Correlation of Internal Crack Healing and Microstructure Evolution in 42CrMo4 Steel during Hot Compression

Owing to its superior mechanical properties and heat treatability, 42CrMo4 alloy steel finds widespread applications. However, the healing of its internal cracks, particularly in large-scale forgings, remains a critical challenge. In this study, internal cracks were introduced into the samples. High-temperature pressure-holding heat treatment was applied to heal internal cracks at temperatures of 900‐1200 °C for 0.5‐5 h. The correlation between the healing process and microstructural evolution was investigated. It indicates that the microstructural evolution of the crack healing zone can be divided into four stages: edge passivation, void segmentation, spheroidization, and final closure via grain boundary migration. Under optimum conditions (1100 °C, 2 h), multiple slip systems activate synergistically, leading to a more uniform distribution of slip systems. Fully recrystallized grains bridged the crack interface, achieving complete metallurgical bonding. The increase in XRD diffraction peak intensity indicates improved crystal integrity. The crack healing mechanism transits from diffusion to recrystallization and eventually leads to grain coarsening with the increase of temperature and duration. These findings provide an experimental and theoretical basis for crack healing of large-scale forgings.