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After training the structure of steel consists of marten site and residual austenite.
Hardness is defined by the hardness of marten site and its quantity. Plasticity of
the tempered steel depends not only on the content of marten site, but also on its
dispersion (the size of needles). For providing a
high complex of mechanical
properties seeks to receive after training fine-needled structure that is reached at
fine-grained structure of austenite before transformation.
Hardness of steel depends on temperature of isothermal disintegration of
austenite. The temperature of isothermal disintegration of austenite is lower, the
dispersion of pearlitic phases and there of the hardness of steel is higher is
higher.
Final operation of heat treatment is drawback. At drawback the
steel product
gains the final properties. The drawback temperature is higher, the durability is
lower and plasticity of steel is higher. The greatest plasticity corresponds to
drawback at a temperature 600-650zs.
Mechanical properties of steel after hardening and high drawback are higher in
comparison with the annealed or normalized steel.
The double heat treatment consisting in hardening with the subsequent high
drawback, conducting to significant improvement
of the general complex of
mechanical properties, is called as improvement and is a main type of heat
treatment of constructional steel.
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