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STUDY OF LAYER COMPOSITION DURING NITRIDING AND STEAM
OXIDATION OF COMPOSITE STEELS.
Sh.A.Berdiev
senior lecturer
Karshi Engineering and Economic Institute, Karshi
https://doi.org/10.5281/zenodo.14557704
Abstract.
The article examines the structural and phase changes in surface
diffusion nitride-oxide coatings obtained during nitriding in a gas environment
followed by oxidation of ferrite-pearlite steels in water vapor, as well as the
influence of phase changes on the corrosion properties.
Key words:
ferrite, pearlite, nitriding, oxidation, diffusion coating, nitride
layer, corrosion resistance.
Introduction.
When the nitride layer is oxidized in the vapor of
oxyethylidene biphonic acid solution by nitriding at a high nitrogen potential, in
addition to iron, in the presence of carbon in the steel matrix, a high nitrogen
nitride layer
ε
-phase
(Fe
2−3
N)
can be obtained. As a result, the carbonitride
εʹ
phase
(Fe
2−3
(NC))
is formed, and at the next stage, due to the dissociation of
high nitrogen nitride by oxidation, creating a barrier oxide layer, the nitrogen
layer carbonitride
-phase
(Fe
2−3
(NC
, oxycarbonitride) consists of a mixture of
εʹʹ-
phase
(Fe
2−3
(NSO))
,
-phase
(
Fе
4
N
)
(Fig. 1 ).
Methods.
As the duration of oxidation increases, the thickness of the oxide
layer due to the release of dispersed nitride particles in the inner nitride zone
and dissociation with the nitride zone leads to a mixture of -
-,
-
phases and
-
decrease in the thickness of the nitride layer (Fig. 1).
Figure 1. Diffractogram of the nitride-oxide coating surface
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Nitriding and oxidation of Stal 45 at
580
0
С
for 3 hours, and nitriding and
oxidation at
550
0
С
for 2.5 hours.
FеКα
radiation.
The desired structure and composition of the phase composition of the nitride
phases in the nitride-oxide coating can be achieved by changing individual
technological conditions at each stage of the nitro-oxidation process.
Results and its discussion
Dissociation of the
-phase takes place through the partial separation of the
-phase and the formation of a low-nitrogen
-phase at the expense of carbon
from the matrix, as well as the formation of the
-phase at the expense of
carbon and saturated atmospheric oxygen.
Picture. 2. Changes in the intensity of the ε(101), εʹ+γʹ (101), γʹ(200)
and Fе
3
О
4
(110) lines of the nitride layer depending on the duration of
oxidation.
Nitriding of steel 45 at a temperature of
580
0
С
for 3 hours and oxidation at
a temperature of
550
0
С
. FеКα radiation.
By forming a barrier oxide film on the surface, it is possible to ensure
interdiffusion of nitrogen in the nitride layer, which allows changing the
concentration of nitrogen between the phases, thereby achieving the desired
composition and structure of the composition of the nitride phases.
The nitrided part of the nitride-oxide coating can obtain an oxide layer with
the desired composition of
Fе
3
O
4
, which is responsible for the formation of anti-
corrosion oxide coatings on the surface of metal objects.
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As a result, the ε -phase is present in the concentration range of 4.55-11%
with the γ
ʹ
-phase, and undergoes dissociation of nitrides during isothermal
exposure. [1].
Currently, there is very little information about the effect of the quantitative
ratio of ε- and γ
ʹ
-nitrides on the physico-chemical and physico-mechanical
properties of the joint zone. At the same time, for the layers of compounds with a
large proportion of the γ
ʹ
-phase, it is possible to determine a higher corrosion
resistance than the layers with a higher proportion of ε-nitrides in the composite
nitride-oxide coating [2].
In the first stage of nitrification, nitrogen dissociation rate (nitrogen
potential) during nitrogen oxidation at a temperature of
〖
580
0
С
according to
the regime of two-stage change: α =30-45% in 1 hour and α =45-60% in 2 hours
(total duration 3 hours) and in the second step, after oxidation at 550
0
C for 1
hour, a nitride layer is obtained, which is as follows:
- ε -phase, ε+γʹ -phase, γʹ-phase, as well as the surface layer of Fе
3
О
4
oxide
(Fig. 2). [3].
A wide range of individual nitride phases lies between the saturation
temperature and the eutectoid temperature of nitridation. By changing the
degree of dissociation of ammonia on the surface of the treated part, it is
possible to achieve the formation of high-nitrogen ε -phase, γ
ʹ
-phase and
supersaturated or low-nitrogen α-solid solution. In particular, stable nitrides
consisting of separate phases are formed at wide dissociation levels in the
temperature range of 560-
580
0
С
. [4].
Figure 3. Diffractogram of the nitride-oxide coating surface
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Nitriding at a temperature of
580
0
С
in a two-step change in the degree of
dissociation of Stal 45: 2 hours at α =30-45% and 1 hour at α =45-60% (3 hours
in total) and 1 hour at
550
0
С
oxidation. It is cooled in a vessel to
400
0
С
, and then
in oil. FеКα radiation.
Regulating the phase composition of the nitrided layer is also achieved by
changing the nitrogen potential of the saturated atmosphere [5]. By keeping the
nitrogen potential at the level of nitrogen solubility in one or another phase, it is
possible to form γʹ-phase (low nitride) or ɛ-phase with low nitrogen content on
the surface. It is known from the obtained data that the thickness of the formed
diffusion nitride layer increases with the increase of nitrogen potential.
Therefore, the sample saturation conditions can be selected based on obtaining
an optimal non-porous quality e-phase during the nitriding step (Fig. 3) [6].
Nitrogen content
The saturation temperature is
580
0
С
, the duration is 3 hours.
1 – the total thickness of the nitride layer; 2 is the thickness of the high-
nitrogen ɛ-phase.
Figure- 4. Change of the total thickness of the nitride layer
depending on the nitrogen potential of the atmosphere during
nitriding
At
π
Н
>1 values of nitrogen potential, a quality nitride layer consisting
of ɛ- and (ɛ+
γ′
)- phases is obtained on the surface; with a further increase in
nitrogen potential, the nitride and high-nitrogen e-phase in it increase
accordingly. As the nitrogen potential increases to
π
Н
<6, the share of the γ-
phase in the nitride layer decreases, and the ε-phase increases. According to
the phase composition of ɛ- and (ɛ+γ')-phases, the optimal nitride layer is
formed at nitrogen potential values from 2 to 4. [7].
L
ay
er t
hi
ck
ne
ss
, μm
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An increase in the thickness of the nitride layer results in blocking the
diffusion of nitrogen through the nitride layer. If it is necessary to increase
the duration of nitridation, due to the increase in the percentage of nitrogen
potential, when
π
Н
> 5
exceeds the high nitrogen nitride layer, the
brittleness of the layer and the porosity of the ε -phase are determined.
Oxidation in the first stage of nitriding, reducing the duration of the
nitriding process, saving ammonia and reducing the amount of nitrogen in
the e-phase, which increases the density of the nitride layer and reduces its
brittleness.
The atmosphere in the furnace does not directly affect the process of
deep absorption of nitrogen from the nitride layer to the metal surface, in
this case the temperature and composition of the processed material (the
amount of carbon in the steel matrix) are the decisive factors[8].
The potential of the atmosphere is then realized by reducing the amount of
ammonia supplied to the furnace to a minimum and increasing the degree of
dissociation.
During nitrogen saturation, a sufficient concentration gradient of nitrogen
appears in the nitride layer for further diffusion processes to occur, and in the
second stage of saturation, under conditions of reduced nitrogen potential, they
can separate in two directions until equilibrium is reached:
Nitrogen is deeply absorbed from the nitride layer into the metal and
diffuses into the external environment.
The atmosphere in the furnace does not directly affect the process of deep
absorption of nitrogen from the nitride layer into the metal, in this case the
temperature and composition of the processed material (the amount of carbon
in the steel matrix) are the decisive factors [9].
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David Pye. Practical Nitriding and FerriticNitrocarburizing.ASM
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Vanes S. E. The Nitrotec surface treatment process // Met. and Mat. 1984.
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CONMECHYDRO - 2021
