Achieving the hardness-ductility balance of laser quenching process via thermal cycling

Abstract

The imperfect ductility of laser quenched component is challenging the application of laser quenching process ignoring the excellent hardness performance. In the present work, the “drawback” thermal cycling is employed to achieve the hardness-ductility balance via in-situ tempering the as-quenched microstructure. The extremely non-isothermal condition of thermal cycling induces the partial recovery/recrystallization of martensite, precipitation of refined cementite and limited decomposition of retained austenite. A superior combination of the 184% increase of hardness, 28.19% improvement of strength and almost equivalent elongation of the laser quenched component are achieved. The tempered martensite decorated with sub-micron grain size, moderate carbon content and dislocation density contributes to the high hardness and strength. The softening recrystallized martensite and released retained austenite enhance the plastic deformation ability by dislocation absorption. Associated with the enhanced inhibition of microcrack propagation from the fine cementite, the ductility of laser quenched component is greatly improved. The thermal cycling provides an opportunity for the laser quenching process to be applied to the surface strengthening of axle.