Dislocation Density and Reverse Transformation Kinetics in Martensitic Steel Under Cyclic Ultrafast Heating and Cooling

The martensitic transformation, which influences the mechanical properties of metals, is essential for ensuring high strength in steel. Workability is also improved by the formation of ultrafine grains using this reverse transformation. This study evaluated the reverse transformation kinetics during cyclic ultrafast heating and cooling using femtosecond X-ray diffraction to measure dislocation densities in 50% rolled Fe–0.1 mass% C–2.0 mass% Mn martensitic steel. We also developed a unique cyclic ultrafast heating and cooling system to determine the reverse transformation mechanism from martensite (α΄) to austenite (γ). The maximum heating and cooling rates achieved were 1.2 × 10⁴ °C s⁻¹ and 4.0 × 10³ °C s⁻¹, respectively, which were sufficient to avoid diffusive reversion and bainitic transformation. We then measured the dislocation density and reverse transformation during a two-cycle ultrafast heating and cooling process. The reverse transformation under ultrafast heating was massive in the first cycle, while a displacive transformation was exhibited in the second cycle. Additionally, we found that cyclic ultrafast heating and cooling results in a finer microstructure, irrespective of the martensitic transformation mode. These findings represent an advance in our understanding of functional steels.