## 4 Results and Discussion
Therefore, the area of the metal below the strengthened layer after TDT (the white layer) has a lower hardness than the base metal. During etching with a standard etching agent, it becomes darker. Over the course of strengthening non-quench-hardened metal by TDT (after annealing and normalization), the increased etchability zone is not observed. For greater clarity, the location of the zones in the surface layer after TDT using mineral oil (O) is shown in Figure 6.  
This research has shown that carbon significantly influences the formation of the strengthened layer. The strengthened layer thickness and microhardness increases with increasing carbon content. On the other hand, hydrogen also influences the formation of the strengthened layer. To study the influence of hydrogen on the formation of the strengthened layer, pre-hydrogenated samples were strengthened (by TDT). The samples were made of Steel CT80 (QH and LTT), which had a high carbon content (appr. 0.8%) (Figure 7). The hydrogenation of the surface layer of samples was performed using an electrolytic technique at a current density of 0.1 mA/cm\({}^{2}\) in 1% solution H\({}_{2}\)SO\({}_{4}\) + 2.5 g/L thiourea for 60 min. The distribution of hydrogen with depth of the sample was not determined. A metallographic investigation, Figure 8, showed that the strengthened layer thickness and microhardness increased significantly after the TDT of pre-hydrogenated samples compared to the non-hydrogenated samples. Mineral oil was used as the process liquid. The strengthened layer thickness of the hydrogenated samples increased to 280-290 \(\upmu\)m compared to a thickness of 180-200 \(\upmu\)m obtained from the non-hydrogenated samples. Figure 6: Microhardness of Steel 41 Cr4 (QH, LTT) using different process liquids: O—mineral oil; _AAP_—mineral oil with active additives containing polymers; _ASMC_—aqueous solution of mineral salts based on magnesium and calcium chlorides. Both tensile and compressive strains develop in the surface layers of regions of the tool-part contact area during the course of the TDT (compressive deformations arise in the early contact between the tool and part, and at the end of the contact period, there are tensile deformations), which contributes to hydrogen atom diffusion into the metal. As the temperature rises, the hydrogen solubility and diffusion rate increase sharply. Hydrogen solubility in steels increases by an order of magnitude at polymorphic transformation temperatures [59]. According to references [34; 36], the temperature of the martensitic transformation in iron and steels decreases over the course of the QH of Fe-H and Fe-C-H alloys in the presence of hydrogen, the critical cooling velocity decreases during quench-hardening and martensite formation becomes possible even in anhydrous iron at a normal cooling velocity. Hydrogen plasticizes the metal surface layer, improving the processing conditions for its plastic deformation [58; 59]. The prominent feature of white layers obtained in medium-carbon steels with a ferrite-perlite structure is the ferrite area observed within the white layer. This confirms the possibility of converting ferrite into austenite with high-speed heating via a diffusionless mechanism [60; 62; 63; 64]. In the process of TDT, which exhibits both high-speed heating and cooling, there is no time for carbon to dissolve in the austenite formed out of the ferrite, and with the succeeding high-speed cooling, it diffusionlessly generates ferrite once again and not martensite. The perlite component has the same transformations as under normal quench-hardening via a diffusion mechanism [46; 65]. Perlite transforms into austenite when heated, and martensite is formed when it is cooled. Therefore, the ferrite-perlite structure of the initial metal is less desirable in high-quality strengthened layer formation due to the abrupt change in the carbon content of the structural components in relation to the quench-hardened ones.