*3.2. Residual Stresses*

Figures 7 and 8 illustrate the in-depth residual stresses in the circumferential and tangential directions induced by each finishing process. In the circumferential direction, residual stresses induced by grinding and by sequential grinding and honing have similar trends; they start out compressive at the machined surface (−186 and −290 MPa, respectively) and become tensile at the subsurface depth. Then, they are stabilized at around 50 MPa. The affected depth of residual stress is around approximately 15 μm. Mao et al. [15] showed that phase transformation in the white layer formed by grinding plays an essential role in the build-up of tensile residual stresses. This implies that the predominant factor is deformation, which leads to compressive stresses. As shown in the investigations of Pape et al. [36], residual stresses for ground and honed bearings are compressively closely adjacent to the surface (−500 MPa) and eliminated at a depth of about 20 μm. However, residual stresses induced by precision hard turning show significant differences from those induced by grinding and by sequential grinding and honing. Indeed, they exhibit a "hook"-shaped profile along the depth with maximum compressive value (−680 MPa) at the subsurface depth of 25 μm. Additionally, the affected depth of residual stress is greater for precision hard turning (around 50 μm). These results are in agreement with those reported by Smith et al. [21], who revealed similar differences in the stress states generated by hard turning and grinding processes. They found that the overall residual stress state is significantly more compressive on the hard-turned surface than on the ground surface. Matsumoto et al. [23] reported that the depth of compressive residual stresses is the major difference between hard-turned and ground surfaces. Considering the difference between the plastic deformation that takes place on the ground surface and that on the hard-turned

surface, it is reasonable to expect that the compressive residual stress generated by hard turning is deeper, which may improve the fatigue life of rolling bearings. Otherwise, the results show that the white layers induced in both hard turning and grinding processes possess compression residual stresses. Hosseini et al. [35] showed that compressive residual stresses and decreased retained austenite content were found in the plastically created white layer. Residual stresses in the tangential direction (Figure 8) display similar trends to those described in the circumferential direction. Both grinding and sequential grinding and honing induce compressive and maximum residual stresses (respectively, −186 and −290 MPa) at the machined surface and become tensile at the subsurface depth. Residual stresses induced by precision hard turning start out compressive (−437 MPa) at the machined surface and exhibit a maximum compressive value of −800 MPa at the subsurface depth of 30 μm before stabilizing at the level. The residual stresses in the tangential direction are more compressive than those in the circumferential direction.

**Figure 7.** Residual stresses in circumferential direction.

**Depth below surface (μm)**

**Figure 8.** Residual stresses in tangential direction.

#### *3.3. Rolling Contact Fatigue (RCF) Performance*

Surface integrity induced by finishing processes significantly affects the functional performance of machined components. To investigate rolling contact fatigue performance, contact fatigue tests were performed on a twin-disc testing machine. Figure 9 presents RCF life per million cycles as a function of the three finishing processes under investigation. For each finished ring, two tests were carried out on each specimen, i.e., each test on one raceway, under the same conditions. The rings finished by precision hard turning have the longest life (5.2 million cycles), while rings finished by grinding (Ra = 0.2 μm) have the shortest (1.2 million cycles). The rings finished by sequential grinding and honing (Ra = 0.05 μm) reach 3.2 million cycles. This shows that using the honing process after grinding improves the fatigue life of bearing rings by 2.6 times. Grinding and sequential grinding and honing have similar residual stress distributions, maximum and compressive at the machined surface and tensile at the subsurface depth. The enhancement of the lifetime is due to the high quality of surface roughness obtained by the grinding process. Precision hard turning offers better fatigue life due to the low surface roughness, as well as the residual stress state, which is compressive and maximum at the subsurface depth. The residual stress distributions are of considerable importance; several investigations have reported that compressive residual stresses induced by the manufacturing process can extend the fatigue life of bearings up to 2.5 times [20,23,37]. Low surface roughness and subsurface residual stresses are the key parameters for extending bearing fatigue life.

**Figure 9.** RCF life vs. finishing processes.

Furthermore, it is well known that residual stresses can be developed during the RCF test due to cyclic plastically [38]. Thus, changes in in-depth residual stresses during the RCF test of ring specimens finished by grinding were investigated due to their shortest fatigue life. The experimental simulation of rolling contact fatigue has two phases: a runningin phase called break-in and a bearing life phase. The running-in phase is estimated at 30,000 cycles under these test conditions [28]. After the running-in phase, the contact geometry is stabilized for the entire lifetime. Table 2 illustrates the Hertzian pressure for 600 and 1100 daN normal loads used in the experimental simulation and the RCF life of the ring finished by grinding. This table shows decreasing Hertzian pressure from 3.8 to 3.6 GPa for 600 daN and from 4.5 to 3.8 GPa for 110 daN. In addition, increasing the normal load decreases RCF life.


**Table 2.** Hertizian pressure variations after running-in and RCF life of rings finished by grinding.

Figures 10 and 11 show the in-depth residual stresses with varying loads (650 and 1100 daN) after grinding (initial), after the running-in phase, and at the end of the RCF test. Before the RCF test, the ring specimen (initial) exhibits compressive residual stresses at the machined surface, originating from the machining operation, and tensile residual stresses at the subsurface depth. During the RCF test, the residual stresses changed from a moderate level of tensile residual stresses to compressive residual stress at the subsurface depth. Pape et al. [36] found a similar evolution, and the residual stresses remained constant after 10<sup>6</sup> to 108 revolutions.

Figure 10 shows that during the RCF test, the evolution of circumferential residual stresses in the first 140 μm subsurface depth is not noticeable. At a larger subsurface depth, the residual stresses changed from a moderate level of tensile residual stresses (around 50 MPa for initial) to peak compressive values. Indeed, after running-in with 650 and 1100 daN applied load, the maximum residual stresses are −116 MPa at 299 μm depth and −426 MPa at 427 μm depth, respectively. At the end of the RCF test, the maximum residual stresses are −116 MPa at 299 μm depth and −404 MPa at 396 μm depth. As shown in the investigations of Voskamp [39], the compressive residual stress peaks at a depth ranging from 0.1 to 0.5 mm as the cycles increase. This depth coincides with the maximum shear stress [11,39].

Figure 11 shows that at the shallow depth, ranging from 10 to 140 μm, the tangential residual stresses are tensile and become compressive at the subsurface depth with peak compressive values. Indeed, after running-in with 650 daN and 1100 daN normal load, the maximum residual stresses are −188 MPa at 433 μm depth and −527 MPa at 447 μm depth, respectively. After the end of the RCF test, the maximum residual stresses are −380 MPa at 396 μm depth and −700 MPa at 750 μm depth. As can be seen also that in both directions the peak compressive residual stress increases with increasing the normal load.

**Figure 10.** Circumferential residual stresses after different phases.

**Figure 11.** Tangential residual stresses after different phases.
