*3.1. Surface Modifications*

Several representative examples of the surface topology, as well as their respective measured roughness values, are displayed in Figure 1. Despite a low roughness resulting from the surface grinding (Rq = 0.32 μm), some machining grooves still remained visible on the surface of the RB\_G samples (Figure 1a). These grooves were completely removed after 20 min of SMAT at RT (Figure 1b) but not completely for the cryogenic treatment (Figure 1c). Both types of peening conditions induced a considerable increase of the surface roughness (from 0.32 to 3.42 and 1.97 μm). However, SMAT for 20 min at CT significantly reduced the surface roughness compare to the SMAT for the same duration at RT; a reduction of 42% can be noticed (Figure 1c). This is consistent with previous studies [26], which showed that, as the material becomes harder at low temperature, the steel beads impacting the specimen surface generate less pronounced craters. The effect of SMAT is also visible for the TC specimens (Figure 1d,e). Due to mirror-finish polishing, the TC\_P samples exhibited an extremely low roughness (Rq = 0.022 μm). The surface aspect of the SMAT for 60 min at RT sample (Figure 1e) was rather similar to the one obtained after 20 min (Figure 1b), except that the longer peening duration generated a reduction in roughness of about 50% (Rq = 1.755 μm).

**Figure 1.** Observation of the lateral surface of cylindrical samples with their corresponding Rq roughness values: (**a**) ground condition, (**b**) after surface mechanical attrition treatment (SMAT) for 20 min at room temperature, (**c**) after SMAT for 20 min in cryogenic condition and (**d**) polished condition, and (**e**) after SMAT for 60 min at room temperature.

The hardness values are plotted as a function of the depth for different samples in Figure 2. The horizontal black dotted line defines the initial hardness (210 HV). The SMAT treatment increased the surface and subsurface hardness for all treatment duration and temperature conditions; maximum hardness value was located near the treated surface followed by a gradual decrease toward the specimen core until it reached the initial material hardness. The profile of the curve for the cryogenic treatment had some specific characteristics. Compared to the 20 min room temperature treatment, the use of cryogenic temperature substantially increased the hardness along the first 200 μm, while the hardness was comparatively lower at higher depths. In addition, if the hardened depth is defined as the depth at which the initial hardness of the material is reached, one can see that this depth was 30% lower for the sample treated at CT than for the two other conditions.

The EBSD maps (Figure 2b) show that a large amount of martensite was formed in the CT specimen, whereas this phase is hardly detectable in the sample treated for 20 min at RT. The crossover between the hardness evolution curves (seen in Figure 2a) corresponds to the depth at which the martensitic phase transformation was triggered in the CT processed sample (i.e., about 200 μm below the surface).

It is also interesting to highlight the fact that increasing the RT treatment time from 20 to 60 min did not drastically modify the hardened depth (~500 μm) but increased the maximum subsurface hardness by approximately 20%. Ultimately, the same top surface hardness value as for the CT treatment (~550 HV) was achieved.

**Figure 2.** (**a**) Hardness measurements done on a cross-section of the (i) rotating–bending (RB) specimens treated at room temperature (RT; red) and under cryogenic conditions (CT; blue), and (ii) tension–compression (TC) specimens tested at RT (orange) (0 μm corresponds to the outer surface of the sample). (**b**) Corresponding EBSD band contrast maps with α' martensite distributions.
