Effects of cutting conditions on microhardness and microstructure in high-speed milling of H13 tool steel

In the present study, high-speed side milling experiments of H13 tool steel with coated carbide inserts were conducted under different cutting parameters. The microhardness and microstructure changes of the machined surface and subsurface were investigated. A finite element model, taking into account the actual milling process, was established based on the commercial FE package ABAQUS/Explicit. Instantaneous temperature distributions beneath the machined surface were analyzed under different cutting speeds and feed per tooth based on the model. It was found that the microhardness on the machined surface is much higher than that in the subsurface, which indicates that the surface materials experienced severe strain hardening induced by plastic deformation during the milling process. Furthermore, the hardness of machined surface decreases with the increase of cutting speed and feed per tooth due to thermal softening effects. In addition, optical and scanning electron microscope (SEM) was used to characterize the microstructures of cross sections. Elongated grains due to material plastic deformation can be observed in the subsurface, and white and dark layers are not obvious under present milling conditions. The thickness of plastic deformation layer beneath the machined surface increases from 3 to 10 μm with the increase of cutting speed and feed per tooth. The corresponding results were found to be consistent and in good agreement with the depth of heat-affected zone in finite element analysis (FEA).