Hard turning is becoming increasingly considered by industry as a potential substitute for grinding. However, it is greatly hurdled by surface integrity problems such as tensile residual stress and white layer, which are generally found to have negative effects on the stress corrosion, wear resistance, and fatigue life of the machined parts. This paper investigates white layer formation and morphology in hard turning process using various process parameters, taking into account the effects of heat treatment which results in microstructure and hardness differences on bulk materials. Samples undergone three typical heat treatment processes are prepared and then machined using different cutting speeds and radial feed rates. Optical microscope, scanning electron microscope (SEM), and X-ray diffraction (XRD) are employed to analyze the microstructures of white layer and bulk materials after varies heat treatments and cutting processes. Through the studies, we find the existence of a cutting speed threshold, below which no white layer forms for both the low and medium-temperature tempering. The threshold value increases; however, the white layer thickness decreases under the same cutting conditions, for the low and medium-temperature tempering, respectively. Also, we find that the white layer thickness and the scattering of it along the cutting direction on the surface increases with cutting speed and radial feed rate. White layer with wavy morphology can be found in samples after quenching at high cutting speed. We first discover that the pitch of the white layer with wavy morphology is similar to the displacement of tool at the time a segment of the serrated chips forms. Also, the surface residual stresses of the samples are measured. Relationship between white layer and residual stresses is presented. Based on the relationship we reveal that high temperature is more dominant than volume expansion for white layer formation.
Dissolution of Iron-chromium Carbides during White Layer Formation Induced by Hard Turning of AISI 52100 Steel
