Investigation Of White Layer And Residual Stress During Hard Turning Of Hardened Steel

Hard turning is being increasingly considered by industry in the manufacturing of hardened steel. Hard turning has such advantages as higher efficiency, favorable surface finish, greater flexibility in the producing of complex geometric forms, and the ability to be carried out dry to realize green manufacturing. However, high local temperature, high stress, high strain, and high strain rate accompanying the process are likely to result in surface integrity problems such as tensile residual stress and white layer, which are generally found to have great negative effects on the fatigue life of the machined parts. The main research contents are as following:Firstly, in order to study white layer formation and residual stresses, 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 Xray diffraction(XRD) are employed to analyze the microstructures of white layer and bulk materials. Cutting speed thresholds below which no white layer form are found to exist and are different for samples undergone different heat treatments. White layer with wavy morphology can be found in samples after quenching at high cutting speed. It is first discovered 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.Secondly, the relationship between white layer formation and residual stress distribution is analyzed. Through the comparing between white layer thicknesses and surface residual stresses, it is concluded that the effect of high temperature accompanying white layer formation on residual stresses is more dominant than volume expansion. Impacts of cutting sequence on white layer formation, residual stresses, retained austenite as well as the relationship between them are also studied.Finally, the advantages and disadvantages of varies chip separation criteria are analyzed and concluded. A finite element method model is built to simulate multiple feeds during cutting process and to predict the distribution of residual stresses. This model is based on an ALE model contains both Lagrangian boundary and eulerian boundary. This model employs both explicit method and implicit method so as to improve efficiency. Based on the established model, the transformation of residual stress distribution when cutting bearing steel GCr15 with multiple feeds and different cutting parameters are studied.