• Applied in-situ high-temperature EBSD to investigate austenite reversion, with the case of tempered martensite reported for the first time. • Clarified the nucleation and growth mechanisms of austenite reversion from three initial microstructures. • Degenerated pearlite induces the most pronounced refinement, martensite yields only limited refinement, and tempered martensite exhibits an intermediate effect. • The refinement of austenite grains is governed by the nucleation density of the globular austenite. Austenitization behavior significantly influences the high-temperature austenite grain size, as well as the final microstructure and properties of ultra-high strength steels. To clarify the underlying mechanisms, this study investigates how the initial microstructure controls this reversion process using in-situ high-temperature electron backscatter diffraction (EBSD). Crystallographic analysis of austenite reversion from degenerated pearlite, martensite, and tempered martensite reveals that the initial microstructure critically controls the austenite grain size at 860°C. The refinement of high-temperature austenite grains is primarily determined by the nucleation density of globular austenite, which stems from distinct nucleation mechanisms. In martensite, the nucleation of acicular austenite formed by reverse transformation predominates. Globular austenite only nucleates at prior austenite grain boundaries, existing in minimal quantities and ultimately developing into the coarse austenite grains. Tempered martensite follows a similar mechanism, with the addition of nucleation on cementite particles, which increases the density of globular austenite and yields intermediate refinement. Degenerated pearlite exhibits synergistic nucleation at pearlite colony boundaries and cementite particles, generating the highest density of randomly oriented nuclei and achieving the finest austenite grains. This work establishes a clear mechanistic link between the initial microstructure, the reversion pathway, and the final austenite grain size, providing critical insights into how to refine the austenite grain through the initial microstructure design of ultra-high-strength steels.
Zhu et al. (Wed,) studied this question.