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

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The sating element has to interact with the main metal, forming firm solutions 
or chemical compounds, otherwise processes of adsorption and diffusion are 
impossible. 
Results of chemical heat treatment are defined by thickness of a diffusive 
layer and concentration of a sating element in blankets. The major technology 
factors influencing these characteristics, are the structure of the sating 
environment, temperature and hold time. The structure of the environment and 
temperature determine the speed of dissociation and formation of a sating 
element in an atomic condition. Change of thickness of a layer depending on 
time happens under the parabolic law: 
у=К√τ 
where τ- process duration. 
Most intensively depth of a layer increases in the process beginning. Control 
of results of chemical heat treatment make hardness measurement that can 
indirectly testify to extent of saturation. Depth of a layer can be determined by 
the form of a break of the special samples undergoing processing together with 
details. 
Steel cementation. 
Cementation is a process of saturation of a detail surface by the carbon, 
carried out for the purpose of increase of hardness, wear resistance and 
endurance limit at variable loadings. Increase of the listed characteristics is 
reached, however, only if cementation is accompanied by the heat treatment 
consisting in training and low leave. Usually for cementation we take low-
carbonaceous steel with the content of carbon to 0,2%, in this case the hardness 
of not carburized inside layers of a product after hardening doesn't change and 
remains approximately equal to HB 160-170 while the hardness of a product 
surface increases to HB 600. If the increased strength properties in a core are 
required from a product, steel can apply with the big content of carbon (to 0,3), 
however viscosity thus will be a little lowered. Usually thickness of the 
cemented layer doesn't exceed 1-1,5 mm, and concentration of carbon in it – 0,8-
1%. 
Depending on a condition of the sating environment (carburizer) we 
distinguish cementation by a firm carburizer, a liquid carburizer and gas. 
At the cementation by a firm carburizer, products are stacked in metal boxes 
and filled up with a carburizer consisting of charcoal and carbonates which add 
in quantity from 10 to 40% from the weight of coal. Box pressurize, covering its 
cover with refractory clay and place in the furnace. Cementation is made at 
temperatures 900-950ºС, cementation duration depending on the sizes of a box 
changes from 10 to 20 hours. Carbonates play an important role in cementation 
processes, being an additional source of an oxide of carbon and by that 

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considerably activating process. When heating in a cementation box, the 
following processes proceed: 
ВаСО3+С-ВаО+2СО 
where С- charcoal from carburizer., 
2СО-СО2+Сат 
where Sat-atomic carbon formed by the dissociation of carbon monoxide 
Fе+ Sat -Fе(С) 
This reaction leads to increase of carbon concentration in austenite. At heat 
treatment, the details at first are cooled on air (are normalized), and then are 
tempered from temperatures 760-780ºС. After the hardening, the cemented 
details subject to low leave at temperatures of 160-180ºС. 
At cementation in a liquid carburizer, liquid salinas which structure contains 
table salt, carbonic sodium, cyanic sodium and chloride barium are used. 
Gas cementation is widely applied at mass production of the cemented details. 
As a carburizer we apply limit and nonlimiting gaseous hydrocarbons or carbon 
oxide which when heating dissociate with allocation of atomic carbon. Gas 
cementation is made in the hermetically sealed furnaces having special devices 
for supply of gas and its hashing. Hardening is often made directly from the 
cementation furnace at a small subcooling to 840-860ºС. Application of gas 
cementation allows more than twice to reduce duration of cementation process 
as in this case the need for warming up of cementation boxes disappears. 
Steel nitriding. 
Nitriding is made for increase of hardness, wear resistance and a limit of steel 
endurance, thus corrosion resistance in the atmosphere and tap water also 
increases.
Process of nitriding consists in endurance during quite long time (till 60 
o'clock) details in the ammonia atmosphere at a temperature 500-600ºС. 
Ammonia when heating decays into nitrogen and hydrogen. 
The nitrated layer on a surface consists of mechanical mix of firm solutions 
on the basis of two nitrides of iron. Under this layer, nitrogenous ferrite settles 
down. As nitrides and their firm solutions possess high hardness, the high 
hardness of the nitrated layer turns out directly after nitriding without any 
additional heat treatment. Hardness of the nitrated layer strongly depends on 
dispersion of crystals of firm solutions (nitrides) and the higher, if these crystals 
are more disperse. At introduction of alloying elements, the nitrides of these 
elements not inclined to coagulation and keeping dispersion at heating to 600-
650ºС are formed. Aluminum, chrome, molybdenum which form thermally 
resistant nitrides not inclined to coagulation, especially effectively increase the 
hardness of the nitrated layer. At simultaneous presence of these elements in 
steel, the hardness of the nitrated layer can reach HV 1200. To nitriding we 
apply medium-carbon alloyed steels, for example 38ХМЮА. We distinguish 
nitriding for the purpose of hardness increase, wear resistance and fatigue 

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durability (strength nitriding) and nitriding for increase of corrosion resistance in 
the damp atmosphere and fresh water (anticorrosive). 
At strength nitriding we apply alloyed steel and nitriding carry out at rather 
low temperatures: 500-520ºС. 
Anticorrosive nitriding is made at more high temperatures (600-700ºС). If, 
except high corrosion resistance, other demands are requested for the nitrated 
layer, it is possible to nitrate not only alloyed steel, but carbonaceous as well. 
Wear resistance of the nitrated steel is higher, than tempered or cemented. 
Before nitriding we make the heat treatment consisting of hardening and high 
leave, for increase of durability of a detail core and prevention of breakdown of 
the thin nitrated layer at big specific loadings. 
Cyanation. 
Cyanation (nitrocementation) is a process of joint saturation of a surface of 
steel products by nitrogen and carbon. Main objective of cyanation is increase of 
hardness and wear resistance of details. 
Cyanation can be made:
- in the melted salts containing sodium cyanide NаСН or potassium cyanide 
КСН 
- in the gas environment (nitrocementation) consisting of endothermic gas with 
addition of natural gas and ammonia. 
Structure and properties of a cyanated layer depend on temperature of carrying 
out cyanation. With temperature increase, the content of nitrogen in a layer 
decreases, and carbon increases. 
Depending on temperature of process, we distinguish: 
- high-temperature cyanation. It is carried out at 850-950ºС. After cyanation, 
details are cooled on air, and then exposed to hardening and low leave. It is 
applied to details from low - and medium-carbon, and alloyed steel as well 
- low-temperature cyanation. Cyanation temperature 530-560ºС. The blanket is 
sated mainly with nitrogen. It is applied to details from medium-carbon steel and 
the tool from quick cutting steel. 
Advantages of cyanation in comparison with cementation: 
- higher hardness and wear resistance of a cyanated layer. 
- higher resistance of corrosion. 
- deformation and buckling of details of a difficult form is less. 
- higher productivity. 
Cyanation shortcomings in comparison with cementation: 
- high cost 
- high toxicity of cyanic salts. 
Diffusive metallization. 
Diffusive metallization is a process of diffusive saturation of blankets of 
various metals steel (aluminum, chrome, silicon,boron). After diffusive 

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metallization, details get a number of valuable properties, for example, heat 
resistance, resistance to scaling, etc. 
Diffusive metallization can be carried out: 
- in the firm environment. metallizator is ferroalloy (ferrochrome, ferrosilicium, 
etc.) with addition of chloride ammonium. 
- in the melted metal with a low temperature of melting (zinc, aluminum) which 
is carried out by the detail immersion to fusion., 
- in the gas environment containing chlorides of various metals. At firm and gas 
metallization, saturation happens to the help of flying compounds of chlorine 
with metal which at 1000-1100ºС enter exchange reaction with iron with 
formation of active diffusing atom of metal. 
Alitizing is a process of diffusive saturation of a blanket by aluminum. It is 
carried out in powdery mixes or melted aluminum. Thickness of an aluminized 
layer makes 0,2-1mm, concentration of aluminum in it to 30%. Alitizing is 
applied to increase of corrosion resistance and heat resistance of details from 
carbonaceous steel, working at high temperature. 
Chromium plating is a process of diffusive saturation of a surface by chrome. 
Thickness of a layer makes 0,2mm. Chromium plating is used for products from 
steel of any brands. At chromium plating, high firmness against gas corrosion to 
800ºС, a resistance to scaling and wear resistance of details in hostile 
environment (sea water, acids) is provided). 
Siliconizing is a process of diffusive saturation of a surface by silicon. 
Thickness of a layer is 0,3-1mm. Along with the increased wear resistance, 
siliconizing provides high corrosion resistance of steel products in acids and sea 
water. It is applied to the details used in chemical and oil industry. 
Borating is a process of diffusive saturation of a surface by boron. Thickness 
of borated layer reaches 0,4mm. Borating gives to a blanket exclusively high 
hardness, wear resistance and stability against corrosion in various 
environments. 

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