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

v.cojocaru@uem.ro, d.frunzaverde@uem.ro, v.campian@uem.ro,

gabriela.marginean@fh-gelsenkirchen.de, r.ciubotariu@uem.ro, am.pittner@uem.ro,

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