Effects of cutting conditions on the microstructure and residual stress of white and dark layers in cutting hardened steel

The microstructure and residual stresses of white and dark layers formed during a hard-cutting process have significant influence on the surface integrity of a workpiece. Therefore, the microstructure and residual stress of white and dark layers at low and high cutting speeds, along with different levels of flank wear are discussed, based on the experiment and simulation results. The orthogonal hard-cutting experiments on AISI 52100 steel were performed using polycrystalline cubic boron nitride (PCBN) inserts. A finite element model was developed to simulate the hard-cutting process. The microstructure and residual stress of the white and dark layers were analyzed through scanning electron microscopy, transmission electron microscopy, and an X-ray diffraction stress analyzer. The results indicate that the thickness of the white and dark layers is determined by the cutting heat and plastic strain in the subsurface. The grain size of the white and dark layers increases with cutting speed and flank wear. The formation mechanism of the dark layer changes from dynamic recovery to dynamic re crystallization with increase in flank wear. The residual compressive stress in the white layer at low cutting speeds is mainly caused by the microstructure of the white layer and plastic deformation, and the residual tensile stress in the white layer at high cutting speeds is mainly caused by the cutting heat. Tensile stress is induced in the white and dark layers when the workpiece is cut by a worn tool, and the tensile stress is caused by the high cutting heat and microstructure of the dark layer.