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Materials Science, Vol. 46, No. 1, 2010
CAVITATION RESISTANCE OF MODIFIED SURFACE LAYERS OF ALLOYED
STEELS
V. H. Marynin UDC 620.197:667.637.22
The cavitation strength of 15Kh11MF and 15Kh12VNMF steels after
nitriding and deposition of vac-uum arc titanium and titanium
nitride coatings is investigated. Kinetic curves of fracture of
specimens under the action of cavitation and polarization curves in
a 3% aqueous solution of NaCl are obtained, and the microhardness
is measured. A steelcoatings system is thermocycled by the scheme
293 K543 K293 K with cooling in a 3% NaCl solution. The
serviceability of the system is confirmed.
Keywords: cavitation resistance of steels, nitriding, coatings,
serviceability.
For some elements of power equipment that operate at a
temperature of 883 to 850 K and are made of 15Kh11MF and 15Kh12VNMF
steels, it is necessary to extend the overhaul period. Though the
steels, on the whole, have satisfactory mechanical characteristics,
including refractoriness and high-temperature strength, they
exhibit inadequate cavitation resistance and corrode in
chloride-containing media. The possibility to increase, in
particular, their erosion resistance using a number of cold working
processes was investigated earlier [1]. It was shown that it can be
increased by half. However, taking into account that, for many
operated components, these modification processes cannot be used,
the search for other methods of efficient hardening of a steel
surface re-mains an urgent problem. In what follows, we investigate
the resistance of steels after nitriding and application of
coatings.
Methods of Investigations
We tested 25 15 6 mm steel specimens with preliminarily polished
surfaces. Nitriding was carried out at a temperature 833 K for 48 h
in an ammonia atmosphere. Coatings were deposited in an upgraded
Bulat-type unit. The vacuum-arc discharge cathode was made of VT1-0
titanium. In different experiments, the cur-rent strength of arc
discharge was changed from 65 to 135 A. The negative potential on
specimens ranged from 0 to 250 V in deposition of titanium coatings
and was maintained constant and equal to 200 V during formation
of titanium nitride coatings. Air was evacuated from the chamber
with specimens to a pressure of 8105 Pa with two oil diffusion
pumps. The nitrogen pressure was changed to a limit value of 2.6
Pa. We obtained tita-nium coatings at a temperature of specimens of
300 to 1040 K and titanium nitride coatings at 770 K. The ac-tion
of cavitation was investigated in the unit by a technique described
in [2]. A cavitation zone was formed in tap water using an
ultrasonic vibrator of exponential profile with an amplitude of
oscillations of the surface of (30 2) m and a frequency of (21 1.5)
kHz. The distance between the end face of the vibrator and the
sur-face of a specimen was 0.5 mm. We measured weight losses of
specimens with an accuracy of 0.015 mg. The cavitation resistances
of the materials were determined by the relation Zc = V0 /Vi ,
where V0 is the fracture rate of a standard (0.28 nm/sec) and Vi is
the fracture rate in a segment of the kinetic curve where it is
constant or
changes slowly. The abrasive wear was investigated by a
plane-disk scheme. We used 15 15 3 mm experi- Kharkiv
Physicotechnical Institute National Scientific Center, Kharkiv,
Ukraine; e-mail: [email protected].
Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 46,
No. 1, pp. 8689, JanuaryFebruary, 2010. Original article submitted
February 20, 2009.
1068-820X/10/46010097 2010 Springer Science+Business Media, Inc.
97
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98 V. H. MARYNIN
mental specimens (plane) and a disk 15 mm in diameter with
rigidly fixed abrasive grains. The linear speed of the disk near
the surface of a specimen was 4.4 m/sec, and the test time was 300
sec. The wear resistance under abrasive action Za was calculated as
in [3]. The microhardness H was measured with a PMT-3
instrument
under a load of 0.196 and 0.49 N. The corrosion properties were
determined from the results of polarization measurements with a
PI50-1 po-
tentiostat. The potential was changed stepwise by 0.1 V, and
values were maintained for 180 sec. The values of the current
density on polarization curves are presented for the third change
in the potential from negative to positive values.
Results of Investigations
The structures of nitrided layers of the steels (Fig. 1) differ
in character and, in particular, in thickness. The thickness of the
nitrided layer of 15Kh12VNMF steel is about 120 m, and the
thickness of 15Kh11MF steel is 160 m. Their values of the
microhardness at the same distance from the surface of a specimen
differ by no more than 8%, and, for 15Kh12VNMF steel, the values
were systematically higher. In particular, at a distance of 30 m,
its microhardness is 600 GPa, and for 15Kh11MF steel, the
microhardness is 560 GPa. After nitrid-ing, the abrasive resistance
of the surface layers increases 1.52 times. The cavitation
resistance is represented by kinetic curves of the dependence of
the average depth of fracture of the nitrided layers of the steels
on the cavitation time (Fig. 2). For comparison, we show kinetic
curves for 15Kh11MF steel in the as-delivery state and kinetic
curves for 15Kh11MF and 15Kh12VNMF steels after removing layers 0.5
mm thick from their sur-faces. Curve 1 differs from the other
curves by a large time interval with insignificant fracture, i.e.,
by the larger plasticity of the initial specimens of the steels in
comparison with that of specimens of the steels heat-treated during
nitriding.
The cavitation resistances of all specimens were calculated from
the results of the fracture rate, which was deter-mined on
quasilinear segments of the kinetic curves (see Table 1).
Nitriding increases the cavitation resistance of 15Kh11MF steel
specimens 3.6 times and the cavitation resistance of 15Kh12VNMF
specimens 8.1 times in comparison with those of nonnitrided
specimens, though there is a much smaller increase against the
cavitation resistance of the standard.
Fig. 1. Structures of 15Kh12VNMF (a) and 15Kh11MF (b) steels
after nitriding.
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CAVITATION RESISTANCE OF MODIFIED SURFACE LAYERS OF ALLOYED
STEELS 99
Fig. 2. Dependences of the average depth of fracture (h) of
15Kh11MF (13) and 15Kh12VNMF (4, 5) steel specimens on the time of
cavitation tests: (1) steel in the as-delivered state; (2, 4) after
nitriding; (3, 5) without a nitrided layer.
Fig. 3. Polarization curves of nitrided 15Kh11MF (1) and
15Kh12VNMF (2) steels in a 3% NaCl solution.
The corrosion properties of the nitrided steels are described by
polarization curves (Fig. 3). The values of the potential are given
relative to the normal hydrogen electrode.
In a 3% NaCl solution, the intensive dissolution of 15Kh11MF
steel begins at 0.2 V, and 15Kh12VNMF steel begins to dissolve
intensively at 0.1 V with an insignificant interval of the passive
state. Stationary potentials measured after holding in a corrosive
medium for 600 sec are equal to 0.34 and 0.33 V, respectively,
which
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100 V. H. MARYNIN
confirms the low corrosion resistances of the steels. To enhance
their corrosion and erosion resistances, we used coatings of
several layers of titanium and titanium nitride. The titanium
coatings were obtained at a residual
pressure in the vacuum chamber of about 8105 Pa (Fig. 4). As we
can see, under cavitation, the stationary potential, corrosion
current density, and erosion rate change
with increase in the microhardness. Their values pass through an
extremum, which was recorded for a micro-hardness of 1.7 GPa.
Characteristics of TiN-type coatings were presented earlier [3,
57]. It was established that the micro-hardness and erosion rate of
the coatings under cavitation are maximum if the nitrogen pressure
in the vacuum chamber during their formation ranges from 0.03 to
0.05 Pa, and are minimum at a pressure of 0.4 to 0.6 Pa, which
corresponds to the formation of coatings of stoichiometric
composition. Using a model of fracture [8], one can determine the
relationship between the degree of erosion and other
characteristics of the mechanical
properties of coatings by the relation Zc = CW 0.75H 0.5 , where
C is the cavitation power, which is a con-stant under fixed
conditions of the experiment, W is the fracture toughness, and H is
the hardness of the
Fig. 4. Dependences of the stationary potential (1), corrosion
current density (2, 3), and erosion rate (5, 6) on the
microhardness of coatings for a current intensity of 65 A (2, 3, 5,
6) and 135 A (4).
Table 1
The number of the specimen corresponds to the number of the
curve in Fig. 2
Coatings Characteristic
0 (standard) 1 2 3 4 5 Ti TiN Cavitation resis-
tance, Zc 1 0.3 1.1 0.33 2.0 0.24 0.10.4 from 1.1
to >10
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CAVITATION RESISTANCE OF MODIFIED SURFACE LAYERS OF ALLOYED
STEELS 101
material. The corrosion of coatings in media that simulate
operating conditions of elements of power equipment was
investigated. We established that, in a 3% NaCl solution, at 385 K,
pits appear on the steel already after 120 h with subsequent
cracking, whereas titanium-based coatings remain undamaged in tests
for 4000 h in both a 22% NaCl solution and a 28% NaOH solution.
Parameters of the process of deposition of coatings, which
in-creases the corrosion-fatigue resistance and erosion resistance
by more than half, were also determined.
To check the refractoriness of the steelcoatings system and the
adhesion of the coatings, we performed thermocycling with heating
of specimens to a temperature of 543 K and cooling to 293 K in a 3%
aqueous solution of NaCl. Disk-shaped specimens 18 mm in diameter
with coatings deposited on one side and strip-shaped specimens 60
mm long and 7 mm wide made of a stainless steel and bent by an
angle of 180 after deposition of coatings were in-vestigated. It
was established that, after the tests, the specimens were free from
cracks and delaminations, which con-firms the serviceability of the
system consisting of a nitrided steel with combined
titaniumtitanium nitride coatings on the surface.
CONCLUSION
Due to nitriding with subsequent deposition of combined titanium
and titanium nitride coatings on the sur-face, the cavitation and
corrosion resistances of the system have been increased.
REFERENCES
1. V. H. Marynin, Cavitation resistance of cold-worked steels
and copper, Fiz.-Khim. Mekh. Mater., 43, No. 3, 8992 (2007). 2. V.
I. Kovalenko and V. H. Marynin, Equipment for the investigation of
erosion of coatings under microimpact action, Vopr.
Atomn. Nauki Tekh., Ser. Fiz. Radiats. Povrezhd. Radiats.
Materialoved., Issue 5(71), 8385 (1998). 3. V. G. Marinin, Erosion
of PVD coatings under the action of cavitation and vapor-water
condensate, in: Proc. of the Conf. ICVTE-5 [in Rus-
sian], Kharkov Physicotechnical Institute National Scientific
Center, Kontrast Publishing Center, Kharkov (2002), pp. 177180. 4.
V. H. Marynin, Resistance of tungstenrhenium alloy under the action
of impacts, Fiz.-Khim. Mekh. Mater., 41, No. 4, 121123
(2005). 5. V. H. Marynin and V. I. Kovalenko, Development of an
experimentalmanufacturing technology and equipment for the
corrosion
protection of turbine blades operating in the zone of two-phase
flows, in: Collection of Works Problems of Lives and Safety of
Op-eration of Structures, Constructions, and Machines [in
Ukrainian], Paton Institute of Electrowelding, ESSE company, Kiev
(2006), pp. 300305.
6. V. G. Marinin, Protective coatings for increasing the service
life of blades of the phase transition zone of steam turbines of
thermal and nuclear power plants, in: Collection of Scientific
Works of the Sevastopol National University of Nuclear Energy and
Industry [in Russian], Issue 15, Sevastopol National University of
Nuclear Energy and Industry, Sevastopol (2005), pp. 136142.
7. G. N. Kartmazov, V. I. Kovalenko, V. V. Kunchenko, and V. G.
Marinin, Investigation of erosion of titanium nitride coatings
un-der the action of cavitation and airabrasive flow, Vopr. Atom.
Nauki Tekhn., Ser. Fiz. Radiats. Povrezhd. Radiats. Materialoved.,
Issue 5(71), 7174 (1998).
8. A. G. Evans and T. R. Wilshau, Quasi-static solid particle
damage in brittle solids. 1. Observation, analysis and
implications, Acta Met., 24, 936956 (1976).
AbstractMethods of InvestigationsResults of
InvestigationsConclusionReferences