*3.3. Surface Residual Stresses*

The values of the axial surface residual stresses measured before and after fatigue tests (on non-broken specimens) are compared in Figure 4. The residual stress on the surface of the RB\_G sample was approximately −400 MPa (compression) showing that, before SMAT, rather high compressive residual stresses were created by machining followed by grinding, as in the work of Velásquez et al. [32]. After SMAT, the magnitude of the compressive residual stresses was significantly enhanced. However, the temperature at which the SMAT was done only slightly increased the residual stresses, +10% for the γ austenite and +3% for α' martensite, compared to the SMAT done at RT. It is interesting to note that higher residual stresses were reached in α' martensite than in the γ austenite (approximately 200 MPa more in martensitic phase than in the austenitic one) which may be attributed to the difference in Young's modulus of these two phases, as explained in the work of Spencer et al. [28].

For polished TC specimens, due to the fact that the first stages of polishing were similar to grinding in terms of removing the top surface layer, the measured compressive residual stress (−340 MPa) was almost as high as for the ground RB specimens, and no martensite was detected. For the TC sample treated by SMAT for 60 min, no austenite was found, and only the characteristic {211} plan of α' martensite diffracted, which resulted in a measured residual stresses value of −920 MPa. The magnitude of the residual stresses was significantly higher after 60 min of RT SMAT (TC sample) than after 20 min (RB samples).

**Figure 4.** Measured residual stresses in the longitudinal direction on the sample surfaces done by X-ray diffraction. Different colors and textures were used to illustrate austenite or martensite and pre- or post-cycled measurements.

Concerning the residual stresses after rotating–bending fatigue, the comparison of the fine and spare textured bars in Figure 4 clearly indicates that the residual stress relaxation after fatigue loading was rather pronounced in the austenitic phase of the SMAT samples (30% and 60% for the RB\_20 min RT and RB\_20 min CT samples, respectively) but quite limited for the RB\_G sample (only a few tens of MPa). The extent of the relaxation for the samples treated by SMAT at cryogenic temperature was much more pronounced than in the RT processed ones, and this applied to both phases. Indeed, for the RB\_20 min CT sample, the relaxations were roughly twice those recorded for the RB\_20 min RT sample. Decreases of −58% and −36% were measured, respectively, in the austenitic and martensitic phases for the RB\_20 min CT, while they were only −28% and −18% for the RB\_20 min RT condition.

For the tension–compression fatigue tests, a high relaxation of the residual stresses occurred in the austenitic phase of the polished sample (about −75%), and a diffraction peak of martensite was indexed after 2 <sup>×</sup> 106 cycles at a stress amplitude of 207 MPa. For the sample treated at RT for 60 min, approximately half of the residual stress was relaxed in run-out tests, leading to a very similar measured value compared to the initial state run-out specimens.

Finally, a rather large error bar can be seen on the graph for the α' martensite, formed during fatigue loading of TC\_20 min RT. The reason is that the amount of martensite was low, and the number of diffracting planes was not high enough to ensure a good confidence on the residual stress measurement. All other measurements showed an acceptable error level.
