Research On Surface Affected Layer In Grinding Ultra-high Strength Steel

Ultra-high strength steels have been widely used in aerospace, aviation, and other industries to manufacture important structural components such as aircraft landing gear and rocket engine shell for their excellent properties of high strength, hardness and good fracture toughness. Due to high strength and hardness characteristics, greater grinding force and grinding heat will be produced during the material processing. Grinding force can make metal plastically deform, causing surface hardening and compressive residual stress, while grinding temperature will soften the material and cause tensile residual stress, even grinding burn and crack will appear on workpiece’s surface when grinding temperature reaches a threshold, thus affecting the fatigue life of components. In this paper, the study is undertaken on Aermet100 steel in order to investigate the grinding force and temperature, grinding stress and strain field, and affected layer characteristics by finite element simulation(FEM), grinding experiment, test analysis, etc.The main works and results are described as follows:1. Based on the grinding theory, grinding process and grinding essential of single abrasive grain are introduced. Then the structure and the formation of the affected layer are analyzed. Finally analysis method of plastic deformation on grinding affected layer is investigated.2. By FEM method, effects of grinding parameters on grinding force and grinding temperature distribution along the depth are obtained in cutting process of single abrasive grain. Based on probabilistic method of grain distribution on the grinding wheel surface, the corresponding prediction model of grinding force has been built. Finally simulation results are verified through the grinding force and temperature measuring experiment. Results show that either for cubic boron nitride(CBN) wheel or white alumina(WA) wheel, grinding force increases with increasing of both workpiece speed and grinding depth, but declines with the wheel speed increasing. Compared with grinding temperature using a WA wheel, grinding temperature by CBN wheel is lower because of the effective thermal conductivity of the CBN abrasive. The affected depth of grinding heat on surface layer is deeper with the increase of workpiece speed and grinding depth. When wheel speed rising, the falling speed of highest temperature on workpiece surface along the depth direction is increasing, thus the affected depth will reduce.3. By FEM methold, the features of stress and strain field in grinding zone are described in cutting process of single abrasive grain. The influences of thermal-mechanical coupling effect on equivalent stress and the equivalent strain field are achieved under different grinding parameters combination. Results show that equivalent stress field and equivalent strain field increase with increasing of single abrasive cutting intensity. When deformation temperature caused by grinding heat is lower than the material softening temperature, high strain rate and large plastic strain by grinding force lead to equivalent stress increasing. At the same level of processing parameters, plastic deformation on the grinding layer by CBN grinding wheel is much severer, the changes of plastic strain field occur on the grinding surface.4. Combined with FEM and experimental methods, the grinding heat and mechanical plastic strain coupling effect on grinding affected layer is analyzed and the formation mechanism of affected layer is revealed. Results show that the influence of grinding heat on grinding surface is obvious, while effect of plastic tension or compression on grinding subsurface by grinding force is significant. For WA wheel, grinding heat weakening effect dominates in surface grinding, but mechanical reinforcement of plastic deformation caused by grinding force is the main factor for CBN wheel.