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Figure 6 gives an overview of the resulting microstructure for the analyzed process parameters. The cutting edge radius has also the most significant effect on the occurrence and the thickness of white layer. These results correlate very well with the hardness measurements. For an increasing feed, the thickness of the white layer also increases slightly up to 1.5μm, due to higher cutting forces. A white layer of less than 1μm is produced by a cutting speed of 300 m/min. Increasing the cutting edge radius from rβ=40 to 105μm leads from no white layer to a very constant white layer thickness of more than 5μm The hardness of these layers can be seen in Fig. 5. Large radii produce big white layer thicknesses. The material is flowing under the cutting edge, due to the ploughing effect. This causes higher passive forces, as the conducted force measurements proof. Hence, higher friction occurs in the contact zone, which leads to higher temperature. As shown by Hosseini [22] an increasing temperature results in thicker white layers.
Hardness affected by hard turning processes
Influence of process parameters on white layer
4 Conclusion
Improving the endurance of roller bearings by inducing a specific surface integrity design by machining requires a deep understanding of the interaction of the machining process and the fatigue life of roller bearing. Therefore, the effects of process parameters in hard turning on the surface quality, residual stress profile, microstructure and hardness are analyzed. Literature shows that all analyzed surface parameters can increase the endurance of roller bearings. However, these surface parameters have to be balanced, since due to interaction one cannot be improved without another is worsened. For example high residual stresses and a poor surface finish with Rz＞1.5μm will lead to a very short endurance, because of an increasing number of micro contacts. Due to this required adjustments, also the process parameters need to be chosen wisely.
The conducted experiments highlight the most significant parameter to affect the surface integrity of roller bearings, which is the cutting edge radius. Large radii lead to bad surface finish, high maximum compressive residual stresses and big white layer thicknesses. Figure 7 qualitatively summarizes the identified correlations.
Small surface roughness values can be produced by sharp cutting edges and very small feed values. Additionally to the well-known effect of feed for hard turning there is an interaction with the cutting edge radius. Within the conducted experiments the best combination is (rβ=70μm and f = 0.07 mm.
In order to create high compressive residual stresses within the surface in large distances from the surface, large cutting edge radii are necessary. Cutting speed and feed do not affect the residual stress profile for the analyzed parameter scope.
Due to a higher cutting edge radius the thickness of the produced white layer increases which correlates with the hardness changes of the surfaces.
A surface integrity design for an increasing roller bearing endurance cannot be performed by a simple hard turning process. Additional processes have to be applied to create the necessary combination of a high surface quality and high compressive residual stresses. One possible process is deep rolling, which smoothens the surface and creates very high compressive stresses within the surface due to plastic deformations.
Summarizing the effects of process parameters and cutting edge geometry on surface integrity
Acknowledgments The authors thank the DFG (German Research Foundation) for supporting this project in the context of the research program Ressource efficient Machine Elements (SPP1551).
References
1. Toenshoff HK, Denkena B (2013) Basics of cutting and abrasive processes, lecture notes in production engineering. Springer, NewYork
2. Toenshoff HK, Arendt C, Ben Amor R (2000) Cutting of hardened steel. CIRP Ann 49(2):547–566