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: kartmazov@kipt.kharkov.ua.

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

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.

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

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

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.

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AbstractMethods of InvestigationsResults of InvestigationsConclusionReferences