Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02192507 1997-O1-21
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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 and size distribution of inclusions in molten metal, the
apparatus employing what is now sometimes known as the electric sensing zone
(ESZ)
method.
1 o REVIEW OF THE FIELD
The production and refining of metals from their basic ores inevitably
results in what, for convenience in nomenclature, are referred to as
"inclusions", such
as precipitated secondary phase particles, drops of slag and air 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 unavoidable 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
2 o properties such as fatigue life, toughness, corrosion, tearing, splitting,
surface quality,
pinholes, etc., particularly when larger inclusions (e.g., dimensions > 20
microns) are
present. It has therefore become more and more essential to know whether or
not
the metal is sufficiently "clean" for its intended purpose, and also to show
whether
or not the refining processes employed are producing sufficiently clean metal.
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A quantitative measurement method and apparatus for such inclusions
particularly in molten aluminum, has now become well established in the
aluminum
industry, and is known as the LiMCA (Liquid Metal Cleanliness Analysis)
system; these are
described and claimed for example in U.S. Patents Nos. 4,555,662, 4,600,880,
and
4,763,065. The application of the method and apparatus to the detection of
inclusions
during the refining and recycling of other metals is under investigation. The
ESZ method
was used prior to its application to molten metals to measure inclusions in
aqueous
solutions and relies upon the fact that any inclusion is usually of different
conductivity
(usually much lower) 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 in an electrically insulating
material; 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 an electrical potential pulse between two electrodes on opposite sides of
the orifice.
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.
Currently used sensing probes 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 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
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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 outside the sampling tube. The
materials
used and proposed for the fabrication of the sensing probe tubes are all non-
conducting and highly heat-resistant, such as borosilicate glass, alumino-
silicate glass,
fused silica, alumina, magnesia, mullite, boron nitride and other ceramic
materials.
Problems with these materials can be manifold, especially in view of the broad
range
of metals to which the system can be applied, including iron, steel,
aluminium,
copper, titanium, magnesium and alloys thereof. For example, chemical reaction
often occurs between the metal and the material of the tube causing its rapid
disintegration. As a specific example, a fused silica tube placed in molten
magnesium
will be attacked rapidly, since silica (SiO~ is thermodynarriically less
stable than
magnesia (Mg0) and is therefore reduced by the magnesium, leading to
relatively
rapid disintegration of the tube, e.g. about two minutes for one of wall
thickness of
lmm. Molten copper containing dissolved oxygen causes similar problems, with
the
resulting copper oxide fluxing the silica (m.p. 170°C), and again
causing rapid
disintegration of the tube.
Other problems with refractory materials follow from the fact that many
are brittle, making it difficult to produce robust, thin-walled tubes, and
difficult to
2 o assemble, disassemble and maintain the equipment without high risk of
damage and
breakage. Further problems result from the thermal stresses that are generated
in the
tubes when they are immersed in the liquid metal; these stresses are
particularly severe
with liquid steel baths, typically operating at melt temperatures between
1500°C and
1700°C, and it is not uncommon for the sample tubes to crack unless
they are
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carefully preheated to a relatively high temperature. All of the refractory
materials
suitable for use as sample tubes are difficult to fabricate, so that the tubes
made from
them are correspondingly expensive, and the difficulty and expense increase
disproportionately as the size of the sample tube is increased. Finally, all
ceramic
s oxides tend to develop electrical conductivity at high temperatures that can
become
sufficiently high compromise the electrical performance of the probe.
U.S. Patent No. 4,763,065 referred to above discloses apparatus for the
detection and measurement in a molten metal sample of suspended particulates,
the
apparatus comprising a container defined by a composite wall including
electrically
Zo conducting inner and outer walls electrically insulated from one another
and an
electrically insulating barrier including a sensing zone passage. The
electrically
conducting walls comprise two concentric cylindrical metal tubes which also
comprise
the electrodes for the sensing zone. The upper ends of the tubes extend into a
mounting block while the lower ends are provided with inwardly extending
flanges
15 between which the insulating barrier, which comprises a flat disc through
which the
sensing zone passage passes, is held by a pair of insulating discs. 'rhe
specification
states that the annular space between the tubes is filled with densely packed
alumina,
or with some other electrically insulating heat conducting material, or it
could be left
empty, or a heating element could be provided to ensure that the contents of
the
2 0 container remain molten.
DEFINITION OF THE INVENTION
It is the principal object of the present invention to provide inclusion
sensor probes for molten metals of new construction.
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It is another principal object to provide such probes of inexpensive
construction suitable for use for mufti-shot measurements in the less severe
environments of the lower melting point metals, and that can be sufficiently
inexpensive that they are usable for one-shot measurement in higher melting
point
metals during which the probe is destroyed.
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:
an inner metal tube disposed within an outer metal tube, the tubes being
1 o electrically insulated from one another and forming an annular gas
containing space
between them;
a sensing zone member of electrically insulating heat resistant material
mounted by the tubes and having therein an orifice comprising a sensing zone
passage
between the interior of the inner tube and the exterior of the outer tube;
~ 5 spacing means of electrically insulating heat resistant material
maintaining the tubes spaced from one another; and
vent means venting the annular space through the wall of the outer tube
to the exterior of the outer tube, or through the wall of the inner tube to
the interior
of the inner tube.
2 o Such a sensor probe may comprise spring means operative between the
inner and outer metal tubes and the sensing zone member to urge them for
relative
movement such as to maintain sealing contact of the metal tubes with the
sensing
zone member as the temperature of the sensor probe changes. Such spring means
may comprise at least one tension spring connected between the metal tubes and
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urging them for longitudinal movement to maintain sealing contact of the metal
tubes
with the sensing zone member, or alternatively may comprise at least one
compression spring interposed between the metal tubes and urging them for such
longitudinal movement. The spring means may be disposed in the annular gas
s containing space between the inner and outer metal tubes.
The spacing between the two metal tubes may be in the range 2mm-
l0mm (O.OSin-0.40in), preferably in the range 2mm-Smm (0.0$in-0.20in).
The spacing means may comprise the sensing zone member at the ends
of the metal tubes that are inserted into the molten metal and at least one
ring-shaped
1o spacer member at or adjacent to the other ends of the metal tubes; the
sensing zone
member may then have the form of a cup having the sensing zone orifice in the
bottom wall thereof, the cup side wall constituting spacing means between the
metal
tubes. Alternatively the spacing means may comprise at least two ring-shaped
spacer
members at or adjacent to the opposite ends of the metal tubes, and preferably
the
15 total length of all of such spacing means is not more than 10°~~ of
the total length of
the metal tubes.
Preferably the metal tubes are of low carbon steel and the wall thickness
of the metal tubes is in the range 1mm-2mm (0.04in-0.0$in). In a sensor probe
intended for the measurement of inclusions in iron and steel the wall
thickness of the
2 o outer metal tube may be in the range 4mm-l0mm (0.16in-0.40in).
The inner surface of the outer metal tube and the outer surface of the
inner metal tube facing one another across the gas containing space may be
rough
and/or provided with a black coating to increase the efficiency of heat
transfer
between them by radiation.
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DESCRIPTION OF THE DRAWINGS
Molten metal inclusion sensor probes 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 side elevation of electrical equipment as employed by a
currently available LiMCA system for the measurement of inclusions in
aluminium,
and illustrating the use therewith of mufti-shot sensor probes of the
invention for the
measurement of inclusions in lower melting point metals;
Figure 2 is a longitudinal cross-section to a larger scale of the sensor
1 o probe only of Figure 1;
Figures 3-7 are cross-sections similar to Figure 2 of sensor probes which
are another and further embodiments of the invention, the lower portions of
the
sensor probes of Figures 3-5 being shown in side elevation; and
Figure 8 is a cross section of the lower end only of a sensor probe that
is a still further embodiment of the invention to show a still further sensing
zone
member construction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The same reference number is used for similar parts of the different
2 o embodiments wherever that is possible.
The mufti-shot sensor probe of Figure 1 is intended for use in the
measurement of inclusions in metals and alloys of lower melting points such as
magnesium (650°C-750°C), aluminium (660°C-750°C),
and copper (1083°C-1300°C).
The sensor probe is mounted in a sensor head 20 so as to protrude vertically
7
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downward therefrom, the head enabling the probe to be lowered into and raised
from
a bath 21 of the molten metal. The head is connected for such movement by a
linkage 22 to electrical apparatus 24 which receives electric signals from the
sensor
probe and uses them to determine whatever information is required as to the
nature,
size, size distribution and number of the inclusions detected by the sensor
probe.
The head and associated electrical apparatus can for example be the LiMCA II
apparatus as sold by Bomem Inc. of Quebec City, Quebec, Canada, for inclusion
detection in aluminium, such apparatus being shown and described in U.S.
Patent No.
5,130,639, issued 14 July, 1992 to Alcan International Limited, to which
further
1 o specific reference is made below.
Since the two metal tubes are electrically insulated from one another
they could also constitute the sensing electrodes for the sensing system, in
which case
they would be electrically connected to suitable terminal assemblies within
the head
20 by electric leads which are not shown. Even if the metal tubes are not used
as
electrodes they may still be electrically connected into the sensor head 20,
for example
in order to minimise background noise. In this embodiment it is preferred that
the
metal tube assembly only provide the container into which the molten metal is
drawn
through the sensing zone passage, and the required electrical measurements for
the
detection of inclusions are made between a pair of additional rod-shape
electrodes 79
2 0 (Figure 1) and 80. The electrode 79 is mounted directly in the sensor head
20 so as
to extend parallel to the exterior of the outer tube 32, whale the electrode
80 is
mounted within the interior of the inner tube, coaxially therewith, and is
screw
threaded into the metal mounting member 48 by which it is electrically
connected
into the sensor head. Preferably these electrodes are coated with a thin life
extending
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layer 82 of heat and electrically insulating material, such as baron nitride,
except for
small portions at their ends adjacent to the sensing zone passage 42. One of
the
difficulties encountered with ESZ apparatus is the extremely noisy electrical
environments in which they must operate, and the low signal/noise ratios that
are
s obtained, making accurate measurement of the pulses very difficult; it is
also very
desirable to be able to monitor continuously the size of the sensing zone
aperture 42,
since this is enlarged by the abrading action of the molten metal passing
through it,
and this also has a direct effect upon the sensitivity and accuracy of the
measurement.
Additional electrode configurations, employing up to five electrodes, that
facilitate
1 o both of these measurements, and also the minimization of ground loop
currents, is
described in U.S. Patent No. 5,130,639, referred to above.
The sensor probe comprises a cylindrical highly elongated first metal cup
providing an inner cylindrical tube 26, the horizontal bottom 28 of the cup
having
an aperture 30 therein. A cylindrical highly elongated second metal cup of
shorter
15 length than the first cup surrounds the first cup and provides are outer
cylindrical tube
32 that is coaxial with the inner tube, the two tubes together forming a
vertically
elongated annular hollow gas-containing space 34 between them. The horizontal
bottom 36 of the second cup is parallel to that of the first cup and has an
aperture 38
therein that registers with the aperture 30. A sensing zone containing disc 40
of a
2 o refractory material is sandwiched between the two cup bottoms and has
therein an
accurately formed orifice 42, very much smaller than the apertures 30 and 38,
that
constitutes the electric sensing zone passage through which the molten metal
passes
and in which the inclusions are detected. Besides providing this passage the
disc 40
maintains the adjacent lower ends of the two tubes in their caaxial
relationship, and
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to this end upper annular face 44 of the disc that is engaged by the circular
edge of
the aperture 30 is tapered upward to be of frusto-conical shape, while lower
annular
face 46 that is engaged by the circular edge of the aperture 38 is tapered
downward
to be also of frusto-conical shape and a mirror-image of the face 44. The edge
of
s aperture 30 is spring pressed downward, by spring means which will be
described
below, into sliding butting contact with the downward sloping surface 38, and
the
edge of aperture 38 is spring pressed upward into sliding butting contact with
the
upward sloping surface 46, and the adjacent bottom ends of the two tubes 26
and 32
are thereby centred on the disc 40 so as to maintain the tubes coaxial with
one
1 o another, despite changes in their dimensions with changes in their
temperature and
differential changes in their dimensions with differences in their
temperatures. The
inclined surfaces ensure that both longitudinal and radial changes in
dimensions are
accommodated.
This embodiment also relies solely upon the spring-urged contact
1 s between the circular aperture edges and the surfaces 44 and 46 that they
butt against
to seal the bottom end of the chamber 46 against entry of molten metal from
the
bath, especially when the sensor probe first enters the bath and there is
maximum
differential expansion between the relatively low expansion coefficient
refractory
material of the disc 40 and the relatively high expansion coefficient metal of
the tubes.
2 o Maintenance of the seals in this manner is of particular importance in one-
shot
probes, where there is usually no opportunity to lower the probe slowly into
the bath
to try to minimize disruptive differential expansion forces affecting the
seals. The
edges of the apertures 30 and 38 are inclined so that they extend parallel to
the
to
CA 02192507 1997-O1-21.
~ «~5~7
respective disc surfaces 44 and 46 that they engage, so that the contact is
surface to
surface facilitating the sealing action between them.
The sensor probe is provided at its top end with a metal mounting
member 48 of external diameter at its top end such that it fits within the
mounting
in the LiMCA II sensor head 20 for the ceramic tube based sensor probes
currently
used therewith. The bottom end of the mounting member is externally screw
threaded and is screwed into the top end of an internally screw threaded
cylindrical
connector member 50 of electrically insulating material. The upper end of the
inner
1 o metal tube 26 is externally screw threaded and is screwed into the bottom
end of the
connector member, so that this tube is rigidly connected to the' mounting
member 48
while electrically insulated therefrom. The outer metal tube 32 is mounted for
longitudinal movement relative to the fixed inner tube, as will be described
below,
and so as to permit the application to the disc 40 of the centring and sealing
spring
1 s force provided by four tension springs 52 (only two shown) spaced
equidistantly
circumferentially around the tubes 26 and 32. As few as two springs can be
employed, although a minimum of three is preferred to ensure uniformity, and
more
than four can of course be used. An annular spacer ring 54 is a snug fit in
the upper
end of the outer tube 32 and is a close sliding fit on the immediately
adjacent smooth
2 o cylindrical portion of the inner tube 26, this ring thus permitting the
necessary
relative longitudinal movements of the tubes while cooperating with the disc
40 in
maintaining the tubes coaxial along their lengths. A flange 56 extends
radially
outwards from the top end of the outer tube 32 and is fastened to the annular
bottom
surface 57 of an upward opening cup-shaped member 58, the cylindrical body of
11
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~m ~~ul
which is a close sliding fit within the cylindrical body of a downward opening
cup-
shaped member 60. Annular bottom 61 of the member 60 extends radially outward
from the mounting member 48 and is clamped between the connector member 50 and
a radially outward extending locating flange 62, a washer 64 of electrically
insulating
s material being interposed between the members 48 and 60 to maintain the two
tubes
electrically insulated from one another, so that the only electrical path
between the
metal in the bath and that inside the inner tube is through the sensing zone
orifice
42.
The upper cup-shaped member 60 is therefore rigid with the mounting
1 o member 48 and the inner tube 26, while the lower cup-shaped member 58 is
rigid
with the outer tube 32 and the two cup-shaped members can move longitudinally
relative to one another. The tension springs 52 are enclosed within an annular
cross
section chamber 65 formed between the cup-shaped members 58 and 60, and are
connected at their ends between the bottoms of the members so as to urge the
lower
15 member 58 and the outer metal tube 32 to move upward in the direction of
arrows
66, thereby producing a corresponding sealing force between the disc 40 and
the
butting circular edge of the outer tube 32, as indicated by the arrows 66. A
corresponding reaction sealing force is produced between the disc 40 and the
butting
circular edge of the inner tube 26, as indicated by arrows 68. These seals are
2 o maintained by the springs despite the changes in temperature and
dimensions of the
different parts of the probe while it is suspended over the metal bath and as
it is
lowered into the bath, and despite the different rates of change for the parts
of metal,
as compared to those for the parts of refractory materials. Moreover, the
seals are
also maintained when, upon insertion of the sensor probe into the metal bath,
the
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outer metal tube 32 is heated faster by its immediate full contact of its
outer wall with
the molten metal than is the inner metal tube 26, which is heated by radiation
from
the outer tube and by conduction from and direct contact with the molten metal
as
it slowly fills the tube interior through the very narrow sensing orifice.
A gas vent 70 connects the interior of the annular space 34 with the
exterior of the outer metal tube 32, and is provided in the wall of the outer
tube close
to its top end and at a height sufficient to ensure that molten metal of the
bath 21
cannot enter the annular space 34, the vent being essential to accommodate the
1 o relatively large expansion of gas that occurs within the spaces as it is
heated. It is
found with a probe sensor for use with these relatively low melting point
metals that
the connector member 50 and the spacer member 54 can be of relatively low
melting
point material, such as TEFLON (Trade Mark), although for extended life it may
be
preferred to make at least the spacer washer 54 of a ceramic or other
refractory
s5 material. Screw threaded connectors connecting the ends of the springs 52
to the cup
bottoms 57 and 61 permit adjustment of the tensions provided by the springs.
The
springs will weaken as they are heated and to maintain their strength at an
adequate
value without the need for initially greatly oversized springs cooling air may
be
provided which is pumped into the chamber 65 through an inlet 72, the heated
air
2 0 leaving the chamber through an outlet 74. In some embodiments it may be
found to
be sufficient to provide the sensor probe with a radially extending shield 76
(see
Figure 2) of a heat insulating and reflecting material positioned just below
the lower
cup-shaped member 58, the shield protecting the upper part of the probe from
heat
radiated from the bath 21, avoiding the need for an air pump and connecting
tubing.
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In a sensor probe in which the metal tubes 26 and 32 are exceptionally long,
so as
to be able to reach more deeply into the bath, it may be found desirable to
provide
one or more short annular spacer rings, such as a centrally disposed ring 78
as shown
in Figure 8, between the spacer member 54 and the disc 40. Any such
intermediate
spacer rings must provide for free through passage of the gas present in the
space 34.
Since the two metal tubes are electrically insulated from one another
they could also constitute the sensing electrodes for the sensing system, in
which case
they would be electrically connected to suitable terminal assemblies within
the head
0 20 by electric leads which are not shown. Even if the metal tubes are not
used as
electrodes they may still be electrically connected into the sensor head 20,
for example
in order to minimise background noise. In this embodiment it is preferred that
the
metal tube assembly only provide the container into which the molten metal is
drawn
through the sensing zone passage, and the required electrical measurements for
the
detection of inclusions are made between a pair of additional rod-shape
electrodes 79
(Figure 1) and 80. The electrode 79 is mounted directly in the sensor head 20
so as
to extend parallel to the exterior of the outer tube 32, while the electrode
80 is
mounted within the interior of the inner tube, coaxially therewith, and is
screw
threaded into the metal mounting member 48 by which it is electrically
connected
2 o into the sensor head. Preferably these electrodes are coated with a thin
life extending
lay er 82 of heat and electrically insulating material, such as boron nitride,
except for
small portions at their ends adjacent to the sensing zone passage 42. One of
the
difficulties encountered with ESZ apparatus is the extremely noisy electrical
environments in which they must operate, and the low signal/noise ratios that
are
14
CA 02192507 1997-O1-21 7
L w c~J~~
obtained, making accurate measurement of the pulses very difficult; it is also
very
desirable to be able to monitor continuously the size of the sensing zone
aperture 42,
since this is enlarged by the abrading action of the molten metal passing
through it,
and this also has a direct effect upon the sensitivity and accuracy of the
measurement.
s Additional electrode configurations, employing up to five electrodes, that
facilitate
both of these measurements, and also the minimization of ground loop currents,
is
described in U.S. Patent No. 5,130,639, referred to above.
In operation, initially the probe sensor is held above the bath for a
1 o period of at least a few minutes so as to be heated thereby, particularly
the sensing
zone member 40, so as to try to ensure that metal will not freeze in the
aperture 42
as the probe is lowered into the bath. To the same end, as the sensor probe is
lowered into the bath and for a period thereafter, an inert gas, such as
argon, or an
inert gas mixture, is supplied under pressure to the interior of the inner
tube through
15 an inlet/outlet tube 84 that extends through and is mounted by the mounting
member 48. The gas bubbles out through the aperture 42 and maintains it and
the
tube interior free of metal until the sensing zone member and the probe
interior are
hot enough to ensure that the entering metal will not freeze. After a suitable
period
the supply of gas is stopped and instead a vacuum is drawn by a vacuum source
(not
2 o shown) within the probe interior through the inlet/outlet tube 84, the
vacuum
sucking molten metal from the bath into the tube interior through the sensing
zone
passage 42 while the apparatus 24 records the number, size and size
distribution of the
particles detected. The drawing of the molten metal continues either for a
predetermined period of time sufficient for the desired quantity of metal to
be
CA 02192507 1997-O1-21
~1y~~07
sampled, or until the molten metal contacts a level sensing electrode $6
within the
probe, giving a signal to the apparatus that a sufficiently large sample has
been tested.
Since visual observation of the level within the sensor probe is not possible,
in
commercial equipment such a sensing means will usually be provided as a safety
back-
s up system to the timing circuit to ensure that metal is not drawn into the
sensing
head.
Upon immersion of the probe, the outer metal tube 32 will heat very
rapidly to the melt temperature, while heat transfer by conduction, convection
and
particularly by radiation across the gas-filled chamber 34 must be sufficient
to ensure
1 o that the inner tube will also heat up sufficiently rapidly for the
entering metal to
remain molten. This is important for efficient operation, since otherwise the
sensor
probe may need to be preheated for an undue length of time,, or otherwise
brought
very close to its operating temperature before such immersion. If such
blockage does
occur it may be necessary to withdraw the probe from the bath in order to take
the
15 necessary corrective action. This is even more important when the sensor
probe is
intended for one-shot operation, a specific embodiment of which will be
described
below, since once freezing occurs there is usually no opportunity for
correction
be:Eore the probe is destroyed. The most efficient mode of heat transfer from
the
outer tube to the inner tube is by radiation across the gap between them,
since this
2 o is operative along the whole length of the tubes, and particularly over
the immersed
portion of the outer tube. It is therefore important to ensure that the
radiation view
facaor between the tubes is as close as possible to one (1), i.e., "black
body" radiation
levels are being obtained. To this end steps may be taken to make the facing
outer
surface of the inner tube and the inner surface of the outer tube as radiation
efficient
16
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L 1 / LJ
as possible, for example by roughening these surafces if they are not
sufficiently rough
as received from the manufacturer, as for example by scoring, wire brushing,
sand or
shot blasting, and/or by coating them with a layer having a higher radiation
coefficient than the metal, such as a layer of iron oxide, sulfide or carbide,
or any
other adherent refractory black coating.
Ensuring that the entering metal does not freezes is more difficult with
ceramic tubes unless they are slowly and thoroughly preheated to a temperature
close
to the bath temperature, owing to the complete absence of the possibility of
heat
transfer by radiation, and to their very much lower heat conductivity
inhibiting the
to passage of heat from the metal bath to the sample tube interior. In
addition they are
much more sensitive to breakage by thermal shock while being preheated and
upon
entering the bath. Owing to the negligible electrical conductivities at the
operating
temperatures of the gas in the chamber 34, especially as compared to some
ceramics,
correspondingly negligible amount of electrical current v~rill pass between
the
concentric tubes through the chamber 34, and the current that does pass
between
them is correspondingly almost exclusively that passing between the electrodes
79 and
80 through the sensing zone aperture 42.
Since this embodiment is intended for multiple serial test operation, at
the conclusion of each test the metal is driven from the probe interior while
still
2 o molten by again injecting an inert gas, or inert gas mixture, under
positive pressure
through the inlet/outlet passage 84, when the test can be repeated.
The second embodiment shown in Figure 3 functionally is similar in
operation to that shown in Figure 2, the major difference being that the
sealing
butting contact of the metal tube edge surfaces against the disc faces 44 and
46 are
17
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2192507
produced by the action of a single compression spring 52 in place of the
plurality of
tension springs 52 of the first embodiment. The upward opening lower cup-
shaped
member 58 and the downward opening upper cup-shaped member 60 are connected
together by a bayonet connection 88 that permits the probe to be disassembled
when
s required but prevents any relative vertical movement between them. The
compression spring is interposed between the annular radial flange 56 at the
top end
of the outer tube 32 and annular bottom surface 57 of the lower cup-shaped
member
58, so that the relative vertical movement takes place between the outer tube
and the
lower cup-shaped member. Adjustment of the spring force can be accomplished by
1 o use of shim washers of different thicknesses that are interposed between
the spring
and the flange 56 and/or the flange 57. The single compression spring can be
replaced by a plurality of smaller springs spaced circumferentially, and again
a
minimum of three is preferred.
It has been found that such cement-less pressure seals are extremely
15 effective, to the extent that a prototype probe as illustrated by Figure 2
performed
flawlessly, permitting successive tests to be carried out successfully over a
period of
nearly two hours immersion in a magnesium melt. Moreover, when the tests were
ended the tubes 26 and 32 and disc 40 were found to be intact and uncorroded.
This
may be contrasted with the experience with sample tubes of compacted silica
which
2 o were used during initial experiments to test for inclusions in magnesium
when they
were found to be resistant to attack by liquid magnesium. It is believed that
this
resistance resulted from initial reaction of the surface with the melt to form
a
relatively impervious corrosion product layer. The sensing zone orifices were
formed
in inserts of boron nitride which were screw threaded into the tube walls. The
is
CA 02192507 1997-O1-21
L I /LJU~
tubes were found to be slightly porous and tended to crack. easily due to
thermal
shock, the failure rate being from 10% to 50%, depending upon the porosity and
grain
size, so that finding the right batch of tubes to be used was somewhat a hit-
or-miss
problem and the tubes could not reliably be re-used.
Figures 4 and 5 show two further constructions for the upper portions
of the sensor probes, that of Figure 4 employing a plurality of springs, as
with the
embodiment of Figure 2, while that of Figure 5 employs a single spring, as
with the
embodiment of Figure 3. These springs are accommodated in the upper part of
the
chamber 34 between the two tubes and the resulting probes are of smaller
radial
1 o dimension. Such constructions are preferred for one-shot operation. In the
embodiment of Figure 4 the springs are connected between inward extending
flange
56 corresponding to the outward extending flange 56 of the embodiment of
Figure 2,
and outward extending flange 61 at the upper end of the inner tube 26,
corresponding
to the bottom 61 of the cup-shaped member 60 of the embodiment of Figure 2. An
additional annular spacer ring 54 is provided between the flange b1 and the
inner wall
of a cup-shaped extension of the outer tube 32, this extension taking the
place of the
cup-shaped member 60 of the embodiments of Figures 2 and 3. In the embodiment
of Figure 5 a single compression spring 52 is interposed between the annular
washer
64 at the upper end of the outer tube and an annular washer $9 mounted on the
2 o flange 61 at the upper end of the inner tube 26. The different numbers of
springs
usable with the embodiments of Figures 2 and 3 are also usable with these
embodiments.
A very satisfactory material for the tubes 26 and 32 is steel, preferably
a low carbon mild steel (of the order of 0-1°l°, usually about
0.1% carbon), since the
19
CA 02192507 1997-O1-21
solubility of iron in molten magnesium or copper is minimal, and is still very
low in
molten aluminium. As described above, a rough unfinished surface is preferred
to
increase the radiation coefficient, whether or not a black highly radiative
coating is
also employed. It is also a suitable material for the electrodes 78 and 80
when
magnesium is to be tested, since it is not attacked by magnesium. Other high
melting point metals or alloys such as titanium, zirconium a.nd INCONEL (Trade
Mark) may also be employed, provided they are not unduly corroded by the
molten
metal, and provided that the available life justifies the higher cost. Steel
is much
cheaper than the equivalent quantity of a shaped refractory ceramic and the
resultant
1 o sensor probes are more robust, resulting in a longer life during which
they can be
reused. The relatively simple disc 40 required for the sensing :one orifice
42, and the
short, simple, robust spacer rings 54 and 78 require a minimum of material and
are
relatively inexpensive to manufacture, resulting in a sample tube that is
stronger and
more robust than an equivalent ceramic tube, and which is potentially much
less
expensive to manufacture, despite the need for the cup-shaped members 58 and
60 and
the springs 52. A very suitable material for the disc 40 is boron nitride,
which is not
a particularly difficult material to work, the manufacturing operation
required to
produce the sensing zone orifice in a disc being much simples- than that
required to
form it in a long sample tube, while the scrap cost if the orifice is
imperfect is
2 o correspondingly very much less.
Metals are inherently much less susceptible than ceramics to damage or
destruction by thermal shock, even if some deformation results from the sudden
heating. ~tlith the relatively high strength modulus of steel, even at the
high
temperatures of the molten metal, a minimum wall thickness for both tubes of
about
CA 02192507 1997-O1-21
L I l L. J V r
1mm (0.04in) will be satisfactory. The radial dimension of the annular space
34 will
be in the range 2mm-l0mm (0.08in-0.40in), preferably 2mm-5mm (0.08in-0.20in),
the
dimension being as small as possible without the possibility of the tubes
contacting
one another, in the event of any shape distortion produced by the sudden
elevation
in their temperatures. A suitable outside diameter for the mounting member 48
so
that it will fit in the apparatus head 20 is 2.5cm (1in). For measurements
with
magnesium an appropriate size for the molten metal sample s is about 60m1-
100m1,
resulting in a column of metal within the inner tube interior of about 12.5-20
cm (5-
Bins) length, so that an appropriate minimum length of inner tube is about
25cm
(loins), resulting in a probe of about 45cm (l8ins) overall length. If the
metal to be
sampled is iron or steel, as will be discussed below, the gas within the space
34 can
be air. An atmosphere of argon or argon plus sulfur hexafluoride will be
required
for magnesium, while argon or nitrogen will be required for aluminium or
copper.
Boron nitride has been used as the material for the sensing zone disc 40
but it can instead be any one of silicon nitride, aluminium nitride, magnesia
or silica.
The high-temperature electrically insulating materials used for the sensing
disc 40 can
also be used for the connector 50, the spacer ring 54, the washers 64 and 89,
and any
intermediate ring 78, in the event that it is not possible to use, or it is
preferred not
to use, a material such as TEFLON ''~ that is more sensitive to heat. The
lower cost
2 o magnesia and silica are more appropriately used for elements that are not
subjected
to the wearing effect of the direct passage of the hot metal. l:n the
embodiments to
be described below the disc. needs to be only a thickness of 1-5mm (0.04-
0.20in),
preferably 1-2mm (0.04-0.08in), but there is advantage in making it much
thicker, e.g.
in the range 1.5-2.5cm (0.6-l.Ocm) so that, as shown in Figures 2 and 6,
entrance 90
21
CA 02192507 1997-O1-21
L I 7GJU1
to the sensing zone passage, and also exit 92 therefrom, can be tapered
towards the
passage so as to ensure that the flow of metal through the passage is as
streamlined
as possible. Two values of included angle of 81° and 93° were
tested without any
noticeable difference in performance. The size of the sensing orifice 42
depends upon
the metal under test, and the known nature of the inclusions. to be detected,
and is
chosen to be reasonably larger (e.g. 2 times) than the largest inclusion
particle
expected to be encountered in the melt. The orifice will usually be of the
order of
about 300-350 microns for the lower melting point metals, and about 750
microns for
iron and steel. It is found that a somewhat larger aperture is required for
magnesium
1 o than for aluminium owing to the larger inclusions encountered and also the
much
greater number of inclusions found, believed to be due to the much higher
oxidation
potential of magnesium.
The embodiment of Figure 6 is also one in which the metal tubes 26 and
15 32 are not used as the sensing electrodes, and instead they serve merely as
a
replacement for the ceramic sensing tube of the prior art apparatus, the
sensing
function being provided by a pair of electrodes 79 and 80. The inner electrode
is
provided with a splash guard 94 to divert the entering stream of molten metal.
The
sensing zone containing member comprises a thin flat disc 40 which is
sandwiched
2 o between the horizontal bottoms 28 and 36 of the two tubes and extends the
full
width of the interior of the outer tube 32, the joints between the disc and
the bottom
surfaces being sealed with a suitable high temperature cement 96 (for example
saureisen cement). The concentric spacing between the two tubes is maintained
by
an annular spacer ring 54 at the top end of the outer tube 32 and by a similar
annular
22
CA 02192507 1997-O1-21
7 LJLJT
spacer ring 96 at the adjacent bottom ends of the two tubes and butting
against the
upper surface of the sensing zone disc 40. Owing to very different
coefficients of
thermal expansion of steel and most refractory materials suitable for use as
the sensing
zone disc difficulty may be experienced in maintaining the tightness of the
cemented
joints unless the probes are taken carefully through a heating protocol as
described
above to bring them as close as possible to their operating temperature while
maintaining the integrity of the seals before immersing them in the molten
metal
bath. For example, the preferred material boron nitride has a coefficient of
0.87
~,m/m/°C perpendicular to the pressing direction and 2.95
~,m/m/°C parallel to the
to pressing direction, while steel has a coefficient of about 12
~ern/m/°C at 100°C and
about 15 ~,m/m/°C at 700°C. The rings may be maintained
mechanically in
position along the tubes by crimping the tubes at various locations 98, as
shown in
the drawings, or by the use of radially extending fastening pins 100, or by
any other
suitable fastening means available for such high temperature apparatus.
Typically the
spacing rings will be of length in the range 1-2.5cm (0.4-l.Oin), so that with
two rings
used with an inner tube of length proposed above the rings extend over a
minimum
of about 8% of the length of the tube, and a maximum of about 20%. If more
than
two rings are used then preferably their lengths are reduced so that together
they do
not occupy more than 25% of the length of the inner tube, a.nd preferably they
do
2 o not occupy more than 10% of the length.
Figure 7 shows a sensor probe designed and intended for inclusion
sampling in liquid iron and steel. Owing to the high melt temperatures
involved
(1500°C-1700°C), at high superheats (superheat = bath
temperature minus freezing
point or liquidus temperature) the outer tube 32 will quickly be severely
eroded, and
23
CA 02192507 1997-O1-21
~IyL~Ul
consequently such a probe must be considered to be a one-shot device, unless
only
used in steel melts at temperatures at which they are about to freeze. This is
usually
also the case with the known ceramic probes, and the potentially less
expensive
construction characteristic of the probes of the invention therefore makes
them
particularly suitable for such an application. The features of this embodiment
that
adapt it for single-use applications can be employed in the previously
described multi-
use embodiments when they are intended for single-use operation. In this
embodiment the sensing zone member 40 has the form of a shallow ceramic cup or
crucible the bottom of which contains the electric sensing zone orifice 42,
and the
z o side wall of which extends between the inner and outer steel tubes 26 and
32 to
maintain them separated at their lower ends. The member therefore replaces the
separate disc 40 and spacing ring 96 of the embodiment of Figure 4. Such a
shallow
cup-shaped crucible is considerably more robust and is relatively inexpensive
to make,
so that the overall cost of the probe can be competitive with that of the
highly
15 elongated sample tubes required by the prior art probes. The inner metal
tube is
made somewhat thicker, of the order of l.5mm-2mm (0.06in-0.08in) while the
radial
dimension of the annular space 34 remains about the same, and the thickness of
the
outer metal tube is greatly increased in proportion to the amount of superheat
that
is encountered, for example to at least 4mm (0.16in) for 50°C
superheat. This
2 o thickness should allow an immersion time of up to four minutes before the
tubes melt
sufficiently to fail and admit metal directly to the inner tube interior. By
making the
probe tubes of high quality, low carbon steel, contamination of the melt is
not an
issue, even though the probe is completely consumed. For a given superheat
longer
immersion times can be obtained by making the outer tube even thicker, but
there
24
CA 02192507 1997-O1-21
C I 7GJU1
is a practical limit to the potential increase beyond which the possibility of
metal
freezing in the aperture 42 and the lower part of the inner tube becomes too
great,
and the probe becomes too big and heavy. Another possibility is to add a
cylinder
102 of thermal insulation over the outer tube 32 which will delay its
destruction, but
with added expense in manufacture, the possibility of adding undesired
contaminants
to the melt, and again increased possibility of freezing of the entering
metal.
In this embodiment, the two tubes 26 and 32 constitute the sensing
electrodes while the probe is detachably connected to the lower end of an
elongated
support member 104, such as a lance, which enables the probe to be thrust
safely
1o manually by an operator into the furnace. The inner and outer tubes are
therefore
connected to respective electric leads 106 and 108 which pass upward through
the
lance to the measuring apparatus. Since the annular space 34 cannot in such an
embodiment be vented to the ambient atmosphere it is instead vented by the
bore 48
to the interior of the inner tube. The probe inner tube 26 is closed at the
top end
and the sensor is assembled with sufficient vacuum within its interior to
eliminate the
need for an external vacuum source that would otherwise require a tube to
extend
through the lance. In other embodiments which are not specifically illustrated
such
a single-use disposable sensor probe is designed as a "bomb" which is
connected to the
electrical apparatus by a flexible cable, and which is thrown into the furnace
2 o containing the melt. Design requirements for such probes, are described,
for example,
in PCT application No. PCT/CA90/00140, International Publication No.
X1090/13014, to which further reference can be made.
As with the other embodiments described above, the probe will usually
be preheated before it is immersed to ensure that the entering metal does not
freeze.
CA 02192507 1997-O1-21
21 ~~SUI
In addition the aperture 42 may be closed by a thin cap 114 of a material that
melts
much faster than those of the outer tube 32 and the cylinder 102, such as high
carbon
iron or aluminium. The cap closes the aperture until it melts after a period
determined by its thickness, giving an opportunity for the sensing zone member
40
and the inner tube to heat to a suitable operating temperature before the
metal enters
the probe interior under the urge of the internal vacuum.
Provision may be made in such sensor probes for a fixed sample of the
molten metal to be drawn into the probe body by placing a chill block 110 at
the
desired level within the inner steel tube 26, the block dividing the tube
interior into
1 o upper and lower compartments which are connected by a narrow bore 112 of
about
1mm (0.04in) diameter. The bore allows the vacuum in the upper compartment to
be effective in drawing metal into the lower compartment, but the block is
sufficiently large, and the bore is sufficiently small that the molten metal
will freeze
in it and prevent further metal being drawn into the probe when a sample of
the
volume of the lower compartment has entered the probe interior.
Figure 8 shows another construction for the lower end of the probe, in
which the horizontal bottom 28 of the inner metal tube has been eliminated and
the
tube end is closed by the sensing zone disc 40. The disc is provided with a
2 o downward tapered surface 46 which centres the disc in the outer metal tube
aperture
38 and it is cemented to the bottom end of the tube by a fillet of a
refractory cement
96. The spacing means at this end is completed by a ring 116 of a castable
refractory
cement, the material also entering any space that exists between the disc 40
and the
26
CA 02192507 1997-O1-21
L 1 7~~vj
tube bottom 36 to seal against entry of liquid metal from the bath other than
through
the sensing zone orifice 42.
2?
