artigo sobre cavitação

1. Wear 267 (2009) 1954–1960
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Wear
journal homepage: www.elsevier.com/locate/wear
Cavitation erosion resistance of stellite alloy weld overlays
Shuji Hattori∗
, Norihiro Mikami
Graduate School of Engineering, University of Fukui, 9-1 Bunkyo 3-chome, Fukui 910-8507, Japan
a r t i c l e i n f o
Article history:
Received 9 September 2008
Received in revised form 17 May 2009
Accepted 31 May 2009
Available online 11 June 2009
Keywords:
Erosion
Cavitation erosion
Vibratory method
Cavitating liquid jet method
Stellite
Erosion resistance
a b s t r a c t
Stellite alloys have excellent cavitation erosion resistance and are often used for liquid machinery, but
the erosion properties of various stellite alloys have not been evaluated by a standard method. In this
study, we evaluate the erosion resistance for various stellite alloy weld overlays of ST6 and ST21 in a
vibrating method and in a cavitating liquid jet method. The grain size of the Co matrix affects the cavitation
erosion resistance of stellite alloy weld overlays of ST6. The erosion rate of the maximum rate stage of
stellite weld overlay alloys of ST6-1, ST6-2 and ST6-3 were found to be about 1/13 to 1/7 times that of
SUS304. Moreover, we clarified the cavitation erosion mechanism of SUS304 and ST6 by scanning electron
microscopy. Furthermore, by comparing the erosion behavior in a cavitating liquid jet method with that
in a vibratory method, it was found that the erosion rate of the cavitating jet method and the vibratory
method have a good correlation.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Cavitation is defined as the formation and subsequent collapse,
within a liquid, of cavities or bubbles that contain vapor or gas, or
both, in the ASTM G32-03 standard [1]. When the machine com-
ponents were exposed to cavitation, erosion occurs. The erosion is
the progressive loss of original material from a solid surface due
to continued exposure to cavitation [1]. Preece [2], Hammitt [3],
Karimi and Martin [4], and Lecoffre [5] reviewed the material effect
of cavitation erosion, test methods and degradation mechanisms.
One of the present authors constructed the erosion database of car-
bon steels, alloy steels, aluminum alloys, copper alloys, titanium
alloys and so on, and analyzed the erosion data of carbon steels [6].
Recently, cavitation erosion of fluid machinery components has
become more serious problem. Stellite alloys (Co-base alloys) with
high erosion resistance are often used for turbine blades, valves and
the like. Gould [7] carried out a cavitation erosion test with a vibra-
tory method for five kinds of commercially available stellite alloys,
and reported that the erosion resistance of stellite 6B is relatively
independent of hardness or grain size. But the chemical composi-
tion of the five alloys was very similar, so that the comprehensive
material differences were not clear. Antony and Silence [8] used Co-
base cast alloys with a carbon content widely ranging from 0.12% to
1.35% and carried out cavitation erosion tests. They reported that
the erosion resistance increased with the carbon content until 0.2%,
and kept constant between 0.3% and 1.4%. Heathcock et al. [9] used
∗ Corresponding author. Tel.: +81 776 27 8546; fax: +81 776 27 8546.
E-mail address: hattori@mech.u-fukui.ac.jp (S. Hattori).
six kinds of stellite cast alloys and found that the erosion resistance
of a stellite alloy with a carbon content of 2.6% increases twice as
much as that of stellite 6 because of the fine carbide microstructure
in the alloy. However, few studies have been carried out for coatings
such as weld overlay. Moreover, the cavitation erosion mechanism
of stellite alloys has not been clarified yet.
In this study, cavitation erosion tests were carried out on stel-
lite alloy weld overlays which are often used for components of
fluid machineries. A vibratory method based on ASTM G32-03 was
employed for the test. The cavitation erosion–time curves in the
vibratory method are discussed. Three base materials (SUS304,
SUS316 and S15C) were tested and the erosion characteristics of
the test materials are compared. Simultaneously, the cavitation ero-
sion process was observed with an SEM and a model of the erosion
mechanism was established. The result clarifies the cavitation ero-
sion mechanism of stellite. Cavitating liquid jet tests were carried
out to obtain the cavitation erosion–time curves and the erosion
resistance is compared with that in the vibratory test method.
2. Material and experimental procedures
Tables 1 and 2 show the chemical composition and the mechan-
ical properties of the test materials. The stellite overlays used in this
study are stellite 6 (ST6) and stellite 21 (ST21) of cobalt base alloys.
ST6 is a hardfacing material and has resistance both to corrosion and
erosion. ST6 is often used for the components of fluid machineries.
Since ST21 includes Mo, this alloy has better corrosion resistance
and better thermal shock resistance than ST6, which is used for fluid
machineries operating in seawater, or for components operated at
high temperatures. The substrate of the overlay ST6 was a carbon
0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.wear.2009.05.007

2. S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960 1955
Table 1
Chemical composition of materials (mass%).
Material Co Ni Cr W Mo Fe C Si Mn S P
ST6 Bal. ≤3 28 4 ≤3 1 – – – –
ST21 Bal. 2.5 27 – 5 ≤2 0.25 – – – –
S25C – – – – – Bal. ≤0.30 ≤0.60 ≤1.00 ≤0.04 ≤0.04
SUS316 – 9–13 17–21 – 2–3 Bal. ≤0.03 ≤1.50 ≤1.50 ≤0.04 ≤0.04
SUS304 – 8.21 18.47 – – Bal. 0.071 0.48 1.21 0.022 0.025
Table 2
Mechanical properties of materials.
Material Density [g/m3
] Yield point [MPa] Tensile strength [MPa] Elongation [%]
ST6 8.42 – 920 –
ST21 8.30 – 800 –
S25C 7.80 ≤245 ≤480 ≤19
SUS316 7.87 ≤205 ≤480 ≤33
SUS304 7.98 – 618 62
Fig. 1. SEM photographs of the original surfaces of ST6-1, 2 and 3.
Table 3
Vickers hardness of the materials.
Material HV0.2
ST6-1 451
ST6-2 506
ST6-3 567
ST21 381
S25C 166
SUS316 218
SUS304 209
Fig. 2. MDE curves of ST6.
steel S25C with 0.25% carbon content, whereas the substrate of ST21
was an austenitic stainless steel SUS316. For comparison, stainless
steel SUS304 was used as highly erosion resistant material.
ST6 was overlaid with 3–5 mm in thickness on the S25C sub-
strate with three methods, which were called ST6-1, ST6-2 and
ST6-3. But the detailed methods are kept unknown at present. The
hardness and the microstructure of each weld overlay are listed in
Table 3 and Fig. 1(a)–(c), respectively. The hardness was measured
at a load of 200 gf and for a holding time of 15 s at room temperature
with a Vickers microhardness tester. The hardness of ST6-1 is the
Fig. 3. MDE curve of ST21.

3. 1956 S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960
lowest, and it becomes higher in the order of ST6-2 and ST6-3. How-
ever, all materials are almost twice as hard as SUS304. The surfaces
of all overlay ST6 alloys were etched on the mirror-polished test
specimen for 30–40 s in potassium ferricyanide solution and then
observed by scanning electron microscopy (SEM). Fig. 1 shows that
the surface of the overlay ST6 consists of carbide (black) and Co-
matrix (white). The difference in size of the Co-matrix was observed
for the three materials. By comparing Fig. 1(c) with (a) and (b), the
Co-matrix Fig. 1(c) of was clearly smaller than those of Fig. 1(a)
and (b). By comparing Fig. 1(a) with (b), the Co matrix of Fig. 1(a)
looks smaller than that of (b). But, the cobalt grain regions were
elongated, and some of them joined together. On the other hand,
larger round cobalt regions are observed for ST6-2. Thus, the Co
matrix of ST6-1 with the lowest hardness of the three is relatively
large. The Co matrix of ST6-3 with the highest hardness is relatively
small.
Cavitation erosion tests were carried out by using a vibratory
apparatus as specified in the ASTM standard G32-03 [1]. The test
method was a stationary specimen method. A disk of 16 mm in
diameter made of erosion resistant SUS304 steel was screwed
into the amplifying horn of an oscillator, and the test specimen
was placed in close proximity to the vibrating disk. The distance
between disk and test specimen was 1 mm. The resonance fre-
quency of the oscillator was 19.5 kHz, and the double (peak to
peak) amplitude of the disk was 50 ␮m. After using a vibrating
disk for 10 h, it was replaced by a new one. Deionized water was
used as test liquid and kept at 25 ± 1 ◦C with a temperature control
device.
Fig. 4. SEM photographs of eroded surface of SUS304.

4. S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960 1957
Cavitating liquid jet erosion tests were carried out in a test cham-
ber at a constant upstream pressure of 17.4 MPa (the corresponding
velocity was 185 m/s), at a liquid temperature of 25 ◦C, and at a cav-
itation number of 0.025 (the downstream pressure was 0.44 MPa),
which is specified in the ASTM G134 standard [10]. The stand-
off distance between the nozzle inlet and the test specimen was
10 mm.
3. Experimental results and discussion
3.1. Erosion of ST6 and ST21 weld overlays
Fig. 2 shows the MDE (mean depth of erosion) curves obtained
from the cavitation erosion test with ST6 weld overlays and a ref-
erence material SUS304. The key “ST6 overlay (previous)” indicates
the data points of the ST6 alloy weld overlay which have previ-
ously been obtained in our laboratory. Since the material densities
of ST6 and SUS304 are different, the test results are expressed by
the MDE which is the mass loss divided by the material density and
test area (a circular area of 201 mm2 with a diameter of 16 mm).
The MDE curves of all materials pass through a period where the
erosion rates are very low, followed by a liner increase in a max-
imum rate period. The incubation period has been defined as the
value obtained from the intersection of a straight extension line
of the maximum rate period with the time axis. The incubation
periods of ST6-1 to ST6-3 are about 14–17 h, which are about 4–5
times as long as that of SUS304 (which is about 3 h). The peri-
ods are a little shorter than that of the ST6 overlay (about 20 h).
The numbers in Fig. 2 show the slope of the straight line portion
in the maximum rate stage of the MDE curves. The slopes of the
maximum rate stage are 0.28 ␮m/h for ST6-1, 0.22 ␮m/h for ST6-
2, 0.17 ␮m/h for ST6-3, 0.16 ␮m/h for ST6 overlay, and 2.17 ␮m/h
for SUS304, respectively. The slope of the maximum rate stage of
ST6 overlays is about 1/13 to 1/7 times less than that of SUS304,
and it is therefore seen that ST6 overlays have a good erosion
resistance. Among ST6-1, ST6-2 and ST6-3, ST6-3 has the best ero-
sion resistance and the resistance is getting worse in the order
of ST6-2 and ST6-1. The slope of ST6-3 is about 0.6 times that of
ST6-1.
Fig. 3 shows the MDE curves of ST21, ST6-3, the two substrate
materials (S25C and SUS316) and of a reference material (SUS304).
The incubation period of ST21 is about 18 h, which is 6 times as
long as that of SUS304 with about 3 h. The period is lightly longer
than that of ST6-3 with about 18 h. The slope of the maximum rate
period of ST21 is 0.32 ␮m/h, which is one-seventh that of SUS304
with 2.17 ␮m/h. The slope is 2 times steeper than that of ST6-3 with
0.17 ␮m/h. By comparing the slopes of the maximum rate period
of the overlay stellites with that of the substrate materials, ST6-
3 turns out to be about one-27th that of SUS316 with 4.72 ␮m/h
and ST21 is about one-34th that of S25C with 11.1 ␮m/h. Thus, a
ST21 weld overlay has a better erosion resistance than its substrate
material.
3.2. Cavitation erosion mechanism of SUS304 and ST6
Eroded surfaces of SUS304 and ST6 were observed by scan-
ning electron microscopy (SEM) to clarify the erosion mechanism.
Fig. 4 shows the SEM photographs of SUS304. After 45 min (Fig. 4a),
plastic deformation occurred inside crystal grains by cavitation
and accumulated at grain boundaries. After 3 h (Fig. 4b), crack-like
grooves began to occur in the highly deformed area along a gain
boundary near point A . After 4 h (Fig. 4c), material removal (ero-
sion) occurred in the grain. After 5 h (Fig. 4d), the erosion extended
across a wide area inside the grain.
Fig. 5. Model of erosion process on SUS304 specimen.
Fig. 5 is a schematic model of the erosion on the cross section
A–A based on the observation of the eroded surface. The origi-
nal surface was repeatedly exposed to the collapses of cavitation
bubbles, and therefore plastic deformation of the material surface
occurred by shock waves and micro jets in the bubble collapses
(Fig. 5a). Since bubble collapse repeatedly acted on the deformed
area, the area gradually expanded, and plastic deformation accu-
mulated at crystal grain boundaries as shown in Fig. 5(b), which
corresponds to the SEM photograph after 45 min (Fig. 4a). Since the
material surface was plastically deformed, swelled parts appeared
at the crystal grain boundaries. These swelled parts produced a
step relative to the adjacent grain with less plastic deformation and
caused a high stress concentration, resulting in crack initiation as
in Fig. 5(c). Fig. 5(d) shows that the erosion easily occurred at the
crack initiation site. Fig. 5(c) and (d) corresponds to the SEM pho-
tographs after 3–4 h (Fig. 4(b) and (c)), respectively. Fig. 5(e) shows
the more eroded surface after 5 h (Fig. 4d).
Similarly, the erosion mechanism of ST6 was investigated. Fig. 6
shows the SEM photographs of ST6, which have already been
reported [11]. The Co matrix and the eutectic structure of carbide
consist of a virgin surface in Fig. 6(a). After 5 h (Fig. 6b), plastic
deformation occurred in Co matrix. The white part near the car-
bide shows the eroded Co matrix. Eutectic carbides were removed
from the whole area after 10 h (Fig. 6c), and then the matrix near
the carbide was preferentially eroded (Fig. 6d).
Fig. 7 shows the erosion model of ST6 which was newly estab-
lished in this study on the basis of the observation of Fig. 6. Fig. 7(a)
shows the virgin surface of the ST6 eutectic structure consisting
of the Co matrix and carbide. Fig. 7(b) shows that the Co matrix is
softer than the carbide and plastic deformation occurs in the matrix.
Thus, carbide plays the role of a grain boundary in the erosion mech-
anism of SUS304. Plastic deformation occurs in the matrix, and a
swelled part appears near the carbide. The swelled part near the
carbide has high stress concentration which easily initiates cracks.
Fig. 7(c) shows that the erosion proceeds near the interface between
the Co matrix and carbide. And then the carbides falls off, which is
observed as a white portion after the removal of carbides in Fig. 6(b).
Fig. 7(d) corresponds to the SEM micrograph taken after 10–20 h
(Fig. 6c and d), and shows that carbides fell off preferentially. By
comparing the erosion result of ST6-1, 2 and 3 weld overlays, the
Co matrix of ST6-3 turns out to be smaller than that of ST6-1, 2.
Therefore, the matrix of ST6-3 does not easily cause plastic defor-

5. 1958 S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960
Fig. 6. SEM photographs of eroded surface of ST6.
mation. Amongst the three kinds of weld overlays, ST6-3 has the
best erosion resistance.
3.3. Comparison of the erosion behavior in cavitating liquid jet
method with that in vibratory method
Fig. 8 shows the MDE curves in the cavitating liquid jet method
using the same test materials as in the vibratory method. The MDE
was defined as the mass loss divided by the material density and
eroded area. The eroded area was obtained by a surface profile
meter, which was 26–27 mm2. By comparing Fig. 8 and Fig. 2, the
increasing tendency of the erosion for all materials is relatively
similar, but the behavior of the erosion in the incubation period
is different. For example, by comparing the methods for ST6-3, the
MDE curve for the cavitating liquid jet method gradually increases
until 10 h and reaches 2 ␮m, while the MDE curves in the vibra-
tory method reaches less than 0.2 ␮m at 15 h. Thus, the MDE curves
in the vibratory method increase hardly, and the erosion in the
cavitating liquid jet method progresses gradually in the incubation
period.
Fig. 9(a) shows the surface profile of the eroded surface of ST6-
1 tested after 30 h with the vibratory method. The peripheral area
affected by the edge of the vibrating disk was eroded to about 5 ␮m
depth. The eroded area shows that the specimen was uniformly
eroded by about 3 ␮m in mean depth over the whole area with an
unevenness of about 2 ␮m, except for the peripheral area. The mean
depths of ST6-2 and ST6-3 tested after 30 h are different, but the
eroded surfaces exhibit similar shapes. Fig. 9(b) shows the surface
profile of the eroded surface of ST6-1 after 30 h of testing with the
cavitating liquid jet method. The eroded surface of the cavitating
liquid jet method became W-shaped, because the center position
with a high cavitation bubble population is weakly eroded, while
the area between 1 and 3 mm from the center is strongly eroded.
In the cavitating liquid jet method, the cavitation bubble collapse

6. S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960 1959
Fig. 7. Model of erosion process on ST6 specimen.
Fig. 8. MDE curves for cavitating jet method.
pressure depends on the location. The incubation period finished
in some areas while it was not yet finished in other areas, thus the
erosion proceeded in spite of the apparent incubation period.
Fig. 10 shows the relation between Vickers hardness and erosion
resistance for all specimens. The erosion resistance was defined as
the reciprocal of the slope of an MDE curve in the maximum rate
stage. Because the erosion mechanism of stellite has been clarified
in Fig. 7, and the erosion begins normally in the Co matrix, the ero-
sion resistance should be evaluated in terms of the hardness of the
Co matrix. However, the size of the Co matrix was very small and
it was difficult to measure the hardness of the Co matrix only. The
conventional hardness is important to evaluate the erosion resis-
Fig. 9. Profiles of eroded surface.
Fig. 10. Relation between Vickers hardness and erosion resistance.
Fig. 11. Relation between erosion resistances for vibratory method and for cavitating
liquid jet method.
tance, which is shown in Fig. 10. By comparing the differences in the
weld overlay methods, the erosion resistance in both the vibratory
method and the cavitating liquid jet method decreases in the order
of ST6-3, 2, and 1. By comparing the Vickers hardness of ST6-3 and
ST21, these are 567HV and 381HV, respectively. ST6-3 turns out to
be about 1.5 times harder than ST21. The harder the material is, the
better its erosion resistance.
Fig. 11 shows a comparison of the erosion resistance in the
vibratory method and the cavitating liquid jet method. The hori-
zontal axis shows the erosion resistance for the cavitating liquid jet
method, and the vertical axis shows the erosion resistance for the
vibratory method. By taking the slope of the straight line, the test
time of the vibratory method is about 3.5 times that of the cavitat-
ing liquid jet method. It can be seen that cavitating jet method and
the vibratory method have a good correlation.
4. Conclusions
Cavitation erosion tests were carried out on stellite alloy welds
overlays which are often used for the components of fluid machiner-
ies. Eroded surface, MDE curves and SEM observations of each
material were compared with a reference material. The results of
the vibratory method and the cavitating liquid jet method were
compared and discussed. We come to the following conclusions:
1. The harder the material surfaces of ST6 overlays due to different
weld overlay methods, the better becomes their erosion resis-
tance.
2. The erosion mechanisms of SUS304 and ST6 were clarified by
SEM observations of the surface over the test time, and models
of the erosion mechanisms were established.

7. 1960 S. Hattori, N. Mikami / Wear 267 (2009) 1954–1960
3. In case of the cavitating liquid jet method, the surface profile
eroded to a W-shape, and the erosion rate depends on the eroded
area. The erosion proceeds in spite of the apparent incubation
period.
4. The erosion resistance of stellite weld overlays is correlated with
the hardness of the material.
5. The erosion rates in the vibratory method and the cavitation
liquid jet method have a good correlation.
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