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Abstract Hard turning is used as a finishing process to machine hardened parts with very high accuracies. During the last decades it asserted as an alternative to conventional grinding processes due to higher flexibility and productivity. Furthermore, hard turning also increases positive effects on the surface integrity compared to grinding processes. Process parameters such as cutting speed, feed and cutting edge geometry influence the effect on subsurface area as well as the surface roughness. Many researchers have been analyzing these effects during the last years. However, they all cover one or two aspects of the surface integrity. Due to the fact that all researchers applied different experimental conditions it is almost impossible to compare the effects of hard turning on the surface integrity. The presented paper covers the effects of cutting speed, feed and cutting edge radius on the main factors of surface integrity residual stress, roughness, microstructure and hardness of roller bearings in a summarizing overview to identify the optimal parameter values for machining roller bearings with an increased endurance. Hard turning tests are conducted and the effects on residual stresses, surface roughness, hardness and white layers are analyzed. This overall view on surface integrity of roller bearings is necessary to improve the endurance of bearings due to a specific surface integrity design. The interactions between the surface integrity and the expected resulting endurance are discussed at the end of this article.47191
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Keywords Hard turning Surface integrity Roller bearings
1 Introduction
In the past, materials with a hardness above 47 HRC were machined only by grinding. Due to the development of new cutting materials such as pCBN, ceramics or coated carbide, these hard materials can be machined with defined cutting edge, e.g. turning or milling. These processes are defined as hard machining [1]. Highly stressed components are often hardened to increase their fatigue life. In most cases, the applied stress is close to the physical limit of the material properties. Examples for hardened parts are bearings, gears or guides [2–4]. Hard turning is more flexible and even more ecological due to the resignation of coolant, compared to grinding processes [2]. Another major advantage of hard turning is the larger effect on the surface integrity, which can influence the performance of roller bearings in different ways. The endurance of bearings is described by the classical Lundberg–Palmgren theory as an effect of the maximum shear stress within the contact area [5]. However surface roughness and residual stresses can affect the maximum shear stress in a positive or negative way. Due to surface roughness micro contacts occur, which lead to very high local stresses [6, 7]. Therefore a machining process achieving very low surface roughness values and the material ratio curves is aspirated. An increased loading of roller bearings within the shake-down phase can induce high compressive residual stresses, due to higher hertzian stresses. Results from Neubauer et al. showed that a higher loading during the first 1.5 million revolutions leads to an increased endurance from L10=25×106to L10=45×106 revolutions [4]. This effect can be included into the machining process, if hard turning is applied. Due to high mechanical and thermal loads in hard machining or even in rolling contacts, the microstructure of the workpiece changes. One example for microstructural changes are white shining areas after a metallurgical etching process, which is responsible for their name white layers. White layers are described as martensitic structures with a very high hardness [8]. However, rolling contact fatigue often occurs as a results of white etching cracks [9]. On the other hand studies show, that white layers, created by a hard turning processes, do not influence the endurance of roller bearings [10]. The effects of surface integrity on the endurance of roller bearings are summarized in Fig. 1.