US3459255A - Graphite continuous casting mold - Google Patents

Description

INVENTOR GEORGE FREDi'R/C KOLLE' ATTORNEYS.

Aug. 5, 1969 a. F. KOLLE GRAPHITE CONTINUOUS CASTING MOLD Filed Dec. 7. 1966 United States Patent 3,459,255 GRAPHITE CONTINUOUS CASTING MOLD George Frederic Kolle, Yardley, Pa., assignor to Ascast Corporation, Riverton, N.J., a corporation of New Jersey Filed Dec. 7, 1966, Ser. No. 599,920 Int. Cl. B22c 3/00; B22d 11/02 US. Cl. 164-283 4 Claims ABSTRACT OF THE DISCLOSURE A continuous casting mold made from low density graphite is coated with a thin layer of copper or silver to obtain advantages of dense graphite which is substantially more expensive and not commercially available in large pleces.

This application is an improvement over the mold disclosed in copending application Ser. No. 376,006 filed June 18, 1964 and entitled Graphite Continuous Casting Mold and now Patent No. 3,304,585. The disclosure therein is incorporated by reference.

This invention relates to an improved apparatus and method for producing cast metals, such as copper, copper base alloys, aluminum, aluminum alloys, and the like.

In the production of cast metals, it is common practice to pour molten metal through a die or mold. A portion of the mold is cooled. As the molten metal reaches the cooled portion of the mold, the metal solidifies, and the casting which is for-med is withdrawn from the bottom of the mold.

The selection of the mold material used in the casting process is extremely important. The mold material must have a high heat conductivity so that heat may be withdrawn from the molten metal through the mold in as rapid a fashion as possible.

The mold material should have a lubricating action on the cast metal as it flows through it. This will not only effect a rapid transfer of the cast metal through the mold, but will decrease the probability of surface defects forming in the cast metal. The friction between the metal and the mold walls tends to cause fracturing of the newly solidified metal.

Another important factor in selecting a suitable mold material is its ready machinability. The mold, in certain instances, must be machined so as to be employed in different type casting machines. It has been found that graphite fulfills the above requirements.

A good casting mold should also be formed with a heat sink above the solidifying zone of the molten metal. The heat sink will allow a pool of fluid to be maintained on the solidifying metal beneath it so that the pool will readily feed the shrinking metal below it to prevent shrinkage defects. Further, since crystallization takes place from a comparatively small volume of molten metal, segregation will not take place. The pool of metal will prevent or relieve all shrinkage stresses.

While graphite has the above-noted advantages for use as a casting mold, it has the inherent disadvantage of tending to burn away rather rapidly since temperatures employed in the casting operation are inherently high. Hence, prior casting devices and methods have employed a relatively low casting rate. This was done to prevent excessive heat transfer through the mold walls at any given time.

As set forth in the above-mentioned application good results can be obtained by using dense graphite having a fluid permeability of l to 100 millidarcys. It has now been found that low density graphite available at a price of about one-twentieth of that for said dense graphite can be used if the graphite is provided with a thin coating of a 3,459,255 Patented Aug. 5, 1969 conductive metal such as copper or silver. Low density graphite to 500 millidarcys) is more readily available commercially and in larger pieces.

This invention relates to a casting apparatus and method for metals which employs a low density graphite mold having a thin layer of metal metallurgically bonded to surfaces thereof to render the same impervious to fluids.

A further object of the present invention is to provide an apparatus for casting metals which employs a low density graphite mold which is directly cooled by a coolant whereby its heat transfer characteristics are greatly improved.

A still further object of this invention is to disclose an apparatus and method for casting metals which employs a low density graphite mold coated so as to be substantially impervious to a coolant fluid.

Other objects will appear from the disclosure which follows hereinafter.

For the purpose of illustrating the invention there is shown in the drawings forms which are presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.

FIGURE 1 is a fragmentary sectional view through a continuous casting machine illustrating as a component thereof a cooled mold used in the continuous casting of metals by said machine.

FIGURE 2 is a cross-sectional view taken substantially along the plane indicated by the line 22 of FIGURE 1.

Referring now to the drawings in detail, wherein like elements are designated by like numerals, FIGURES l and 2 illustrate apparatus for continuously casting metal which is generally designated by the numeral 10.

The apparatus 10 includes a molten metal receptacle 12 formed from refractory material. Molten metal 14 is poured from a suitable source into the receptacle 12. The molten metal 14 flows from the receptacle 12 through a mold 18 positioned in the opening 16 in the side of the receptacle. The molten metal 14 solidifies in the mold 18 as shown at 20. The solidified cast metal 20 is disposed between rollers 22 and 24 which continuously withdraws it from the mold 18.

The mold 18 includes an annular chamber 26 surrounding the bore in the mold through which the molten metal flows. A coolant, such as water 28, is fed to the annular chamber 26 by means of an inlet conduit 30. The water 28 continuously circulates in the annular chamber 26 and is withdrawn through an outlet conduit 32.

The mold 18 is formed from low density graphite having a fluid permeability of 100 to 500 millidarcys. A darcy is defined as the flow of a cubic centimeter per second per square centimeter per atmosphere per centimeter for a gas of one centipoise viscosity. Hence, it will be understood that the graphite mold 18 is not impervious to the coolant 28 within the chamber 26. Hence, there is a substantial chance of Water penetrating the mold walls and mixing with the molten metal flowing through the mold bore.

In order to prevent such water penetration which could result in an explosion, all exposed surfaces of mold 18 including those defining chamber 26 are coated with a thin layer 27 of copper or silver. Copper is preferred because of the higher cost for silver. A thin layer 27 of copper or silver is one having a thickness of .0005 to 0.25 inch. The layer 27 on the surfaces defining chamber 26 and directly in contact with the coolant 28 may be thinner than the layer 27 contacting the casting 20.

Low density graphite as described above costs about one-twentieth the cost of the dense graphite in the above copending application. The layer 27 is metallurgically bonded to the graphite to make it non-permeable to liquids and gases. The application of the layer 27 by flame or plasma arc spraying onto the outer surface of mold 18 is done with such force that the particles of copper become intermingled with the graphite particles such that no true boundary line exists between the graphite and the copper. The lack of a boundary line enhances heat transfer. Since copper has penetrated the graphite and is a better heat conductor, heat transfer from casting 20 to mold 18 is improved. Layer 27 does not interfere with the self-lubricating property of the graphite mold 18, reduces oxidation, and affords abrasion resistance that is lacking when pure low density graphite is used.

The thusly coated graphite mold is easier to handle and less likely to be gouged or nicked. The mold can include a natural heat sink merely by elimination of the layer 27 around the cooling zone and substituting a layer 29 of low thermal conductivity of the same thickness such as a layer of alumina. Layer 29 is co-extensive with chamber 26. Attachment of copper pipes for coolant 28 to the mold 18 is much easier, more reliable, and quicker since the junction is copper to copper. The layer 27 need not be applied by plasma spraying but can be applied as a colloidal suspension sprayed on mold 18 or into which mold is dipped. Also, the metal could be applied in the form of silver halide and then processed in accordance with standard photographic processes to provide a layer 27 of silver.

Due to the fact that the coolant 28 contacts the molding surface surrounding the flowing molten metal, the solidified metal casting 20 is produced at a high casting rate or speed. Heat is withdrawn from the molten metal through the layer 27 and the mold surface surrounding the bore in the mold and passed directly to the coolant 28. The withdrawn heat need not be passed through an additional surface as has previously been proposed. The direct cooling of the mold effects a greater heat transfer rate thereby allowing for faster solidification of the molten metal, as well as preventing localization of the heat in the graphite mold which will have the tendency to burn away portions of the graphite mold.

The mold 18 may also be provided with a heat sink 31. The heat sink 31 is upstream from the coolant chamber 26. Hence, a pool of molten metal is maintained in the mold above the solidified casting 20. The pool of metal will readily feed the shrinking metal downstream from it to prevent shrinkage effects in the casting. The presence of the pool of metal prevents and relieves all shrinkage stresses. Further, even if some coolant should penetrate the mold, the coolant will not enter the mold in the region of the liquid metal, but rather where the metal has solidified. This moisture will evaporate, and in so doing, will increase the cooling effect. The heat sink 31 has a length from ten percent to sixty percent of the mold length depending on the metal to be cast.

The mold 18 is also preferably slotted, as shown at 36. The slots 36 permit contraction of the bore surrounding the solidified casting 20. This will permit the mold 18 to maintain intimate thermal contact with the casting 20.

The present invention may be emboded in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

I claim:

1. Apparatus for continuous casting of metals comprising a graphite mold having a bore therethrough adapted to receive a stream of molten metal, said bore having an upstream portion and a downstream portion, said graphite mold comprising a graphite material having a fluid permeability of to 500 millidarcys, means for increasing heat transfer from metal being cast to said mold and for rendering said mold impervious to fluids, said means including a layer of copper bonded to the surfaces of said mold and having a thickness substantially thinner than the thickness of the walls of said mold, mold cooling means adjacent the downstream portion of bore, a portion of said mold defining a heat sink adjacent the upstream portion of said bore for maintaining metal in the upstream portion in the molten condition, said heat sink extending between 10 and 60 percent of the overall mold length, said downstream mold cooling means including means for applying a fluid coolant to a copper-coated surface of said mold, said copper-coated surface of the cooling means surrounding the downstream portion of said bore and being spaced from said bore by a wall of said graphite material.

2. A mold for casting metals comprising a mold body made from low density graphite, said body having a bore extending therethrough, means for increasing heat transfer to said mold and for rendering said mold impervious to fluids, said means including a thin layer of good heat conducting metal bonded to surfaces on said body and selected from the group consisting of copper and silver and halides of silver, said layer having a thickness of between .0005 and .02 inch with the particles of said layer intermingled with the particles of the graphite mold body, mold cooling means adjacent said bore but spaced therefrom by a wall of said body for cooling said bore, said mold cooling means including an annular chamber in said mold body surrounding at least a portion of the downstream end of said bore, said chamber being coated with a layer of said metal.

3. A mold in accordance with claim 2 wherein said mold body is made from graphite having a fluid permeability of 100 to 500 millidarcys, said layer of metal having a thickness substantially thinner than the thickness of the wall defining said annular chamber, and said bore being coated with said layer of metal to reduce oxidation of the mold body and provide abrasion resistance.

4. A mold in accordance with claim 2 including a natural heat sink surrounding said chamber, said heat sink including a layer of material having a thermal conductivity substantially lower than copper bonded directly to said mold body in intimate contact therewith.

References Cited UNITED STATES PATENTS 2,367,148 1/1945 Smart et al. 164-283 X 3,059,295 10/1962 Vosskuehler 164-283 3,076,241 2/1963 Simonson et al. 164274 X 3,302,251 2/1967 Speith et al. 164-283 3,304,585 2/1967 Marchlik 164281 FOREIGN PATENTS 708,632 5/1954 Great Britain. 379,698 8/ 1964 Switzerland.

J. SPENCER OVERHOLSER, Primary Examiner R. SPENCER ANNEAR, Assistant Examiner US. Cl. X.R. 164-273; 249-114

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