## 3 Results and Discussion  
### Effect of DB and LTGN on the Microstructure  
The initial microstructure and its evolution due to DB intervention were studied previously [2; 3]. In the present study, a comparison was made of the microstructures of the surface layers in correlation with the phase composition obtained after the following: (1) turning and LTGN; (2) smoothing DB and LTGN and (3) hardening DB and LTGN. Thus, the influence of the degree of plastic deformation (due to DB) and LTGN on the obtained strengthened layers was evaluated. Phase analyses were performed on the cylindrical surfaces of the three specimens, each with dimensions of \(\phi 20\times 30\) mm, processed as previously stated. The established phases and their intensities are shown in Figure 2. The surface layers of the first two samples processed, i.e., after treatment options (1) and (2), besides a very high-intensity S-phase, also contained \(\gamma\)-phase and \(\varepsilon\)-phase (the chemical compound Fe\({}_{2-3}\)N) in different proportions. The surface layer of the third sample (processed by hardening DB and LTGN) does not contain \(\gamma\)-phase. The severe surface plastic deformation introduced by hardening DB [2; 3] is the reason for the transformation \(\gamma\rightarrow\alpha^{\prime}\). The intensity of the \(\varepsilon\)-phase is significant, while the S-phase is weakly expressed. Figure 2: Phase analysis results: 1—turning and LTGN; 2—smoothing DB and LTGN and 3—hardening DB and LTGN. Figure 3 shows the microstructure near the surface layer after turning, polishing and LTGN. In this study, the affected layer (AL) refers to the strengthened layer (nitrogen-rich layer) resulting from LTGN. The AL is 10-12 \(\upmu\)m thick (Figure 3a). Based on the relatively large thickness, it can be assumed that the three established phases (S, \(\varepsilon\) and \(\gamma\)) are in the form of a phase mixture in the AL. This means that nitrogen-bearing austenite with a \(\gamma\)-cubic lattice partially exists in this layer. A relatively large amount of Fe\({}_{2-3}\)N with a high degree of homogeneity was formed in the AL. Based on the (011) and (002) planes, the degree of tetragonality of the S-phase was determined: a = 2.655 A, c = 4.286 A and c/a = 1.6. The EDX analysis close to the boundary with the bulk material showed a local nitrogen content of 27.33 at% (Figure 3b). No nitrogen was detected under the AL, i.e., the structure is the same as before the LTGN intervention. Figure 4 shows the microstructure near the surface layer after smoothing the DB and subsequent LTGN. Fine slip lines are observed below the AL, a consequence of the smoothing of the DB. The amount of the chemical compound, Fe\({}_{2-3}\)N, in the AL is less compared to that of the first sample (see Figure 2). The local nitrogen content measured at this point is 17.2%. From the crystallographic planes (011) and (002) established by the X-ray diffractometer, the degree of tetragonality of the S-phase was determined (a = 2.7042 A, c = 3.94508 A and c/a = 1.458), less than that of the first sample. Figure 5 shows the microstructure near the surface layer after hardening DB and subsequent LTGN. The structure under the AL has a well-defined deformation band (Figure 5a), a consequence of severe surface plastic deformation due to the hardening DB. There is no indication of recrystallization processes. The AL consists of S-phase and \(\varepsilon\)-phase with a high degree of homogeneity and strain-induced \(\alpha^{\prime}\)-martensite introduced by hardening DB before LTGN. The technological temperature of 420 \({}^{\circ}\)C is not high enough for the reverse transformation, \(\alpha^{\prime}\rightarrow\gamma\), at which point the already formed martensite is stabilized by dissolving nitrogen in its crystal lattice. Thus, the \(\gamma\to S\)-phase transformation in the surface layer is hindered to some extent, confirmed by the fact that compared to the first two samples, the S-phase is of the lowest intensity.