Fergana polytechnic insitute mechanical engineering faculty department of mechanical engineering and automation

Figure 5.3. Start (Mb) and end temperatures of martensite formation
graph of 
changes in steel carbon content. 
Carbides of strong carbide-forming elements (NiC, NgC, ZrC) slow down 
austenite grain growth, and the martensite formed during cooling of steels alloyed with 
such elements has a fine needle-like appearance. 
Martensite creates internal stresses in the steel, the internal energy of the steel 
increases, and the steel undergoes an unstable state. Achieving such a state is called 
heat treatment. When forging steels, it is necessary to cool them at a rate greater than 
Vk. Then the austenite state at high temperature remains at low temperature, and the 
supercooled austenite transforms into martensite without diffusion. This condition is 
unstable, and over time the steel tends to a more stable condition, a natural 
phenomenon. 
4) Changes in steel release. When martensite in an unstable state is heated, it 
decomposes to form a mixture of ferrite and cementite. Such a phenomenon is called 
discharge. 
Annealing of hardened steel is divided into four stages: initiation of 
carburization, formation of carbides, growth and coarsening of carbides. 
According to the intended purpose, the following three types of release are used: 
a) low-temperature discharge (t 

200 
0 
C). In this case, released martensite is 
formed, the plasticity (viscosity) of steel increases; 

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b) medium temperature release (t=300...500 
0 
C). At such temperatures, tr o ssite 
is formed from martensite, the elastic properties of steel increase. c) High 
temperature discharge (t=550-650 
0 
C) 
At such temperatures, sorbite h is formed from martensite and all properties of 
steel increase. 
Resistance to brittle corrosion of steels is their most important, reliable indicator. 
At 200-300 
0 
C and 500-550 
0 
C, where
hardened steels are released, the impact 
viscosity of steel decreases sharply. Such a phenomenon is called embrittlement. 
Brittleness at 300 
0 
C is caused by the growth of carbides of alloying substances. 
0 C 
, which occurs in all steels , can be avoided, or if it cannot be avoided, 
embrittlement at 500 
0 
C can be prevented by rapidly cooling the steel at these 
temperatures. If this is done, the carbides of the alloying substances do not have time 
to grow, and the second type of embrittlement does not occur in steels. 
Chemical thermal processing of steels is a method of achieving the required properties 
by changing the composition and structure of their surface at a certain depth. 
Depending on what element the steel surface is enriched with, there are the 
following chemical thermal treatments: cementiting, nitrocementing, nitriding, 
diffusion metallization. 
In chemical thermal processing, the element absorbed on the surface of steel 
must be atomized and it must interact with iron. 
In chemical thermal operation, the following processes must be performed: 
decomposition of a chemical substance (dissociation), concentration of active atoms 
on the surface (adsorption) and absorption of adsorbed substance atoms into the metal 
sphere (diffusion). 
The depth of diffusion of active atoms into the metal sphere depends on 
temperature, time and concentration of atoms on the surface. 

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