*2.2. Laser Shock Peening*

The heat-treated and polished specimens of maraging steel (l = 80 mm × w = 20 mm × t = 9.5 mm) were exposed to LSP in a confined mode using water as a transparent overlay and without an absorbent coating. In comparison to the conventional LSP with an absorbent coating, this technique uses lower laser pulse energy in order to avoid surface melting. This permits laser treatment to be performed without a sacrificial layer and allows for higher overlapping rates between laser spots, in our case between 78% and 92%. The LSP path mode on the specimens is presented in Figure 1. The direction of the laser processing was parallel to the rolling direction. Several specimens were exposed to LSP simultaneously. The width of the LSP area on each specimen was 16 mm. We used di fferent combinations of laser pulse parameters. We changed the laser PD in a range from 900 to 2500 cm<sup>−</sup><sup>2</sup> with a step of 100 cm<sup>−</sup><sup>2</sup> at three di fferent laser spot diameters (SDs; i.e., 1.5, 2.0, and 2.5 mm). The LSP processing parameters used in the experiment are presented in Table 3. The Laser pulse energy and the laser pulse duration were constant, i.e., 2.8 J and 10 ns, respectively. Therefore, the laser hit the specimen surface with a power density in a range between 5.7 and 15.8 GW·cm<sup>−</sup>2. We used a Q-switched Nd: YAG laser (model: Brilliant B, wavelength: 1064 nm, Gaussian spatial distribution; Quantel, Lannion, France) operating at 10 Hz.



**Figure 1.** Principle of the LSP path mode on the specimens.
