Abstrict
An electric heater assembly suitable for heating molten metal,
the electric heater assembly having a sleeve comprised of a closed
end suitable for immersing in the molten metal. The sleeve is fabricated
from a composite material comprised of titanium alloy and having
an outside surface to be exposed to the molten metal coated with
a refractory resistant to attack by the molten metal having a sealant
metal applied thereto; and an electric heater located in the sleeve
in heat transfer relationship therewith.
Claims
What is claimed is:
1. A method of forming an electric heater suitable for heating
molten metal, the electric heater comprised of a tube having a closed
end for immersing in the molten metal, the tube comprised of a composite
material, the method comprising the steps of: (a) providing a tube
having a coefficient of thermal expansion less than 8.times.10.sup.-6
in/in/.degree. F. and having an outside surface; (b) applying a
bond coat to the outside surface; (c) applying a refractory coating
to said bond coat; (d) applying a metal sealant to said refractory;
(e) converting said metal sealant to provide a transformed metal
sealant resistant to reaction by said molten metal; and (f) locating
an electric heating means in said tube.
2. The method in accordance with claim 1 including applying said
metal sealant by vapor deposition.
3. The method in accordance with claim 1 including applying said
metal sealant by immersing in molten metal sealant.
4. The method in accordance with claim 1 wherein said metal tube
is fabricated from a metal selected from the group consisting of
titanium, titanium alloys, stainless steel, nickel based alloys
and iron based alloys.
5. The method in accordance with claim 1 wherein said bond coating
comprises an alloy selected from the group consisting of Cr--Ni--Al
alloy, a Cr--Ni--Al--Y alloy and a Cr--Ni alloy.
6. The method in accordance with claim 1 wherein said refractory
coating is selected from the group consisting of one of Al.sub.2O.sub.3,
ZrO.sub.2, Y.sub.2O.sub.3 stabilized ZrO.sub.2, SiAlON and Al.sub.2O.sub.3--TiO.sub.2.
7. The method in accordance with claim 1 wherein said metal sealant
is selected from the group consisting of Mg, Al, Zn, Ca and Y.
8. The method in accordance with claim 1 wherein said oxidized
metal sealant is magnesium oxide.
9. The method in accordance with claim 1 including converting the
said metal sealant to a metal oxide.
10. An electric heater assembly suitable for immersion heating
molten metal, the electric heater assembly comprised of a tube having
a closed end suitable for immersing in molten metal, the tube fabricated
from a composite material comprised of a case having a coefficient
of thermal expansion of less than 10.times.10.sup.-6 in/in/.degree.
F. and having an outer surface coated with a refractory coating
having a coefficient of thermal expansion less than 10.times.10.sup.-6
in/in/.degree. F.; the refractory coating having a metal sealant
applied thereto, the metal sealant oxidized to provide an oxidized
metal sealant in pores of the refractory coating, the oxidized metal
sealant resistant to attack by said molten metal.
11. The electric heater in accordance with claim 10 wherein the
metal case is selected from a metal selected from the group consisting
of titanium, titanium alloys, stainless steel, nickel based alloys
and iron based alloys.
12. The method in accordance with claim 10 wherein said metal sealant
is selected from the group consisting of Mg, Al, Zn, Ca and Y.
13. The method in accordance with claim 10 wherein said oxidized
metal sealant is magnesium oxide.
14. A method of forming an electric heater suitable for heating
molten metal, the electric heater comprised of a tube having a closed
end for immersing in the molten metal, the tube comprised of a composite
material, the method comprising the steps of: (a) providing a metal
tube having a coefficient of thermal expansion less than 8.times.10.sup.-6
in/in/.degree. F. and having an outside surface; (b) applying a
bond coat to the outside surface; (c) applying a refractory coating
to said bond coat; (d) heating said refractory coating to cause
formation of micro cracks and to form oxides of metals comprising
said bond coat and metal tube in said micro cracks to render said
refractory coating resistant to corrosive attack by said molten
metal; and (e) locating an electric heating means in said tube.
15. The method in accordance with claim 14 including heating said
coating to a temperature range of 400.degree. to 2200.degree. F.
16. The method in accordance with claim 14 wherein said metal tube
is fabricated from a metal selected from the group consisting of
titanium, titanium alloys, stainless steel, nickel based alloys
and iron based alloys.
17. The method in accordance with claim 14 wherein said bond coating
comprises an alloy selected from the group consisting of Cr--Ni--Al
alloy, a Cr--Ni--Al--Y alloy and a Cr--Ni alloy.
18. The method in accordance with claim 14 wherein said refractory
coating is selected from the group consisting of one of Al.sub.2O.sub.3,
ZrO.sub.2, Y.sub.2O.sub.3 stabilized ZrO.sub.2, SiAlON and Al.sub.2O.sub.3--TiO.sub.2.
19. An electric heater assembly suitable for immersion heating
molten metal, the electric heater assembly comprised of a tube having
a closed end suitable for immersing in molten metal, the tube fabricated
from a composite material comprised of a metal case having a coefficient
of thermal expansion of less than 10.times.10.sup.-6 in/in/.degree.
F. and having an outer surface coated with a refractory coating
having a coefficient of thermal expansion less than 10.times.10.sup.-6
in/in/.degree. F.; the refractory coating oxidized by heating in
a temperature range of 400.degree. to 2200.degree. F. to provide
an oxidized metal in micro cracks of the refractory coating, the
oxidized metal rendering the refractory coating resistant to attack
by said molten metal.
20. The electric heater in accordance with claim 19 wherein the
metal case is selected from a metal selected from the group consisting
of titanium, titanium alloys, stainless steel, nickel based alloys
and iron based alloys.
21. The method in accordance with claim 19 wherein said refractory
coating is selected from the group consisting of one of Al.sub.2O.sub.3,
ZrO.sub.2, Y.sub.2O.sub.3 stabilized ZrO.sub.2, SiAlON and Al.sub.2O.sub.3--TiO.sub.2.
22. The method in accordance with claim 19 wherein said bond coating
comprises an alloy selected from the group consisting of Cr--Ni--Al
alloy, a Cr--Ni--Al--Y alloy and a Cr--Ni alloy.
Description BACKGROUND OF THE INVENTION
[0001] This invention relates to electric heaters, and more particularly,
it relates to electric heaters suitable for use in molten metals
such as molten aluminum.
[0002] In the prior art, electric heaters used for molten aluminum
are usually enclosed in ceramic tubes. Such electric heaters are
very expensive and are very inefficient in transferring heat to
the melt because of the air gap between the heater and the tube.
Also, such electric heaters have very low thermal conductivity values
that are characteristic of ceramic materials. In addition, the ceramic
tube is fragile and subject to cracking. Thus, there is a great
need for an improved electric heater suitable for use with molten
metal, e.g., molten aluminum, which is efficient in transferring
heat to the melt. The present invention provides such an electric
heater and provides a refractory coating resistant to attack by
the molten aluminum.
SUMMARY OF THE INVENTION
[0003] It is an object of the invention to provide an improved
electric heater assembly.
[0004] It is another object of the invention to provide an improved
electric heater assembly for use in molten metal such as molten
aluminum.
[0005] Yet, another object of this invention is to provide an improved
electric heater assembly for use in molten metal, the electric heater
assembly having a protective sleeve that has intimate physical contact
with the heating element, thereby substantially eliminating the
air gap between the heater and sleeve.
[0006] And yet, another object of the invention is to provide an
improved electric heater assembly for use in molten metal, the electric
heater assembly having a protective sleeve having a thermal conductivity
of less than 30 BTU/ft hr.degree. F. and having a thermal expansion
coefficient of less than 15.times.10.sup.-6 in/in/.degree. F. and
having a chilling power of less than 5000 BTU.sup.2/ft.sup.4 hr.degree.
F.
[0007] And yet, it is a further object of the invention to provide
an improved electric heater assembly for use in molten metal, the
electric heater assembly having a protective sleeve comprised of
a material resistant to erosion or dissolution by molten metal such
as molten aluminum.
[0008] These and other objects will become apparent from the specification,
drawings and claims appended hereto.
[0009] In accordance with these objects, there is disclosed an
improved electric heater assembly suitable for heating molten metal.
The electric heater assembly is comprised of a tube having a closed
end suitable for immersing in molten metal, the tube fabricated
from a composite material comprised of a metal case having a coefficient
of thermal expansion of less than 10.times.10.sup.-6 in/in/.degree.
F. The refractory coating has a metal sealant applied thereto, the
metal sealant oxidized to provide an oxidized metal sealant in pores
of the refractory coating. The oxidized metal sealant in combination
with the refractory coating is resistant to attack by the molten
metal.
[0010] The invention also includes a method of forming an electric
heater suitable for heating molten metal wherein the electric heater
is comprised of a tube having a closed end for immersing in the
molten metal. The tube is comprised of a composite material. The
method of forming the electric heater comprises the steps of providing
a metal tube having a coefficient of thermal expansion less than
8.times.10.sup.-6 in/in/.degree. F. and having an outside surface.
A bond coat is applied to the outside surface and a refractory coating
is applied to the bond coat. A metal sealant is applied to the refractory
coating and then the metal sealant is oxidized to provide a metal
sealant resistant to reaction by said molten metal. An electric
heating means is located in the tube.
BRIEF DESCRIPTION OF THE FIGURES
[0011] FIG. 1 is a cross-sectional view of an electric heater assembly
in accordance with the invention.
[0012] FIG. 2 is a curve of Mg vapor versus temperature.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] Referring to FIG. 1, there is shown a schematic of an electric
heater assembly 10 in accordance with the invention. The electric
heater assembly is comprised of a protective sleeve 12 and an electric
heating element 14. A lead 18 extends from electric heating element
14 and terminates in a plug 20 suitable for plugging into a power
source. A suitable element 14 is available from International Heat
Exchanger, Inc., Yorba Linda, Calif. 92687 under the designation
P/N HTR2252.
[0014] Preferably, protective sleeve 12 is comprised of titanium
tube 30 having a closed end 32. While the protective sleeve is illustrated
as a tube, it will be appreciated that any configuration that protects
or envelops electric heating element 14 may be employed. Thus, reference
to tube herein is meant to include such configurations. A refractory
coating 34 is employed which is resistant to attack by the environment
in which the electric heater assembly is used. A bond coating may
be employed between the refractory coating 34 and titanium tube
30. Electric heating element 14 is seated or secured in tube 30
by any convenient means. For example, swagelock nuts and ferrules
may be employed or the end of the tube may be crimped or swaged
shut to provide a secure fit between the electric heating element
and tube 30. In the invention, any of these methods of holding the
electric heating element in tube 30 may be employed. It should be
understood that tube 30 does not always have to be sealed. In a
preferred embodiment, electric heating element 14 is inserted into
tube 30 to provide an interference or friction fit. That is, it
is preferred that electric heating element 14 has its outside surface
in contact with the inside surface of tube 30 to promote heat transfer
through tube 30 into the molten metal. Thus, air gaps between the
surface of electric heating element 14 and inside surface of tube
30 should be minimized.
[0015] If electric heating element 14 is inserted in tube 30 with
a friction fit, the fit gets tighter with heat because electric
heating element 14 expands more than tube 30, particularly when
tube 30 is formed from titanium.
[0016] While it is preferred to fabricate tube 30 out of a titanium
base alloy, tube 10 may be fabricated from any metal or metalloid
material suitable for contacting molten metal and which material
is resistant to dissolution or erosion by the molten metal. Other
materials that may be used to fabricate tube 30 include silicon,
niobium, chromium, molybdenum, combinations of NiF (364 NiFe) and
NiTiC (40 Ni 60 TiC), particularly when such materials have low
thermal expansion and low chilling power, all referred to herein
as metals. For protection purposes, it is preferred that the metal
or metalloid be coated with a material such as a refractory resistant
to attack by molten metal and suitable for use as a protective sleeve.
[0017] Further, the material of construction for tube 30 should
have a thermal conductivity of less than 30 BTU/ft hr.degree. F.,
and preferably less than 15 BTU/ft hr.degree. F., with a most preferred
material having a thermal conductivity of less than 10 BTU/ft hr.degree.
F. Another important feature of a desirable material for tube 30
is thermal expansion. Thus, a suitable material should have a thermal
expansion coefficient of less than 15.times.10.sup.-6 in/in/.degree.
F., with a preferred thermal expansion coefficient being less than
10.times.10.sup.-6 in/in/.degree. F., and the most preferred being
less than 5.times.10.sup.-6 in/in/.degree. F. Another important
feature of the material useful in the present invention is chilling
power. Chilling power is defined as the product of heat capacity,
thermal conductivity and density. Thus, preferably the material
in accordance with the invention has a chilling power of less than
5000 BTU.sup.2/ft.sup.4 hr.degree. F., preferably less than 2000
BTU.sup.2/ft.sup.4 hr.degree. F., and typically in the range of
100 to 750 BTU.sup.2/ft.sup.4 hr.degree. F.
[0018] As noted, the preferred material for fabricating into tubes
30 is a titanium base material or alloy having a thermal conductivity
of less than 30 BTU/ft hr.degree. F., preferably less than 15 BTU/ft
hr.degree. F., and typically less than 10 BTU/ft hr.degree. F.,
and having a thermal expansion coefficient less than 15.times.10.sup.-6
in/in/.degree. F., preferably less than 10.times.10.sup.-6 in/in/.degree.
F., and typically less than 5.times.10.sup.-6 in/in/.degree. F.
The titanium material or alloy should have chilling power as noted,
and for titanium, the chilling power can be less than 500, and preferably
less than 400, and typically in the range of 100 to 300 BTU/ft.sup.2
hr.degree. F.
[0019] When the electric heater assembly is being used in molten
metal such as lead, for example, the titanium base alloy need not
be coated to protect it from dissolution. For other metals, such
as aluminum, copper, steel, zinc and magnesium, refractory-type
coatings should be provided to protect against dissolution of the
metal or metalloid tube by the molten metal.
[0020] For most molten metals, the titanium alloy that should be
used is one that preferably meets the thermal conductivity requirements,
the chilling power and the thermal expansion coefficient noted herein.
Further, typically, the titanium alloy should have a yield strength
of 30 ksi or greater at room temperature, preferably 70 ksi, and
typical 100 ksi. The titanium alloys included herein and useful
in the present invention include CP (commercial purity) grade titanium,
or alpha and beta titanium alloys or near alpha titanium alloys,
or alpha-beta titanium alloys. The titanium-base alloy can be a
titanium selected from the group consisting of 6242, 1100 and commercial
purity (CP) grade. The alpha or near-alpha alloys can comprise,
by wt. %, 2 to 9 Al, 0 to 12 Sn, 0 to 4 Mo, 0 to 6 Zr, 0 to 2 V
and 0 to 2 Ta, and 2.5 max. each of Ni, Nb and Si, the remainder
titanium and incidental elements and impurities.
[0021] Specific alpha and near-alpha titanium alloys contain, by
wt. %, about:
[0022] (a) 5 Al, 2.5 Sn, the remainder Ti and impurities.
[0023] (b) 8 Al, 1 Mo, 1 V, the remainder Ti and impurities.
[0024] (c) 6 Al, 2 Sn, 4 Zr, 2 Mo, the remainder Ti and impurities.
[0025] (d) 6 Al, 2 Nb, 1 Ta, 0.8 Mo, the remainder Ti and impurities.
[0026] (e) 2.25 Al, 11 Sn, 5 Zr, 1 Mo, the remainder Ti and impurities.
[0027] (f) 5 Al, 5 Sn, 2 Zr, 2 Mo, the remainder Ti and impurities.
[0028] The alpha-beta titanium alloys comprise, by wt. %, 2 to
10 Al, 0 to 5 Mo, 0 to 5 Sn, 0 to 5 Zr, 0 to 11 V, 0 to 5 Cr, 0
to 3 Fe, with 1 Cu max., 9 Mn max., 1 Si max., the remainder titanium,
incidental elements and impurities.
[0029] Specific alpha-beta alloys contain, by wt. %, about:
[0030] (a) 6 A, 4 V, the remainder Ti and impurities.
[0031] (b) 6 Al, 6 V, 2 Sn, the remainder Ti and impurities.
[0032] (c) 8 Mn, the remainder Ti and impurities.
[0033] (d) 7 Al, 4 Mo, the remainder Ti and impurities.
[0034] (e) 6 Al, 2 Sn, 4 Zr, 6 Mo, the remainder Ti and impurities.
[0035] (f) 5 Al, 2 Sn, 2 Zr, 4 Mo, 4 Cr, the remainder Ti and impurities.
[0036] (g) 6 Al, 2 Sn, 2 Zn, 2 Mo, 2 Cr, the remainder Ti and impurities.
[0037] (h) 10 V, 2 Fe, 3 Al, the remainder Ti and impurities.
[0038] (i) 3 Al, 2.5 V, the remainder Ti and impurities.
[0039] The beta titanium alloys comprise, by wt. %, 0 to 14 V,
0 to 12 Cr, 0 to 4 Al, 0 to 12 Mo, 0 to 6 Zr and 0 to 3 Fe, the
remainder titanium and impurities.
[0040] Specific beta titanium alloys contain, by wt. %, about:
[0041] (a) 13 V, 11 Cr, 3 Al, the remainder Ti and impurities.
[0042] (b) 8 Mo, 8 V, 2 Fe, 3 Al, the remainder Ti and impurities.
[0043] (c) 3 Al, 8 V, 6 Cr, 4 Mo, 4 Zr, the remainder Ti and impurities.
[0044] (d) 11.5 Mo, 6 Zr, 4.5 Sn, the remainder Ti and impurities.
[0045] When it is necessary to provide a coating to protect tube
30 of metal or metalloid from dissolution or attack by molten metal,
a refractory coating 34 is applied to the outside surface of tube
30. The coating should be applied above the level to which the electric
heater assembly is immersed in the molten metal. The refractory
coating can be any refractory material, which provides the tube
with a molten metal resistant coating. The refractory coating can
vary, depending on the molten metal. Thus, a novel composite material
is provided permitting use of metals or metalloids having the required
thermal conductivity and thermal expansion for use with molten metal,
which heretofore was not deemed possible.
[0046] When the electric heater assembly is to be used for heating
molten metal such as aluminum, magnesium, zinc, or copper, etc.,
a refractory coating may comprise at least one of alumina, zirconia,
yittria stabilized zirconia, magnesia, magnesium titanite, or mullite
or a combination of alumina and titania. While the refractory coating
can be used on the metal or metalloid comprising the tube, a bond
coating can be applied between the base metal and the refractory
coating. The bond coating can provide for adjustments between the
thermal expansion of the base metal alloy, e.g., titanium, and the
refractory coating when necessary. The bond coating thus aids in
minimizing cracking or spalling of the refractory coat when the
tube is immersed in the molten metal or brought to operating temperature.
When the electric heater assembly is cycled between molten metal
temperature and room temperature, for example, the bond coat can
be advantageous in preventing cracking, particularly if there is
a considerable difference between the thermal expansion of the metal
or metalloid and the refractory.
[0047] Typical bond coatings comprise Cr--Ni--Al alloys and Cr--Ni
alloys, with or without precious metals. Bond coatings suitable
in the present invention are available from Metco Inc., Cleveland,
Ohio, under the designation 460 and 1465. In the present invention,
the refractory coating should have a thermal expansion that is plus
or minus five times that of the base material. Thus, the ratio of
the coefficient of expansion of the base material can range from
5:1 to 1:5, preferably 1:3 to 1:1.5. The bond coating aids in compensating
for differences between the base material and the refractory coating.
[0048] The bond coating has a thickness of 0.1 to 5 mils with a
typical thickness being about 0.5 mil. The bond coating can be applied
by sputtering, plasma or flame spraying, chemical vapor deposition,
spraying, dipping or mechanical bonding by rolling, for example.
[0049] After the bond coating has been applied, the refractory
coating is applied. The refractory coating may be applied by any
technique that provides a uniform coating over the bond coating.
The refractory coating can be applied by aerosol, sputtering, plasma
or flame spraying, for example. Preferably, the refractory coating
has a thickness in the range of 0.3 to 42 mils, preferably 5 to
15 mils, with a suitable thickness being about 10 mils. The refractory
coating may be used without a bond coating.
[0050] In another aspect of the invention, boron nitride may be
applied as a thin coating on top of the refractory coating. The
boron nitride may be applied as a dry coating, or a dispersion of
boron nitride and water may be formed and the dispersion applied
as a spray. The boron nitride coating is not normally more than
about 2 or 3 mils, and typically it is less than 2 mils.
[0051] The refractory coating may be sealed to provide additional
protection against penetration of molten metal, e.g., molten aluminum
that will react with metal tube 30. Thus, in yet another aspect
of the invention, a sealant agent such as magnesium oxide may be
intruded into the pores or micro cracks in refractory coating 34.
The actual sealant agent may be directly intruded, such as magnesium
oxide particulate, or result from an in-situ reaction that occurs
within the pores or micro cracks. A necessary condition for direct
intrusion of a sealant agent is that the particle size of the agent
is less than the effective diameter of the pore or discontinuity.
Available sealant agents due to these size incompatibilities cannot
necessarily intrude pores or micro cracks with an exceptionally
small effective diameter. It has been found that certain liquids
or even vapors can easily intrude pores or micro cracks without
being limited by size. These liquids or vapors may not be acceptable
as a sealant agent, alone, however, they can be reacted or transformed
in the pore or micro crack itself to become an acceptable agent.
An example of such a transformed sealant is magnesium oxide that
originated as magnesium metal and was transformed to the oxide within
the pore or micro crack. In the case of magnesium, it can be intruded
into the refractory coating, subjected to an oxidation process to
form the oxide, e.g., magnesium oxide, of the intruded metal to
provide a compound such as magnesium oxide that is resistant to
attack by molten metal such as molten aluminum.
[0052] Any process that intrudes the sealant metal into the pores
of refractory coating 34 may be used. For example, the refractory
coated tube may be dipped in molten magnesium to intrude metal into
the pores. Excess metal adhering to refractory coating 30 is removed
leaving only metal intruded in the pores.
[0053] In another method, sputtering of a metal onto the refractory
coating and into the pores can be used.
[0054] In yet another method, a form of vapor deposition may be
used to intrude the metal. The refractory coated tube is placed
in a retort in such a manner as to seal the outside diameter of
the tube to the atmosphere, while allowing access to the inside
diameter of the tube. Under an inert gas backfill, e.g., argon,
the retort is heated above the melting point of the metal being
intruded, e.g., about 1200.degree. F. for magnesium. The argon atmosphere
is removed and a vacuum is applied sufficient to vaporize the molten
magnesium in the retort at that particular temperature. At this
point, the tube is internally cooled to a temperature sufficient
to permit condensation of magnesium vapor. This temperature can
be obtained by using an equation of the form:
ln P=-A/T+B,
[0055] where: P=absolute pressure
[0056] A,B=constants for specific metal
[0057] T=absolute temperature
[0058] In the case of magnesium, this equation becomes:
In P(mm Hg)=-15829/T(.degree. K)+17.99
[0059] This equation is represented in graphical form in FIG. 2
that becomes an operating curve for a magnesium retort. Successful
magnesium vaporization requires the maintenance of a temperature/pressure
condition that falls under the illustrated curve.
[0060] An air jet or air/fluid mist directed inside the tube can
be used for this purpose. The vaporized magnesium condenses out
on the refractory coating and within the pores and micro cracks.
Thereafter, the intruded magnesium is oxidized. Any process that
transforms the intruded magnesium metal to magnesium oxide may treat
the magnesium. For example, a thermal conversion process can be
used where the refractory coated tube is heated in an air furnace
to convert the intruded magnesium metal to magnesium oxide. The
tube should be heated to a temperature of at least 600.degree. F.
for a minimum duration of 30 minutes to form magnesium oxide. Metals
other than magnesium a can be used for this purpose, provided that
it will form an oxide that is resistant to attack by molten metal,
such as aluminum. In the thermal conversion method, it is also necessary
for the oxide formed by this metal to be incoherent and be able
to grow to completion.
[0061] The refractory coating has also been shown to develop improved
resistance to molten metal, e.g., molten aluminum, by subjecting
it to a specific thermal practice. It is believed that heating the
refractory coated tube prior to immersion in molten metal develops
micro cracks that help to accommodate thermally induced mechanical
stresses. If these micro cracks develop after the tube is exposed
to molten metal, failure may occur if the molten metal is permitted
to react with the bond coat that would now become exposed as a result
of the micro cracks. Allowing the micro cracks to develop before
molten metal exposure and subsequently sealing the micro cracks
has been shown to improve resistance of the refractory coating to
molten metal. Sealing can be accomplished by one of the previously
described methods. An additional method can be used if the bond
coating can be converted to a material resistant to attack by molten
metal. In cases where the bond coat consists of elements that can
form stable and resistant oxides, a thermal treatment sufficient
to produce such oxides may be used. Other conversion treatments
to form stable reaction products (e.g., nitrides and/or carbides)
with the bond coat are possible with the appropriate atmosphere.
In all cases, however, such treatment must be performed after the
formation of micro cracks. In the case of exposure to molten aluminum,
a bond coat material containing aluminum, chromium, and/or titanium
can be pre-oxidized by heating to an elevated temperature following
the formation of micro cracks. It can be shown that cyclic heating
and cooling of the tube will further encourage micro crack formation.
Still further, it can be shown that heating the tube internally
from the interior wall, rather than through the exterior, will promote
micro crack formation and development.
[0062] The following practice has been shown to be beneficial in
the case of a refractory coated tube that is intended to be exposed
to molten aluminum. Such a tube will use a bond coat material that
preferably contains yttrium, and/or chromium, and/or aluminum. Following
application of the refractory top coating, the tube is heated to
1350.degree. F. from within using an inserted cartridge heater.
Micro cracks are permitted to form. The tube is then cooled to a
temperature of 500.degree. F., and then reheated to 1350.degree.
F., followed by cooling. This thermal cycle may be repeated for
three total cycles. Then, the tube is held at a temperature of 900.degree.
F. in air to allow stable oxides of yttrium, chromium, and aluminum
to form. The oxides formed by this process provide significantly
improved resistance to molten metal attack. Temperatures that may
be used are those sufficient to form stable oxides from the element(s)
in the bond coat, for example, 400.degree. to 2200.degree. F.
[0063] The heater assembly of the invention can operate at watt
densities of 40 to in excess of 140 watts/in.sup.2.
[0064] The heater assembly in accordance with the invention has
the advantage of a metallic-composite sheath for strength and improved
thermal conductivity. The strength is important because it provides
resistance to mechanical abuse and permits an ultimate contact with
the internal element. Intimate contact between heating element and
sheath inside diameter provides for substantial elimination of an
annular air gas between heating element and sheath. In prior heaters,
the annular air gap resulted in radiation heat transfer and also
backs radiation to the element from inside the sheath wall, which
limits maximum heat flux. By contrast, the heater of the invention
employs an interference fit that results in essentially only conduction.
[0065] In conventional heaters, the heating element is not in intimate
contact with the protection tube resulting in an annular air gas
or space there between. Thus, the element is operated at a temperature
independent of the tube. Heat from the element is not efficiently
removed or extracted by the tube, greatly limiting the efficiency
of the heaters. Thus, in conventional heaters, the element has to
be operated below a certain fixed temperature to avoid overheating
the element, greatly limiting the heat flux.
[0066] The heater assembly of the invention very efficiently extracts
heat from the heating element and is capable of operating close
to molten metal, e.g., aluminum temperature. The heater assembly
is capable of operating at watt densities of 40 to 120 watts/in.sup.2.
The low coefficient of expansion of the composite sheath, which
is lower than the heating element, provides for intimate contact
of the heating element with the composite sheath.
[0067] In another feature of the invention, a thermocouple (not
shown) may be inserted between sleeve 12 and heating element 14.
The thermocouple may be used for purposes of control of the heating
element to ensure against overheating of the element in the event
that heat is not transferred away sufficiently fast from the heating
assembly. Further, the thermocouple can be used for sensing the
temperature of the molten metal. That is, sleeve 12 may extend below
or beyond the end of the heating element to provide a space and
the sensing tip of the thermocouple can be located in the space.
[0068] While the invention has been described in terms of preferred
embodiments, the claims appended hereto are intended to encompass
other embodiments, which fall within the spirit of the invention.
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