381
Wear, 151 (1991) 381-390
The effect of erodent
erosion of metals*
Markus
Liebhard
particle characteristics
on the
and Alan Levy
Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 (U.SA)
(Received
July 21, 1991)
Abstract
The effects of several particle variables on the erosivity of particle streams were determined.
They included shape, mass, size impact velocity and feed rate. Tests were carried out at
room temperature
on 1018 steel using spherical glass beads and angular SIC, neither of
which shattered
upon impact. The ranges of the test variables, particularly
the particle
velocities, used in the investigation
were considerably
different from those reported in the
literature.
It was determined
that the particle size effect was different for spherical and
angular particfes. The angular particles were much more erosive than the spherical particles.
Lower feed rates caused more mass loss than higher feed rates when tests with large
differences
in the feed rate were compared.
1. Introduction
An investigation to determine the sensitivity of the erosion rate of ductile metal
alloy to variations in particle size or feed rate at particle velocities of 60 m s-’ or
less has been conducted.
It had been determined
in earlier work [l] that at high
particle velocities, 240 m s-i, a decrease in particle size below 100 Frn resulted in a
decrease in the erosion rate, while there was no effect of particle size on erosion for
particles from 100 to 200 pm diameter. That work was limited to a relatively small
range of particle sizes and other test conditions.
The reason for the decrease in
material removed by the smaller particles had been ascribed to lower impact stresses
resulting from lower particle kinetic energy. While a given mass of erodent of small
particle size contains a larger number of particles than the same mass of erodent of
larger particle size, it appears that the resulting increased number of particle impacts
does not compensate for the smaller kinetic energy of each particle,
Several factors were found to influence the erosion of ductile metals by larger
particles of different shapes in the current work. They included the increasing kinetic
energy of the Iarger particles, the decreasing number of particles for a given total
erodent weight, the decreased amount of physical interference
between particles at
lower feed rates and the decreased ability of round particles as compared to angular
particles to penetrate and deform the surface of the specimens. This work examined
the erosion behavior of an alloy as affected by changes in the erodent particle size
of two different erodents at two different particle velocities and at three different feed
rates.
*Paper presented
at the International
U.S.A., April ‘7-11, 1991.
0043-1648/91/$3.50
Conference
on Wear of Materials,
0 1991 -
Orlando,
FL,
Elsevier Sequoia, Lausanne
382
2. Test conditions
The erosion
tests were carried
out on 1018 steel at room temperature
in the
Lawrence Berkeley Laboratory
(LBL) nozzle erosion tester described in detail previously
[Z]. Spherical
glass beads of four different
diameter
ranges between
53 and 600 pm
and angular SIC of nine different
diameter
ranges between
44 and 991 pm were the
erodents.
The particle velocities
were 20 and 60 m s-l as determined
by a computer
model [3]. All the tests were carried out at an impact angle of 30”. The feed rate
The total mass of erodent
used to achieve
was varied between
0.6 and 6 g min-‘.
steady state erosion,
as determined
in calibration
tests, was 100 g for the Sic, 200 g
for the glass beads in the higher particle velocity tests and 400 g for the glass beads
in the lower particle
velocity tests. Mass loss was determined
using a balance
that
measured
to 0.1+ 0.3 mg.
3. Results
and discussion
3.1. Reproducibility
tests
The results
of a spherical
particle
reproducibility
test are listed in Table 1.
Reproducibility
test results using angular particles are listed in Table 2. The standard
deviations
obtained were deemed acceptable
for the experiments
reported
herein. The
source used to accelerate
the
variable
air pressure
from the “shop air” pressure
particles
was found to be the main reason for the scattering
of the results. A water
manometer
was used to measure
and adjust the air pressure
during the tests. The
higher and lower particle velocities
caused by the pressure
variation
are responsible
for the differences
in the resulting
mass losses because
of the dependence
of mass
loss on particle velocity. The reproducibility
of the basic blast-type
erosion tester is
considered
acceptable
for the types of experiments
performed
using [4].
3.2. Spherical erodent
The mass losses for both the high and low velocity tests
are listed in Table 3. While the mass losses from the 60 m s-l
TABLE
1
Spherical
particle
Specimen
Mass loss (mg)
Average
Standard
Test
value
deviation
reproducibility
test
1
2.7
2
3.3
2.96 mg
0.24 mg (8%)
conditions
Material
Erodent
V
T
a
Feed
Mass
using the glass beads
tests were measurable
rate
of erodent
1018 steel
250-355
pm
60 m s-’
25 “C
30”
6 g min-’
200 g
glass
beads
3
2.8
4
2.9
5
3.1
383
TABLE
2
Angular
particle
reproducibility
test
20 m SK’
Specimen
Mass loss (mg)
1
1.3
Average value
Standard deviation
1.58 mg
0.27 mg (17%)
2
1.6
3
1.4
4
1.6
5
2.0
2
27.9
3
24.1
4
24.5
5
30.9
60 m SK’
Specimen
Mass loss (mg)
1
31.1
Average value
Standard deviation
27.7 mg
3.4 mg (12%)
Test conditions
Material
Erodent
V
T
(Y
rate
1018 steel
250-300 Frn Sic
20, 60 m s-r
25 “C
30”
2.5
min-’
384
0
100
200
Particle
Fig. 1. Effect of particle
300
Size
size of spherical
Partlcle we
glass beads at V=20 m s-l on metal wastage.
[pm]
Fig. 2. Kinetic energy of glass beads of different
Ekin=
$ = volume
600
500
400
(pm)
particle
X density X v2 = (diameter/2)
2
sizes.
X $X
density X vz
2
the variation of the kinetic energy with the third power of the particle size can be
seen. A similar cubic curve can be found between the mass loss and the particle size
up to 300 pm in Fig. 1. The increasing number of particles per second travelling
toward the specimen with decreasing particle diameter at a given rate is shown in
Fig. 3. The calculation
385
Glass beads, density
2.250 g/cm3
_
SIC particles,
3.217 g/cm3
I
denslty
Glass
beads:
. Feed rate 6 g/mln
o Feed rate 0 6 g/mln
0 Feed rate 6 g/mln
A Feed rate 2.5 g/mln
2
Feed rate 0 6 g/min
Particle size [pm]
Fig. 3. Number of particles striking surface per second for different particle sizes.
particles
p=
seconds
feed rate
particle mass per second
=
mass per particle
(diameter/2)3 X 7X$X density
shows the dependence
of the number of particles on the negative third power of the
particle size. This factor appeared to have, at most, a minor effect on the erosion
behavior for the smaller particles. This might be explained by the fact that the number
of particles actually striking the surface does not increase in the same way as the
number of particles travelling toward the specimen. This is due to a shielding effect
effect provided by the rebounding particles [S]. It seems that the decreasing kinetic
energy dominates the erosion behavior and the increasing number of particles does
not result in higher mass loss as the particle size gets smaller.
At particle sizes greater than 300 pm another factor became dominant in controlling
the mass loss to make it decrease as dramatically as it did. The decrease in mass loss
above this particle size might, in part, be due to the decrease in the number of
particles striking the surface. However, the major factor relates to the particle’s ability
to penetrate the target surface. While the larger diameter glass beads had more mass
and hence more kinetic energy, the particle diameter became large enough to markedly
decrease the ability of the spheres to penetrate the target surface and cause the severe
plastic deformation, i.e. platelet formation [6], required for effective removal of material.
At 60 m s-l there was a considerably
higher mass loss for the 300 pm glass
beads that impacted the surface at the feed rate of 0.6 g min-’ compared with 6 g
min-‘. This indicates that there was particle-to-particle
interference
at the higher
solids loading that reduced the effectiveness of the particles to erode the surface. The
primary mode of this interference
was probably particles rebounding
up from the
surface deflecting incoming particles in the downward-moving
stream.
3.3. Angular erodent
The mass losses in the erosion tests carried out with angular Sic are listed in
Table 4. Figure 4 shows the mass loss US. particle size curve at a particle velocity of
TABLE 4
Mass Ioss by angular particles
Particle
(fim)
size
Feed rate
(g min-‘)
44- 62
149-177
250-300
250-300
250-300
30&355
425-495
495-600
600-701
701-850
85Ck991
0
Mass loss (mg)
2.5
2.5
2.5
6.0
0.6
2.5
2.5
2.5
2.5
2.5
2.5
200
400
Particle
Fig. 4. Effect of particle
600
Size
600
20 m SK’
100 g
60 m s-l
100 g
0.6
1.9
1.6
1.6
2.0
1.5
1.5
2.0
2.0
1.6
1.5
9.6
22.5
27.7
28.0
32.7
27.0
37.0
42.4
45.9
49.5
67.5
1000
bm)
size of angular SC at V=20 m s-l on metal wastage.
20 m s-‘. The shape of the curve is the same as that determined over a much shorter
range of particle sizes at a much higher velocity for a much stronger alloy by Tily
[l]. The steep slope of the curve at small particle diameters can be interpreted
as
the same steep increase in particle kinetic energy with increasing particle size as was
found in the tests using spherical particles (see Figs. 1 and 2). The more or less
constant mass loss with increasing particle diameter above 200 pm particle size is
probably due to a combination
of the relation between four characteristics
of the
particle stream that appear to influence mass loss, i.e. (a) the particle size, (b) the
number of particles striking the surface, (c) their kinetic energy and (d) the interference
between incoming and rebounding particles.
387
Metallographic observation of particles of various sizes indicates that the sharpness
of the Sic particle edges does not change with increasing particle size and therefore
there is no decrease in their ability to penetrate
and plastically deform the metal
surface as occurred with the spherical particles. Thus the curve in Fig. 4 remains flat
over a wide range of particle sizes. Note that only the largest difference in particle
loading at the 250 pm angular particle size resulted in an increased metal loss, as
occurred with the spherical particles (see Fig. 1). There was no difference in metal
loss between the 2.5 and 6.0 g min-’ flow rates.
The results of the 60 m s-l velocity tests are plotted in Fig. 5. A relatively steady
increase of mass loss with increasing particle diameter occurred. The four particle
stream characteristics
mentioned
earlier that are thought to influence the mass loss
are combined in a manner that caused a nearly linear relation between particle size
and mass loss. The change in slope in the range 150-300 pm may relate to the
flattening of the curve that Tilly found [l]. Why the marked change in the shape of
the curves for the 20 and 60 m s-l velocities occurred is not known.
3.4. Particle shape
The effect of the shape of the erodent particles on the mass loss is shown in
Table 5 for two size ranges of particles. It can be seen that the shape of particles is
a major factor in establishing their erosivity [S]. The difference is nearly a factor of
10 for the smaller particles and up to almost 40 times for the larger particles. There
is very little difference between a feed rate of 2.5 and 6 g min-‘, so the last two
rows of Table 5 can be directly compared. The shape factor is not grossly affected
by differences in the feed rate, comparing 6.0 and 0.6 g min-’ rates, but there is the
tendency for the lower flow rate to cause larger mass loss, as is discussed below.
3.5. Solids loading
The results of the variation of
the surface per unit time, of feed
Table 3 for glass beads and in Table
in Figs. 1, 4 and 5. In general, the
solids loading, i.e. the weight of particles striking
rate for the 250-355 pm particles are listed in
4 for Sic particles. The points can also be found
mass loss at the lowest solids loading was higher
0
Fig. 5. Effect of particle size of angular Sic at V=60 m s-r on metal wastage.
388
TABLE 5
Effect of particle
Particle size
(pm)
shape
Feed rate
(g min-‘)
Mass loss (mg)
20 m s-t
250-355
2.50-355
495-600
495-600
6.0
0.6
6.0
2.5
TABLE
6
Average
distance
Particle
(Crm)
size
between
particles
60 m SK’
Spherical
Angular
Spherical
Angular
0.2
0.2
0.1
_
1.6
2.0
2.0
3.0
4.5
1.2
-
28.0
32.7
_
42.4
(one dimension)
Feed rate
(g min-‘)
Velocity
(m s-t)
Distance
(mm)
6
0.6
6
0.6
20
20
60
60
3
29
20
196
6
2.5
0.6
6
2.5
0.6
20
20
20
60
60
60
7
17
70
21
50
210
Glass beads
212-250
212-250
250-355
25&355
Sic
250-300
250-300
25&300
25&300
250-300
250-300
at the higher solids loadings. The average distance between two particles, which
depends on the particle size, the feed rate and the density of the particle material,
is listed in Table 6. As long as the distance between the particles is large enough, a
low number of collisions between particles occurs. Most particles are able to strike
the surface and leave the area before the next particle strikes the same area. With
decreasing distance between particles, more and more rebound particles collide with
incoming particles and slow them down and/or change their trajectories. Both factors
decrease the force or angle with which they strike the surface and, in some instances,
prevent the incoming particles from even striking the target.
The effect of flow rate is shown for the 60 m s-l spherical particles in Table 3
and Fig. 1. The difference in the distance between the particles for solids loadings
of 0.6 and 6.0 g min-’ was one order of magnitude (see Table 6). There was a 50%
greater metal loss at the lower solids loading. The greater distance between particles
prevented much of the interference
between particles from occurring.
than
389
The results from the angular Sic particle tests listed in Table 4 show that there
was relatively little difference in mass loss as a function of solids loading, with only
a small increase in loss for the smallest solids loading, in spite of the large difference
in particle separation listed in Table 6. Why there is such a large difference between
the solids loading effect for spherical particles and for angular particles can only be
speculated. It is probably related to the greatly increased erosivity of the angular
particles (compare Tables 3 and 4) which reduces the effect of interference
due to
particle separation.
4. Conclusions
(1) There was a major difference in the erosivity of the spherical and angular
particles as a function of particle size.
(2) The erosivity of spherical particles increased with particle size to a peak and
then decrease at still larger particle sizes.
(3) The angular particle erosivity increased with particle size to a level which
became more or less constant with size at lower velocities, but increased continuously
at higher velocities.
(4) Lower solids loading caused more mass than higher solids loading for both
spherical and angular particles when the feed rate difference was large.
(5) The shape of particles was a major factor in determining their erosivity, angular
particles generally being an order of magnitude more erosive than spherical particles.
(6) There appeared to be combinations of four particle stream characteristics that
determined
the erosivity of a particle stream for a given particle shape. These were
(a) particle size, (b) the number of particles striking the surface per unit time, (c)
particle kinetic energy, i.e. mass and velocity, and (d) the interference between particles
striking and rebounding
from the surface.
Acknowledgment
This research was sponsored by the U.S. Department
of Energy, Fossil Energy
ARKI’D Materials Program, DOE/FE AA 15 10 10 0, Work Breakdown Element
LBL3.
References
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Wear,
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Wear, 108 (1) (1986) 1-22.
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and I. Finnie,
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