## 3 Materials and Methods
### The Technology of TDT
All the specified treatments refer to strengthening methods using highly concentrated sources of thermal energy of various types. There is a cyclic effect of the intense shear deformation of the surface layer, which contributes to the grinding of the layer structure and is absent in the other indicated processing methods, in addition to the action of a high-concentration thermal energy source during thermo-deformation treatment. For the practical implementation of thermo-deformation treatment, inexpensive tool machines are used, which are available at every enterprise, in comparison with laser, plasma, and ion beam processing, which require expensive equipment. It is possible to more easily micro-alloy the surface layers of the metal of the treated surfaces with various chemical elements, using various technological liquids, pastes, solid elements, etc., during thermoforming treatment. During laser, plasma, and ion beam treatments, surface alloying of the treated surfaces is performed, using coatings with certain chemical elements, pastes, and other solid or plastic substances. It is not possible to use liquids. _The process liquids_ tested were as follows: mineral oil (O); mineral oil with active additives containing polymers (AAP); water (W); and an aqueous saturated solution of mineral salts based on magnesium and calcium chlorides (ASMC) (Bischofite). All of these process liquids used during the processing contain hydrogen. To study the impact on the formation of the strengthened layers, the TDT was also carried out without use of a process liquid, as well as with pre-hydrogenated samples. The process liquids are used as a source of chemical elements (carbon, hydrogen, nitrogen, chlorine, silicon and others) to saturate the surface layer during processing and improve the quality of the treated surface. They affect the process of grain crushing and the formation of a strengthened layer. _Machine and tool._ TDT of the working surfaces of samples was performed on a modernized flat-grinding machine KNUTH, HFS 3063 VS (Wasbek, Germany) (Figure 1). The engine was arranged to allow the possibility of changing the rotation frequency of the spindle. A metal tool disc is set instead of an abrasive wheel, made of Steel C45 (after normalizing) [55]. _Samples._ Flat samples with the dimensions 10 \(\times\) 20 \(\times\) 100 mm were made of the following materials: Armco-iron (microhardness is 1.5 GPa); Steel C22 (microhardness of 2.51 GPa); Steel C45 (quench-hardening (QH), low-temperature tempering (LTT), microhardness of 4.06 GPa); Steel 41 Cr4 (QH, LTT, microhardness of 5.05 GPa); and Steel CT80 (QH, LTT, microhardness of 6.14 GPa) (Figure 2). The chemical composition of samples is as follows: mass. %: Steel C22 (EN): 0.22 C, 0.50 Mn, 0.27 Si, 0.25 Cr, 0.04 S, 0.3 Ni, 0.035 P, 0.3 Cu, 0.08 As, Fe: balance; Steel C45 (EN): 0.45 C, 0.70 Mn, 0.27 Si, 0.25 Cr, 0.04 S, 0.3 Ni, 0.035 P, 0.3 Cu, 0.08 As, Fe: balance; Steel 41 Cr4 (EN): 0.40 C, 0.78 Mn, 0.26 Si, 1.12 Cr, 0.01 S, 0.01 P, Fe: balance; Steel CT80 (EN): 0.8 C, 0.25 Mn, 0.25 Si, 0.2 Cr, 0.028 S, 0.25 Ni, 0.03 P, 0.25 Cu, Fe: balance; Armco-iron (the total impurity content is about 0.16%): 0.025 C, 0.035 Mn, 0.05 Si, 0.025 S, 0.015 P, 0.05 Cu, Fe: balance. The samples were strengthened by TDT under the following processing conditions: the relative linear velocity of the working part of the tool--70 m/s; the velocity of sample movement--4 m/min; and the pressing force of the tool to the sample--760 N. A double stroke of the tool was used on the treated surface.