Continuous Casting Furnace and Die System of Modular Design

This invention relates to continuous casting furnaces and to dies suitable for use in the continuous casting of high melting temperature metals such as: iron, nickel, nickel-chromium, palladium, platinum and cobalt. More particularly, the present invention relates to vertical and horizontal continuous casting furnaces and dies made of a modular construction wherein the crucible section, the feed section and the die section are each separable from each other and each of these sections is in turn made up of various individual components.

Continuous casting is a well known method for converting liquid metals into solid materials of constant cross-section in continuous or semi-continuous lengths, convenient for use as cast or suitable to further forming by well known metal working procedures.

Equipment for continuous casting is well known in the metals industry. Several schemes are in common usage. For the casting of relatively low melting metals and alloys, (below a melting point of about 1200.degree. C.) a type of system using solid crucibles and dies is often used.

In this system the solid crucible is usually constructed of graphite. The metal may be introduced as solid alloy or components and melted directly in the crucible, or may be added as molten metal from an exterior melting and/or alloying source.

The crucible is generally maintained at some temperature above the metal melting point via externally applied heat, as by induction, electrical resistance heaters, gas flames, or other means well known in the industry, and is connected with an orifice or feed section that allows molten metal to flow into a casting die. The die is most often machined from graphite, and is held tightly in contact with, inserted in, or in some fashion attached to the orifice or feed section so as to form a leak proof seal.

The die can be placed at an angle, generally 90.degree., to the crucible, in which case the method of casting is referred to as "horizontal" casting, or may be placed below the crucible, in which case the method of casting is referred to as "vertical" casting.

Molten metal feeds by gravity and/or pressure through the feed section and freezes to solid metal at some point. Freezing is controlled by cooling devices such as water cooled plate coolers, attached or pressed against the outer surface of the die or adjustable water or gas cooling R.W. 7 July 1987 by 15 July 87 probes, disposed in the die. Casting is accomplished in a continuous manner by withdrawing the solidified metal via some withdrawal mechanism.

Silver, gold, copper, and aluminum and their alloys are commonly produced by both horizontal and vertical casting, using graphite dies and crucibles. A die used in such a furnace is shown in U.S. Pat. No. 4,295,516, the furnace being of a small size suitable for installations in existing facilities.

It is desirable to produce other alloys by the continuous casting process, most particularly, for instance, alloys based on nickel (Ni), nickel-chromium (NiCr), palladium (Pd), platinum (Pt), iron (Fe) and cobalt (Co). However, it is not possible to use a graphite die and particularly graphite crucible systems for these alloys, as they all readily dissolve carbon (graphite). This dissolution not only changes the properties of the metals; it erodes the crucible and die, rendering them almost immediately unusable and ultimately resulting in metal leaking through the system, producing a partial or total metal loss into the heating portion of the melting system.

U.S. Pat. No. 4,175,611 discloses plasma coating graphite dies with various materials for use when casting the above materials, which various materials may improve the wearing characteristics of the die.

This invention overcomes the problems associated with the continuous casting of alloys of Ni, NiCr, Pd, Pt, Fe, and Co in graphite systems, by use of modular lining components that are resistant to dissolution and/or erosion by the contained metals. A graphite holding system may be used for the furnace or die if it is properly isolated from the metals in question. This can be done using non carbon containing refractory materials such as known refractory ceramics, which are not subject to attack by the metal. Representative materials include aluminum oxide, magnesium oxide, zirconium oxide, calcium oxide, beryllium oxide, aluminum nitride, boron nitride and titanium boride (titanium diboride). It will be understood that various combinations of these compounds and/or these compounds with small additions of other materials as "binders" are included within the scope of this invention.

In accordance with the present invention there is provided a continuous casting furnace for use with metals capable of disolving carbon, the furnace comprising a crucible assembly provided with a graphite crucible and a graphite feed housing, a non-carbon refractory liner disposed within the graphite crucible, and a feed tube system formed of non-carbon refractory material, the feed tube system extending substantially from the liner through the crucible assembly; a die section including a tubular graphite die carrier, and a non-wetting non-carbon refractory die disposed within the tubular graphite die carrier; flexible sealing means extending between the die section and the crucible assembly; and holding means capable of holding the parts together in juxtaposition relationship.

Thus, in accord with one form of the present invention there is provided a continuous casting furnace made up of three distinct sections, namely an upper or crucible section, a lower or feed section, and a die section. Each section includes a graphite holding system and refractory inserts or liners. These inserts are in turn made of an appropriate non carbon containing refractory material which is not subject to attack by the particular metal or metal alloy system being cast.

The above will become more apparent from a consideration of the following detailed description taken in conjunction with the accompanying drawings in which the preferred embodiments of this invention are illustrated.

FIG. 1 is a section view through a first embodiment of the continuous casting furnace of this invention.

FIGS. 2 and 3 are sections taken generally along the lines 2--2 and 3--3 in FIG. 1.

FIG. 4 is a partial sectional view of a second embodiment of this invention.

FIGS. 5, 6 and 7 are sectional views taken generally along the lines 5--5 6--6, and 7--7 in FIG. 4.

FIG. 8 is a partial sectional view of a third embodiment of the present invention.

FIG. 9 is a sectional view through another form of die which may be used in a fourth modification of the present invention.

FIGS. 10, 11 and 12 are sectional views taken generally along the lines 10--10, 11--11 and 12--12 in FIG. 9.

Referring first to FIG. 1, a first embodiment of the continuous casting furnace of this invention is illustrated, the furnace being indicated generally at 10. The furnace includes a die section indicated generally at 12, and a crucible assembly indicated generally at 14, the crucible assembly in turn being made up of a crucible section indicated generally at 16, and a feed section indicated generally at 18. The entire furnace is disposed within a metal housing 20.

The crucible section 16 includes a monolithic graphite crucible 22 provided with a suitable cylindrical aperture or orifice at its lower end. Disposed within the monolithic graphite crucible 22 is a refractory liner 24. The refractory liner is preferably made of a non carbon containing material such as aluminum oxide, with or without binders. However, it could also be made of other suitable materials such as magnesium oxide, zirconium oxide, calcium oxide, beryllium oxide, aluminum nitride, and boron nitride. The actual selection of the liner material will depend upon the material being melted within the furnace, costs and availability.

It should be noted that the preferred refractory material may not be the same for crucible and die. For example when the metal to be cast is palladium, nickel-chromium, nickel, or an alloy having a high nickel content, the crucible liner will preferably be made out of aluminum oxide, zirconium oxide or magnesium oxide. The liner for the die in casting the palladium or nickel-chromium alloy could be made of either beryllium oxide or boron nitride. However, the liner for nickel alloys would be preferably beryllium oxide.

As can be seen from FIG. 1, the refractory liner is provided with an orifice 26, which orifice is concentric with the cylindrical aperture in the graphite crucible 22. The liner may bear directly against the walls of the graphite crucible, or alternatively it may be supported by a suitable refractory cement 28. A feed tube system extends from the orifice in the liner, the feed tube system including an open feed tube 30 disposed within the cylindrical orifice within the graphite crucible, the feed tube also being made of a suitable refractory material other than graphite. The outer diameter of the feed tube is properly sized with respect to the orifice in the graphite crucible so that there will be as little clearance as possible between the parts and which will still permit disassembly of the feed tube from the crucible. Disposed between the top surface of feed tube 32 and the bottom of the liner 24 is a flexible or soft seal 34. The soft seal is so designed that it will maintain a fluid tight relationship between the liner 24 and the top surface of the feed tube 30 and yet will permit thermal expansion of one part with respect to the other as is more fully brought out below. The bottom of the feed tube 30 is provided with a conical surface 36. Although the four corners of the bottom of the graphite crucible are depicted as having threaded apertures 38, any sutiable fastening or positioning device will do such as a peg, wedge, etc. The top of the liner 24 may be closed by any suitable closure 40. The closure could be a spring loaded plate. Optionally, it may be held in place by gravity. Closure 40 is shown as having a flange 42 which rests upon the metal housing 20. Surrounding the sides and the top of the crucible 22 may be suitable insulation 44. The thickness of the insulation may vary considerably from that shown in FIG. 1. In addition, a suitable heating element, a portion of one being indicated at 46, is disposed about crucible 22 for the purpose of heating and maintaining the contents of the crucible in a liquid stage. Any known heating apparatus will suffice including standard gas, electric or induction heating elements.

The feed section 18 includes a tubular feed system 48, which forms the feed tube system 30. In the embodiment shown in FIG. 1 the tubular feed system is formed from a single piece of machined refractory material, such as boron nitride. The tubular feed system 48 is in turn is supported within a monolithic graphite feed housing 50, the parts 48 and 50 being so machined that they will closely interfit each other. As can be seen from FIG. 1 the tubular feed system has a vertically extended aperture 52 and horizontal aperture 54 which intersects the lower end of the vertical aperture 52. This form of design is called a horizontal casting continuous furnace. However, it should be noted that the aperture 52 could extend downwardly to the housing and that the die section 12 could be disposed below the feed section, in which case the furnace would be referred to as a vertical casting continuous furnace. Many of the principles of this invention are applicable to both vertical and horizontal continuous casting furnaces.

Vertical aperture 52 is shown in the drawing figure as being provided with a top bevel edge which meets with the bottom bevel edge 36 of the feed tube 30. Although this configuration is preferred, flat mating surfaces may also be employed where the feed tube 30 meets the tubular feed system 48. The feed tube 30 as well as the tubular feed system 48 are preferably formed of the same materials so that they will have the same rate of thermal expansion and contraction and therefore will maintain a tight seal adjacent to the machined meeting surfaces.

Graphite holding means are provided to hold graphite crucible 22 and the graphite feed housing 50 together in juxtapositioned relationship at all times so that the lower end of the feed tube will be maintained in contact with one end of the tubular feed system. To this end, triangular corners 56 are machined out of the lower surface of the feed housing 50 and suitable vertical apertures are provided therein, which vertical apertures are in concentrical alignment with the threaded apertures 38 in the crucible 22. Bolts 58, which are formed of graphite, are then passed through the apertures and snuggly secured within the threaded apertures 38 to maintain the parts together. The sides and the bottom of the feed housing 50 are also surrounded by suitable insulation 44 although one side of the feed housing 50 will abut against a stop 60 or the purposes which will be brought out below. Again, although graphite threaded bolts are depicted, any known suitable internal or external holding means may be substituted for the threaded system.

The die section includes as its principal component a graphite die carrier 62 and a tubular die 64 formed of a non carbon refractory material. In the embodiment illustrated in FIG. 1 the graphite die carrier is formed from a single piece of machined monolithic graphite. A suitable cavity is machined within the graphite die carrier 62 and the tubular die is inserted therein. It should be noted that the tubular die need not extend the full length of the die carrier, it only being needed for that portion of the length of the die carrier wherein the metal being cast may be still in liquid form. However, once the metal being cast has been transformed to a solid, it will no longer dissolve the graphite and thus that portion of the graphite die carrier which surrounds solid metal need not be provided with the tubular die 64, the tubular die being of a non carbon refractory material which is not wet by the metal being cast.

Some of the refractory materials may be "wet" by the metal being cast, but not eroded by it. Such a refractory is suitable for containing the liquid metal, but is not suitable as a "casting" surface in the die. In this regard, it should be noted that if the metal wets the die surface it will adhere to the surface as it freezes, causing the solidified metal or die to be torn apart as the solidified metal is withdrawn from the die section, rendering it unusable.

A soft seal 66 is disposed between the graphite die carrier and the tubular die on one side and the graphite feed housing and the crucible on the other side. Holding means, indicated generally at 68, are provided to maintain the parts together in their desired assembled relationship. The holding means includes a metal plate 70 which is passed over studs 72 carried by one end 74 of the metal housing 20, the metal palte being brought to bear against the end of the graphite die carrier remote from the feed section by nuts 76. When the nuts are brought down to bear onto the metal plate it will tend to force the graphite carrier to the left as viewed in FIG. 1 bearing in turn against the soft seal, shifting movement of the graphite feed housing 50 to the left being prevented by stop 60. It should be noted that the metal plate will be provided with a suitable aperture for the passage of the metal which is being cast. As the metal is being cast it is caused to be brought out of the furnace by rollers 78 which are of conventional construction. Disposed about graphite die carrier is insulation 44.

While not shown in FIG. 1 the graphite die carrier is preferably provided with cooling means of the type shown U.S. Pat. No. 4,295,516, the subject matter which is incorporated herein by reference thereto. In addition, the graphite die carrier may further be provided with a thermocouple as is well known in the art. The cross section of the die may be of any desired configuration and in the embodiment shown in FIG. 1 it is of a rectangular cross section.

It can be seen that the design shown in FIG. 1 is of rather simple construction requiring neither difficult casting nor machining of the parts to produce the desired apparatus. In addition, by using interchangeable liners, tubular feed systems, and dies many differing materials may be produced in the furnace of this invention.

One form of the invention has been illustrated in FIGS. 1 through 3. Another form is illustrated in FIGS. 4 through 7. In the design shown in FIGS. 4 through 7, a somewhat different construction of feed section and die section is shown. In this design a two section tubular feed system may be utilized, the first section being a generally square block 48a which is suitably machined to provide intersecting passageways for the flow of metal from the crucible to the die. The feed system further includes a tubular member 48b which abuts one surface of the square block 48a to provide a liquid tight passageway. The tubular portion is in turn received within a tubular hole drilled within the monolithic graphite feed housing 50 of this figure. A soft seal of the same type as is shown in FIG. 1 is provided between the upper end of feed tube 30 and the lower end of liner 24. The die section in FIGS. 4 through 7 is formed of discrete graphite die carriers and tubular die sections, the parting lines of which are not coextensive with each other as can been seen from the FIG. 4. Thus, the graphite die carrier in this embodiment is formed of three discrete sections 62a, 62b, and 62c and the tubular die is also formed of three sections 64a, 64b and 64c. In addition, an orifice 80 is provided through the tubular refractory dies 64, the orifice terminating within the aperture within the tubular portion 48b. The purpose of the orifice 80 is to introduce an inert gas, such as dry nitrogen, into the liquid metals for the purpose of flushing away undesirable gases and also for the purpose of agitating the metals as they are being maintained within the crucible. While not shown in FIGS. 4 through 7 the graphite sections 62a-c may be held together by suitable graphite bolts or other means. In addition, cooling means and thermolcouples are also provided. Concentric annular soft seals 66a, 66b, and 66c are provided as shown in FIG. 4.

The design shown in FIG. 8 differs from the preceding designs in that a signle unitary graphite feed housing is utilized, as in the design of FIG. 1 and a multiple section die carrier and die are illustrated as shown in FIGS. 4 through 7. In addition, because of the differing geometry it is not possible to have the bubbler orifice 80 terminate at the junction of the left hand die section 64a R.W. 7 July 1987 by 15 July 1987 and the feed tube within the tubular feed system adjacent to the die. Therefore, in this design the graphite sections are provided with a cylindrical bore 82, an enlarged portion of which receive a refractory bubbler tube 84 which extends through the various sections 62a-62d of the sectional graphite die carrier. The refractory bubbler tube 84 abuts against the outer surface of the tubular feed system 48, which tubular feed system is provided with a further cylindrical aperture 85 for the passage of inert gases. A single soft seal 66 is utilized in this design, the seal being provided with an additional orifice (no number) for the passage of inert gasses.

FIGS. 9-12 show a further die design which may be utilized for casting simultaneously two separate rods. In this design a multiple section die carrier 62a-c is utilized however only a single die 64 is provided. The graphite die carriers 62a-c are held together by suitable graphite bolts 86 and suitable apertures 88 are provided for the reception of cooling means. An aperture 90 is provided for the reception of a thermocouple. In addition, a further aperture 92 is provided for the passage of inert gasses. The apertures 90 and 92 are only within the right hand section 62c of the sectional graphite die carrier and are concentric with further apertures 94 and 96 within the die 64. In this design the graphite die carrier will abut against a graphite feed housing and crucible, as shown in FIG. 1 and it is only necessary to provide a soft seal within the annular cavity 98.

An important consideration in using a graphite crucible and die system with refractory linings is the difference in thermal expansion between materials. Allowance must be made for differential dimensional changes that occur between the components. If this is not done, cracking of the components may occur due to large stresses caused by one component expanding more than another. Conversely, if too much allowance is made, the components will not mate properly, allowing molten metal to leak through to the graphite, causing adverse reactions and possible destruction of the holder and/or die.

This may be prevented by use of "soft" or flexible refractory seals or washers which compress during heating. The use of flexible sealing means 34, between crucible liner 24 and feed tube 30 allows the use of different crucible and feed tube materials. In the case of the feed tube 30 being fabricated out of an expensive material like boron nitride, a much less expensive material, e.g., aluminum oxide, would then be preferred for the crucible liner 24. However, aluminum oxide has a much higher expansion rate.

Similarly, the use of flexible sealing means 66 between die 64 and feed section 48 is desirable to keep die holder 62 and die 64 under pressure against feed section 48.

The use of a soft or flexible sealing means will preferably be required wherein it is desirable to allow for expansion differences between parts, to maintain a leak proof seal or to prevent excessive compression forces.

Flexible sealing means 34 and 66 are preferably made of aluminum oxide or zirconium oxide fibers fabricated into a paper, cloth or felt-like "soft" consistency. Although aluminum oxide is preferred, any suitable non carbon containing refractory fibrous material may be utilized provided it does not melt within the operating temperature range of the furnace. The seals may optionally be impregnated with a boron nitride paste or slurry to improve their flexibility and sealing ability. The boron nitride paste or slurry prevents sintering and/or hardening of the flexible sealing means at elevated temperatures, (i.e. above about 1100.degree. C.) thereby maintaining the soft consistency of the seal or washer.

Suitable boron paste is available commercially from ZYP Coatings Incorporated sold under the name "TYPE BN PAINT" or from SOHIO under the designation "BN NITRIDE COATING."

While preferred structures in which the principles of the present invention have been incorporated are shown and described above, it is to be understood that this invention is not to be limited in the particular details shown and described above, but that, in fact, widely differing means may be employed in the broader aspects of this invention.