Effect of martensitic variant selection on the crystallographic features of bainite/martensite multiphase structure of 42CrMo steel

The multiphase structure of bainite and martensite with different proportions was obtained in 42CrMo steel by controlling the isothermal temperature and isothermal time, and the mechanism of martensitic variant selection on the crystallographic features of materials was studied. The results show that the final microstructure after austempering is a multiphase structure of bainite and martensite, which conforms to the K–S orientation relationship with the parent phase austenite. Under the redistribution of carbon elements and plastic confinement effect, the martensite formed by the post-transformed austenite has stronger variant selectivity than bainite. As the isothermal temperature decreases, the dislocation density of the material increases, which improves the strength of the material. At the same time, the increase of phase transformation driving force and austenite strength weakens the variant selection, resulting in the decrease of grain size and the increase of high angle grain boundaries (HAGBs), further improving the strength and plasticity of the material. At a shorter isothermal time, more martensite with stronger variant selectivity reduces HAGBs and decreases the plasticity of the material. Finally, the best matching of strength and plasticity was obtained under the heat treatment process of isothermal temperature and 30 min isothermal time. This study provides a theoretical basis for further improving the product of strength and elongation of 42CrMo multiphase steel.