## 3 Results and Discussion
### Effect of DB and Heat Treatment on the SI of 304 SS  
The effects of DB and initial heat treatment on the roughness height parameter, R\({}_{\text{a}}\), are shown in Figure 11. The turning after initial heat treatment (HT7) showed higher roughness (R\({}_{\text{a}}\)) compared with the as-received state (AR1), while the trend for DB was the opposite. Both DB processes, smoothing and hardening, led to lower roughness (R\({}_{\text{a}}\)) after initial heat treatment (HT8 and HT9), a consequence of the increased plasticity of the steel after quenching. Smoothing DB provided significantly lower roughness compared with hardening DB. Furthermore, the higher number of tool passes (n = 5) reduced the roughness in comparison with single-pass hardening DB. The influence of DB and heat treatment on the micro-hardness is visualised in Figure 12. All DB implementations increased the surface micro-hardness compared with turning. This increase was greater for the initial heat treatment state because of the greater plasticity of 304 SS after quenching. DB caused greater plastic deformation of the surface and subsurface layers and, thus, greater strain hardening. Expectedly, the single-pass hardening DB (AR3 and HT9) led to greater micro-hardness in comparison with smoothing DB (AR2 and HT8), and this effect was more pronounced with a higher number of tool passes (n = 5) (AR4 and HT10). After DB, heating at 350 \({}^{\circ}\)C for 3 h significantly increased the micro-hardness. A greater increase was observed for single-pass hardening DB (AR6 and HT12), with the micro-hardness being significantly greater compared with five-pass hardening DB (AR4 and HT10). Therefore, when the goal is to maximise the surface micro-hardness, it is appropriate to combine single-pass hardening DB and subsequent heat treatment. Figure 11: Dependence of the R\({}_{\text{a}}\) roughness height parameter on DB and heat treatment. Figure 13 shows the influence of DB and heat treatment on the percentage content of \(\alpha^{\prime}-\)martensite induced in the surface layer. The initial heat treatment was a prerequisite for a higher content of \(\alpha^{\prime}-\)martensite because of the increased material plasticity. The five-pass hardening DB for the initially heat-treated state (HT10) induced 100% \(\alpha^{\prime}-\)martensite content in the surface layer. The heating at 350 \({}^{\circ}\)C for three hours after DB led to partial reverse \(\alpha^{\prime}-\)martensite transformation. The distributions of the residual axial stresses introduced by turning and DB are shown in Figure 14 for the two initial states: as-received and initially heat-treated. Since DB causes \(\alpha^{\prime}-\)martensite transformation in the surface layer (see Figure 13), the residual stresses in the surface layer and the nearby subsurface layers introduced by hardening DB were measured separately for the two phases: austenitic and martensitic. As expected, the stresses measured for the martensite phase were greater in absolute value because of the greater hardness of the martensite. Turning introduced residual tensile stresses in the  
Figure 12: Dependence of the micro-hardness on DB and heat treatment. Figure 13: Dependence of the strain-induced \(\alpha^{\prime}-\)martensite in the surface layer on DB and heat treatment (Note: for samples HT11, AR6, and HT12, the \(\alpha^{\prime}-\)martensite is tempered, i.e., transformed into \(\alpha^{*}-\)martensite). surface layer, but DB introduced significant compressive stresses with a compressive zone depth of up to 0.6 mm. For both initial states, smoothing DB introduced lower residual stress in absolute value compared with hardening DB. This trend was more pronounced when 304 SS was initially heat-treated. The higher number of passes (n = 5) increased the absolute residual stresses in the surface and the nearby layers for the martensitic phase when the steel was initially heat-treated. In this case (HT10), the \(\alpha^{\prime}-\)martensite content in the surface layer was 100% (see Figure 13).