## 5 Conclusions  
1. These studies have shown that in the process of the TDT of steel samples, a strengthened (reinforced) nanocrystalline layer is formed in the surface layer. The grain size of the strengthened layer was 12-20 nm near the treated surface, and the misorientation angle was more than 10\({}^{\circ}\). With an increasing depth in the strengthened layer, the size of nanocrystalline grains increases, and at a depth that is close to the thickness of the strengthened layer, the grain size becomes 54-60 nm.
2. The nature of the process liquid used during the TDT significantly affects the thickness of the strengthened layer. The studies have shown that during the TDT using mineral oil with AAP, a uniform strengthened white layer with a nanocrystalline structure of the greatest thickness and hardness is formed on steel samples. Strengthened white  
Figure 8: Microstructure of steel, which was formed after TDT: normalized Steel C22, process liquid—(_O_) mineral oil (**a**); normalized Steel C22, process liquid—(_ASMC_) aqueous solution of mineral salts based on magnesium and calcium chlorides (**b**); Steel C45 (QH, LTT), process liquid—(_O_) mineral oil (**c**); Steel C45 (QH, LTT), process liquid—(_ASMC_) aqueous solution of mineral salts based on magnesium and calcium chlorides (**d**).  
layers significantly improve the operational properties of the working surfaces of parts, especially wear resistance.
* It is speculated that molecules of the process liquid, which is fed into the treatment zone, under the action of high temperatures and pressures decompose into component chemical elements which diffuse into the surface layers of the metal. It is considered likely that carbon, which diffuses into the surface layer, significantly affects the formation of a strengthened layer of greater thickness. It may also be that this strengthening is also caused by thermal or mechanical effects, and an avenue of further work would be to determine which of these mechanisms is the primary cause of the strengthening.
* Hydrogen, which is evolved in the contact zone from the process liquid due to the friction of the tool on the treated surface of the part, likely diffuses into the surface layer of the part, thus significantly affecting the formation of the strengthened layer. Investigations into the strengthening of Steel CT80 samples showed that after treatment of pre-hydrogenated samples, the thickness of the strengthened layer increased 1.5 times, and the microhardness increased from 9.1 GPa to 10.4 GPa.  
Investigation, I.H., A., P.M., A.F, V.G., V.D. and O.P.; conceptualization, I.H., V.G. and V.D.; supervision, I.H.; data collection, I.H., A.A., P.M., A.F., V.G. and V.D.; formal analysis, V.D.; methodology, A.A., P.M. and A.F.; visualization, V.G. and A.A.; data curation: A.F. and V.D.; writing--draft, I.H., A.A., P.M., A.F., V.G. and V.D.; writing--reviewing, I.H., V.G. and O.P. All authors have read and agreed to the published version of the manuscript.  
We gratefully acknowledge funding in the form of EU Grant Nos. [2017-1-UK01-KA107-036029], [2019-1-UK01-KA107-061076] and [2020-1-UK01-KA107-078410], which permitted this collaborative work to take place.  
Not applicable.  
Data availability Statement: Data is contained within the article.  
The authors declare no conflicts of interest.