## 4 Results and Discussion
Hydrogen increases the plasticization of the surface metal layer, facilitates plastic deformation, and improves the quality of the treated surfaces. During the interaction of hydrogen with metal, interconnections between the surface metal atoms are weakened, which promotes the creation of a positive strength gradient from the surface into the deeper layers of the metal. Such a gradient in the surface layer also reduces the coefficient of friction, as well as facilitating the formation of the parts' working surfaces over the course of cutting and pressure treatment [34; 59; 61]. Conducted studies have shown that during TDT without supplying a process liquid to the processing zone of samples made of steel with a low carbon content (for example, Steel C22), an intermittent low-quality strengthened layer is formed., The layer is practically not formed when processing Armco-iron samples without a process liquid. During the treatment with the supply of water (H\({}_{2}\)O) to the treatment zone, a continuously strengthened layer of small thickness is formed, which is much thinner than the layers obtained using the other investigated process liquids. When mineral oil is employed as the process liquid in the contact area, it decomposes into atomic carbon and hydrogen, which diffuse into the part's surface layers and facilitate the formation of the strengthened layer. The formation of a qualitative strengthened layer is mainly influenced by carbon that diffuses from the process liquid. The layer thickness is about 10-15 \(\upmu\)m (Figure 3). When water is employed as the process liquid, there is no carbon diffusion source from the process liquid, though a strengthened layer is nonetheless formed. Its thickness is the smallest of any of the layers formed by the addition of the process liquids and it is about 5-10 \(\upmu\)m. When ASMC is used as the process liquid, the strengthened layer is thicker (15-20 \(\upmu\)m) compared to treatment with mineral oil. ASMC contains no carbon compounds either. The strengthened layer of the largest thickness was obtained during the treatment using mineral oil with the AAP process liquid. The layer thickness was about 20-25 \(\upmu\)m. During the treatment of the Armco-iron samples without using any process liquids, the strengthened layer is formed intermittently, with an insignificant thickness (less than 5 \(\upmu\)m). The investigation of the microhardness of Armco-iron samples following TDT using different process liquids showed that the greatest value of microhardness (2.51 GPa) was obtained near the treated surface after mineral oil with AAP was used as the process liquid. A slightly lesser microhardness (2.43 GPa) was obtained after using mineral oil (O). The smallest microhardness was obtained after treatment using water alone as the process liquid. Following the TDT of Armco-iron without any process liquid, the microhardness of the treated layer differs only slightly from that of the untreated layer. Under the influence of high temperatures, the polymer is depolymerized (organic polymers are formed from C-H radicals), i.e., it decomposes into its components, carbon, hydrogen and other elements, which then diffuse into the metal surface layer. The temperature of polymer depolymerization is approximately 300-350 \({}^{\circ}\)C, which is not as low than the temperature of the decomposition of mineral oil. Polymers are an additional source of the diffusion of carbon and hydrogen into the metal surface layers of the treated surfaces. The use of ASMC as the process liquid during the TDT contributes to a significant increase in the thickness and microhardness of the surface layer. Mineral oil with AAP has the greatest impact on the formation of the strengthened layer. The process liquid decomposes into its constituent chemical elements (atomic carbon, hydrogen and others) over the course of the treatment, which diffuse into the surface layer.