Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Molten Metal Inclusion Sensor Probes
Field of the Invention
This invention is concerned with improvements in or relating to molten
metal inclusion sensor probes, namely sensor probes that are used in apparatus
for
detecting the number, size and size distribution of inclusions in molten
metal, the
apparatus employing what is now known as the ESZ (Electric Sensing Zone)
method.
The invention is also concerned with improvements in or relating to methods of
making molten metal inclusion sensor probes. Such sensor probes are used in
the
quality control of liquid metals, such as aluminium, magnesium and steel, and
are
particularly valuable for this purpose in that they permit rapid on-line
monitoring of
flowing molten metal.
Review of the Field
The production and refining of metals from their ores inevitably results in
what, for convenience in reference, are called herein "inclusions", such as
pre-
cipitated secondary phase particles, drops of slag and gas bubbles, all of
which have
a more or less deleterious effect upon the technical properties of the metals.
An
even greater quantity and variety of inclusions may be found when scrap metal
is
being recycled and refined, either alone or as an addition to virgin metal,
owing to the
presence of various products of oxidation and corrosion, dirt, oils, paint,
etc, on the
scrapped articles. The presence of such inclusions within the resultant rolled
or cast
products is generally undesirable from the point of view of properties such as
fatigue
life, toughness, corrosion, tearing, splitting, surface quality, pinholes,
etc., particularly
when larger inclusions (e.g., dimensions > 15 microns) are present. For
example,
the production of aluminium beverage can bodies is very sensitive to the
presence of
any inclusions within the can walls, whose thickness is of the order of 80
microns;
large inclusions, which can be as large as 60 microns, can cause the metal to
tear
during deep drawing, or the can to perforate when its content is pressurized.
Other
applications in which cleanliness is critical are the production of thin
sheets and
lithographic plates. It is therefore essential to know whether or not the
metal is
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sufficiently "clean" for its intended purpose, and also to show whether or not
the
refining processes employed are producing sufficiently clean metal.
A quantitative measurement method and apparatus for such inclusions,
particularly in molten aluminium, that can be operated on-line has now become
wet!
established in the aluminium industry, and is known as the LiMCA system
(Trademark of Limca Research Inc.); these are described and claimed for
example in
U.S. Patents Nos. 4,555,662, 4,600,880, and 4,763,065. Commercial equipment is
manufactured under license by Bomem, Quebec City, Quebec, Canada. The
application of the method and apparatus to the detection of inclusions during
the
l0 refining and recycling of other metals is under development.
The ESZ method was used prior to its application to molten metals to
measure inclusions in aqueous solutions in what was known as the Couiter
counter,
and relies upon the fact that any inclusion usually is of different
conductivity (usually
much tower) than the highly electrically conductive liquid metal in which it
is
entrained. A measured volume of the molten metal is passed through a sensing
zone consisting of an orifice of specific size (usually 300 microns diameter
for
aluminium) in the wall or bottom of a tube of an electrically insulating
material, usually
be connecting a vacuum to the tube interior, while a constant current is
maintained
through the sensing zone between two electrodes disposed on opposite sides of
the
orifice. As an inclusion particle passes through the orifice the electrical
resistance of
the current path through the orifice changes in proportion to the volume of
the
inclusion, and this change is detected as a voltage pulse between the two
electrodes,
or more usually between finro other electrodes in the current path provided
for this
purpose. The amplitude of each pulse indicates the size of the respective
inclusion,
while the number of pulses indicates the number of inclusions in the sample
volume.
Besides monitoring the quality of liquid metals in terms of the number and
size
distribution of lower conductivity inclusions, the LiMCA system can also be
used for
the detection and analysis of titanium diboride (TiB2) particles that have
been added
to aluminium silicon casting alloys as grain refining agents. Titanium
diboride is
more conductive electrically than molten aluminium and voltage pulses of
opposite
polarity were observed.
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Currently used on-line sensing probes for testing aluminium employ a
sampling tube of electrically-insulating, heat-resistant material that is
lowered into the
metal, the tube forming a chamber into which the molten metal is sucked
through a
sensing zone orifice in or near to its lower end. The usual method employed at
this
time for forming the orifice is to drill a hole of suitable diameter through
the wall of the
tube, and then to heat the inlet opening using an intense micro-flame of
sufficient
temperature to melt the material, whereupon it flows to form a rounded edge
under
the action of the surface energy force that becomes operative. The current-
supplying
electrodes and/or the sensing electrodes may take the form of two rods
disposed one
inside and one outside the tube, or concentric tubes of a suitable conductive
material
applied to the inner and outer walls of the tube. In order for the ESZ method
to
operate successfully it is necessary that the electrical current path pass
entirely
through the electric sensing zone, and there should be no unwanted leakage
between the liquid metal inside and that outside the sampling tube.
Since every particle registers a pulse when passing through the ESZ, and
nonconductive particles of the same size but of different type, e.g. different
density,
give rise to voltage pulses of the same magnitude, it was initially impossible
to
discriminate between different types of inclusions within a melt. In the
aluminium
industry proprietary degassing units generate microbubbles and microdroplets
of salt
in the molten aluminium. These microbubbles and microdroplets interfere with
the
LiMCA probe and cause inaccuracies in its inclusion counts. In practice,
microbubbles are relatively harmless compared to hard solid inclusions, and
one
therefore needs to distinguish one from the other from the metal quality
control point
of view. Better particle discrimination can be obtained by the application of
DSP
technology (Digital Signal Processing), which permits more information to be
extracted from the signals by consideration of other parameters besides pulse
height.
Using the McGili DSP system each pulse can also be characterized by six other
pulse parameters, namely start slope, end slope, time to maximum voltage,
total
signal duration, start time and end time.
Very early on the successful continuous in-line operation of the LiMCA
system was found to depend on a procedure termed "conditioning", which
involves
passing an electric current of 200-300 amperes, compared to the sensing
current of
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about 60 amperes, through the orifice for about 300ms before taking a new
sample
when it is observed that the inflow rate of the molten metal has decreased, or
when
instabilities are observed in the voltage baseline. The application of this
high current
usually is found to correct these problems, it is presumed by removing
particles that
have stuck to the orifice walls and are obstructing the flow of the molten
aluminium
and other particles through the orifice. The mechanism for this conditioning
effect is
a key to LiMCA's successful implementation in melts of aluminium, and probably
also
in melts of other metals, but still needs be clarified. It is believed that
the main
mechanism has been identified and that the new sensing probe structures now
provided and methods for their production renders its implementation even more
effective than hitherto.
Definition of the Invention
It is the principal object of the present invention to provide inclusion
sensor probes for molten metals of new construction.
1 S It is another principal object of the present invention to provide new
methods of making inclusion sensor probes for molten metals.
It is a further principal object to provide such probes, and methods of
making such probes, with sensing orifices of improved profile which
facilitates the
monitoring of the particles passing in the sensing zone, and also permits
prior
determination of the conditioning current required, consisting of a short
pulse of high
current passed through the sensing zone.
In accordance with the present invention there is provided a molten metal
inclusion sensor probe of the type which is immersed in the molten metal and
detects
inclusions therein by the electric sensing zone method, the probe comprising a
sensor probe body of electrically insulating heat resistant material having in
at least a
part of a wall thereof a sensing passage for the flow of molten metal from one
side of
the body to the other, the sensing passage providing therein an electric
sensing zone
and extending about a longitudinal axis;
wherein the sensing passage decreases progressively in flow cross
section area from its entrance to the electric sensing zone; and
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wherein the profile of the sensing passage from its entrance to the electric
sensing zone is selected from, a parabola having the parabola determinant
coincident with the sensing passage longitudinal axis and the focus within the
sensor
probe body and spaced from the sensing passage, and an ellipse having one axis
parallel with the sensing passage longitudinal axis and within the body and an
extension of its other axis passing through the electric sensing zone.
Also in accordance with the invention there is provided a method of
making a molten metal inclusion sensor probe as specified in the immediately
preceding paragraph, the method including the step of forming the sensing
passage
in the wall to the specified profile by a machining operation that will
provide a
passage wall smoothness of predetermined value.
Further in accordance with the invention there is provided a molten metal
inclusion sensor probe of the type which is immersed in the molten metal and
detects
inclusions therein by the electric sensing zone method, the probe comprising a
sensor probe body of electrically insulating heat resistant material having in
at least a
part of a wall thereof a sensing passage for the flow of molten metal from one
side of
the body to the other, the sensing passage providing therein an electric
sensing zone
and extending about a longitudinal axis;
wherein the sensing passage decreases progressively in flow cross
section area from its entrance to the electric sensing zone; and
wherein the passage has been produced by a machining operation to have
a wall surface smoothness of less than 1.016 micrometers (40 microinches).
Description of the Drawings
Molten metal inclusion sensor probes, and methods for the production of
sensing passages therein, which are preferred embodiments of the invention,
will
now be described by way of example with reference to the accompanying
diagrammatic drawings, wherein:
Figure 1 is a schematic representation of prior art apparatus employing the
LiMCA system for the measurement of inclusions in aluminium;
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Figures 2-4 are longitudinal cross-sections through the lower part of a
prior art sampling tube container as used in the apparatus of Figure 2 to show
different prior art profiles of the electric sensing zone sampling passage;
Figure 5 shows to an enlarged scale a sensing passage in the side wall of
a sampling tube wherein in accordance with the invention the sensing passage
has a
parabolic profile;
Figure 6 shows to an enlarged scale a sensing passage in the side wall of
a sampling tube wherein in accordance with the invention the sensing passage
has
an elliptic profile;
Figure 7 is a plot of the electric potential distribution within the
computation field;
Figure 8 is a plot of the electric current density within the field ;
Figure 9 is a plot of the self induced magnetic flux density within the field;
Figure 10 is a plot of the specific electromagnetic force distribution within
the field;
Figure 11 is a plot of the computed metal flow vectors;
Figure 12 shows the axial flow velocities at selected axially spaced points;
Figure 13 shows the radial flow velocities at the same axially spaced
points; and
Figure 14 shows the metal flow vectors when the current is above a
threshold value resulting in a conditioning flow of the metal.
Description of the Preferred Embodiments
A prior art apparatus employing the LiMCA system is illustrated very
schematically in Figure I. For example, a trough 10 conveys the molten metal
12 to
be tested from the furnace in which it has been melted to subsequent treatment
stages such as a degasser, filter bed and caster. The cleanliness of the
molten
metal, either in the flowing stream, or in a stationary test vessel (not
illustrated), can
be examined immediately and on line by drawing a sample, usually by means of
reduced pressure, into a sample receiving test container 14, usually an
elongated
replaceable tube, which is closed at its lower end, and is removably mounted
via its
open upper end in an end cap 16. The cap is mounted for vertical up and down
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movement so that the tube can be dipped at will into the flowing stream 12 and
with-
drawn therefrom, the mounting means for this being shown diagrammatically
herein
as a standard 18. The end cap has four electrodes protruding downwardly
therefrom,
three of which enter the tube interior while the fourth is outside . One of
the three
internal electrodes is a current-carrying electrode 20 consisting of a metal
rod, the
upper part of which is encased in a heat insulating material 22, so that only
the
exposed lower tip 24 immediately adjacent to a sensing passage 26 in the
container
wall will be in electrical contact with molten metal that enters the
container. The
outer electrode 28 is also a current-carrying electrode and is mounted by the
end cap
l0 16 so as to extend parallel to the first electrode 20 with its bare lower
tip 29 also
immediately adjacent to the passage 26. The resultant current path between the
electrodes 20 and 28 and through the sensing passage 26 is supplied with
current
from a battery 30 via a ballast resistor 32 that can be shunted when required
by a
switch 33 to increase the current flow to a "conditioning" value. One of the
battery
I S leads includes an on/off switch 31 and an ammeter 34. The end cap 16 also
provides a fluid connection from the interior of the test container to a three-
way valve
36, which permits the interior to be connected alternatively to a source of
reduced
pressure, or to a source of a suitable shielding inert gas, such as argon, or
to the
atmosphere. The reduced pressure source consists of a reservoir 38 which is
20 exhausted as required in between tests through valve 36 by a pump 40. The
two
electrodes 20 and 28 are connected to a differential amplifier 42 and thence
to a
logarithmic amplifier 44, a peak detector 46 and multichannel analyser 48,
which can
also serve as a recorder.
Before use the interior of the container 14 is flushed with a gas such as
25 argon or nitrogen and the container is then lowered into the metal12. The
valve 36
is then operated to connect the container interior to the reduced pressure
reservoir,
whereupon the molten metal is drawn smoothly and rapidly through the passage
26.
As soon as enough metal has entered the container to touch and immerse the tip
24
of the electrode 20 a current path is established between the two electrodes
20 and
30 28 and through the passage. The analyser/recorder 48 is switched on when
sufficient metal has entered the container to contact the lower level
electrode 50 of a
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metal level detector 52, and is switched off when the metal contacts an upper
level
detection electrode 54.Since the area of contact between the liquid metal 12
and the
electrodes 20 and 28 is limited to the tips 24 and 29, the only changes in
voltage that
are measured are those arising from the displacement of metal by inclusions
passing
through the sensing passage 26. Each of these inclusions when sensed produces
a
voltage pulse above or below the steady state value. Thus, as each particle
passes
through the passage 26 it displaces its own volume of the liquid metal and
causes a
change in the electrical resistance between electrodes 20 and 28, the
magnitude of
the pulse being related to the ratio between the cross section flow area of
the
passage and the size of the particle by a known relation. The voltage pulses
are of
relatively low amplitude superimposed on a large D.C signal and these are fed
to the
differential pre-amplifier 42 and filtered to remove the large D.C. component
and
inevitable high frequency noise. The accurate and reliable detection and
measurement of these pulses among the high level noise is very difficult and
limits
the size of inclusion that can positively be detected to about 15-20
micrometers. The
logarithmic amplifier 44 extends the dynamic range of the signal, and its
output is fed
to the peak detector which samples the signal and produces discrete pulses of
fixed
length that can be handled by the analyzer 48. The analyzer counts the number
of
these pulses and also analyzes them as to size. The output of the analyzer is
therefore a histogram of particle number from which the particle concentration
and
particle size distribution in the specimen can be determined.
The suggested range of sensing passage diameters is 100 to 5000
micrometers, more usually for the lower melting point metals such as aluminium
and
magnesium of 200 to 500 micrometers, the value chosen depending primarily on
the
size of the inclusions in the melt and of typical inclusions to be measured.
For
sampling in aluminium the container 14 typically is a tube of a refractory
material, for
example a boro-silicate glass with a wall thickness of 1 mm to give a bore 26
of the
same length. When used to sample aluminium the sensing passage is of diameter
about 300 micrometers. Figure 2 shows one form taken by the passage in a prior
art
apparatus, e.g. for testing molten steel, consisting of a drilled hole of
uniform
diameter along its length. Figure 3 shows another version in which the
entrance to
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the drilled hole of Figure 2 has been enlarged by drilling a conical entrance
portion
extending into the passage, in order to give a more laminar flow into and
through the
passage. Figure 4 shows the type of passage usually employed in a sensor probe
for aluminium, where the tube is of a material that can readily be softened by
heating.
Thus, a flame is applied to a cylindrical hole as shown in Figure 2 of
sufficient
temperature and heat capacity to melt the material around the hole, so that
the
material flows under the action of surface energy until its edges,
particularly the
circular entry edge, are converted to more or less smoothly randomly rounded
respective profiles. The part of the passage of smallest diameter, or
equivalent
diameter if it is not truly circular, due for example to irregularity in the
melting,
constitutes the sensing zone proper. The diameter of the sensing zone can
easily
be measured by a gauge rod inserted into the passage, but the sensing passage
profile obtained will vary in dependence upon the material from which the tube
is
made and the specific heating conditions, including the skill of the
fabricator, under
which each passage has been formed, there being no guarantee that the wall is
truly
smooth and uniform about the flow axis. Examination of the orifices of
commercial
versions of such tubes shows that their shapes are somewhat irregular, and
attempts
to develop a mathematical term for use in analysis of flow through such
orifices has
required, as a practical compromise, the fitting of at least a second order
polynomial
to the observed shape. Since it was found in practice that such rounding did
enable
relatively stable signals to be obtained, it appears that this was then deemed
to be all
that was required.
The effective cross section area of the passage should be as small as
possible, so that passage of the smaller inclusions therethrough will produce
clearly
detectable pulses, but cannot be too small, since it is then found that inflow
rates
quickly decrease and the voltage baseline quickly becomes unstable. It is
believed
that the cause of this instability etc. is most likely that the inclusions of
non-
conducting material are directed toward the passage wall by the
electromagnetic
force generated therein, as will be described in more detail below, and may be
sticking to the wall, reducing and disrupting the desired smooth laminar flow
of the
metal through the passage. A large inclusion may of course be unable to pass
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through and almost block the bore. A gradual deterioration in accuracy of the
testing, and instability of the baseline readings, from test to test, and even
during a
test, was found from the start of the LiMCA system. Fortunately it was also
found
early on that it was possible to restore the apparatus virtually to its
original accuracy
and stability before a test by the "conditioning" process as described above,
which is
presumed to operate by removing any such obstructions. The mechanism by which
this occurs has not been known for certain.
One proposal that has been made to alleviate this problem is contained in
US Patent Serial No. 5,834,928 to Doutre, whereby the metal is conveyed
through a
wider passage upstream of the sensing passage immediately before discharging
into
the sensing passage, this wider passage being defined by an electrically non-
conductive surface positioned in the current path and providing a flow region
having
a constant hydrodynamic diameter of between 2 and 10 times that of the sensing
passage. The invention depends upon the fact that since the inclusions are
particles
of conductivity different from the molten metal they are subjected to a self-
induced
magnetic flux and resultant electromagnetic force. When the particles are of
lower
conductivity (the usual situation) this electromagnetic force urges them
radially
outward away from the flow axis, while particles of higher conductivity are
urged
toward the flow axis. The provision of such an initial wider passage has the
effect of
removing substantially all liquid and gaseous inclusions from the molten metal
before
it passes through the sensing passage; generally such liquid and gaseous
inclusions
do not deleteriously affect the metal quality as much as the solid inclusions.
The
initial passage should also trap larger particles and prevent them from
entering the
sensing passage, so that a sampling tube with a smaller orifice can be used to
more
accurately detect the smaller inclusions, while a sampling tube with a larger
orifice
and without the initial passage can be used in another test to measure the
larger
inclusions
A detailed mathematical discussion of the motions of particles entrained in
a flow of liquid metal while subjected to a co-current flow of electric
current is the
subject of a paper entitled "Numerical Studies of the Motion of particles in
Current-
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Carrying Liquid Metals Flowing in a Circular Pipe" by the inventors herein Mei
LI and
Roderick R.L. GUTHRIE, published in Pages 357-364, Volume 31 B, April 2000 of
Metallurgical and Materials Transactions B, to which reference may be made.
The
following non-mathematical discussion of these motions is believed to be
sufficient to
enable a person skilled in the art to understand and apply the teachings of
the
present invention, and reference can be made to the paper above if a more
detailed
mathematical explanation is required.
Because of the inherent difficulty of accurate detection and measurement
of the small signal pulses generated by the passage of an inclusion through
the
sensing zone against the relatively large steady D.C. signal needed for
successful
operation, great care has had to be taken in the design of the accompanying
equipment to try to reduce the background "noise" as much as possible. For
example, a pre-evacuated vacuum reservoir 38 is provided, so that the pump can
be
turned off during the test and not cause electrical interference as it
operates. For
the same reason the battery 30 is rechargeable, instead of using an on-line
D.C.
supply from an A.C. source. Great care is taken in the design of the
electronic
circuits to provide as much electronic smoothing as possible and to reduce to
the
minimum any ground loops that are otherwise a major source of noise.
As will be demonstrated in more detail below, and as is well known, for
example from the Doutre patent referred to above, inherently the sensing
passage
itself constitutes an electromagnetic circuit element. Thus, the metal under
test is a
highly conductive "wire" moving through the sensing passage in an intense
electric
field and consequently generates a correspondingly relatively high magnetic
flux and
resultant strong electromagnetic force which mechanically affects the
particles and
the flow field within the ESZ. Unless therefore the utmost care is taken to
ensure
that the rapid flow of the molten metal through the very narrow sensing
passage is as
smooth as possible the sensing passage itself can be a generator of
deleterious
background noise. It is believed that such noise may be generated for example
as a
result of vortices and fluctuations in the flow generated as a result of the
action of the
electromagnetic field on the fluid flow within the ESZ, the passage profile
and the
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surface conditions of the passage wall, any such vortex and fluctuation
potentially
constituting a minuscule but potent electrodynamic noise generator. It will be
noted
that the flow rates of the molten metal are relatively high, e.g. 2-5 m/s,
which is
required for satisfactory operation of the system, and these high rates are at
values
at which the flow can easily become completely disrupted and turbulent, at
which
point useful readings can no longer be obtained. It is known for example that
a
stirred fluid contains vortices of size determined by the viscosity and
temperature of
the fluid, and such vortices can be of size the order of that of the particles
to be
detected, i.e. about 15-50 micrometers.
l0 It is believed therefore that previous suggestions simply to make the entry
to the sensing passage "smooth" to obtain a "smooth" flow are inadequate, and
instead at least the entry portion of the sensing passage wall, and preferably
also the
exit portion thereof, must be shaped to a precise progressive profile selected
either
as parabolic or elliptical, so that a flow that is as laminar as possible is
achieved with
the possibility of the generation of fluctuations and internal vortices
minimized. With
the high rates of flow employed the stagnant boundary layers that normally are
present with slower flowing columns of fluids are vanishingly thin, and owing
also to
the operating temperatures involved, there is relatively high wear by abrasion
of the
passage wall that increases the size of the passage until the tube becomes
unusable. It is also believed therefore that the initial smoothness of the
passage
wall is also unexpectedly important, and even the standard roughness of a so-
called
"smooth" wall, as previously employed, may be sufficient to generate
fluctuations and
vortices contributing to the background noise as the metal passes over it. To
this
end a preferred method of producing the sensing passage 26 is to first drill a
pilot
hole of appropriate diameter and then enlarge the hole to the required maximum
starting diameter, while at the same time producing a sufficiently precisely
formed
parabolic or elliptical profile. This can be done, for example, by use of a
rotary
grinding tool 84 of complementary profile which is fed into the pilot hole
first from one
side and then from the other, the grinding produced by the tool being
supplemented,
if necessary, by a finishing operation with a similarly profiled lapping tool.
Thus, a
standard surface finish for high quality machining is about 1.016 micrometers
(40
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microinches), but this is believed to be insufficient in that microscopic
inspection of
such a finished surface will reveal ridges and protrusions that are potential
generators of noise-inducing fluctuations and vortices, and it is preferred
instead to
achieve surface finishes of at least 0.254 micrometers (10 microinches), and
more
preferably 0.127 micrometers (5 microinches). Such finishes are relatively
readily
attainable with currently available grinding and lapping tools. It is believed
that the
matter of an ultra-smooth surtace for the sensing passage wall is of
sufficient
importance to be applicable also to any other profiles that are not
necessarily
parabolic or elliptical and to the "smooth" randomly curved surfaces that have
been
proposed hitherto. Since the surfaces are most conveniently produced by use of
a
precisely formed rotary tool they will inherently be symmetrical about the
flow axis,
and the most likely alternative profile is a semi-circular one, even though it
not
expected to be as efficient in the suppression of noise as parabolic and
elliptical
surfaces.
The curvature of a parabolic curve can be expressed as a coefficient of
the polynomial:-
Y - ,4x2 + R
where A is the coefficient and R is the radius at the sensing zone throat.
When A is
zero the passage is cylindrical, while when it is very large the throat
becomes a knife
edge; a preferred range of values for the coefficient is from 1.0 to 5.0, more
preferably from 2.0 to 4Ø The value for the curves in the discussion below
is 2.15.
Similarly, the curvature of an elliptical curve can be expressed as a
coefficient a = b/a of its respective polynomial:-
(~a)2 + lYfb~2= 1
where a and b are the half axes lengths respectively in the x and y
directions. A
preferred range of values for the coefficient J~ is from 0.2 to 2.5, more
preferably from
0.5 to 2Ø The specific value employed for the curve evaluated is 0.5 .
Figure 5 shows to an enlarged scale the portion of the wall of the sensing
container 14 that includes the sensing passage 26 and, in accordance with this
invention this has a parabolic profile, the coefficient in this embodiment
being of the
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preferred value 2.15, the directrix of the parabola being coincident with
central
longitudinal flow axis 58, about which the parabola is rotated to generate the
wall,
while the focus of each parabola is a point 60 lying on a circle within the
container
wall. The liquid metal 12 flows into the inlet 62 of the sensing passage along
the flow
axis 58, and also in the general direction of the arrows 64, the rate at which
the metal
is drawn into the container interior being such that the flow is smooth and
laminar and
is directed in such flow to the central part 66, or throat, of the passage
which is of
minimum cross sectional flow area and constitutes the electric sensing zone
proper.
The molten metal discharges through the passage exit and is found in practice
to be
more of a jet flow, rather than following closely the part of the parabolic
profile
between the zone 66 and the outlet 68. The current I flows in the direction of
the
arrow 70 between the electrodes 20 and 28.
Figure 6 also shows to an enlarged scale the portion of the wall of the
sensing container 14 that includes the sensing passage 26 and, also in
accordance
with this invention, this has an elliptical profile with the major axis 80
parallel with the
sensing passage longitudinal axis 58 and within the container wall and having
an
extension of its minor axis 82 passing through the electric sensing zone 66.
As with
the parabolic profile the elliptical profile is rotated about central
longitudinal flow axis
58 to generate the wall 56 of the sensing passage.
The physical observation of the flow of molten metals is extremely difficult,
since there are no transparent materials that can be used for making pipes
through
which the metals flow while under observation. The mathematical computation of
the
dynamic motion of particles in fluids also is notoriously difficult, and
simplifying
assumptions are essential; accordingly the treatment employed a two
dimensional
simulation using a cylindrical coordinate system. The computation domain was
taken to be that between an inlet boundary 76 which is an inlet spherical cap
centred
at point C on the axis 58, this point being the intersection of axis 58 with a
cone
tangential to the passage wall 56 at the passage inlet edge; a corresponding
outlet
spherical cap 78 centred at point C' which is apex of a cone tangential to the
passage
wall at the passage outlet edge establishes the effective length of the
sensing
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passage for computational purposes. It will be noted that in practice the
transient
time through the ESZ passage is not the time taken by a particle to go through
the
physical length of the passage, but rather the time it takes to go through the
region
where the pulse height generated is higher than the thresholds set by the
electronic
circuits for adequate detection and recognition. This computation length
increases
with particle size. The outlet boundary for the flow of the metal and the
sensing
current was taken to be the sensing passage throat 66. In addition, in order
to
simplify and enhance the accuracy of the calculations a non-orthogonal grid of
variable spacing within the computational boundary was set up while time steps
as
small as 10-5 ms were employed.
At the inlet boundary 76 the liquid velocity and the electric current density
were both assumed to be uniform and normal to the boundary, while at the
outlet
boundary 66 the electric potential was assumed to be constant and the exit
velocity
gradient zero; iterative corrections were made in any numerical calculations
to match
inflow and outflow rates so as to respect continuity. Jet flow of the molten
metal was
assumed beyond the throat 66, i.e. the diverging sidewalls at the outlet side
were
ignored and it was assumed that the liquid simply passes on with an axial
velocity
distribution of the same as that at the throat. Experiments with an aqueous
system
(which can be conducted at room temperature) showed that in practice a jet-
type flow
was obtained, and it seems safe to assume that the same will be true for the
molten
metal flow. The boundary conditions applied were zero slip along the passage
wall
and zero electric current flux across the tube between its inner and outer
walls. The
calculation was made assuming that the media is composed of one continuous
material (the molten metal) of resistivity Pe and sparsely distributed
spherical
inclusions of resistivity Peff, the particles being considered as sufficiently
scattered so
that the distance between each other is large enough so as to not disturb the
course
of the surrounding current.
The LiMCA system usually is operated with molten aluminium while at a
temperature of about 700°C, at which temperature its density is
2.368X10 kg/m~, and
its electrical' resistivity (gym) is 0.25X10-6. The most common inclusions are
alumina
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CA 02342297 2003-12-31
and gas micro-bubbles which have densities respectively of 3.8X103 kg/m3 and
zero,
and which essentially are electrically non-conducting. Inclusions of titanium
diboride
were considered since its density is 4.5X103 kg/m3, while its electrical
resistivity is
lower than that of alumina at 0.09X10-6. It is found that the voltage pulse
generated
by a perfectly conducting particle is negative, opposite to that of a non-
conducting
particle, and has a peak resistive height two times that of a non-conducting
particle of
the same size, while aTiB2 particle in molten aluminium also produces a
voltage
pulse of opposite sign because of its greater conductivity, the height of the
voltage
peak being about three fourths of that for a non-conductive particle of the
same size.
In order to predict the flow behavior of molten aluminium entering the
converging section of the sensing zone the metal was taken to be
incompressible,
with constant properties, and the flow was considered lamina and steady, which
are
the ideal practical operating conditions. The metal flows at a speed of 4.6m/s
and the
Reynolds number through the ESZ orifice is about 400, based on orifice
diameter.
The current generates a self-induced magnetic field with the result that all
particles
entrained in the moving metal are subjected to a corresponding electromagnetic
field
and resultant electromagnetic force. Figure 7 shows the distribution of the
electric
potential within the computation field, Figure 8 shows the electric current
density,
Figure 9 shows the self-induced magnetic flux density, while Figure 10 shows
the
specific electromagnetic force. As can be seen from Figure 7, which shows the
calculated isopotential contours in the computational field and their
respective
values, the isopotential along the central cross section of the orifice has
its highest
value at contour L where the current flow from the inner positive electrode 22
enters
the throat of the ESZ. The electrical potential gradient is very high near the
throat of
the orifice and drops gradually towards the entrance or exit of the orifice.
With the
usual measuring current of 60 amperes the voltage drop over the whole orifice
is
approximately 0.105 volts. This potential distribution gives rise to the
electric current
density distribution shown in Figure 8 and therefore corresponds with it.
Thus, as
with the potential distribution the current density is very high near the
central region
of the orifice, and decreases with increasing distance from the throat. Figure
9
shows the isodensity contours for the self-induced magnetic flux within the
orifice and
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CA 02342297 2003-12-31
it will be seen that this increases from the central axis 58 to the passage
wall 56.
The interaction of this standard testing value electric current (60 amperes)
and its
induced magnetic flux results in an electromagnetic force whose distribution
is shown
in Figure 10.
Figure 11 shows the metal flow velocity vectors as computed, while Figure
12 shows the profiles of the axial velocities at three selected axially spaced
positions
R, S and T in the sensing passage. Figure 14 shows the equivalent profiles of
the
radial velocities at the same three selected axially spaced positions R, S and
T. It
can be seen that the stronger electric current density and magnetic flux
density near
the throat 66 of the orifice give rise to much stronger electromagnetic forces
there
than in the entrance or exit regions 62 and 68. The electromagnetic force is
higher
near the passage wall, but decreases toward the central axis 58, becoming
virtually,
and theoretically, zero along the central axis. In this force field particles
suspended
in molten aluminium that are electrically non-conductive experience an axial
force in
the opposite direction, urging them backwards against the metal stream, and a
radial
force that urges them toward the wall 56 out of the main flow of the molten
metal.
Particles that are electrically more conductive (e.g. TiB2) than the aluminium
experience an axial force in the same direction as the metal flow, and are
pushed
towards the central axis by the radially acting component of the force. With
the
current I at the usual measuring value the axial flow force produced by the
vacuum
source 38 is much higher than the opposed electromagnetic force, which is
therefore
relatively ineffective in opposing it. However, the radially acting forces are
virtually
unopposed and therefore are very effective.
The magnitude of these axial and radial electromagnetic forces are
dependent directly on the value of the current I and increase therewith.
Further
computations, whose results are as shown in Fig.14, show that at a threshold
value
of this current the increasing backwardiy acting axial force, being
increasingly
stronger toward the passage throat, can reverse the metal flow and move the
metal
from the throat 66 to the entrance 62 with the result that a strong annular
vortex flow
is established in the entrance to the passage. The metal can now only flow
into the
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CA 02342297 2003-12-31
sensing zone between the vortex and the wall and the velocity of the vortex
flow is
added to the metal flow at the passage wall; it has been found in practice
that with a
conditioning current of 250 amperes the maximum velocity at the wall increases
from
4.64m/s to at least 6.15m/s. It seems reasonable therefore to ascribe the
conditioning effect to this high fluid velocity near the wall, which is
sufficiently high
that it is able to scour away any debris or residual buildup of inclusions
that have
adhered to the wall entrain them in the vortex and then expel them back into
the main
metal stream.
The phenomenon is therefore a result of the magnetic pinch effect
to interacting with a precise shape of the entrance to the ESZ, the
combination creating
a pressure buildup in the orifice that at a critical current value causes a
strong flow
reversal at the entrance to the ESZ. There is therefore also a strong and
definite
advantage to the use of the precisely formed sensing passages of the invention
in
place of the randomly formed "smooth" rounded passages, or the cylindrical, or
conical entrance passages proposed and used hitherto. As described above, at
this
time it is customary to use a conditioning current of 250-300 amperes in a
LiMCA
system intended for aluminium, since this is found to work under most
circumstances.
It can be shown however that with a parabola profile of a coefficient of 1.0,
the lower
end of the preferred range, the critical current value above which vortex flow
is
obtained is about 165 amperes, while the value is even lower for a profile of
coefficient 5.0, which is at the upper end of the preferred range. It is
possible
therefore with these new profiles to operate with a battery system of lower
capacity
when These lower conditioning currents are able to establish a conditioning
vortex
flow.
Another general problem encountered with the measurements made in
flowing liquid metals is the tendency as described for particles to be
repelled radially
outward and also to some extend axially backwards along the flow axis. It is
known
that this does reduce the total number of particles that pass through the
sensing zone
during a standard test" as can be determined testing the same melt by the
slower
3o and more expensive, but more comprehensive prior art methods employed
before the
LiMCA system became available. The proportion that passes through is known as
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CA 02342297 2003-12-31
the pass-through fraction and if the operating parameters are not sufficiently
carefully
chosen it can be as low as 50%. Since the rejection of the particles clearly
is due to
the random motions of the particles under the action of the electromagnetic
force as
the metal stream passes through a rough walled sensing passage of
indeterminate or
unsuitable profile, then the adoption of the progressive parabolic or
elliptical profiles
of the invention, togther with the adoption of the much smoother machining
proposed
for the passage wall, will result in a substantial decrease in the fraction
that is
rejected. For example, if a non-conductive particle is considered to be
collected by
the passage wall when it assumes a radial velocity toward the central axis 58,
and its
centre is a radius away from the wall, then the calculated collection
coefficient for
particles of size 20-240um is only 5% in a parabolic profile passage with a
polynomial coefficient of 2.15, and is still only 8% when the polynomial
coefficient is
1Ø
A problem encountered with the application of the system to the analysis
of magnesium is to find a material for the tube 14 that is resistant to attack
by the
molten metal. The physical properties of liquid magnesium are not very much
different from those of aluminium, with a liquid temperature of 700°C,
a density of
1.577x10-3 kg/m3, a viscosity of 1.23X10'3 kg/ms and an electrical resistivity
of
0.28X10'6. Compacted silica tubes are resistant to such attack but tend to
crack
easily due to thermal shock and had a high failure rate. A more recent
proposal is to
form the sensing passage in a disc of boron nitride which is then held between
two
steel tubes that support the disc with the inner one forming the sampling
chamber. In
neither of these constructions is it possible to smooth the entrance by a
heating
procedure, as with aluminium, and instead the passage has been formed with a
cylindrical bore at the ESZ throat and conical openings at the entrance and
exit, it
was found that the background noise encountered is significantly higher than
the
value of about 10uV obtained with aluminium, namely usually about 30-50uV, and
the
sensing orifice size required is also significantly larger to avoid blockage,
namely
about 400-500pm, both of which increase the minimum size of particle that can
be
reliably detected. It is found by computation that even at the standard
operating
current for aluminium of 60 amperes a large toroidal vortex or recirculation
zone is
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CA 02342297 2003-12-31
established inside the conical entrance, again generated by the strong
electromagnetic pinch force at the passage entry and the consequent strong
axial
backpressure it produces. Such a vortex would itself cause an increase in
background noise, such as is encountered. It is also believed that it may be
an
explanation for the requirement for a larger passage in that particles are
entrained in
the vortex and cannot reach the sensing zone throat. Instead they are swept
around
in the vortex giving them an opportunity to coalesce and become larger
particles that
more easily block the passage, or if able to pass through the passage are then
counted as such, giving an inaccurate count. It will be seen that the
provision of an
accurately formed passage of the profiles of the invention and of the surface
smoothness specified will be able to raise the current flow at which vortex
formation
takes place above the value of 60 amperes, which is needed for accurate and
reliable
testing, while also making it possible to predict the higher current value at
which a
vortex of sufficient axial velocity will form for adequate conditioning as and
when
required.
Steel is another metal for which fast and reliable inclusion detection since
many characteristics of a steel product can be badly compromised by the
presence of
inclusions, such as ductility, toughness, drawability, machinability,
weldability, H.I.C.,
and fatigue strength, as well as surtace characteristics such as paintability,
pitting,
corrosion and reflectivity, all of which can be critically affected by the
nature, size and
spatial density of such inclusions. Current methods employed are relatively
time
consuming and costly and inherently cannot be operated on-line. Depending on
the
grades of the steel being produced, and attendant processing operations, large
inclusions, typically in the range of 50-200Nm diameter can be present. Since
the
successful application of the LiMCA system to aluminium melts much effort has
been
devoted to the development of sensor probes for liquid steel. An initial
design
employed a composite boron-nitride/silica (fused quartz) tube that is usable
as long
as the steel is properly deoxidized beforehand. The part of the tube of boron
nitride
was immersed in the melt and remained chemically stable therein while the
quartz
upper body was above the melt and provided the visibility needed to control
successive filling and emptying operations. This design was found to be prone
to
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CA 02342297 2003-12-31
leaks at the boron nitride/silica joint, and consequently was abandoned in
favor of a
one-piece silica tube supported by a graphite reinforcing inner electrode and
using a
cylindrical ESZ. Although it has been successful sometimes the operations are
troubled by the relatively high background noise, and furthermore, it is
observed that
the high background noise cannot always be improved by conditioning. The
pertinent properties of liquid silicon-boron steel are melt temperature
1350°C, a
density of 7.0x10'3 kg/m3, a viscosity of 7.0X10-3 kg/ms and an electrical
resistivity of
1.40X10 .
The velocity vectors for flow of liquid steel within the ESZ were examined
l0 with an operating current of 20 Amperes, and a conditioning current of 200
Amperes,
the adoption of lower current values than those for aluminium being indicated
by the
much higher resistivity of the steel. It was found that the maximum velocities
are
along the central axis for both cases, but surprisingly the maximum velocity
is lower
in the conditioning operation than that in a typical testing operation. This
is in
contrast to the conditioning effect in molten aluminum, where the
recirculation zone
formed in the inlet region with an increased current dramatically increases
the fluid
velocity near the ESZ wall. It can be inferred that the ineffective
conditioning
operation in steel with the higher current is the direct result of the
cylindrical shape of
the ESZ and use of a sensing passage of either a parabolic or elliptical
profile of the
invention, together with the adoption of the preferred highly smoothed wall
surface for
the passage, would remove this anomaly and result in an overall operation
similar to
those obtained with aluminium and magnesium. Calculations for steel employing
a
parabolic profile passage of 2.15 coefficient gave a threshold current value
of 184
amperes at 4 m/s and 86 amperes at 2 m/s, as compared with equivalent values
for
aluminium of 110 and 58 amperes respectively, showing that with the correct
design
of passage the critical current value for steel is higher, as should be
expected.
A major advantage of the use of sensing passages of precisely formed
profile and surface smoothness is the ability to determine and adjust the
values of the
sensing and conditioning currents to be employed for a procedure. Thus, it is
essential for accurate measurement that the testing current be sufficiently
high
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without producing disruption of laminar flow, and conversely that the
conditioning
current does produce the required strong vortex flow. The threshold current
value
at which vortex flow is produced also depends upon other parameters, such as
the
flow rate of the metal and the diameter of the passage throat, both of which
are inter-
s related. The choice of passage diameter has been described above. The choice
of
flow rate is dictated by the need for a large enough sample flowing into the
sample
chamber to give meaningful results, without the Pest taking an unduly long
period of
time as the metal flows into the chamber. Owing to the small passage diameters
involved the flow rate must be relatively large, and usually is in the range 2-
4 mls.
Since the transition from laminar to turbulent flow is dependent upon the
current at
which the electromagnetic force overcomes the flow rate the threshold value is
also
dependent on the flow rate.
The following Table shows examples of this interdependence of the value
of I for threshold current for parabolic profile sensing passages of 300pm
diameter,1 mm length and of polynomial value A from 0.1 to 10, the metal being
aluminium. It will be noted that the value decreases progressively until it
becomes
impractically low at the highest values :-
Value A Flow 4 m/s Flow 3 m/s Flow 2 m/s
0.1 450 amps 340 amps 231 amps
1.0 165 amps 126 amps 87 amps
2.15 110 amps 84 amps 58 amps
3.8 70 amps 54 amps 38 amps
10.0 11 amps
An equivalent table for an elliptical profile passage gives the following
results:-
~ Value Flow 4 m/s
0.2 370 amps
0.5 175 amps
1.0 140 amps
2.0 47 amps
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