The effect of high-temperature ECAP on dynamic recrystallization behavior and material strength of 42CrMo steel

With the development of mechanical equipment towards long life and high reliability, the traditional heat treatment method is challenging to meet the high strength requirements of 42CrMo steel for mechanical equipment. High-temperature equal channel angular pressing (ECAP) of 42CrMo steel was carried out above austenite transformation temperature, and the microstructure and material strength of the specimen at different radial positions were characterized and tested. The dynamic recrystallization (DRX) kinetic model of 42CrMo steel was established. The distribution of strain, strain rate, temperature and DRX volume fraction in high-temperature ECAP was studied by coupling finite element software and DRX kinetic model, and the formation mechanism of microstructure and material strength difference was revealed. The results show that the different degrees of DRX caused by the uneven distribution of strain and strain rate on the specimen is the main reason for the gradient distribution of microstructure and material strength in the radial direction of the specimen. From the top to the bottom of the specimen, with the decrease of strain and strain rate, the degree of DRX increases continuously, the microstructure inhomogeneity decreases, the size of the martensite packets and blocks in-creases gradually, and the strength decreases gradually. The grain boundary strengthening caused by the decreased size of martensite packets and blocks is the main reason for the radial difference in material yield strength. Grain boundary strengthening and the coordinated deformation of geometrically necessary dislocations at the soft-hard grain interface of heterogeneous materials are the main mechanisms leading to the tensile strength gradient distribution. This study reveals the dynamic recrystallization behavior and material strength variation of high-temperature ECAP, which contributes to the qualification of the ECAP process and promotes the development of severe plastic deformation on the further strength enhancement of high-strength steel.