Research On Formation Mechanism Of White Layer In Grinding Of Harden Steel

Hardened steel has been widely used in industry due to its high strength and high hardness (HRC>55). Since good surface quality can be achieved by grinding, it is commonly used for surface finishing of hardened steel. During grinding process, most of specific energy is converted into heat. Therefore, temperature of the grinding zone increases sharply. At the mean time, severe plastic deformation happpens on the workpiece surface due to sliding, plowing and cutting. A fine metamorphic microstructure layer or ground white layer is produced in ground surface. White layer is more brittleness, and has more serious plastic deformation and more micro-cracks than original surface. The fatigue strength of mechanical part is reduced due to the ground white layer. The ground white layer is one of factors which affect product performance and life. Therefore, it is necessary to research the formation mechanisms of ground white layer during the grinding process of hardened steel. In this paper, the grinding surface temperature, grinding force, ground white layer structure, plastic deformation and stress are investigated. The purpose is to reveal the formation of white layer, and analyze influencing factors. Grinding conditions on white layer such as grinding parameters, states of grinding wheel and the properties of workpiece material are all considered. The research findings will provide theorical basic for surface quality control of products and grinding process parameters optimization. The research includes:(1) There is thermal inertia during temperature measurement of grinding process by using tthermocouple technology. The dynamic error certainly will exist. The factors affecting dynamic error are analyzed and a compensation method about thermocouple dynamic error is proposed. Time constant of the chromel-silicel thermocouple is experimentally measured. The temperature is calculated by dynamic error compensation. Results indicate that the compensated temperature reflecst the true surface temperature more closedly.(2) Ground white layer properties are studied during wet grinding conditions, which include organization structure, micro-hardness, and residual stress distribution. Experimental results indicate ground whiter layer is made up of cryptocrystalline martensite, retained austenite and carbide. Higher hardness of white layer is resulted from consolidation through extremely smaller grain size and higher dislocation density. The residual stress in ground white layer exists in the form of tensile stress. Additionally, the value of the tensile stress increases with grinding depth.(3) The finite element model of single grain cutting processes is established. According to the analysis of cutting force model, the effective stress acting on workpiece can be calculated by the grinding force. According to a single grit cutting of physical model, a single grit cutting finite element model is established, including the grain geometrical model, material constitutive model of hardened steel GCrl5. The stress and strain of grinding surface is simulated by three-dimensional finite element method.(4) Formation condition of ground white layer is established. The roles of the stress, the strain and grinding temperature in the ground white layer are discussed. It is found that ground white layer is a product of phase transformation, and grinding heat provides energy basis for white layer formation. White layer can form due to phase transformation at temperatures below the nominal austenitization temperature of the steel. Plastic deformation and contact stress in grinding process play a leading role in phase transformation. With contact stress increased, phase temperature will be reduced. In the same time, due to plastic strain, the carbon atoms diffusion in austenite is strengthened and the transformation of martensite structure is enhanced. Crystal grain refining process is also accelerated due to strong plastic deformation and fast cooling in ground white layer.(5) The role of grinding parameters and material characteristics are investigated. These include carbon content, heat treatment on white layer formation. Researchs show that the wheel speed, grinding depth and workpiece speed will affect white layer formation. Above all, grinding depth is the most influential factor for white layer formation. White layer thickness increases with the increase of carbon content and hardness of workpiece material, especially at higher wheel speed. Microhardness of white layer also increases with the increase of carbon content of workpiece material.