Research On Microstructure Formation Mechanism Of White Layer In High Speed Dry-hard Cutting

High speed dry-hard cutting technology is the combination of high speed cutting technology and dry-hard cutting technology, which have merits that high processing efficiency, good machined surface quality, and environment protection, etc. Machine tools can cut hardened steel precisely with ultra-hard cutting tools by using the cutting technology. However, machined surface can produce white layer, it has great influence on physical and use properties of parts. There is a lack of systematic and thorough study about formation mechanism of white layer that prevents popularization of the high speed cutting. Thus studying formation mechanism of white layer is significant to improve machinability and performance of parts.In this paper, machined surface can be obtained through orthogonal cutting experiments of cutting 45 steel,40CrNiMoA steel and GCr15 steel. Microscopic experiments, in which the methods of optical microscope, XRD, SEM, EPMA, nano indentation et al. are used. These methods are conducted to observe microstructure and analyze the elements distribution of white layer. These works also research formation mechanism of white layer and optimization of cutting parameters to control the formation of white layer and popularize high speed dry-hard cutting technology. The XRD analysis results show that, the microstructure 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 content of retained austenite in white layer is higher than that in matrix material. The segregation of element C and element Cr are discovered also in white layer area by means of EPMA analysis. It is noticed that the white layer has a hardness harder than the matrix material. White layer is hardened remarkably after being machined. Rapid heating and quenching, sever plastic deformation, are considered to be the major causes of ultrafine grain structure and high hardness of white layer.The influence of the cutting parameters on the formation of white layer are studied through changing cutting velocity, feed rate, et al, during orthogonal experiment. It was found that the thickness and residual austenite content of white layer are greatly affected by the cutting velocity and the flank wear of cutting tools. Thickness and residual austenite content of white layer increase as tool flank wear increases. Thickness and residual austenite of white layer increase first and then decrease with the increasing of cutting velocity. White layer thickness also increases with feed rate increasing. The thickness of black layer, that is over-tempered layer, is mainly affected by tool wear, its thickness increases with the increasing of heat input into the subsurface of matrix material.Finally, the influence of matrix material characteristics on formation of white layer is studied through experiments about cutting different matrix materials. It is found that the white layer thickness increases with carbon content of matrix material increasing, hardness of high carbon content material's white layer is higher than hardness of light carbon content material's white layer. White layer thickness also increases with the increasing of hardness of machined material after heat treatment, it is certainly affected by alloying element content of matrix material.