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NITROGEN HELPS THE CULTIVATOR WORKING OF THE ORGANS
OF THE SIRTED COATING, HARVEST SHARING, LEARNING
MICROSTRUCTURES.
Sh.A.Berdiev
senior lecturer
Karshi Engineering and Economic Institute, Karshi
https://doi.org/10.5281/zenodo.14557710
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.
During nitriding of iron and its alloys in an environment
containing ammonia at the saturation temperature of the eutectoid according to
the "iron-nitrogen" phase diagram, the surface nitride zone consists of
successive layers:
Fe
2
N (
- phase)
Fe
2-3
N (
- phase)
Fe
4
N (
- phase). The
-phase is
formed as a result of recrystallization of the
-phase during cooling in regions
with nitrogen concentration of 11.0-11.35% (by weight).
Methods.
In the first stage of nitrooxidation for oxidation in water vapor,
the processes of obtaining a nitride layer by nitration in ammonia were studied.
As mentioned above, the pre-eutectoid and eutectoid temperature ranges were
studied for the "iron-nitrogen" system. The microstructure of the nitride layers
obtained after nitriding at a temperature of 620
0
C (eutectoid temperature) is
presented in Fig. 1.
At the nitriding temperature above the eutectoid temperature (at 620 ° C), a
columnar structure of the surface zone is observed in the nitrogen layer, its large
crystals have a value equal to the thickness of the ɛ -phase.
[1]
A small number of
pores and cracks are noted between the crystals, the boundaries between the
crystals are very open.
- and
-phase layers have a lot of pores, the walls of
which are oxidized (Fe
2
O
3
) upon cooling.
Results and its discussion
The formation of pores is related to the metastability of nitrite phases, from
which nitrogen tends to be released in free form. This leads to the formation of a
volume of gaseous nitrogen in dislocations, grain boundaries and developed
voids, which, under high pressure, is first in the atomic and then in the
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molecular state. Probably, under the influence of this pressure, pores are formed
in dislocations (grain boundaries) in the nitride coating
[2]
.
х1000 х1000
а) b)
х1000 х1000 (РЭМ)
c) g)
a – Steel 20, ammoniacal dissociation of degree acid nitrogenated a = 30-
45%;
b - Steel 40X after softening ammonia a = 30-45% dissociation of degree
acid is nitrogenized;
c - Steel 20, ammoniacal dissociation of nitrogenous acid a = 70-85%;
g - Steel 45, ammonia a = 45-60% dissociation of degree of acid is
nitrogenized
Figure 1 shows the formation of microstructures of the nitrogen layer after
nitriding at a temperature of 620
0
C.
Carrying out the nitriding process at a high eutectoid temperature with a
nitrogen-rich environment and subsequent rapid cooling leads to the formation
of a γ-eutectoid phase that enters the nitrogen layer (Fig. 1, a and b). In nitriding
of low-carbon steels, due to the small amount of carbon in the steel matrix, the
formation of γ-eutectoid is slightly brighter (Fig. 1, a), and in carbon steels, the
unabsorbed nitrogen layer is darker in color (Fig. 1, b).
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At a low nitrogen potential eutectoid temperature, the nitride layer can be
obtained by nitriding with slow cooling before the γ- eutectoid (Fig. 1, v), but the
columnar structure and porosity of the ε- phase remain unchanged (Fig. 1, g.)
[3]
When studying the nitride layer obtained at temperatures below the
eutectoid temperature, it is characterized by the overall small thickness of the
insufficiently developed ɛ-phase (Fig. 2).
х500 х500
а) b)
a – Stal 20 (after annealing); b – Stal 40X (after improvement).
Figure 2 - Microstructure of steel after nitriding at
550
0
C
for 3 hours
If the nitriding process is at a temperature closer to the eutectoid
temperature, obtaining a nitride phase with a very small number of pores and
the required thickness of the ɛ-phase can be in a very short temperature range
(from 570 - 590 ° C) (Fig. 3)
[4]
.
In the microstructure of the nitride layers taken at the dissociation level of
ammonia α = 30-45%, there are some pores in the saturated zone in the form of
accumulation of matrix carbon and free nitrogen (Fig. image, b)
[5]
.
х1000 (РЭМ) х1000 (РЭМ)
а) b)
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a - ammonia dissociation level α = 30-45%;
b - ammonia dissociation level α = 45-60%.
Figure 3 - Microstructure of Stal 45 after nitriding at 580
0
C for 3 hours.
The formation of pores in the nitride layer occurs when the atmosphere
is saturated with this medium at a high nitrogen potential; in particular,
accumulation of nitrogen from the atmosphere in the nitride layer and carbon
from the steel matrix prevents the incorporation of carbon into the ε-phase with
the formation of high-nitrogen nitride. The layer isomorphous to the ε-phase is
the carbonitride phase - Fe(N,C) (εˊ-phase)
[6]
.
Nitrite layer of the required thickness or ε+εˊ-phase mixture can be
obtained by nitriding the steel by setting a certain value of ammonia α = 45-60%
(nitrogen potential of the atmosphere) of the dissociation level in the
atmosphere of the more developed ε-phase for pre-oxidation and in the furnace
atmosphere
[8]
.
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CONMECHYDRO - 2021
