleted lath martensite are hereditary obtained [3, 5, 9, 11], respectively. Research by Razik et al. [12] has demonstrated that the lamellar spacing of pearlite increases continuously with pearlite transformation temperature. Therefore, by adjusting the pearlite formation temperature, it is possible to control the thickness of the Mn-enriched cementite and Mn-depleted lamellar ferrite in pearlite, thereby directly influencing Mn distribution in high-temperature austenite. In this study, we investigate the role of the cementite lamellar width on microstructures and tensile properties in Mn-heterogeneous martensitic steels. We believe this study will stimulate further research into chemical heterogeneity as a microstructure-tailoring tool. 2. Experimental details A hot-rolled steel of 7mm thickness is received with a nominal composition of 0.39 wt. % C, 3.69 wt. % Mn and balanced Fe. In order to form full pearlite with different cementite widths, the samples are first austenitized at 800 °C for 10 min in a resistance furnace and then transferred to a salt bath furnaces held at 540 °C and 570 °C for 12 h each. The pearlite samples with dimensions of 40 mm × 20 mm ×1.3 mm 3 are fast heated at a rate of 80 °C/s to 750 °C in a salt bath and held for 50 s, subsequently water quenched to room temperature. To soften the martensitic matrix and improve ductility, the samples are further tempered at 200 °C for 30 min. The samples are mounted, polished and etched, and then characterized using a scanning electron microscope (SEM, Zeiss SUPRA55). The samples are subjected to mechanical grinding, followed by meticulous polishing and etching with a solution containing 3 vol % nitric acid in ethanol. 3 mm diameter foils are mechanically polished down to 50 μm thickness and then twin-jet polished using an electrolyte containing 10 vol.% perchloric acid in methanol. Their investigation is carried out using a transmission electron microscope (TEM, FEI Talos F200X) equipped with electron dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) patterns are obtained using a Bruker D8 Advance equipped with Cu- Kα radiation. The XRD patterns are recorded over a range of 40 to 105 ° at a scanning rate of 0.30 °/min. The RA volume fraction is quantified using direct comparison method based on the intensities of (200) α , (211) α , (200) γ and (220) γ peaks [13]. The uniaxial tension with dimension of 8×2×1.2 mm 3 (gauge length×width×thickness) is carried out using a Instron 5565 universal tensile testing machine equipped with a high-precision video extensometer at an initial strain rate of 4×10-4 s -1 . At least, three samples were tested for each condition.