Maksymiv O. The regularities of formation of surface layers nanostructured by mechanical-pulse treatment on steels, resisting to mechanical and corrosion-hydrogen fracture.

The dissertation is devoted to determination of structural parameters of formation of nanocrystalline structures on carbon and low-alloyed steels by surface mechanical-pulse treatment, investigation of their influence on tribological properties, corrosion resistance and hydrogen embrittlement, and on workability of steels under the mutual action of mechanical loading and corrosion-hydrogenating environments. It has been established that parameters of nanocrystalline surface layer and its physical and mechanical properties depend on treatment regimes and type of technological environment, which enables forming a surface layer with adjustable structural state and properties. It has been shown that size of crystallites in the surface layer of the treated steel influences on the surface microhardness: it increases with decreasing crystallite size. It has been found that the surface layers, formed on the 40X and 65Г steels by mechanical-pulse treatment, had nanocrystalline structure even under heating up to a temperature of 500 °C. The regularities of changing the size of crystallites in the surface layer in a nano scale range with an increase of heating temperature have been established: the size of crystallites is decreased at increasing temperature up to 300 °C and it is increased at higher temperatures. Friction coefficient of steels decreases significantly after their mechanical pulse treatment, which correlates with reducing crystallite size of the steel surface layer. It is reduced in almost four times for the 45 steel with nanocrystalline surface layer in a pair of friction with the ШХ15 steel under oil wear. It has been found that the surface layer with nanocrystalline structure formed on the 45 steel by mechanical-pulse treatment is characterized by lower hydrogen permeability (hydrogen diffusion coefficient is in 1.3–4 times lower) and higher in 1.5–4.4 times efficiency of hydrogen trapping in comparison with the untreated steel. Therefore, it serves as a barrier for hydrogen penetration into the bulk material. It has been established that nanostructurization of the steel surface using mechanical-pulse treatment by multidirectional deformation in an oil technological medium provides the highest resistance of the steel to hydrogen embrittlement. It has been shown that alloying the surface layers of the 35 and 45 steels by nickel, boron and nitrogen during mechanical-pulse treatment can offset the negative influence of intensive plastic deformation on their corrosion resistance. The nanocrystalline surface layer is characterized by high wear resistance under oil and oil-abrasive wear and under the action of corrosion-hydrogenating medium of diethylene glycol, as well. It significantly increases limits of fatigue and corrosion fatigue, and also contact fatigue of treated steels. The method of mechanical-pulse treatment of equipment components made of carbon and low-alloyed steels with formation of surface nanocrystalline structures have been implemented at MCC “Lvivvodokanal” and PJSC “Kohavynska Paperova Fabryka”, showing increase in service life of the treated components in 2.5–3 times. Key words: nanocrystalline structure, mechanical-pulse treatment, friction coefficient, mechanical properties, wear resistance, hydrogen embrittlement, fatigue, corrosion fatigue, contact fatigue.