*3.2. Phase Composition, Structure and Microhardness of the Surface Layer*

According to the metallographic analysis, because of the plasma electrolytic treatment, the diffusion saturation of the surface occurs with the formation of modified layers detected under the surface oxide layer (Figures 6–8).

**Figure 6.** Microstructure of cross-section of the steel surface after PEN at 850 ◦C. 1—oxide layer; 2—modified layer; 3—initial structure.

**Figure 7.** Microstructure of cross-section of the steel surface after PEB at 850 ◦C. 1—modified layer; 2—initial structure.

**Figure 8.** Microstructure of cross-section of the steel surface after PEC at 850 ◦C. 1—modified layer; 2—diffusion layer (N and C solid solution); 3—initial structure.

Depending on the type of processing, the modified layers will have a different phase composition and structure. According to X-ray analysis, after PEN, the oxide layer consists of α-Fe2O3 and γ-Fe2O3, and the modified layer includes phases FeN0.0939 and CrFe7C0.45, which form due to diffusion processes, and intermetallides of the initial components of the alloy (Figure 9).

**Figure 9.** X-ray diffraction patterns of the steel surface layer after PEN at different treatment temperatures with the indication of ICDD card number.

After PEB, a porous FeO oxide is detected in the oxide layer, except for γ-Fe2O3, and no inclusion compounds were detected in the modified layer, except for intermetallides (Figure 10).

**Figure 10.** X-ray diffraction patterns of the steel surface layer after PEB at different treatment temperatures with the indication of ICDD card number.

As a result of the PEC, a more complex structure comprising three subsequent layers is formed: a surface oxide layer consisting of Fe2O3 and Fe3O4 phases; an outer modified layer, including intermetallides and chromium carbide; and a diffusion layer (a solid solution of diffusion atoms in the initial matrix) (Figure 11).

**Figure 11.** X-ray diffraction patterns of the steel surface layer after PEC at different treatment temperatures with the indication of ICDD card number.

Measurements of the microhardness of the surface layers showed that after PEN the surface is hardened to the depth of its modification, reaching 1400–1450 HV after nitriding at 650–700 ◦C (Figure 12). With an increase in the PEN temperature, the hardness of the nitrided layer decreases, which is associated with the coagulation of nitride particles and the breakdown of coherence. PEN at the maximum temperature of 850 ◦C nearly doubles the hardness on the surface compared to that in the core.

**Figure 12.** Microhardness distribution in the surface layer after PEN at different treatment temperatures.

After PEB, surface hardening does not occur (Figure 13).

**Figure 13.** Microhardness distribution in the surface layer after PEB at different treatment temperatures.

After PEC, the microhardness increases with the rise in saturation temperature, similarly to during the carburizing of carbon steels [36,37], reaching 700 HV, and the thickness of the hardened layer correlates with the thickness of the modified and diffused layers (Figure 14).

**Figure 14.** Microhardness distribution in the surface layer after PEC at different treatment temperatures.
