Prediction And Experimental Research Of White Layer And Residual Stress In High Speed Machining Of Hardened Steel

High speed machining, which is one of the most efficient advanced manufacturing technologies, is developing rapidly in recent years. It is of special advantage in precision machining of difficult-to-process materials such as hardened steel. High dimensional accuracy can be achieved in high speed machining. However, white layer and residual stress produced in machined surface, which have a significant influence on usability and service life of parts, become the key factors of impeding the development and popularization of high speed machining. Therefore, a systematic research on the mechanism and prediction of white layer and residual stress in high speed machining, are of great theoretical significance and practical value.Discussions in this paper are based on GCrl5hardened steel (HRC62). A finite element model of high speed machining is developed. The formation mechanism of white layer in high speed machining is studied, and also, a prediction model of white layer thickness based on the formation mechanism is proposed. A prediction model of residual stress in machined surface is also presented based on thermo-elastic-plastic finite element theory. These works not only provide theoretical basis for the research of cutting mechanism and optimization of cutting parameters, but also provide technical support for the popularization of high speed machining.In this paper, a finite element model of high speed machining of GCr15hardened steel (HRC62) is developed. This model is validated by comparing cutting force and chip morphology between predicted values and experimental values.Microscopic experiments, in which the methods of XRD, SEM, EPMA et al. are used, are conducted to study the formation mechanism of white layer in high speed machining. The result shows that, the micro structure of white layer consists of cryptocrystalline martensite, retained austenite and carbide. It is the result of rapid secondary quenching of the material in machined surface.The real phase transformation temperature in cutting process is derived mathematically based on the theory of materials science and physical chemistry, in which the effects of alloying elements, stress and strain on the transition temperature are taken into account. It indicates that the real phase transformation temperature is lower than the nominal phase transformation temperature during high speed machining. According to the calculated phase transformation temperature and the result of finite element simulation, a prediction model of white layer thickness in high speed machining is proposed. The influences of cutting parameters on the thickness of white layer are also analyzed. The result indicates that, with the increase of cutting speed, the thickness of white layer increases at first, then, there is a slight of diminution after reaching the peak value and reaches the balance at last. The increases of cutting depth and flank wear promote the formation of white layer. In a certain range, the thickness of white layer increases with the increase of chamfer length. However, once chamfer length is larger than cutting depth, the thickness of white layer keeps almost constant.The mechanism of residual stress in high speed machining is analyzed based on the thermo-elastic-plastic theory. A prediction model of residual stress is developed based on ABAQUSTM, in which, dynamic and static analysis, explicit and implicit analysis are conducted simultaneously. In this way, the time consuming problem encountered in the calculation of residual stress based on explicit analysis purely is solved. At last, the influences of cutting speed, flank wear and cutting depth on residual stress are discussed.