5. 4 - Fig.
Cementation of steel.
Cementation is the method of enriching the surface layer of steel with carbon
atoms. The purpose of cementation is to increase the hardness and strength of the steel
surface and maintain the viscosity of the core.
This method is performed in solid and gas environments. In the form of
carburetors, pistachio coal, methane, natural gas and other carbon-rich substances are
used. Solid cementation is carried out in steel boxes, gaseous cementation is carried
out in steel boxes, gas (retorts). Due to the high solubility of carbon in iron ,
cement
grinding is carried out at 930..950
0
C. At high temperature, atomic carbon is formed
as follows:
60
S+
2
1
0
2
SO,
2 So
So
2
+S
at
Steel products are enriched with carbon to a depth of 0.5...1.5 mm, the amount
of carbon in this layer should be 0.8...1.0%.
Cementation in a solid environment is usually used in granular production, while
gas cementing is used in serial and mass production .
Cemented parts (low-carbon steels) are released at low temperatures. Such a
treated surface increases hardness and corrosion resistance.
The chemical thermal process in which the surface of steel is enriched with
carbon by diffusion in a certain medium called carburizer is called cementation. The
result of the properties of the surface layer of steel is determined thermally after
chemical thermal treatment. After cementing steels, the purpose of heat treatment is to
achieve greater surface hardness and increase surface friction resistance, on the other
hand, surface contact strength and resistance to bending and twisting also increase.
Usually low carbon (0.1-0.18%C) steels are cemented. In mechanical engineering,
alloyed low carbon steels are also often cemented. For cementation of large-sized parts,
the carbon content should be slightly higher (0.2-0.3%).
Because after such details are found, the middle part of the steel should be
viscous. Usually, the part must be machined for cementing, only after cementing, there
should be about 50-100 mm of clearance left for clean machining. But in most cases,
only a certain part of the detail is required to be cemented. In that case, the part of the
part that will not be cemented is covered with a thin layer of copper (20-40) by the
electrolytic method or coated with another mass.
Structure and formation mechanism of cementite layer.
Diffusion saturation of the surface layer of steel with nitrogen is called nitriding.
For this, the temperature of steel 700°Cis heated to 480-, and ammonia (NS
3
) is
dripped into the furnace.
If nitriding is applied to parts working at a relatively high temperature and in the
environment created after gas ignition, the service life of the part will increase. In the
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car, cylinder liners, valves, gears, shafts and similar details are nitrogenized. The
reason for the improvement of detail properties during nitriding is that metal nitrides
are formed in its part, that is, chemical compounds of nitrogen (Cr, Fe, Al, Mo, U, etc.)
with metals are formed. The hardness of these nitrites reaches 600-1200 Mn/m
2 .
Only
the hardness of iron nitrite does not exceed 500-600 Mn/m
2
. Therefore, nitriding is of
great importance only for alloy steel. In alloy steels that are often nitrogenized, the
elements are as follows: 0.3-0.4% S, 1.35-1.65% SCH, 0.2-0.3% Mo, 0.7-1, 2% A.
Then stable nitride compounds can be formed. In production, rust-resistant and fire-
resistant steels, tool steels and stamping steels, as well as structural steels with high or
low alloying are subjected to nitriding.
The method of enriching their surface with atomic nitrogen to a certain depth is
called nitriding of steels.
ammonia environment at 500-600
0 C
(NH
3
3N+N). Nitrogen atoms absorb
into the steel core and form dispersed nitrides, and nitrates give a good result in
strengthening the enriched layer in steels (35XMyuA) with strong nitride-forming
substances (Al, Gr, M h , Ti , V, Zr) (hardness
increases to 100 NV). Nitriding is
performed on finished parts and parts are not welded after nitriding.
Other types of chemical thermal treatment include nitrocementing, sulphation,
chalking, metallization, and diffusion methods.
Nitriding is carried out after all mechanical and thermal treatment of details.
Nitriding is the last operation, only after that it is possible to carry out polishing to
obtain a very low layer (0.01-0.02). Nitriding is carried out in hermetically sealed
muffle furnaces. Factors affecting this process include the heating temperature, the
time of holding at the heated temperature, and the rate of decomposition of ammonia
into nitrogen. If we increase the temperature, the diffusion of nitrogen increases, but
the growth of hardness decreases. Depending on the temperature, the rate of
decomposition of ammonia can be from 15% to 45%. Nitriding temperature is usually
chosen in the range of 500-620
0
C, the process is very slow. It takes 20-60 hours to
reach a layer of 0.25-0.65 mm and a nitrogen concentration of 3-4%. In the technology
of nitriding, the hardness will not be less than 1200 km/m according to Vickers.
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If the nitriding temperature increases, the hardness will increase as well as the
brittleness will increase , but the diffusion rate will be higher at higher temperature.
Therefore, it is better to carry out the nitrification process in two stages . Nitriding is
carried out at 620°C500- to ensure the necessary hardness , and then the temperature
520°Ccan be increased to 600- to increase the diffusion layer. Doing so will speed up
1.5-2 times. For example, if a thin-walled part is made of 38X2MUa , it is heated to
500- and 520°Csaturated with nitrogen. How long the process should last depends on
the thickness of the diffusion layer. The higher the temperature, the greater the
thickness of the diffusion layer, but the lower the hardness. Typically, the thickness of
the nitrogen diffusion layer is 0.3-0.6 mm (300-600 μm). If the temperature 520°Cis
500-, it takes 24-60 hours to get this layer.
Next, nitriding 570°Cby adding ammonia (metal) at temperature begins to be
used. After this treatment, a carbonitride layer Fe
3
( N,C) is formed on the steel surface.
Such a layer is less brittle than the nitrogen layer and has a greater abrasion resistance.
The hardness of the carbonitride layer in alloyed steels is 6000-11000 MPa according
to Vicker s .
Such treatment increases the durability of the part and also increases the resistance to
corrosion.
Nitrogenation is also carried out in a liquid environment. In the process 570°C,
for 0.5-3.0 hours, ts ionic salt solutions or 55% carbonide. Dry air is carried out. Due
to the low temperature, mainly nitrogen diffuses on the steel surface as a result of the
decomposition of ionic salts . As a result, the iron nitride Fe
3
( N,C) layer (7-15 mm)
is highly resistant to abrasion and less susceptible to brittle erosion.
Diffusion metallization . Enrichment of the steel surface with aluminum, chromium,
ammonia and similar elements is called diffusion metallization. Metallized details have
a number of valuable properties.
Diffusion metallization can be done in the following ways:
1. The part to be subjected to diffusion metallization can be placed in liquid metal,
if it is a metal that melts easily (aluminum, zinc).
2. It is possible to saturate the surface by diffusion using the electrolyte method .
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3. A surface can be saturated using a sublimation phase of a diffusible element.
4. Diffusion saturation is possible in a gas environment.
For example, in chrome plating, complex carbides (Cr, Fe) formed on the surface
become highly heat-resistant due to C 800°C
3 -
it is possible to work up to. Rust
resistance is high in water, sea water, and nitric acid.
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