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Thesis for the candidate’s degree in engineering sciences by speciality

05.02.01 – Materials Science. – Karpenko Physico-Mechanical Institute of the

National Academy of Sciences of Ukraine, Lviv, 2019.

The dissertation is devoted to determination of structural parameters of

formation of nanocrystalline structures on perlitic 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. A surface layer with a gradient nanocrystalline

structure with a crystallite size on the surface in the range of 12 – 60 nm with an

angle of intergranular disorientation of more than 10 degrees is formed on pearlite

carbon and low-alloy steels by mechanical-pulse treatment. The size value of

crystallites is the smallest and the angle of intergranular disorientation is the

greatest on the surface. The size of crystallites of the nanocrystalline structure

depends on regimes of mechanical-pulse treatment (specific pressure in the zone of

friction contact, feed of the hardening tool, rotational speed of the hardened

sample) and the type of cooling tachnological medium; it directly affects the

microhardness of the treated surface, which increases with decreasing size of

crystallites.

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.