Effect of rapid heating on the microstructure and toughening mechanisms of medium carbon steel

Abstract Achieving a reliable strength–toughness synergy in medium-carbon steels is crucial for lightweight and safety-critical components. Here, we compare a rapidheating route (RH) with a conventional furnace route (CH) to clarify how heating rate governs toughness in a Fe-0.3C-0.9Mn-0.8Si steel. Microstructural features were quantified by SEM and EBSD, and mechanical responses were evaluated through tensile testing, sub-zero Charpy impact, and J-based fracture toughness measurements. Despite nearly identical strength levels, RH steel exhibits a pronounced increase in both impact energy and fracture resistance. The improvement is associated with (i) a small fraction of ferrite retained after rapid austenitization, which introduces mechanical heterogeneity, and (ii) refined martensitic packets/grains accompanied by a higher population of high-angle boundaries. These microstructural changes jointly promote crack-tip plasticity, enhance crack deflection, and slow down crack advance. The present results highlight rapid heating as a viable, energy-efficient pathway to produce high-strength steels with superior toughness.