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

Heat-resistant steels . Heat resistance criteria (criteria) of materials.
The fact that the material withstands deformation (mechanical loads) for a long 
time above its melting temperature by 0.3 parts and does not break (does not erode) is 
called heat resistance. Modern machine parts operate at high temperatures under great 
forces: metallurgical furnaces, gas pipes, aircraft engines, internal combustion engines, 
etc 
When choosing a material, the length of time of working under force and the 
acting forces are decisive. 
Heating reduces interatomic bonding forces, elastic modulus decreases at high 
temperatures, temporary resistance also decreases, both yield strength and hardness 
decrease. The lower the melting temperature (t 
er 
) of the base of the alloy, the lower its 
limited operating temperature (Fig. 6. 2 ) . 
, the state of the material during long-term loading is determined by diffusion 
processes in it. These conditions are characterized by yield processes and stress 
relaxation processes. 
6. Fig. 2 Temperature dependence of yield strength: 
1-Al; 2-Cu; 3-Ti; 4-To; 5-W. 

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gradual increase in plastic deformation under the influence of forces below the 
yield point is called 
yielding 
. 
A specific variation of the deformation with the length of time of application of 
the force is shown. The yield curve consists of three periods. In the 1st period, the 
deformation begins well and slowly fades away - the rate of deformation is not 
constant; In the 2nd period, the rate of deformation stabilizes; In the 3rd period, the 
deformation accelerates and the metal breaks. It is impossible to bring the detailed 
operation to the 3rd period, it will break, break, etc. 
The yield deformation develops as a result of displacement of dislocations in 
grains, displacement of grain boundaries and diffusion migration. 
Migration of dislocations (above the melting temperature -0.3T 
earth 
) takes place 
in two ways: sliding, jumping. 
To ensure heat resistance, it is necessary to limit the mobility of dislocations and 
slow down diffusion. This is achieved by increasing the interatomic binding forces : 
obstacles are placed on the migration of dislocations between the grains, and the grain 
sizes are increased. 
The strength of interatomic forces is increased by alloying: by changing the 
crystal lattice, by changing from a metallic bond to a stronger covalent bond. 
The feasibility of alloying is alloying with hard-to-melt metal, heat-resistant 
steel with a centered crystalline lattice is alloyed with molybdenum (up to 1%), and 
heat-resistant steel with a centered crystalline lattice is alloyed with tungsten, 
molybdenum, cobalt (up to 15-20% in total). 

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Figure 6.3 Yield curve. 1st non-stagnant period; 2nd stagnant period; 3rd decay 
period. 
A small amount of alloying elements (0.1-0.01%) are introduced to increase the 
strength of the grain boundaries of heat-resistant steels. These accumulate at the grain 
boundaries and slow down the grain boundary movement. These are boron and cerium 
elements. Thermo-mechanical processing also increases the heat resistance of steel. 
increases.