AbstractThe formation of austenite during both slow and fast heating (0.25–100 °C s−1) was investigated for different microstructures of a selected low-alloy steel. With the simultaneous use of dilatometry and high-energy X-ray diffraction, it was possible to follow not only the global progress of the austenitization, but also the individual evolutions of each phase (ferrite, cementite and retained austenite if present in the initial microstructure). The results confirm earlier published data regarding the ease of austenitization of different initial microstructures (ferrite–pearlite, bainite and tempered martensite). More importantly, two stages were clearly identified, corresponding to the simultaneous transformation of ferrite and cementite, followed by the progressive disappearance of the remaining ferrite. While this is well known for ferrite–pearlite microstructures, it is not yet documented for bainite and tempered martensite. Microstructure evolution calculations based on a diffusion-controlled mechanism helped rationalize the differences observed between the three initial microstructures. In addition, they also strongly suggested the existence of a critical carbide size beyond which the second austenitization phase would correspond to carbide dissolution instead of ferrite transformation.