Cavitation erosion investigations on thermal spray coatings
VASILE COJOCARU, DOINA FRUNZAVERDE, CONSTANTIN VIOREL CAMPIAN,
GABRIELA MARGINEAN, RELU CIUBOTARIU, ANA-MARIA PITTNER
Faculty of Engineering
“Eftimie Murgu” University of Resita
No. 1-4, P-ta Traian Vuia, 320085, Resita
ROMANIA
[email protected], [email protected], [email protected],
[email protected], [email protected], [email protected],
http://www.uem.ro
Abstract: The cavitation erosion is a major problem for the hydro turbine components. This phenomenon
which consists in the progressive loss of material from a solid surface, affects the runner blades and adjacent
areas of the runner. Usually, the repairs of affected areas are made by welding. In order to increase the
cavitation resistance, new repair methods are tested. The goal of this study was to investigate the cavitation
resistance of thermal spray coatings. Two laboratory samples were realized by thermal spraying of wolfram
carbide powder onto martensitic stainless steel substrates. The cavitation investigations were made using the
ultrasonic method. Metallographic investigations and hardness results are also presented.
Key-Words: Cavitation, Erosion, Thermal Spray Coatings, Wolfram Carbide, Martensitic Stainless Steel
1 Introduction The cavitation phenomenon consists in formation
and collapse of vapor bubbles in a fluid due to
decreasing of local pressure under the equilibrium
vapor pressure of water [1]. The effects of cavitation
are: noises, vibrations and cavitation erosion of
runner blades and adjacent areas.
Cavitation erosion affects the components of
mechanical systems which work in liquid
environment. The biggest economic impact is found
in hydraulic turbines, were runner blades and
adjacent areas of the runner are submitted to
cavitation.
Up to now the best results regarding repair
techniques applied to turbine runner blades were
obtained by overlay welding of cold hardening
austenitic stainless steels [2,3,5
]. The main problems
in case of repair welding in situ are connected to the
residual stresses and to the important structural
modifications of the base and filler materials, which
are appearing during the welding process. These
effects, especially when extensive welding is
applied, can lead to the damage of the repaired
component during following operation.
In order to enhance the lifetime of the turbine,
the interval between two welding repairs must be
extended. To increase the cavitation erosion
resistance, new repair methods are tested [4]:
reinforced epoxy coatings and thermal sprayed
coatings.
Thermal spraying is a process by which finely
divided metallic or nonmetallic materials, in molten
or semi-molten conditions are deposited to a surface.
Different thermal spray processes are applied:
combustion powder flame spray, combustion wire
flame spray, wire arc spray, plasma spray, high
velocity oxyfuel (HVOF).
2 The research program The research [
6] was carried out on two samples
(table 2) realized by HVOF thermal sparying of
WC-CoCr powder on a martensitic stainless steel
base material (0.03%C, 12.6%Cr, 3.63%Ni). The
martensitic stainless steles are used in the
manufacturing of the turbines rotor blades. WC-
CoCr-based high velocity oxy fuel (HVOF) coatings
are used for components which are exposed to
severe erosion or abrasion
A liquid penetrant inspection was applied on the
base material, before thermal spray. After this
inspection, made in accordance with ISO 4987, the
samples did not present any cracks.
Table 1
Sample Base material HVOF coatings
1 Martensitic
stainless steel
type 1.4313
WC-CoCr
micropowder
2 WC-CoCr
nanopowder
Latest Trends on Engineering Mechanics, Structures, Engineering Geology
ISSN: 1792-4294 177 ISBN: 978-960-474-203-5
Samples were subjected to the next laboratory
investigations:
- Chemical analysis;
- Metallographic investigations;
- Microhardness tests;
- Cavitation erosion tests.
3 Chemical analyses The chemical composition was determined using a
laboratory spectrometer with optical emission.
3.1 Base material The chemical composition of the base material is
presented in table 2.
Table 2
Martensitic stainless steel 1.4313. Chemical
composition [%]
C Si Mn Cr Ni Mo P S 0,03 0,46 0,71 12,6 3,63 0,53 0,04 0,01
From the chemical composition were calculated the
Creq and Nieq equivalents. Using these equivalents
the base material was positioned in the Schaefler
diagram (figure 1).
bioreq NSMCCr %5.0%5.1%% ×+×++= (1)
nieq MCNNi %5.0%30% ×+×+= (2)
955.13=eqCr
035.5=eqNi
Fig.1 Schaeffler diagram
Figure 1 shows that the base material is a soft
martensitic stainless steel (containing up to 10%
ferrite). This type of material is used in the
manufacturing of the rotor blades at Kaplan
turbines.
3.2 Thermal sprayed material The chemical composition of the thermal sprayed
WC-10Co4Cr powder is presented in table 3.
Table 3
WC-CoCr Chemical composition [%]
WC Co Cr
86 10 4
4 Metallographic investigations The metallographic investigations were carried out
using a light microscope equipped with digital
camera and image processing system.
Figure 2 presents the structure of layers obtained
by thermal spraying of micro powders. The total
thickness of these layers is about 140 µm.
1 (200x)
2 (500x)
Fig.2 Sample 1. Base material: martensitic
stainless steel. Layer: WC-CoCr (micropowder).
Cross section
Latest Trends on Engineering Mechanics, Structures, Engineering Geology
ISSN: 1792-4294 178 ISBN: 978-960-474-203-5
1 (200x)
2 (500x)
Fig.3 Sample 2. Base material: martensitic stainless
steel. Layer: WC-CoCr (nanopowder). Cross
section
Figure 3 presents the structure of layers obtained
by thermal spraying of WC-10Co4Cr nanopowders.
In this case, the total thickness of layers is about 110
µm.
The layers applied by thermal spraying are
generally limited to a maximum thickness of 0.5
mm. For this reason the cavitation repair of eroded
areas in hydro turbine can not be done only by
thermal spraying. Thermal sprayed layers are
applied over previously welded layers. It is expected
that these layers have a high resistance to cavitation
erosion.
The micrographs show that the layers obtained
by spraying of nano powder (figure 3) are less
porous and have a lower oxide content than the
layers obtained by spraying micro powders (figure
2). The structure of these layers shows also that is
unlikely to have a good resistance to cavitational
erosion. The main cause is the porosity of the
sprayed powder. This porosity facilitates the
appearance of cavitation erosion craters.
5 Hardness For the two samples microhardness tests were made
on the layers deposited by thermal spraying. The
average values were:
Sample1 (micro powder) - HV0,3 1069;
Sample 2 (nano powder) - HV0,3 1009.
6 Cavitation For the cavitation erosion tests was used the
ultrasonic method. The experimental installation use
a piezoelectric device to produce a high-frequency
vibration (20 kHz) in a test piece immersed in water
(figure 4). The variation of mass was measured
using a precision balance.
Fig.4 Ultrasonic cavitation method
Figure 5 shows the evolution of eroded mass on
the samples made by thermal spraying. In both cases
it was found that in the first minutes of testing, the
superficial layer of the nanopowders was pierced by
the micro jets of water. After breakthrough of
nanopowders layers, began the erosion of base
material. The erosion rate of this material was about
1 mg / 10 minutes.
Fig.5 Weight loss variation on thermal sprayed
samples
Latest Trends on Engineering Mechanics, Structures, Engineering Geology
ISSN: 1792-4294 179 ISBN: 978-960-474-203-5
For comparative analysis, figure 6 present the
cavitational erosion behavior of a austenitic stainless
steel (0.24% C, 16.24% Cr and 12.37% Ni). This
material is used for welding repairs of areas affected
by cavitational erosion in hydraulic turbines.
Fig.6 Weight loss variation on a welded sample.
Base material: martensitic stainless steel, Filler
material: austenitic stainless steel
Large differences are observed between the rate
of cavitation of austenitic stainless steel welded and
the rate of cavitation of the layers obtained by
thermal spraying. In 75 minutes of cavitation the
total weight loss for sample 1 was about 0.018
grams and for sample 2 about 0.013 grams. In 120
minutes of cavitation the total loss for welded
sample was about 0.0008 grams.
The losses of material from thermal sprayed
coatings showed that these samples do not resist to
cavitational erosion.
Analyzing the samples under a electronic
microscope it was observed that the thermal sprayed
layers began to be removed from the base material
(Figure 7 and 8).
Fig.7 Sample 1 surface after cavitation
Fig.8 Sample 2 surface after cavitation
7 Conclusion The coatings obtained by thermal spraying, used
with success for resistance against erosion, didn’t
lead to acceptable results in case of cavitation.
Aspects such as insufficient adhesion of the
deposited layer to the substrate and porosity
determined an unacceptable cavitation behavior.
Further research will be made on remolten thermal
sprayed coatings.
References:
[1] Anton I., Cavitatia, vol. I+II, Ed. Tehnica
Bucharest, 1983;
[2] Bordesau I., Eroziunea cavitationala a
materialelor, Ed. Politehnica Timisoara, 2006;
[3] Hart D., and Whale D., A review of cavitation-
erosion resistant weld surfacing alloys for
hydroturbines, Eutectic Australia Pty. Ltd.,
Sydney, 2007;
[4] Boy, J., and others, Cavitation- and Erosion-
Resistant Thermal Spray Coatings, US Army
Corps of Engineers, Construction Engineering
Research Laboratories, USACERL, Technical
Report 97/118, July 1997;
[5] Frunzaverde D., Campian V., Nedelcu D.,
Gillich G.R., Marginean G. Failure Analysis of a
Kaplan Turbine Runner Blade by
Metallographic and Numerical Methods,
Proceedings of the 7th WSEAS International
Conference on FLUID MECHANICS (FLUIDS
'10), University of Cambridge, UK, February 23-
25, 2010, WSEAS Press, pp. 60-67;
[6] Campian V. and others Optimization of the
repair technology of runner blade's
anticavitational lips on Iron Gates I hydropower
plant, “Eftimie Murgu” University of Resita,
CCHAPT, Internal research report U-09-400-
287, Resita, 2009.
Latest Trends on Engineering Mechanics, Structures, Engineering Geology
ISSN: 1792-4294 180 ISBN: 978-960-474-203-5