Preparation of bainite phase microstructure by laser-electromagnetic induction hybrid solid-state phase transformation

Increasing the proportion of bainite and martensite phases is one of the effective ways to enhance the toughness of medium-carbon low-alloy steel(42CrMo). In this paper, the surface heat treatment of 42CrMo steel is carried out by using a laser heat source to achieve complete austenitization, then with electromagnetic induction heating for several times to implement insulation, and eventually to promote bainite phase transformation. In conjunction with the temperature history curves, OM, SEM, and EBSD are used to characterize the bainite morphology and percentage of bainite via different parameters of the hybrid process. The results demonstrate that the hybrid process combined with twice electromagnetic induction heating can yield a considerable proportion of bainite phase in 42CrMo steel. The resulting duplex structure, consisting of bainite and a minor fraction of martensite, exhibits a significant increase (81.1%) in tensile strength, while maintaining similar impact toughness. The elongation after fracture only has a slight reduction of 5.5% compared to that of the substrate. This innovative process provides a practical alternative to traditional bainite isothermal treatment method which must perform in a heat treatment furnace. Therefore, it is presented as a promising solution for enhancing the mechanical properties of medium-carbon low-alloy high strength steels.