Evolution of Mo-Rich Carbides in H13 Hot-Working Die Steel During Spheroidizing Annealing

In this study, the evolution behavior of Mo-rich carbides in typical hot-working die steel (H13) during spheroidizing annealing at solution temperatures of 950–1150 °C was systematically investigated by means of thermodynamic calculations combined with OM, SEM, TEM, and XRD characterization, as well as V-notch Charpy impact and Rockwell hardness tests. The aim was to reveal the morphological characteristics, size distribution, and evolution mechanisms of Mo-rich carbides during spheroidizing annealing of H13 steel and to avoid the formation of large irregular M6C carbides. The results indicate that the dissolution difficulty of carbides follows the order: V-rich carbide > Mo-rich carbide ≈ Cr-rich carbide. Following low-temperature solution treatment (950 °C), the Mo-rich carbide precipitates uniformly in a dispersed manner during spheroidizing annealing. In contrast, high-temperature solution treatment (1150 °C) leads to the formation of a network structure along the grain boundaries after spheroidizing annealing. The evolution of the Mo-rich carbide is determined by a combination of thermodynamic conditions, the low diffusivity of Mo atoms and nucleation sites. Taking into account the morphology, size, distribution and spheroidization quality of the Mo-rich carbides, 1000 °C provides the most favourable combination of the investigated microstructural characteristics among the conditions examined. After quenching and triple tempering, the Rockwell hardness of all the samples lies within a narrow range of 44–46 HRC, indicating comparable strength levels. The V-notch impact energy also reaches its highest value of 18.3 J at 1000 °C (13.0 J at 1150 °C). This variation correlates closely with the grain size and the distribution of the Mo-rich carbides. This study provides a reliable theoretical basis for optimizing the post-forging heat treatment process of hot-working die steel.