Perkov Oleg Nikolaevich, PhD (Engin.), Senior Researcher, Iron and Steel Institute named after Z.I. Nekrasov of the NAS of Ukraine, Academic square Starodubova, 1, Dnipro, Ukraine,49107; ORCID 0000-0002-8101-4054
Vakulenko Igor Alexandrovich, Dr. Sci., (Engin.), Senior Research Scientist, Iron and Steel Institute named after Z.I. Nekrasovof the NAS of Ukraine, Academic square Starodubova, 1, Dnipro, Ukraine, 49107;ORCID 0000-0002-7353-1916
Kuzmichev Vyacheslav Mikhailovich, Head of Department; Iron and Steel Institute named after Z.I. Nekrasov of the NAS of Ukraine, Academic square Starodubova, 1,Dnipro, Ukraine,49107; ORCID 0000-0003-2057-3001

THERMOCYCLIC TREATMENT (TCТ) OF METALS – WAY FOR
GETTING OPTIMUM STRUCTURES AND PROPERTIES

Summary. The aim of this work is to study the basic principles of thermal
cyclic processing (TCТ) of metals to obtain structures that determine the
optimal complex of mechanical properties. The basic provisions of metal
heating centers using periodically repeated heating and cooling cycles are
given. The TCТ method, as a heat treatment method, is based on constant
accumulation from cycle to cycle of positive changes in the structure of metals.
Studies have shown that with rapid heating, the growth of austenitic grain
occurs slowly and, therefore, heating to high temperatures (up to 10000C) does
not lead to an intensive increase in grain. It has been established that grain size
increases at a variable heating temperature 3 times slower than under
isothermal conditions at the corresponding temperature. Provided that the
growth rate of the new phase (austenite) is small and the nucleation rate of
grains is significant, it turns out that by the end of the  transformation, a
fine-grained structure is retained. Further heating or holding at a constant
temperature leads to a rapid coarsening of austenite grains. If cooling (for
example, in air) of rapidly heated steel is performed 10–150C higher than the
temperature of the Ас1 point, then fine perlite grain is formed due to reverse
recrystallization. With one thermal cycle, ferrite in subeutectoid steels almost
does not undergo changes. But if several such heating and cooling are
performed, then the entire ferrite-pearlite structure undergoes a change. It has
been established that the higher the heating rate during heating and heating and
the less overheating above Ас1, the finer the grain in carbon structural steel.
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However, this increases the need to increase the number of heat treatment
cycles. The mechanism of structure formation explaining these phenomena and
practical recommendations on the implementation of the process of the
technical and economic process are presented. This approach makes it possible
to form the optimal metal structure. At the same time, opportunities can be
significantly expanded in terms of obtaining materials with desired properties
and improving on this basis machines, structures, individual units and parts. All
this puts TCТ in the category of promising areas in metalworking.
Keywords: thermal cyclic treatment, steel, structural transformations,
austenite, perlite, ferrite
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