Non-graphite crucible for high temperature applications

A multi-piece crucible for high temperature applications comprises a tubular side wall member having a lip on the inside surface and a bottom member or members forming a container for containing a melt of a material during a high temperature melt-casting operations. The multi-piece design prevents cracking of the crucible or leakage of the melt from the crucible during the melt-casting operation. The lip of the tubular member supports the bottom member. The contacting surfaces where the lip of the tubular side wall member contacts the bottom member of the multi-piece crucible contains a ceramic sealing material. The ceramic sealing material forms a seal sufficient to prevent the melt of the material from leaking out of the multi-piece crucible during the melt-casting process. The multi-piece crucible is made of a material which is chemically inert to the melt and has structural integrity at the melting point temperature of the melt, or of a material coated with such a material. The multi-piece crucible is contained in a thermal can assembly of a high temperature induction furnace during a high temperature melt-casting operation. One embodiment of the multi-piece crucible comprises a tubular member having a vertical slot filled with a ceramic sealing material to provide expansion of the tubular member without cracking during the high temperature melt-casting operation.

FIELD OF THE INVENTION 
The present invention relates to a crucible, more particularly, to a 
crucible for high temperature applications. 
BACKGROUND OF THE INVENTION 
Large non-graphite crucibles utilized for melting high melting point 
materials have a tendency to crack during the melt-casting process because 
of excessive mechanical stresses that develop within the crucible due to 
nonuniform heating of the crucible. The cracks occur primarily at the 
juncture of the bottom and the side wall of the crucible as well as radial 
cracks emanating from the center of the crucible bottom. The larger the 
crucible the more susceptible it is to cracking. It is desirable to reduce 
the labor and energy costs of the melt-casting process by reducing the 
time required to complete the process and to maximize the service life of 
the crucible. Therefore, it is very important to provide a non-graphite 
crucible which will not crack during the melt-casting cycle of the 
material. In addition, carbon contamination caused by the crucibles used 
in reactive melt processing is a serious problem. Therefore, it is 
important to prevent such contamination. 
OBJECTS OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
non-graphite crucible for high temperature applications which will not 
crack or leak during the melting processing of a high melting point 
material. 
Further and other objects of the present invention will become apparent 
from the description contained herein. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a new and improved 
multi-piece crucible for high temperature applications comprises a tubular 
member and a bottom member. The tubular member has a centerline, an inner 
side wall, a lower portion, a thickness, and a lip. The lip is located on 
the inner side wall of the lower portion of the tubular member. The lip 
has a tapered side tapered in a downward direction toward the centerline 
of the tubular member. The bottom member has an outer side tapered in an 
upward direction toward the tubular member at an angle that provides a 
matching fit with the tapered side of the lip for enclosing the lower 
portion of the tubular member to form the crucible for containing a melt 
of a material in high temperature casting operations. The outer side of 
the bottom member contacts the tapered side of the lip to form a seal 
sufficient to contain a melt of a material used in high temperature 
casting operations. The lip of the tubular member supports the bottom 
member. The crucible or a coating on the crucible is made of a material 
chemically inert to the melt and has structural integrity at the melting 
point temperature of the melt. 
In accordance with another aspect of the present invention, a new and 
improved multi-piece crucible for high temperature applications comprises 
a tubular member and a bottom member. The tubular member has a centerline, 
an inner side wall, a lower portion, a thickness, and a lip. The lip is 
located on the inner side wall of the lower portion of the tubular member. 
The lip has a tapered side tapered in a downward direction toward the 
centerline of the tubular member. The bottom member has an outer side 
tapered in an upward direction toward the tubular member at an angle that 
provides a matching angle with the tapered side of the lip for enclosing 
the lower portion of the tubular member to form the crucible for 
containing a melt of a material in high temperature casting operations. 
The outer side of the bottom member contacts a ceramic sealing material 
and the tapered side of the lip contacts the ceramic sealing material to 
form a seal sufficient to contain melts of materials used in high 
temperature casting operations. The lip of the tubular member supports the 
bottom member. The crucible or a coating on the crucible is made of a 
material chemically inert to the melt and has structural integrity at the 
melting point temperature of the melt. 
In accordance with another aspect of the present invention, a new and 
improved multi-piece crucible for high temperature applications comprises 
a tubular member and a bottom member. The tubular member has an inner side 
wall, a lower portion, and a lip. The lip is located on the inner side 
wall of the lower portion of the tubular member. The lip has a side and a 
top portion. The bottom member has an outer side and a bottom portion. The 
outer side of the bottom member has a periphery. The inner side wall has a 
periphery. The periphery of the outer side of the bottom member is smaller 
than the periphery of the inner side wall. The bottom portion of the 
bottom member contacts the lower portion of the tubular member to form a 
seal sufficient to contain a melt of a material used in high temperature 
casting operations. The lip of the tubular member supports the bottom 
member. The crucible or a coating on the crucible is made of a material 
chemically inert to the melt and has structural integrity at the melting 
point temperature of the melt. 
In accordance with another aspect of the present invention, a new and 
improved multi-piece crucible for high temperature applications comprises 
a tubular member and a bottom member. The tubular member has an inner side 
wall, a lower portion, and a lip. The lip is located on the inner side 
wall of the lower portion of the tubular member. The lip has a side and a 
top portion. The bottom member has a side and a bottom portion. The lip of 
the tubular member supports the bottom member. The side of the bottom 
member contacts a ceramic sealing material and the side of the lip 
contacts the ceramic sealing material to form a seal sufficient to contain 
melts of materials used in high temperature casting operations. The 
crucible or a coating on the crucible is made of a material chemically 
inert to the melt and has structural integrity at the melting point 
temperature of the melt. 
In accordance with another aspect of the present invention, a new and 
improved multi-piece crucible for high temperature applications utilizing 
large diameter crucibles comprises a multi-piece crucible having more than 
two pieces. The crucible comprises a tubular member and two bottom 
members, a first bottom member and a second bottom member. The second 
bottom member fits inside the first bottom member. The tubular member has 
a centerline, an inner side wall, a lower portion, a thickness, and a lip. 
The lip is located on the inner side wall of the lower portion of the 
tubular member. The lip has a tapered side tapered in a downward direction 
toward the centerline of the tubular member. The first bottom member has 
an outer side and an inner side. The outer side of the first bottom member 
is tapered in an upward direction toward the tubular member at an angle 
that provides a matching fit with the tapered side of lip. The inner side 
of the first bottom member is tapered in a downward direction toward the 
centerline of the tubular member. The second bottom member has an outer 
side. The outer side of the second bottom member is tapered in an upward 
direction toward the tubular member at an angle that provides a matching 
fit with the inner side of the first bottom member. The first and second 
bottom members enclose the lower portion of the tubular member to form the 
multi-piece crucible for containing a melt of a material in high 
temperature casting operations. The outer side of the first bottom member 
contacts the tapered side of the lip of the tubular member and the outer 
side of the second bottom member contacts the inner side of the first 
bottom member both forming a seal sufficient to contain a melt of a 
material used in high temperature casting operations. The lip of tubular 
member supports the first bottom member and the first bottom member 
supports the second bottom member. The crucible or a coating on the 
crucible is made of a material chemically inert to the melt and has 
structural integrity at the melting point temperature of the melt. 
For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims.

For a better understanding of the present invention, together with other 
and further objects, advantages and capabilities thereof, reference is 
made to the following disclosure and appended claims in connection with 
the above-described drawing. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Ceramic crucibles are utilized for melting refractory metals and alloys in 
high temperature casting operations. However, ceramic materials are 
inherently brittle and relatively poor conductors of heat. Consequently, 
mechanical stress is generated during the heating cycle by temperature 
differentials that occur within the sidewalls and the bottom of the 
crucible. As a result, cracks are formed usually in the bottom of the 
crucible. To overcome the cracking problem the crucible of the present 
invention is made from more than one piece to accommodate the stresses 
that are generated in the crucible during the high temperature heating 
cycles of the melting and casting process. 
Several crucible designs were prepared and tests were run to evaluate the 
effectiveness of the concept of a multi-piece crucible to solve the 
cracking problems encountered with single-piece crucibles. 
Shown in FIG. 1 is a cross-sectional view of one embodiment of the present 
invention. Crucible 10 comprises tubular member 20 and bottom member 30. 
Tubular member 20 has centerline 40, inner side wall 50, lower portion 60, 
thickness 70, and lip 80. Lip 80 is located on inner side wall 50 of Lower 
portion 60 of tubular member 20. Lip 80 has tapered side 90 tapered in a 
downward direction toward centerline 40 of tubular member 20. Bottom 
member 30 has outer side 100 tapered in an upward direction toward tubular 
member 20 at an angle that provides a matching fit with tapered side 90 of 
lip 80 for enclosing lower portion 60 of tubular member 20 to form 
crucible 10 for containing a melt of a material in high temperature 
casting operations. Outer side 100 of bottom member 30 contacts tapered 
side 90 of lip 80 to form a seal sufficient to contain a melt of a 
material used in high temperature casting operations. Lip 80 of tubular 
member 20 supports bottom member 30. 
Shown in FIG. 2 is another embodiment of the present invention. Crucible 
110 comprises tubular member 120 and bottom member 130. Tubular member 120 
has centerline 140, inner side wall 150, lower portion 160, thickness 170, 
and lip 180. Lip 180 is located on inner side wall 150 of lower portion 
160 of tubular member 120. Lip 180 has tapered side 190 tapered in a 
downward direction toward centerline 140 of tubular member 120. Bottom 
member 130 has outer side 200 tapered in an upward direction toward 
tubular member 120 at an angle that provides a matching angle with tapered 
side 190 of lip 180 for enclosing lower portion 160 of tubular member 120 
to form crucible 110 for containing a melt of a material in high 
temperature casting operations. Outer side 200 of bottom member 130 
contacts ceramic sealing material 205 and tapered side 190 of lip 180 also 
contacts ceramic sealing material 205 to form a seal sufficient to contain 
a melt of a material used in high temperature casting operations. Lip 180 
of tubular member 120 supports bottom member 130. 
Shown in FIG. 3 is a further embodiment of the present invention. Crucible 
210 comprises tubular member 220 and bottom member 230. Tubular member 220 
has inner side wall 240, lower portion 250, and lip 260. Lip 260 is 
located on inner side wall 240 of lower portion 250 of tubular member 220. 
Lip 260 has side 270 and top portion 280. Bottom member 230 has outer side 
290 and bottom portion 300. Outer side 290 of bottom member 230 has a 
periphery. Inner side wall 240 has a periphery. The periphery of outer 
side 290 of bottom member 230 is smaller than periphery of inner side wall 
240. Bottom portion 300 of bottom member 230 contacts lower portion 280 of 
lip 260 to form a seal sufficient to contain a melt of a material used in 
high temperature casting operations. Lip 260 of tubular member 220 
supports bottom member 230. 
Shown in FIG. 4 is still another embodiment of the present invention. 
Crucible 310 comprises tubular member 320 and bottom member 330. Tubular 
member 320 has inner side wall 340, lower portion 350, and lip 360. Inner 
side wall 340 of tubular 320 has lower wall portion 365. Lip 360 is 
located on inner side wall 340 of lower portion 350 of tubular member 320. 
Lip 360 has side 370 and top portion 380. Bottom member 330 has side 390 
and bottom portion 400. Bottom portion 400 and side 390 of bottom member 
330 contacts ceramic sealing material 405 and top portion 390 of lip 380 
and lower wall portion 365 of inner side wall 340 contacts ceramic sealing 
material 405 to form a seal sufficient to contain melts of materials used 
in high temperature casting operations. Lip 360 of tubular member 320 
supports bottom member 330. 
Large multi-piece crucibles of the present invention can have more than two 
pieces. Shown in FIG. 5 is a cross-sectional view of one embodiment of a 
large multi-piece crucible of the present invention. Crucible 410 
comprises tubular member 420 and bottom members 430 and 432. Tubular 
member 420 has centerline 440, inner side wall 450, lower portion 460, 
thickness 470, and lip 480. Lip 480 is located on inner side wall 450 of 
lower portion 460 of tubular member 420. Lip 480 has tapered side 490 
tapered in a downward direction toward centerline 440 of tubular member 
420. Bottom member 430 has outer side 500 and inner side 502. Outer side 
500 is tapered in an upward direction toward tubular member 420 at an 
angle that provides a matching fit with tapered side 490 of lip 480. Inner 
side 502 of bottom member 430 is tapered in a downward direction toward 
centerline 440 of tubular member 420. Bottom member 432 has outer side 
504. Outer side 504 of bottom member 432 is tapered in an upward direction 
toward tubular member 420 at an angle that provides a matching fit with 
inner side 502 of bottom member 430. Outer side 500 of bottom member 430 
contacts tapered side 490 of lip 480 and outer side 504 of bottom member 
432 contacts inner side 502 of bottom member 430. The contacts of outer 
side 500 of bottom member 430 with tapered side 490 of lip 480 and outer 
side 504 of bottom member 432 with inner side 502 of bottom member 430 
form a seal sufficient to contain a melt of a material used in high 
temperature casting operations. Lip 480 of tubular member 420 supports 
bottom member 430 and bottom member 430 supports bottom member 432. 
Shown in FIG. 6 is furnace 510 for making melt-casting metals and alloys. 
The refractory compositions used in the stack typically have melting 
points greater than 1650.degree. C. Furnace 510 contains cast refractory 
materials: ceramic crucible 520, ceramic bottom insert 530, ceramic 
rupture disc 540, ceramic pour plug 550 all sitting on ceramic transition 
ring 560 which is sitting on ceramic billet mold 570 which is setting on 
nesting stack plate 580 which is setting on ceramic spill dish 590. 
Furnace 510 comprises steel base support 600, alumina bricks 610, 
susceptor coil 620, ceramic susceptor support ring 630, bubble zirconia 
insulation 640, 1425.degree. C.-1650.degree. C. firebrick 650, top alumina 
brick 660, molded 1650.degree. C. insulating lid 670, ceramic pour rod 
680, pour rod connector 690, and plasma sprayed tungsten susceptor 700. 
Shown in FIG. 7 is another embodiment of the present invention. Tubular 
member 710 of a multiple-piece crucible has inner side wall 720, lower 
portion 730, length 740, vertical slot 750 and lip 760. Vertical slot 750 
has width 770, shown in FIG. 8. Vertical slot 750 traverses length 740 of 
tubular member 710. Vertical slot 750 is filled with ceramic sealing 
material 780 to form a seal sufficient to contain melts of materials used 
in high temperature casting operations and to provide for thermal 
expansion of tubular member 710 without cracking during the casting 
operations. 
Shown in FIG. 8 is a cross-sectional top view of FIG. 7. Shown in FIG. 8 is 
vertical slot 750 in tubular member 710. Tubular member 710 has a diameter 
715. 
Several tests were run to check the effectiveness of different multi-piece 
crucible designs and materials of construction. 
Large multi-piece ceramic crucibles with sizes ranging from 19" OD, 23" 
high to 30" OD, 22" high were successfully tested. The typical wall 
thickness of the crucibles ranged from 17/8" to 2". 
EXAMPLE I 
A two-piece crucible of graphite was prepared by bonding a graphite tubular 
sidewall piece with a graphite bottom piece with a ceramic sealing 
material. The bonding was accomplished by coating the tapered side of the 
lip of the sidewall of the tubular sidewall piece and the tapered side of 
the bottom piece with a paste-like ceramic sealing material consisting of 
yttria mingled in an aqueous solution of sodium carboxymethyl cellulose 
and upon drying formed a seal and bond between the graphite tubular side 
wall piece and the graphite bottom piece of the two-piece crucible. The 
graphite bottom piece has a pour plug seated on a ledge-insert in the 
center of the graphite bottom section to facilitate the discharge of the 
melt into a mold upon the melting of the material charged in the crucible. 
The two-piece crucible was painted with an yttria wash coat, dried and 
then loaded with 180 kilograms of depleted uranium and placed in a vacuum 
furnace. The loaded crucible was heated in a vacuum to a temperature of 
1315.degree. C. for 0.5 hour to melt the depleted uranium. Then, the 
molten charge of depleted uranium was poured into a casting mold. 
Examination of the two-piece crucible after the charge was poured out of 
the crucible indicated that none of the uranium charge had leaked from the 
crucible and that the crucible was free of cracks. 
The test also indicated that the seal between the bottom piece and the 
tubular sidewall piece of the crucible held and no leakage of the molten 
charge was evident. 
EXAMPLE II 
A two-piece crucible was fabricated from a ceramic material containing the 
following constituents: 48 wt. % silicon carbide; 48 wt. % aluminum oxide; 
and 4 wt. % silicon oxide. The tapered side of the crucible bottom piece 
and the tapered side of the lip of the tubular sidewall piece were sealed 
together with the paste-like ceramic sealing material described in Example 
I and coated as described in Example I. 
The two-piece crucible was loaded with 120 to 240 kilograms of uranium-2 
wt. % niobium alloy and processed through a casting operation as described 
in Example I. Except, the alloy was heated to a temperature of 
1385.degree. C. in the casting operation with a six hour run time. A total 
of 12 melt/cast tests were run with the two-piece crucible. The two-piece 
crucible was examined for cracks and leaks after each test. These 
examinations indicated that none of the uranium alloy had leaked from the 
crucible and that the crucible was free of cracks. Consequently, the 
stresses created in one-piece ceramic crucibles by temperature differences 
during heating cycles can be alleviated by using the multi-piece crucible 
of the present invention. 
EXAMPLE III 
A two-piece crucible was fabricated using a different refractory ceramic 
material for the bottom plate. The tubular sidewall piece with the lip was 
fabricated from graphite and the bottom piece was fabricated from a 
refractory material containing the following constituents: 50 wt. % 
niobium; 30 wt. % titanium; and 20 wt. % tungsten. The bottom piece was 
then heavily nitrided by heat treating at about 1600.degree. C. to about 
1850.degree. C. in nitrogen. The tapered side of the lip of the tubular 
sidewall piece and the tapered side of the bottom piece were sealed with a 
paste-like ceramic sealing material consisting of titanium nitride mingled 
in an aqueous solution of carboxymethyl cellulose and the assembled 
crucible was coated as described in Example I. 
The two-piece crucible was loaded with uranium-2 wt. % niobium alloy and 
processed through a casting operation as described in Example II. 
Examination of the crucible after the casting operation indicated that 
none of the uranium-2 wt. % niobium alloy leaked from the crucible and 
that the crucible tubular sidewall piece and the bottom piece were free of 
cracks. 
Crucibles having bottom pieces 20" in diameter did not crack. The tubular 
side wall piece of the crucible works best if it is as close to being a 
free-standing cylinder as possible. The preferred crucible design seems to 
be the small-ledge crucible design rather than the "valve seat" design. 
However, all the variations tested worked well: valve seat long taper, 
valve seat truncated taper on housing, small-ledge, clover-leaf taper all 
work to reduce thermally induced stresses and prevent crucible cracking. 
One of the causes of crucible cracking is the use of a cylindrical 
susceptor to drive the heat into and through the crucible to the charge of 
material to be melted. The susceptor can be graphite, plasma-sprayed 
tungsten, molybdenum (i.e., a riveted, dove-tailed interlocking solid 1/4" 
thick metallic molybdenum susceptor worked well), or any refractory metal 
or conductor. The susceptor picks up the induction field and heats. The 
heat is transmitted, generally by radiation (or convection if argon or 
other gas is used for the environment; by radiation alone in vacuum) to 
the crucible and charge. Since the susceptor designs are right-circular 
cylinders that are larger in diameter than the crucibles, the heat is 
transmitted to the outer diameter of the crucible where it is conducted 
through to the inside of the crucible. The charge is heated by radiation 
from the inner wall of the crucible and through the bottom by conduction. 
However, the bottom middle of the crucible is the farthest from the 
heat-source susceptor and thus is the last to heat, generally very 
sluggish in attaining temperature. The studies indicate that cracking 
occurs very early in the heating cycle, while the interior is cool and the 
outside is several hundreds of degrees higher in temperature, creating a 
large temperature differential, delta T, from the outside wall to the 
center of the crucible. Generally the multi-piece crucible prevents the 
temperature differential from causing enough stress to crack or break the 
crucible. During the heating cycle the tubular side wall of the crucible 
expands, and the bottom piece slides on the ledge or sloped section, 
thereby reducing or eliminating any stress caused by the expansion. 
With single piece crucibles cracking usually occurs at the point where the 
bottom of the crucible meets the side wall of the crucible and/or radial 
cracks emanating from the center of the crucible bottom. 
The use of silicon carbide-loaded ceramics (essentially mullite-aluminum 
oxide formulations loaded with silicon carbide) will reduce the 
temperature differentials, since the combination of a good thermal 
conductor (silicon carbide) with the poorly conducting oxide ceramic 
yields a better overall thermal conductor, thus reducing the temperature 
differentials. However, high-alumina compositions (such as the 92.5 to 95% 
alumina, remainder silica) appear to still work well in the multi-piece 
crucibles; thus, if, for some reason, silicon carbide is not desired, it 
is not required. Silicon carbide materials react with oxides in vacuum to 
yield silicon monoxide at 1450.degree. to 1550.degree. C., limiting the 
upper use-temperature utility in vacuum to about 1500.degree. C. with 
those materials. 
A multi-piece crucible made from a refractory metal alloy composition was 
tested. The refractory metal alloy called Tribocor 532N, a trademark of 
Fansteel, Inc., is a nitrided refractory metal alloy consisting 
essentially of 50 wt. % titanium, 30% niobium, and 20% tungsten, generally 
nitrided at 1875.degree. C. for 4 hours to yield a surface of titanium 
nitride. The Tribocor 532N is a heavily nitrided metal (1.6 mm 
nitride-affected depth, with 0.25 mm of a mostly nitride outer layer 
consisting mostly of titanium nitride). In the initial test a straight 
edged (non-tapered) Tribocor bottom piece having a thermal expansion 
similar to alumina, 8.times.10.sup.-6 /.degree. C. was used with a 
graphite tubular side wall piece having a straight edged lip and a thermal 
expansion of about 4.times.10.sup.-6 /.degree. C. The graphite tubular 
side wall piece broke, spilling the melt and damaging the Tribocor bottom 
piece, when the crucible was heated. The bottom piece was repaired by 
brazing and the edge was tapered. The Tribocor bottom piece having the 
tapered edge was then tested with a graphite tubular side wall piece 
having a tapered lip which matched the tapered edge of the bottom piece. A 
ceramic sealing paste used for this test comprised a binder/suspension of 
50 wt. % titanium nitride powder in an aqueous solution of 6 wt. % sodium 
carboxymethyl cellulose. The multi-piece crucible using the tapered edges 
worked well, with no cracks developing or spilling of the melted material. 
This test demonstrates that the tapered edges "valve seat" arrangement 
works well for a multi-piece crucible utilizing materials with dissimilar 
thermal expansions for the crucible bottom and tubular side wall. 
The ceramic sealing paste used for grouting the pieces of the multi-piece 
crucible together is a precaution to insure the molten material will not 
leak out of the crucible during the melting operation when melting 
materials which have a low viscosity and a high density. 
One of the ceramic sealing pastes used comprises a binder/suspension of 50 
wt.% yttrium oxide powder in a aqueous solution of 6 wt. % sodium 
carboxymethyl cellulose. The ceramic sealing paste is used for sealing 
areas between the pieces of the multi-piece crucibles. For very large 
gaps, the yttria based paste can be modified by adding zirconia bubbles 
(typically 1/16" in diameter) as a filler. The ceramic sealing paste forms 
a layer of weakly-bonded yttria particles that allows movement of the 
joined sections while not sintering to a hard, dense ceramic. The dried 
paste is easily scraped off after a run if needed. Generally, however, the 
joined sections can be reused as is by applying another coating of ceramic 
sealing paste without any further changes. 
The multi-piece crucible system allows vacuum induction melting of reactive 
metals without the use of graphite components. There is no known 
production scale graphite-free system for induction melting/casting 
reactive metals (including specialty steels, titanium and titanium alloys, 
zirconium and zirconium alloys, beryllium and beryllium alloys, or uranium 
and uranium alloys). The multi-piece crucible enables these reactive 
metals to be processed by vacuum induction melting, a standard, economical 
technique that is used for less-reactive metals and alloy materials that 
can utilize graphite. The uniqueness of the non-graphite multi-piece 
crucible is that it withstands thermal stresses that must be withstood 
when the process cycle time is minimized in order for vacuum induction 
melting to be economically competitive on a production basis. 
Additionally, the multi-piece crucible can be readily substituted for 
graphite in normal vacuum induction melting operations. For alloys that 
react with carbon or graphite, there has been no alternative but to are 
melt them, a very expensive processing method compared to vacuum induction 
melting. Thus the present invention opens up the reactive metal 
melting/casting area for utilization of standard, economical vacuum 
induction melting. 
Other materials can be combined to provide nonreactive surfaces for 
containing a specific material during induction-heating operations. For 
example, silicon carbide at levels of to 85 wt. % can be combined with 
ceramic oxides (primarily alumina with some silica) to improve the thermal 
conductivity of the ceramics for induction heating operations. Various 
materials for the construction of the multi-piece crucibles were used in 
the tests. For example the tubular side wall piece and the bottom piece of 
the non-graphite crucibles were made from a material selected from the 
group comprising graphite; silicon carbide; ceramic composition comprising 
48 wt. % silicon carbide, 48 wt. % aluminum oxide, and 4 wt. % silicon 
oxide; ceramic composition comprising 90 % alumina and 9.5% silicon 
dioxide; ceramic composition comprising 61.5% alumina, 33% silicon 
carbide, and 4.7% silicon dioxide; ceramic composition comprising 48% 
alumina, 48% silicon carbide, and 4% silicon dioxide; ceramic composition 
comprising 35% alumina, 59% silicon carbide, and 5% silicon dioxide; 
ceramic composition comprising 29.4% alumina, 67.5% silicon carbide, and 
3.7% silicon dioxide; and refractory metal alloy composition from 
Fansteel, Inc. called Tribocor 532N comprising 50% niobium, 30% titanium 
and 20% tungsten in which the outer layer of the piece made from such 
composition consisted essentially of titanium nitride and combinations 
thereof. Consequently, a crucible of the present invention can be used for 
heating specialty steels, titanium metal and titanium alloys, zirconium 
metal and zirconium alloys, and beryllium metal and beryllium alloys. 
The multiple-piece crucible can be utilized in the induction-melting of 
uranium metal and uranium alloys. Usually, induction melting operations 
are more efficient than arc-melting operations. Also, the multiple-piece 
crucible can lessen the carbon contamination of uranium metal and uranium 
alloys during casting operations. As a result, the recycle of uranium 
scrap can be increased and waste minimization would be enhanced. 
In addition, a multiple-piece crucible of the present invention can be used 
in casting specialty steels, titanium metal and titanium alloys, zirconium 
metal and zirconium alloys, and beryllium metal and beryllium alloys. 
The problem in heating foundry crucibles is to heat ceramic crucibles 
uniformly so as to reduce or eliminate the thermal stresses or temperature 
differentials (delta Ts) that occur with induction heating. Melt-cast 
operations generally use induction heating with a suscepting crucible: the 
crucible picks up the induction field and heats inductively, thereby 
heating the metal charge by conduction and radiation from the crucible. 
When graphite is used for the crucible, it is also a susceptor--thus 
absorbing the induction field and heating directly. When ceramic crucibles 
are used, they do not suscept. Therefore, a separate susceptor is 
generally used. This susceptor heats up and radiates the heat to the 
crucible and metal charge contained therein. Generally, the susceptor is a 
cylinder of an electrically conductive material, such as refractory metal 
(molybdenum, tungsten, tantalum, or niobium) or graphite. Also, generally, 
the susceptor is fixed in the furnace actually bricked into place on the 
sidewall of the furnace. Typical of such a design is the carbon-free 
induction furnace (U.S. Pat. No. 4,550,412). Typical of the improvements 
necessary to reduce the thermal stresses on the crucible is the use of a 
multiple-pieced ceramic crucible. 
However, there is a need to expeditiously heat crucibles to conduct the 
melt-cast operations in a timely manner: that is, an infinitely slow 
heating to a casting temperature is unacceptable (although ceramic 
crucibles would not crack if heated in that manner); likewise, using 
multiple-piece crucibles allows faster heating, but still can crack if 
sufficient thermal stresses (delta Ts) develop. Thus, a manner was sought 
to minimize the thermal stresses on a ceramic crucible while allowing 
heating of the crucible as fast as possible. The ideal way to heat a 
ceramic is to envelop (or bathe) the crucible in a totally uniform thermal 
environment--where the crucible is heated the same from the top, bottom, 
and sides. In practice, inside a large-sized crucible that is heated by 
radiation from a cylindrical susceptor, the heat travels inward towards 
the center of the crucible: the center is the coolest spot, and there is a 
constant temperature differential from the crucible sidewall to its 
middle. The findings of this disclosure show that a "thermal can" will 
provide almost ideal uniform heating conditions, with all the thermal 
differentials being &lt;200.degree. C. even as the system (susceptor, 
crucible, and metal charge) heats upwards to the casting temperature. 
Results indicate that two-piece ceramic crucibles perform well, yet may 
exhibit barely discernable cracks on the inside only when tested with 
embedded susceptors of tungsten or molybdenum. The embedded susceptors are 
those which are actually permanently mounted on refractory brick ledges in 
the furnace and with refractory fiber or bubble insulation behind the 
susceptor adjacent to the brickwork. These embedded susceptors are not in 
any direct contact with the crucible/mold assembly, and all heat is 
directed radially inward from the susceptor towards the crucible/mold 
assembly. 
The first test with a two-piece ceramic crucible was done with a graphite 
susceptor which had a bottom graphite plate to support it on the 
crucible/mold stack and which had a graphite top plate used for supporting 
the refractory insulating brick. This system performed excellently, 
leading to 12 melt/pour cycles with the same two-piece ceramic crucible 
with absolutely no cracking (internal or external). What was not realized 
at the initial testing was that the system was actually a "thermal can." 
The "thermal can" basically allowed the heat to uniformly bathe the 
crucible as it heats up. Without the "thermal can," we had demonstrated 
many times that some degree of cracking commonly occurs when reasonable 
heating times were utilized (i.e., 3 to 4 hours to 1000.degree. C.). 
However, we have also demonstrated that even one-piece crucibles of these 
large sizes (up to 23-24" OD) CAN be heated slowly enough to prevent 
cracking; i.e., heating at 100.degree. C./h up to 1000.degree. C. 
generally resulted in no cracks and led to successful melting of uranium 
alloy (U- 2 Nb) at 1385.degree. C. (30 min. hold [or "soak"]) metal 
temperature. Likewise, two-piece crucibles exhibited no cracks when an 
8-10 hour heatup to 1385.degree. C. was used. A minimum casting cycle type 
has an economic advantage and, for general foundry operations, an &lt;8-hr. 
melt/cast time is desirable. 
The "thermal can" plus the two-piece crucible is thought to be the best 
means to minimize the casting cycle time and reduce the tendency of large 
ceramic crucibles to crack. The use of ceramics that are all [or nearly 
all] alumina (90-95 wt. % alumina, re. silica) appears OK when both 
two-pieced crucibles and the "thermal can" are used together. There is a 
somewhat improved performance in better distributing the temperature if 
silicon carbide-mullite formulations are used for the ceramic components 
of the crucible: this is because of the enhanced thermal conductivity of 
the ceramic from the mixture of silicon carbide and mullite as opposed to 
all mullite or mullite-alumina. There is reason to eliminate the silicon 
carbide if possible for vacuum-induction-melting (VIM) operations: vacuum 
enhances the production of silicon monoxide from the silicon carbide plus 
silicon oxide (or aluminum oxide), so that the practical upper limit of 
operation is about 1500.degree. C. with a silicon carbide-mullite 
crucible, whereas alumina or alumina-mullite crucibles are operable to 
&gt;1700.degree. C. The "thermal can" appears to be a more-useful way of 
evening out the temperature around the crucible than addition of silicon 
carbide in the ceramic. 
Actual experimental results with graphite top and bottom plates but using 
an embedded molybdenum susceptor [24"OD, 40"L] (not touching the graphite 
plates) led to very large thermal differentials, since the gap between the 
graphite and the Mo did not allow direct conduction of the susceptor heat 
to the top and bottom of the crucible. 
An experiment was conducted using a ring of stacked molybdenum susceptors 
(each ring about 6"H, using 4 rings) with graphite top and bottom plates 
in contact with the susceptor creating the "thermal can". A Blasch 
Precision Ceramics Co. two-piece crucible with minimal-ledge design 
(BP-67/8-SC: composition 67.5 wt. % SiC, 29.4% Al.sub.2 O.sub.3, 3.7% 
SiO.sub.2) was used for the test. Thermal differentials from 
exterior-to-interior sidewall of the crucible were quite low, generally 
below 150.degree. C. This 125 kg. melt test further demonstrated that the 
"thermal can" was useful. 
An experiment was undertaken to see if the "thermal can" could be "tweeked" 
to modify the thermal differentials. Using graphite susceptor and top and 
bottom plates, the diameter of the bottom plate was increased over that of 
the cylindrical graphite susceptor, thus making the bottom plate closer to 
the induction coils. The bottom plate was 25" OD as compared to the 
previously used 23" OD bottom plate and the 23" cylindrical graphite 
susceptor. Temperatures were measured on the crucible outer and inner 
walls, on the graphite lid (supporting the insulting brick), on the bottom 
graphite support flange, on the graphite susceptor, and on the bottom 
stack support (which was a graphite crucible for this test). The thermal 
profile, followed to 1000.degree. C., established that the temperature 
differentials from the susceptor to lid or to bottom were held to 
&lt;120.degree. C.--a remarkable event. This experiment established that 
thermal differentials can be minimized by changing the components of the 
"thermal can". With the large .(25" OD) graphite bottom plate, temperature 
differentials from the susceptor to the bottom plate were only half as 
much (&lt;120.degree. C.) as those observed for the small-diameter bottom 
plate (250.degree. C.). 
An all-metal "thermal can" was the next experiment for thermal profiling. 
Molybdenum stacking/interlocking rings were used as the susceptor. The 
bottom and top plates were made of niobium metal. The top plate supported 
two layers (ea. 1 1/2) of molded-ceramic fiber insulation. Throughout the 
testing (to 1000.degree. C.), the top plate and susceptor were &lt;50.degree. 
C. apart. However, the bottom plate temperature differential from the 
susceptor increased over the run, finally reaching somewhat over 
300.degree. C. This differential is expected to be lowered by increasing 
the diameter of the niobium plate in the same fashion as for the graphite 
plate. In any case, the differentials between the outside and inside of 
the crucible walls were kept below 200.degree. C. The "thermal can" of 
all-metal has been established for reducing thermal differentials and 
heating ceramic crucibles in a way that will minimize the thermal 
stresses. 
Further testing has showed that adding a bottom section onto the "thermal 
can" better distributes the heat in the critical bottom region. Using 
molybdenum stacking/interlocking rings for the system, the bottom region 
looks like two washers. Both washers were made of niobium (not because of 
any reason other than availability) and were spaced apart with ONE 
molybdenum ring. The crucible was supported by the top washer. The bottom 
washer rested on insulating pads on a mold support ceramic. The molybdenum 
ring provided about 6" of gap underneath the crucible, between the 
crucible and the mold into which the metal was to be cast. 
This setup allowed the best thermal conditions of any tested to date. 
A Tribocor (50 wt. % niobium, 30% titanium, 20% tungsten--nitrided for a 
titanium nitride surface) susceptor was used to make a "thermal can". 
Because of its height (dimensions were 24" OD, 20" high), a molybdenum 
ring was placed on top to clear the crucible. The interesting observation 
was that Tribocor heats inductively better than graphite. And, with the 
"thermal can" arrangement, the temperature exposure of the crucible is 
uniform--a bathing of the crucible in uniform temperature as it heats up 
to the melt temperatures. 
The degree of uniformity of the temperatures can, as mentioned above, be 
varied somewhat by changing the diameters of the washers on the bottom and 
top as well as the thicknesses of the refractory metals utilized and with 
the type of susceptor materials used (molybdenum, niobium, Tribocor, 
etc.). Also, certain applications (load [support] areas as on the bottom) 
may require the best hot strengths, probably suggesting Tribocor. But, 
these can be designed around by using ceramic brick spacers (as between 
the bottom washers, etc.) or some other means. The important discovery is 
that a can-shaped susceptor/enclosure allows ceramic crucibles to be 
heated inductively in a uniform manner, preventing cracking problems and 
essentially allowing the ceramic systems to behave like graphite when 
being heated in a rapid manner up to melting temperatures. 
The invention can allow very large carbon,free systems for melting/casting 
of special reactive metals (uranium, titanium, beryllium, lithium, and 
alloys thereof). Previously, systems for heating the ceramics to avoid 
cracking have not existed, so the carbon-free systems are not generally 
used. This development should allow large ceramic crucibles to be used in 
foundry practice essentially as graphite would be used--similar heating 
rates, etc. Also, since the "thermal can" is affixed to the melt/cast 
stack, a furnace can be run with a graphite system one day and with a 
carbon-free system the next day--providing considerably versatility. The 
systems generally can be changed by changing the refractory materials--as 
mentioned, using molybdenum, tungsten, niobium, tantalum, Tribocor. Also, 
interlocking rings can be used or a single cylinder of material to make 
the cylindrical section of the "thermal can". 
This system will likely be used for melting and casting high-purity alloys, 
where carbon contamination is deleterious to the alloy properties. For 
reactive metal melting, carbon contamination is generally not 
desirable--often "killing" the cast parts. Thus, carbon-free melt-cast 
systems are sought. Arc melting is one carbon-free-alternative, but it is 
much more costly than vacuum induction melting (VIM), which is utilized 
with the "thermal can" to allow ceramic (noncarbon) crucibles to be 
effectively used. Uses in VIM operations with high purity alloys could 
nearly eliminate carbon contamination, allowing for recycle of reactive 
(high-surface-area) scrap such as chips as well as massive scrap. 
Shown in FIG. 9 is a cross-sectional view of thermal can assembly 800 in 
foundry furnace 810. Thermal can assembly 800 comprises thermal can 820, 
thermal can middle susceptor plate 850 with aperature 855, thermal can 
spacer blocks 870, multiple-piece susceptor plate 850 with aperature 855, 
thermal can spacer blocks 870, multiple-piece crucible 910, transition 
ring 890 with aperature 895, billet mold 900 with cavity 905 open a top 
902 of billet mold 900 to accept molten material from multiple-piece 
crucible 910, mold stack block 920, and spill dish 930. Thermal can 820 
comprises thermal can top susceptor plate 830 with aperature 835, first 
thermal can susceptor ring 840, tubular shapped, second thermal can 
susceptor ring 860, tubular shapped, and thermal can bottom susceptor 
plate 880 with aperature 885. Thermal can top susceptor plate 830, first 
thermal can susceptor ring 840, thermal can middle susceptor plate 850, 
second thermal can susceptor ring 860, and thermal can bottom susceptor 
plate 880 are made from susceptor material. Multiple-piece crucible 910 
comprises multiple-piece crucible slotted tubular member 950 having a 
vertical slot 955, rupture disc 960, and multiple-piece crucible bottom 
970 with aperature 975. Rupture disc 960 coacts with aperature 975 to 
prevent molten metal from running out of multiple-piece crucible until 
rupture disc 960 is ruptured to allow the molten metal to flow into cavity 
905 of billet mold 900. Thermal can 820 is in contact with and sets on top 
of transition ring 890 which is in contact with and sets on top of billet 
mold 900 which is in contact with and sets on top of mold stack block 920 
which sets on top of spill dish 930. Multiple-piece crucible 910 is 
contained in thermal can 820 and sets on top of thermal can middle 
susceptor plate 850. Thermal can top susceptor plate 830 is in contact 
with and sets on top of first thermal can susceptor ring 840 which is in 
contact with and sets on top of thermal can middle susceptor plate 850 
which is in contact with and sets on top of second thermal can susceptor 
ring 860 which is in contact with and sets on top of thermal can bottom 
susceptor plate 880 which is in contact with and sets on top of transition 
ring 890. Transition ring 890 is in contact with billet mold 900 and 
thermal can bottom susceptor plate 880. Thermal can spacer block 870 is 
sandwiched between and in contact with thermal can middle susceptor plate 
850 and thermal can bottom susceptor plate 880. Side insulating bricks 980 
of foundry furnace 810 are supported by support bricks 990. Insulating 
bricks 1000 set on top of thermal can top susceptor plate 830. Thermal can 
spacer block 870 and transition ring 890 are made of a refractory ceramic 
material and will not yield to the loading forces caused by the 
multiple-piece crucible containing a charge of material for melting even 
at the operating temperatures of foundry furnace 810. Thermal can spacer 
block 870 and transition ring 890 as well as transition ring 890, billet 
mold 900, mold stack block 920, and spill dish 930 support thermal can 
middle susceptor plate 850 and multiple-piece crucible 910. 
While there has been shown and described what is at present considered the 
preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the scope of the invention as defined by 
the appended claims.