Effect of Spheroidizing Annealing Treatment on Carbide Evolution and Impact Toughness of Vacuum‐Melted H13 Steel

The evolution of carbides and the impact toughness of a vacuum‐melted H13 steel were studied under different spheroidizing annealing conditions. The main carbide types identified after annealing were Cr 7 C 3 , Cr 23 C 6 , Mo 2 C, Mo 6 C, and VC. Prolonging the holding time led to a gradual increase in the average carbide size, indicating that excessively long holding times should be avoided. During isothermal holding, carbide growth followed the Ostwald ripening mechanism. The impact toughness exhibited an initial increase followed by a decrease with extended isothermal time. After 2 h of isothermal treatment, the formation of chain‐like carbides along grain boundaries resulted in lower toughness. At 3 h, these chain‐like carbides gradually dissolved, yielding carbides with moderate size and a relatively uniform distribution, corresponding to an average impact energy of 22.1 J. In contrast, further extension to 4 and 5 h promoted pronounced carbide coarsening and the dissolution of numerous fine carbides, leading to a considerable deterioration in impact toughness.