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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.