The Research On The Temperature Field And Thermal Damage In The Surface Grinding

Grinding process is often selected for the final machining in the production of components because of its ability to obtain a high surface quality with fine tolerance and roughness. Compared with other machining processes, grinding process requires extremely high specific energy, and almost all of the energy is converted into heat which is conducted into the workpiece, the wheel, the chips and the coolant if used. This usually results in rise for the temperature of both the wheel and workpiece. The high temperature in the grinding zone will cause various types of thermal damage to the workpiece, such as burning, retempering, white layer, residual tensile stresses, etc. Therefore, the investigation of the grinding temperature is necessary for controlling the high temperature and preventing the thermal damage of the workpiece. In this paper, the temperature field and the thermal damage in surface grinding are investigated deeply through experimental analysis, theoretical calculation, and numerical simulation.(1) The effects of the grinding parameters, the grinding manners, the specific grinding energy and the grinding wheel properties on the grinding temperature are analyzed deeply on the basis of the experiment. The effect of the grinding temperature on the workpiece surface quality is also discussed. It is found that the grinding temperature increases with the cutting depth, the wheel speed and the table speed. The cutting depth is primary and the wheel speed is secondary factor for the increasing of the grinding temperature. The temperature of the down-grinding is higher than that of the up-grinding, and the temperature increases with the specific grinding energy. The temperature in the workpiece surface which is ground by CBN wheel is higher than that by aluminium oxide wheel. The relation between the grinding temperature and the morphology of the ground surface is discussed, as well as the relation between the grinding temperature and the surface roughness of the ground surface. The grinding temperature has little effect on the surface roughness of the ground surface, provided that the grinding temperature is not high enough to cause an evident burnout on the ground surface. The temperature, thereby the thermal expansion in the central zone of the workpiece is higher than that in the side zone under grinding fluid. Therefore, the actual workpiece material removal layer in the central zone is thicker than that in the side zone, and the workpiece surface is concave after grinding. (2) The contact length between the wheel and workpiece is studied. It is found that the real contact length is larger significantly than the geometric contact length for the deformations of the workpiece, the wheel and the wheel grains caused by grinding forces and heat, especially at a small cutting depth or a high table speed. The contact length increases on the conditions of larger cutting depth, higher table speed or bigger wheel diameter. The relations among the grinding force, grinding temperature and the contact length are discussed. A high temperature makes the wheel and the workpiece soften seriously, consequently the real contact length increases. At the same time, with the increase of the contact length, the dissipate heat condition of the grinding zone deteriorates, thus the grinding temperature increases. Wheel, workpiece and wheel grains deform subjected to the grinding force, thus the real contact length increases.(3) The theoretical calculations of the grinding temperature field and energy partitioning are carried out. Based on the interaction of the wheel grains with the workpiece, the grinding heat flux integrated model is discussed. The effects of the parameters in the model on the temperature field, the rate of the temperature change and the position of the maximum grinding temperature are described. The theoretical model of the grinding temperature is analysised on the basis of the moving heat source model which is proposed by Jaeger. A new mathematic model of energy partitioning in the grinding zone is proposed, and the effects of the material properties for the grinding wheel and the workpiece, the grain effective contact flat radius, the grinding parameters, grinding fluid, grinding forces and equivalent diameter of grinding wheel on the energy partitioning are considered in the model.(4) A new three dimensional numerical simulation technique of the temperature field in surface grinding is proposed. According to the contact condition between the grinding wheel and the workpiece, the physical properties of the workpiece material are considered non-linear according to the temperature and the heat flux in the grinding zone is considered proportional to the undeformed chip thickness in this technique. The circular arc heat source model in which the heat flux entering the workpiece is assumed to have a parabolic distribution is used to simulate the temperature field. A good agreement is found between the simulational results and experimental observations. The heat affected grade of the workpiece can be predicted from the temperature fields derived from the simulation, considering the critical temperatures for tempering, martensitic and austenitic transformation.(5) The properties and the influence factors of the white layer in the grinding are investigated. The influences of heat treatment, carbon content and grinding conditions on the white layer formation are discussed. It is found that the white layer in the hardened steel is thicker and harder than that in the annealed steel, and there is not a softer transition zone in the annealed steel. Higher carbon content tends to increase white layer thickness at the larger cutting depth, while no difference is observed at the smaller cutting depth. Increased carbon content tends to increase white layer hardness. The white layer thickness increases as the cutting depth increases. The table speed increases the white layer thickness at the low table speed. However, it is contrary at the high one. The white layer in the central zone of the workpiece is thicker than that in the entrance zone, but it is thinner than that in the exit zone.