Carbon Redistribution in a Martensitic Medium Mn Steel During Heating to Intercritical Region: An In-Situ Synchrotron XRD Study

This study investigates the microstructural evolution and carbon redistribution during the heating stage prior to intercritical annealing treatment (IAT) in a medium manganese steel (MMnS) with the nominal composition of Fe-0.40C-6Mn-2Al-1Si-0.05Nb (wt.%). The material was characterized using high-energy X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The initial microstructure primarily consisted of tempered martensite containing nano-sized plate-like η carbides and 7 vol.% retained austenite (RA) with thicknesses of 10–20 nm and 300 nm in average, respectively. During heating, carbon partitioning caused an increase in carbon content within the RA up to 530 °C, rising from 0.4 wt.% to 1 wt.%. η-carbides initially coarsened and subsequently transformed into cementite with an average diameter of ~20 nm. Above 530 °C, RA began to decompose, resulting in the formation of a pearlite-type microstructure. Concurrently, the carbon content in the remaining RA decreased, facilitating further growth of cementite formed in the earlier stages. The microstructure at the onset of IAT at 640 °C consisted of tempered martensite with nano-sized spherical cementite, 9 vol.% RA with >1 wt.% carbon and a small fraction of pearlite-type decomposed RA. The study highlights the complex interplay between carbon redistribution, carbide formation, and RA stability during the heating stage of MMnS and emphasizes the importance of accurately characterizing the initial microstructure to tailor the properties of these advanced high-strength steels.