## 3 Results and Discussion
### Failure Mechanisms in RCF Tests  
The S-N curves and the fracture images of some RCF tests are show in Figure 6 where the arrows mean that the specimen did not fail, that is, it reached at least 10 million cycles. There are two groups of fractured specimens: one with a high value of Hertz pressure (P\({}_{\rm H}\)? 3 GPa) and short life regime (\(<\)10\({}^{5}\)) and the other with low Hertz pressure (P\({}_{\rm H}\) \(\leq\) 3 GPa) and long-life regime (\(>\)10\({}^{6}\) cycles). Therefore, it seems that a contact fatigue of about 3 GPa exists, for the 32 CrMoV 13 nitrided steel studied. Figure 7 shows the surface's micrographs corresponding to the second mentioned behaviour (P\({}_{\rm H}\)\(\leq\) 3 GPa). The rolling contact track surface can be seen in the Figure 7a). Two magnifications of this rolling track show a flattening of the white layer (Figure 7b) and the beginning of a superficial circumferential crack (Figure 7c). All specimens subjected to a Hertz pressure below the referred contact fatigue limit (3 GPa) have presented a similar track morphology and very high service life. When the Hertz pressure increases, the contact stress exceeds the contact fatigue limit, and a distinct behaviour appears. Specimens present a short life, and the crack nucleation happens at the surface at the border of the rolling contact surface. This is probably caused by the tensile stress that is expected in this region, in accordance with the results of [PERSON] (2018) and [PERSON] (2018). At the surface, the crack propagates in the circumferential direction (rolling direction, Figure 8a), probably related to the tensile stresses, while in depth it grows perpendicular to the rolling track (Figure 8b,c). After a given small depth, cracks bend almost 45 degrees (Figure 8c) in a direction to the rolling track centre, probably due to the action of the shear stress and the material's softening induced by the plastic deformation. The spalling phenomenon occurs when two or more cracks find each other, or a critical strength area is attained, which is a characteristic brittle fracture of the nitrided surfaces. Figure 6: S-N curve and fracture images of the nitrided specimens in RCF tests. Figure 7: (**a**) Rolling track’s SEM micrographs after RCF tests (P\({}_{\rm H}\) = 1.9 GPa; N = 9 \(\times\) 10\({}^{6}\) cycles; \(\Lambda\) = 0.407); (**b**) flattening of the white layer (300\(\times\)); (**c**) beginning of superficial circumferential crack (1000\(\times\)). To better understand the evolution of this process, one test was continued, after crack detection, until the spall appearance. Figure 9 shows optical micrographs of the rolling contact surface track of this specimen tested at P\({}_{\rm H}\) = 3.7 GPa and 1.8 \(\times\) 10\({}^{5}\) loading cycles. The observation of Figure 9 leads to the conclusion that the spalling phenomenon can be described in three stages. A first stage, characterised by the circumferential surface crack nucleation at rolling contact track border (Figure 9a), where the appearance of tensile residual stress in the axial/transverse direction should potentiate the process, is followed by a second stage where, simultaneously, cracks propagate in circumferential and in-depth directions (Figure 9b). The in-depth propagation near the surface, i.e., in the hardened layer, occurs perpendicularly to the surface, which is a typical feature of brittle fracture behaviour occurring in planes perpendicular to the maximum principal stress. A third stage begins when a given critical depth is attained. This stage is probably related to the depth where the maximum hertzian shear stress occurs (which for this contact conditions is 1180 MPa at a depth of 0.453 mm) and with the softening material due to the plastic deformation, where the cracks bend in the direction of the rolling track centre, with a slope of \(\approx\)45 degrees as  
Figure 8: Surface morphology after RCF tests.