## 3 Results and Discussion  
### Mechanical Characterisation of Nitrided Surface Layers  
Figure 2a presents the optical micrograph of the cross-section of the nitrided specimens. According to the XRD results, the white layer (also called compound layer) presents, mainly, \(\gamma^{\prime}\) nitrides (Fe4N) and only vestiges of \(\varepsilon\) nitrides (Fe2,3N). The white layer is 18 \(\upmu\)m thick and the diffusion zone has about 380 \(\upmu\)m depth (according to DIN 50190). Microstructure detail of the white layer surface, obtained by SEM, is presented in Figure 2b). As can be seen in Figure 3, Vickers microhardness continuously increases from the bulk material to the nitride top surface of the specimens, where maximum values around 800 HV were found, twice those obtained for the bulk material. An affected depth of about 400 \(\upmu\)m is also observed. It means that the surface layers were strongly work hardened due to the nitriding treatment. Figure 1: Residual stresses measuring spots at the specimen surface. (**a**) Three zones A, B and C at 120 degrees and (**b**) XRD spots equally spaced along the width of the specimens. The central point shows the circumferential or hoop residual stress (\(\sigma_{11}\)) and axial residual stresses (\(\sigma_{22}\)). Figure 2: 32 CrMoV 13 nitrided steel sample: (**a**) Optical micrograph of cross-section; (**b**) SEM micrograph of the white layer surface. The residual stresses results at the specimen surface before and after RCF tests are presented in Table 2. Table 3 shows the residual stresses results obtained for specimen A in three points equally spaced along the width of the contact surface (Figure 1b) before RCF tests. The in-depth residual stress distribution at the central point of contact, before (specimen B) and after RCF tests (specimen A), is presented in Figures 4 and 5, respectively. Tables 2 and 3 show that, before RCF tests, the initial residual stresses at the material surface are generally highly compressive due to nitriding treatment. Some scattering on the residual stress values can be observed between each specimen and between different track zones on each specimen. The measurements in three points throughout the track width (Table 3) show that the compressive residual stresses at the centre of the contact surface are a little higher than those obtained at edges near zones. However, the results reveal a good homogeneity of the nitriding treatment in the width of the specimens. After RCF tests, it was observed that there was no substantial change in the compressive residual stress in the circumferential direction. However, in the axial direction a strong relieving was observed, including reversing to tensile stress in several cases (such as in specimen A, as shown in Figure 5). Figure 4 shows the in-depth residual stress profiles obtained at two different zones A and B of specimen B, before RCF tests. In-depth compressive residual stress distribution, induced by the nitriding surface treatment, before RCF tests, presents a maximum compressive stress value between \(-500\) and \(-600\) MPa at a depth of about \(200\) um, followed by a decrease of the compressive residual stresses up to a depth of about \(400\) to \(600\) um. Figure 4: In-depth residual stress profiles, before RCF tests, in the central point for zones A and B of specimen B.  
Figure 5: In-depth residual stress profile after RCF tests (P\({}_{\rm H}\) = 3.1 GPa, N = \(6\times 10^{4}\) cycles, \(\Lambda\) = 0.367), of specimen A. The in-depth residual stress distribution, after RCF, in specimen A, can be observed in Figure 5. Low stress relaxation in the longitudinal direction after the RCF tests, expressed by the circumferential compressive residual stresses (\(\sigma_{11}\)), is observed, as also reported by [PERSON] et al. (2018). These stresses are relieved near the surface and present a constant value around \(-400\) MPa in the first \(300\)\(\upmu\)m depth.