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

Figure 2. Brinell press. 
 
Content of the report . 
1. Name of the work. 
2. Purpose of work. 
3. Control questions. 
4. Conclusion from work . 
Control questions 
1. Describe the main differences between steel and cast iron. 
2. What types of furnaces do you know that are used in steel production? 
3. What types of steel do you know? 

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Practical exercise-5 
Topic: Establishment of thermal treatment of steels 
The purpose of the work: 
to study the structure of the thermal operation 
mode of steels 
Theoretical information 
Diffusion enrichment of the steel surface with various chemical elements 
under the influence of temperature is called chemical-thermal treatment (CTT). In 
this process, quantitative changes in the surface lead to qualitative changes. The 
change in the chemical composition of the surface layer leads to the creation of 
properties such as increased hardness, increased resistance to friction or corrosion, 
and fatigue. In CTT, the composition can change to a certain extent, that is, the 
mechanical property does not depend only on the change of structure. CTT is a 
diffusion process. The temperature is determined by the density of the elements that 
diffuse around the surface and the time of their exposure. The most commonly used 
types of CTT in current practice are carbon and nitrogen enrichment of the steel 
surface. The surface is also enriched with elements such as silicon, boron, nickel, 
aluminum, and chromium. The process of changing the composition of the surface 
layer of steel machine tools consists of three stages, and in the first stage, diffusible 
element atoms are activated. In this case, the temperature is crucial. Additional 
elements that increase activity can also be used. 
2. In the second stage, the diffusible elements molecularly approach the 
microplanes on the surface, in other words, they are absorbed into the surface. This 
situation is called adsorption of the modifying element on the surface. In the third 
stage, active atoms that are molecularly close to the surface are adsorbed on the 
surface, as a result, the composition of the layer of elements diffused on the surface 
of the zagotovka becomes different. Then the active atoms begin to diffuse into the 
inner layers of the metal. The state diagram of the composition of the surface layer 
of materials after CTT is necessarily different from the state diagram of the initial 
(main) metal composition. If the temperature, the interaction of the element being 
absorbed into the surface layer with the main metal atoms, and the amount of the 

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element being absorbed in the surface layers are known, it is possible to know in 
advance what phases will be formed on the surface. 
Nitriding of steels 
Diffusion saturation of the detailed surface zone with nitrogen is called 
nitrification. Nitriding is used to increase the wear and tear resistance and endurance 
limit of machine parts: crankshafts, cylinder liners, worms, gear wheels, etc. The 
parts are polished, high-release, clean (finishing) before nitriding. Nitriding is 
carried out in hermetically closed muffle furnaces or containers at a temperature of 
500-6000 C0. Ammonia (NH3) is injected into the muffle at a certain rate: ammonia 
decomposes: 
2NH3=2N+3H2; 
Separated atoms are adsorbed on the surface of the ammonia metal and diffuse 
into its crystal lattice. As a result of the formation of nitrides, the structure and 
mechanical properties change. In nitriding of carbon steels, as the carbon content of 
the steel increases, the rate of nitrogen diffusion decreases. During nitriding, the 
following phases can be formed in the Fe – N system. 
1. α -phase is a solid solution of nitrogen in α -iron. 
2. g - phase is a solid solution of nitrogen in g - iron. 
3. g 1 - phase - iron nitride - Fe4 N 
4. e - phase is a nitride of iron with Fe2N composition. 
the nitrided steel surface layer is e -phase, it will rust in water and atmospheric 
conditions. e and g 1-phase layers do not have relatively high hardness (HV=450-
550). It is obtained by nitriding special alloyed steels (with Sr, Mo, Al, V, Ti) of 
high hardness. These alloying elements are nitride forming elements. After nitriding 
steel 38X2MUa steel, high hardness and resistance to frictional corrosion is 
obtained: HV=1200. Nitriding is a long-term operation. For example, in order to 
obtain a nitrided layer thickness of t=0.5 mm in 38X2MyuA steel, it is necessary to 
keep the temperature at 500-520 0 C for 55 hours. 
The process of simultaneous diffusion saturation of the surface of a steel detail 
with both carbon and nitrogen is called cyanizing. This method has several 

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advantages. For example, nitrogen enhances the diffusion of carbon: it lowers the 
diffusion temperature from 930-9500 Co to 8500 and saturates with carbon as if it 
were cemented. In this case, the growth of austenite grains is reduced, and it is 
possible to find this place immediately with a little airing. This process is called 
nitrocementing because the initial medium is a mixture of 3-5% NH3 with cementing 
gas; carbonitrides are formed in the diffusion zone. The nitrocemented layer has 
good resistance to corrosion and corrosion. Nitrocementing is widely used in 
automotive and tractor manufacturing: it is used to strengthen the surfaces of non-
rotating parts. Cyanization has a serious disadvantage: - toxicity of cyanide salts. 
This requires labor protection, environmental protection - preservation. In this 
regard, the low-temperature process of saturation with nitrogen and carbon is used, 
that is, it is carried out in a solution of non-toxic salts. Non-toxic salts are: cyanites 
and carbonates. High speed steel chisels are carbonitrided after quenching to 
increase stability. Process mode: solution temperature – 530-5700 Co; holding time 
is 5-30 minutes. 
Diffusion saturation of details with metals and non-metals 
In the industry, various requirements are placed on detail surfaces. To do this, 
the detail surface layers are diffusely saturated with different materials. The process 
of impregnation of detailed surface layers with various metallic and non-metallic 
materials (Cr, Al, Mo, W, Si, S) is called diffusion impregnation or diffusion 
alloying. If the surface layer of the detail is saturated with chromium, this process is 
called diffusion chroming, and if it is saturated with aluminum, it is called diffusion 
aluminumization. 
Chrome plating 
The chromium-saturated layer is resistant to severe corrosion, corrosion and 
fire. Chromation is also carried out in 3 different environments: solid, liquid, gas. In 
the case of diffusion saturation with chromium in a solid environment, the parts are 
placed in a bag containing a mixture (60-65% ferrochrome, 30-35% clay-alumina 
oxide, 5% hydrochloric acid). The lid is tightly closed, watered and heated: 1050-
11500 Co, kept for 12-15 hours, then cooled together with the furnace to a 

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temperature of 700-6000 Co, after which it is cooled in air. A mixture of 20% 
chromium chloride CrCl2 and 80% barium chloride VaCl2 is used as a mixture for 
chroming in a liquid medium. Chromium released from decomposition diffuses into 
the surface layer at a temperature of 950-11000 Co. 0.04- 0.1 mm for 4 hours. 
Diffuse in thickness. In gas chroming, the parts are placed in a chrome retort. After 
the retort is heated to 950-10500 Co, a mixture of hydrogen chloride is passed 
through it. The hardness of the chromium layer of low-carbon steels is not very high: 
HV=200-250. However, that of multi-carbon steels is much higher HV=1380. 
Silicification (saturation with silicon), sulfidation ( saturation with sulfur), boron 
saturation and other chemical-thermal processing methods are widely used in the 
industry.