Конструкциялық материалдар және термоөңдеу Конструкционные материалы и термообработка Constructional materials and heat treatment Учебное пособие для специальности: 5В071200– «Машиностроение»


Influence of heat treatment on the mechanical



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Influence of heat treatment on the mechanical 
properties of steel. 
Heat treatment is carried out for change of mechanical properties of steel 
(durability, hardness, plasticity, viscosity). These properties depend on steel 
structure after heat treatment. 
After annealing, drawback, normalization (drawback with cooling on air) the 
structure of steel consists of plastic ferrite and cementite, possessing the high 
hardness and fragility. Inclusions of carbides have strengthening effect on steel. 
At small number of cementite inclusions of steel are plastic and have low 
hardness. Crushing of cementite particles leads to steel hardening at heat 
treatment. When strengthening cementite particles ability of steel to plastic 
deformation increases. 
Temperature increase of drawback of the tempered products, conducting to 
integration of cementite particles, reduces durability. Durability decreases at 
reduction of speed of cooling in the course of training or temperature increase 
from thermal disintegration. 


204 
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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