Refinement-to-Coarsening Transition During Multiple Quenching of SCM440 Steel: Competing Roles of Austenite Reformation and Carbide Dissolution

This study investigated the grain refinement-to-coarsening transition in SCM440 steel during multiple quenching, focusing on repeated austenite reformation, carbide dissolution, and grain-boundary migration. Specimens were austenitized at 850 °C for 20 min and water-quenched for one to four cycles (AQ-1–AQ-4), while AH-80 was continuously held at 850 °C for 80 min and quenched once. The prior-austenite grain size decreased from 12.2 ± 4.6 μm in AQ-1 to 8.5 ± 2.5 μm in AQ-3, but increased to 15.2 ± 4.1 μm in AQ-4 and 22.6 ± 5.4 μm in AH-80. EBSD showed that AQ-3 also exhibited the minimum effective grain size of 0.78 μm and the highest average KAM of 1.62°. TEM revealed a progressive reduction in the carbide population observed after quenching, with the projected area fraction decreasing from 7.39% in AQ-1 to 0.23% in AH-80, consistent with progressive carbide dissolution during austenitization. Based on simplified assumptions regarding carbide geometry, volume fraction, and grain-boundary energy, the estimated Zener pinning pressure decreased from 0.429 to 0.045 MPa, while the estimated PZ/PD ratio decreased from 2.616 to approximately 0.51. Despite progressively weakened carbide pinning, grain refinement continued up to AQ-3, indicating that enhanced pinning alone cannot explain the refinement and suggesting an additional contribution from repeated austenite reformation. AQ-3 exhibited the highest yield strength and ultimate tensile strength and Charpy impact energy, whereas subsequent grain coarsening reduced the mechanical performance. These findings are consistent with a possible competition among repeated austenite reformation, residual-carbide pinning, and grain-boundary migration, although further time-resolved investigations are required to validate their relative contributions.