## 3 Results and Discussion
### Effect of DB and LTGN on the SI Physical-Mechanical Characteristics
#### 3.2.2 Residual Stresses  
As found previously [3], single-pass hardening DB introduces about 50% strain-induced \(\alpha^{\prime}\)-martensite into the surface layer, which quickly decreases to zero at a depth of up to 0.1 mm [2]. Therefore, in the present study, the residual stresses were measured for the austenite phase. The distribution of residual stresses introduced by turning, smoothing DB and hardening DB, i.e., before LTGN, is shown in Figure 7. The turning introduces tensile residual stresses (axial and hoop) in the surface layer. Conversely, both DB variants, smoothing and hardening, introduce significant residual compressive stresses. The axial residual stresses are significantly larger in absolute value compared to the hoop stresses at a depth of approximately 0.1 mm, after which they decrease to zero (in contrast to the hoop stresses). As can be expected, the hardening DB introduces larger compressive residual stresses in the surface and the closely spaced subsurface layers. Figure 8 depicts the residual stress distribution after LTGN and following different initial treatments (turning, smoothing DB or hardening DB). LTGN (after turning and polishing) introduces maximum residual compressive stresses in the surface layer and in the closely spaced subsurface layers at a depth of up to 10 \(\upmu\)m. However, the compressive zone depth is only 50 \(\upmu\)m. The surface residual stresses introduced by the static SCW method on external cylindrical surfaces are characterized by the fact that the axial surface stresses are much larger in absolute value than the surface hoop stresses [2,3]. However, the surface axial stresses introduced through LTGN are 2.43 times lower than the hoop stresses,  
Figure 6: Microhardness profile: (**a**) before LTGN; (**b**) after LTGN. Figure 7: Residual stresses before LTGN (1—turning; 2—smoothing DB and 3—hardening DB): (**a**) axial; (**b**) hoop. whose absolute value reaches 6.6 GPa. The combination of smoothing DB and subsequent LTGN introduces compressive residual stresses at 6 times greater depth (0.3 mm) compared to turning and LTGN, but the surface axial and hoop residual stresses are smaller in absolute value, at 2.1 and 3.94 GPa, respectively. However, the difference between the surface axial and hoop stresses decreases, with the axial stresses being 1.88 times lower than the hoop stresses. The combination of hardening DB and subsequent LTGN intensified the observed trend. The surface residual axial and hoop stresses are even smaller (1.12 and 1.22 GPa, respectively), and the compression zone increases its depth to more than 0.4 mm. The surface axial and hoop stresses are almost equal. The comparison between Figures 7 and 8 shows the final result of the evolution of the residual stresses introduced by smoothing and hardening DB, and this is illustrated in Figure 9. The chemical-thermal impact during LTGN simultaneously causes two effects: (1) relaxation of residual compressive stresses introduced by surface cold work that occurs at a depth greater than 0.02 mm; (2) occurrence of maximum surface residual compressive stresses at a depth of up to 10 \(\upmu\)m as a result of the formed new phases having an increased volume of crystal lattice in the AL. The second effect is mainly due to the S-phase formed. The S-phase has the lowest intensity after hardening DB. Therefore, the residual compressive stresses in the AL are significantly smaller (Figure 9b) compared to smoothing DB (Figure 9a). To evaluate the effect of only the thermal impact when heating up to 420 \({}^{\circ}\)C as a function of time, Figure 10 shows the alteration of the residual stresses introduced by smoothing and hardening DB. The thermal impact of 420 \({}^{\circ}\)C for 20 h causes a relaxation of the residual compressive stresses introduced through the DB and, as a consequence, a reduction of the depth of the compressive zone (Figure 10).