**1. Introduction**

In the field of bearing manufacturing, the finishing process chain of the bearing ring consists of grinding and raceway honing. Honing is applied post grinding to achieve the necessary surface characteristics. Hard turning is emerging as a competitive finishing process, offering potential benefits over the grinding process such as cost-effectiveness, manufacture of complex shapes, dry machining, and high flexibility [1,2]. The surface integrity induced by finishing processes significantly affects the functional performance (e.g., friction, corrosion, and fatigue) of machined components [3,4]. In order to improve the fatigue life of bearing rings, innovative hard machining processes have been developed. Hashimoto et al. [5] provide guidelines for the selection of fine finishing processes in order to obtain the desired performance of functions such as fatigue life. Maiß et al. [6] used a

**Citation:** Jouini, N.; Revel, P.; Thoquenne, G. Investigation of Surface Integrity Induced by Various Finishing Processes of AISI 52100 Bearing Rings. *Materials* **2022**, *15*, 3710. https://doi.org/10.3390/ ma15103710

Academic Editors: J. Antonio Travieso-Rodriguez and Gilles Dessein

Received: 6 April 2022 Accepted: 20 May 2022 Published: 22 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

new hybrid process of hard turn-rolling to manufacture a bearing inner ring. As a result, the bearing fatigue life could be increased by a factor of 2.5. Revel et al. [7] proposed precision hard turning for the finishing of bearing steel components to improve the rolling fatigue life of bearing steel components. Arsalani et al. [8] used sequential hard turning, grinding, and ball-burnishing operations. They showed an improvement in the endurance limit with burnished pre-turned and burnished pre-ground samples compared to turned samples.

Rolling contact fatigue (RCF) has been commonly known as a key factor limiting bearing life [9]. Subsurface-originated spalling is recognized as one of the dominant mechanisms of failure in rolling element bearings [10]. Subsurface cracks are more likely to be initiated in the region of maximum shear stress under rolling contact below the surface and propagate toward the surface to form a surface spall [10,11].

Surface integrity is of particular importance when machining high-performance metals used for high-performance components in industries such as aerospace, biomedical, and automotive [3,12]. Surface integrity is characterized in terms of surface topography, as well as mechanical and metallurgical states of surface and subsurface layers.

Both hard turning and grinding processes introduce microstructural alterations in the surface layer known as white and dark layers [13–15]. These layers are visible under optical microscopy after being polished and etched or featureless under scanning electron microscopy (SEM). The white layer microstructure consists of nanocrystalline grains with grain size smaller than 100 nm [16,17] and is harder than the bulk material. The dark layer, which is located beneath the white layer, is generally softer than the bulk material. Two mechanisms, i.e., thermal and mechanical effects, are considered to be the major causes of white layer formation. Hosseini and Klement [18] investigated the characteristics and the formation of white and dark layers induced by hard turning. They showed that two different types of white layers exist: those that are either predominantly thermally or mechanically induced. Zhang et al. [19] showed that the white layer was formed by rapid austenite transformation and quenching process, and the dark layer was formed by the tempering process. Mao et al. [15] investigated the formation mechanisms and properties of the affected layer formed in grinding. They found that white layers may be generated when the grinding temperature is below the nominal phase transformation temperature. There are controversial opinions in the literature regarding how the white layer may impact the fatigue life of bearing steels. Guo et al. [20] showed that the machining-induced white layer can reduce the fatigue life of AISI 52100 steel by as much as 8 times. Opposite conclusions were achieved by Smith et al. [21], who carried out fatigue tests on AISI 52100 hardened bearing steel and concluded that there was no conclusive evidence suggesting that hard turning with a continuous white layer had a negative impact on axial fatigue performance.

Residual stresses can be either beneficial or detrimental for the components subjected to rolling contact fatigue life, depending upon their nature (compressive or tensile) and distribution at the subsurface depth. Shen et al. [22] used a three-ball-on-rod RCF test to experimentally assess the effects of retained austenite and residual stresses on the RCF of carburized AISI 8620 steel. Their results showed that the presence of compressive residual stresses is beneficial and increases RCF life. Matsumoto et al. [23] compared the fatigue life of ground and hard-turned bearing assembly. They found that the depth of compressive residual stresses is the major difference between hard-turned and ground surfaces. Indeed, hard turning produces compressive residual stresses in a deep subsurface, which improves the fatigue life of rolling bearings. Wang et al. [24] investigated the grinding mechanism of a bearing ring raceway and performed integrated modeling to control stress grinding. They showed that grinding can produce compressive residual stresses in the surface layer of the bearing ring raceway. Pape et al. [25] investigated the effect of residual stresses induced through production processes. They showed that the bearing fatigue life is improved due to the induced residual stresses on the bearing's inner ring by sequential hard turning and deep rolling processes.

Rolling contact fatigue has been commonly known as a key factor limiting bearing life. Thus, it is necessary to improve the fatigue life of bearings by controlling the surface integrity of bearing raceways. In this respect, this paper investigates the surface integrity of AISI 52100 bearing rings finished by three kinds of finishing processes, i.e., precision hard turning, conventional grinding, and sequential grinding and honing, as well as the functional performance of the finished bearing rings. As shown in Figure 1, the surface integrity induced by these finishing processes was investigated via scanning electron microscopy investigations and in-depth residual stress measurements. Furthermore, contact fatigue tests were carried out on a twin-disc testing machine under two loading levels.

**Figure 1.** Workflow of the present study.
