Hard Turning of Bearing Steels. Controlling residual stress and improving fatigue life
There are many different precision mechanical components such as bearings, fuel injectors, gears and hydraulic components, which are made of hardened steels, and have to be machined to a high degree of precision. Traditionally these components are being ground. Due to the development of ceramic and cubic boron nitride tools for turning this process are now being challenged by hard turning. In the metal cutting industry, machining with a single point tool in heat-treated products with a hardness of above HRC 55 is normally considered as hard turning. Important factors such as quality, lead-time and cost can be improved thanks to hard turning. To ensure the quality of hard turned products, many different parameters must be considered. One of these parameters is residual stress generated in the work piece. Residual stresses have a great impact on the fatigue life of the product and must therefore be controlled. The knowledge of residual stress in terms of how it affects the component and how it is generated and measured is not widespread within the industry. In order to have a controlled implementation of hard turning in the industry, the effect of cutting conditions on the residual stresses must be clarified.
In prior research there have been several reports on the improvement of the fatigue life of hard turned products. There have been several explanations for the enhanced product performance. One reason for the improved fatigue life has been explained by the fact that hard turning produces a better product in terms of roundness. Another idea is that hard turning generates an untempered martensitic layer which is harder than the bulk material and which has good wear resistance. However, most reports attribute improved fatigue life to the compressive stresses generated by hard turning. It has been suggested that the cutting conditions should be chosen on the basis of their effect on residual stress.
In this work, the effect of cutting conditions impact on generated residual stresses has been measured using the X-ray diffraction method. The approach has been empirical and statistical methods have been used. The effects on residual stress from different input factors such as different cutting conditions have been investigated. It was found that the residual stress is generated in two different ways. The creation of stresses is mainly controlled by temperature on the surface and under the surface the mechanical generation is the dominating process. Both a summary table explaining how cutting parameter affects the residual stress and a program called cutting advisor have been developed. A fatigue test on bearings showed that the life can be doubled with stresses generated in hard turning.