| High hard metal materials are widely used in some imporltant parts,such as gear,bearing and blade.Hard-cutting,which has the advantages of high quality,high efficiency and environmental protection,is being widely used in finishing hardened materials.Nevertheless,the hard-cutting process is accompanied by high heat and severe plastic deformation,resulting in differences in the microstructure of machined surface compared to that of the bulk material.The metamorphic layer exists in the machined surface and determines the wear resistance of workpieces.Accurate prediction of the Metamorphic layer has significant effect on controlling the surface quality of the workpiece,and plays an important role in promoting the development and application of the hard-cutting technology.Therefore,in this work,a hardened GCrl5 bearing steel was used as the experimental material,and the microstructure,formation mechanism,prediction and wear resistance of the metamorphic layer were expernmentally and theoretically explored.The main items are as follows:(1)Study on the microstructure and formation mechanism of the metamorphic layer.Orthogonal cutting experiments were employed,then the microstructure and the formation mechanisms of the white and dark layers were analyzed via scanning electron microscopy,(SEM),X-ray diffraction(XRD),electron probe micro analysis(EPMA),transmission electron microscopy(TEM)and nanoindentation et al.The results demonstrated that the white layer is formed by the interaction among phase transformations and plastic deformation.The white layer is formed by a rapid austenite transformation and quenching process,and the plastic deformation provides the phase transformation driving force of the austenite transformation,which can accelerate the formation of the white layer;the dark layer is formed owing to the interaction between the high-temperature tempering process and plastic deformation,the heat that transfers into the subsurface and the stress induced by the plastic deformation provide the drinving force for dynamic recovery and dynamic recrystallization in the dark layer and promote the formation of the dark layer.Meanwhile,based on a Johnson-Cook(J-C)constitutive equation,a finite element model was developed to simulate the hard-cutting process.The effects of cutting speed and flank wear on the microstructure and formation mechanism of the white and dark layers were investigated by combining experiments with simulations.The results indicated that the thickness of the white layer increases with cutting speed at low speeds and decreases with cutting speed at high speeds;the grain size of the white and dark layers increases with cutting speed and flank wear;the thickness of the white and dark layers increases significantly with flank wear;the formation mechanism of the dark layer changes from dynamic recovery to dynamic recrystallization with increase in flank wear.(2)Study on the prediction of the critical cutting speed and martensite volume fraction of the white layer.A austenite transformation driving force calculation model in the white layer under the influence of the cutting temperature,stress and strain was deduced based on the coupled thermo-mechanical characteristic of the hard-cutting and the austenite transformation thermodynamics theory.Then,a prediction model of critical cutting speed of the white layer formation was developed in combination with the austenite transformation driving force model and the hard-cutting FE simulations.The critical cutting speeds of the white layer formation at different chip thickness,flank wear and tool rake angles were predicted.Meanwhile,a calculation model of martensite phase transformation temperature M,under the influence of the cooling rate and stress was deduced based on the coupled thermo-mechanical characteristic of the hard-cutting and the martensite transformation thermodynamics theory.A theoretical model of martensite volume fraction fM,of the white layer was developed based on the M.s and the FE simulations.The fM,at different cutting speeds,feed rates,flank wear and tool rake angles were predicted.(3)Effects of the white layer micro structure and the surface residual stress on the wear resistance of the machined surface.The residual stresses in the machined surfaces were measured,the results showed that the residual stresses in the white and dark layers are compressive stress and tensile stress,when the cutting speed is low and high,respectively;the residual tensile stress exists in the white and dark layers when the workpiece is cut by the worn tools,increasing with flank wear.Meanwhile,the wear experiments were employed on the workpieces with white layer and different residual stresses distributions,and the influences of the white layer and the residual stress on the wear resistance of the machined surfaces were revealed.The results showed that under lubricated condition,the white layer with the characteristics of high hardness and grain refinement improves the resistance of abrasive wear;the high retained austenite content improves the resistance of fatigue wear.The residual compressive stress in the white layer inhibits the initiation and propagation of the cracks,thus increasing the fatigue wear resistance,while the residual tensile stress increases the speed of crack propagation in the white layer,which decreases the fatigue wear resistance.Under dry condition,the high hardness and grain refinement of the white layer improve the adhesive wear resistance.. |
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