In Situ Synchrotron X-ray Diffraction Study of Flash Austenitization and Process Design Insights in Medium-Manganese Steels for Energy Applications

Medium-manganese steels (MMnSs) are promising materials for energy infrastructure because tailoring their multiphase microstructures and austenite stability can improve failure resistance. Flash austenitization (FA) rapidly forms austenite while limiting prior austenite grain coarsening and substitutional solute homogenization, but its short-time kinetics remain insufficiently quantified. In the present study, two initial states of an Fe-6Mn-1.5Si-1Cr-0.3Mo-0.05Nb-0.2C (wt.%) MMnS, obtained by austenite reversion treatment (ART), were heated at 100°C/s to 850°C, 900°C, or 950°C and held isothermally while phase evolution was tracked by dilatometry-integrated in situ synchrotron X-ray diffraction. Neither passing the reference Ac3 determined under slow heating nor reaching the FA temperature was sufficient to achieve near-complete austenitization, defined as f$_α$ < 1 wt.%. Holding times of approximately 8 s, 4 s, and 2 s were required at 850°C, 900°C, and 950°C, respectively. Despite differences in the initial austenite fraction, morphology, and Mn enrichment, both ART states showed comparable transformation progress during rapid heating. Increasing the FA temperature shifted more transformation into the heating ramp, thereby reducing the residual bcc fraction at the beginning of holding and changing the austenitization path during holding. These results demonstrate that rapid heating and isothermal holding form a kinetically coupled flash austenitization process.