Investigation of the homogenization–refinement heat treatment process for 45CrNi Steel

This study investigates the microstructural evolution of 45CrNi medium-carbon low-alloy steel under a combined normalizing and subcritical annealing pretreatment process. A series of normalizing experiments were first conducted at various temperatures and gas quenching pressures to evaluate grain refinement and homogenization effects. The results indicate that a normalizing temperature of 1, 113 K and a gas quenching pressure of 0.4-0.5 MPa yield uniform microstructures with ASTM grain sizes of 7-8 and minimal grain size deviation. On this basis, subcritical annealing was introduced to further optimize the structure. The effects of annealing temperature and holding time were analyzed. It was found that annealing significantly promotes carbide spheroidization and ferrite equiaxiality, improving microstructural stability. Among the tested conditions, 953 K provided the best balance between uniformity and grain size control, while higher temperatures such as 983 K resulted in excessive grain growth despite better spheroidization, leading to increased microstructural inhomogeneity. These findings help define the optimal processing window for the homogenization–refinement pretreatment of 45CrNi steel and offer a stable structural foundation for subsequent quenching and surface hardening processes.