*3.1. Microstructure Analysis*

In order to highlight the metallurgical transformations, micrographic analyses were carried out on the cross-sectional samples of the three finishing processes under investigation. SEM observations show that precision hard turning and grinding introduce microstructural changes in the subsurface (Figure 6). Indeed, as shown in Figure 6a, after precision hard turning, a very thin white layer (<1 μm thickness) appears on the top surface, followed by a transition zone in the subsurface and then the bulk material. This very thin white layer can be attributed to the use of a new cutting tool for each machining test. Indeed, tool wear noticeably affects the white and dark layer thicknesses; a worn tool would generate a deep white layer [30,31]. This very thin white layer can also be beneficial for rolling contact fatigue performance. Schwach and Guo [32] showed that the white layer induced by hard turning is very detrimental to RCF life. Indeed, a component free of a white layer can have a life six times that of a white layer component. The transition zone was examined using energy-dispersive spectroscopy (EDS) to investigate the chemical composition and to determine the distribution of elements. The precipitated carbides are distributed spherical (Fe,Cr)3C carbides covering the transition zone. These carbides are observed in the white layer and the transition zone, but their number and size are not the same in different regions. Indeed, the carbides spread in the transition zone are more numerous. After grinding, as shown in Figure 6b, only a white layer with 5 μm thickness is observed above the bulk material, which is greater than that found after precision hard turning. Guo and Sahni [33] found that the thickness of the white layer induced by grinding is greater than those obtained by hard turning. In their micrographic analyzes, Barbacki et al. [34] showed that the white layer thickness varies from 0 and 2 μm, and the dark layer thickness varies from 0 to 5 μm in 17 samples machined by grinding. The absence of the dark layer under the white layer supports that the white layer formation is mainly due to mechanical work. Hosseini et al. [35] reported that the predominantly mechanically formed white layer is accompanied by compressive residual stresses. After sequential grinding and honing processes, as shown in Figure 6c, no microstructural changes are observed. Indeed, the honing process removes the white layer induced by the grinding process.

**Figure 6.** *Cont*.

**Figure 6.** Micrographs of AISI 52100 steel after (**a**) precision hard turning, (**b**) grinding, and (**c**) sequential grinding and honing.
