US3225399A - Casting process using borax-silica slag - Google Patents

Description

United States Patent 3,225,399 CASTING PROCESS USING BGRAX-SILICA SLAG George W. Belcher, Norwallr, Conn, assignor to Union Carbide Corporation, a corporation of New York I No Drawing. Filed Nov. 15, 1961, Ser. No. 152,610 5 Claims. (Cl. 22-200) The present invention relates generally to a casting process and, more particularly, to a process for casting copper and copper-containing alloys.

Heretofore, the casting of copper and copper-containing alloys has produced a variety of defects in the cast products. Examples of such defects are porosity from liberaf tion of gases, refractory or dross inclusions, and surface defects such as cold shuts and bleeders. As a result of such defects, a relatively large number of permanent mold castings of copper or co-ppercontaining alloys must be rejected. In the case of ingots, it is often necessary to remove defects by local chipping or grinding or even to remove the entire ingot surface by cutting or machining.

A number of casting processes have been previously proposed for avoiding the formation of the aforedescribed defects in various metals by forming a shell of casting slag on the inner walls of the casting mold either prior to or simultaneously with the pouring of the molten metal. The casting slag has a freezing point below the freezing point of the metal being cast so that the metal freezes before the inner surface of the shell of casting slag. As a result, the metal freezes in contact with a liquid surface rather than the sol-id mold wall, and a smooth Surface is produced on the cast product. Typical exam les of such casting processes are described in detail in U.S. Patent No. 2,631,344 to Kennedy and US. Patent No. 2,493,394 to Dunn et al. When the shell of casting slag is pre-formed over the entire inner surface of the mold, splashings from the molten metal being poured melt the inner surface of the shell at the point of impingement and fall back into the pool of molten metal rather than clinging to the mold wall and forming scabs and slivers on the ingot surface. Moreover, when the molten metal is poured through a pool of the molten casting slag, the stripping action of the casting slag may effect further refining or purification of the molten metal. However, most such casting processes heretofore proposed have employed casting slags which are of little or no value in casting copper or copper-containing alloys.

It is, therefore, the main object of the present invention to provide a process for casting copper or coopercontaining alloys into bodies substantially free of defects.

It is another object of the invention to provide such a process wherein a shell of casting slag is formed on the inner walls of the casting form.

It is a further object of the invention to provide such a process wherein the copper or coppercontaining alloy is cast through a layer of molten casting slag.

Other aims and advantages of the invention will be apparent from the following description and appended claims.

In accordance with the present invention, there is provided a process for casting copper or copper-containing alloys comprising preparing a borax-silica casting slag having a freezing point below the freezing point of the material to be cast; forming a molten pool of the casting slag on the bottom of a casting form; forming a shell of the casting slag on the inner walls of the casting form; and pouring a molten mass of the material to be cast into the casting form and through the pool of casting slag at a rate such that only the inner surface of the shell is melted.

The casting slag employed in the inventive process is a mixture of borax and silica having a freezing point below the freezing point of copper or the particular copper-containing alloy to be cast. As used herein, the term freez- 3,225,399 Patented Dec. 28, 1965 ing point refers to the temperature at which any portion of the molten material is precipitated or converted to the solid state. The amount of silica in the bo-rax-silica slag should be sufiiciently great to produce a glass-like silicate shell which readily adheres to the walls of the casting mold, and it is generally preferred to have a silica content of at least 20 percent by weight for this purpose. Also, the proportions of borax and silica in the slag should be such that the freezing point of the slag is below the freezing point of the particular material to be cast. Since the eutectic composition has the lowest melting point in a given system, the preferred casting slag in the present process is usually the eutectic composition of the boraxsilica system, i.e., a slag of 72 percent by weight RX and 28 percent by weight silica. This slag has a melting point of 1526 E, which is below the melting point of copper and most copper-containing alloys. Of course, the composition of the slag can be varied from the eutectic composition as long as the variation does not increase the freezing point of the slag above the freezing point of the material to be cast. Also, the slag may contain minor quantities of impurities which are substantially inert to the material to be cast and do not substantially increase the freezing point of the slag.

The shell of casting slag can be formed on the inner walls of the casting mold in various ways. For example, the molten metal may be poured into a mold partially filled with the molten slag so that a layer of molten slag rises on top of the pool of molten metal and forms a shell on successive zones of the mold wall. Alternatively, the entire inner surface of the mold may be covered with a shell of casting slag prior to the pouring of the metal by completely filling the mold with molten casting slag and allowing it to stand until a shell of the desired thickness has solidified on the mold walls. A complete shell may also be formed prior to the pouring operation by blowing the casting slag in powdered form through a flame torch and onto the mold wall. A flame torch suitable for such use is described in detail in U.S. Patent No. 2,735,796.

The thickness of the shell of casting slag deposited on the mold walls should be sufficient to prevent penetration thereof by the molten metal being cast. Since a portion of the shell is melted by the introduction of the molten meta-l, the shell should always have internal dimensions smaller than the external dimensions desired in the final ingot or cast product. The slag which is melted from the shell during the pouring operation rides on the rising pool of metal, thereby isolating the metal from the surrounding atmosphere, and allows the metal to solidify without any outside contamination. It is usually preferred to have a slightlv greater shell thickness at the bottom of the mold to provide for the initial impact of the stream of molten metal. Shells of excessive thickness should be avoided, since such shells do not have sufficient body to Withstand the heat shock from the molten metal without :spalling. Also, excessive shell thicknesses result in small ingots and may interfere with the removal of the ingot from the mold. The preferred shell thickness is less than about inch.

A pool of molten casting slag should be present in the bottom of the mold when the molten metal is poured. The stream of molten metal passes through the molten casting slag, and the horax in the slag scavenges dross inclusions and gases from the metal. Thus. the inventive process eliminates both internal and surface defects in the cast product. Since the molten slag rides on top of the rising pool of meta-l, the incoming metal continuously passes through a layer of molten slag. The de th of the molten slag should be sufiicient to effect the desired degree of washing or refin ng in the molten metal. Where the slag shell is formed by freezing from the rising layer of slag. the initial pool of slag should be large enough to provide a layer of slag even when the surface level of the molten metal reaches the top of the mold. When the slag shell is formed over the entire inner surface of the mold, additional slag will be added to the rising layer by partial melting of the shell.

The inner walls of the shell of casting slag, i.e., the walls which contact the molten metal, should be at a temperature above the temperature of the outer walls of the shell, i.e., the walls which contact the casting form. Preferably, the inner walls of the shell should be at a temperature approaching the melting point of the casting slag so that metal splashing occurring during the pouring operation liquefy the surface of the shell at the point of impingement and are not retained on the inner surface of the shell. Since the melting of the inner surface of the shell causes the splashings to fall back into the molten metal being cast, they do not form scabs and slivers on the ingot surface. It is also important that the outer walls of the shell be at a temperature sufiiciently low to freeze the molten metal so that the molten metal solidifies before coming into contact with the casting form. Since the temperature of the air surrounding the casting mold is normally considerably below the temperature of the molten metal being poured, the desired temperature gradient in the shell is usually achieved without the use of cooling or heating devices.

In an example of the inventive process, 1000 grams of a borax-silica casting slag consisting of 72 percent by weight borax and 28 percent by weight silica was poured into a casting mold and allowed to stand until .a slag shell was formed around the inner surface of the bottom of the mold. A molten alloy containing 85 percent copper, 5 percent tin, 5 percent zinc, and 5 percent lead, by weight, was then poured into the mold and through the pool of molten casting slag at a rise rate of 58 inches per minute. The temperature of the casting slag was 1300 C., while the temperature of the molten alloy was 1200 C. After the ingot had solidified, it was removed from the mold and the slag envelope removed therefrom. The ingot was 3 inches square and 16 inches long, weighed 22 pounds, and was substantially free of 'both surface and internal defects.

In a second example of the inventive process, 40 pounds of a boraX-sil-ica casting slag consisting of 72 percent by weight borax and 28 percent by Weight silica was poured into a cylindrical, water-cooled, copper casting mold and allowed to stand until a slag shell was formed around the inner surface of the bottom of the mold. A molten alloy containing 97 percent by weight copper and 3 percent by weight silicon was then poured into the mold and through the pool of molten casting slag at a rise rate of inches per minute. The temperature of the casting slag was 2400 F., while the temperature of the molten alloy was 2200 F. After the ingot had solidified, it was removed from the mold and the slag envelope removed therefrom. The ingot was 7.25 inches in diameter and 100 inches long, weighed 11 pounds, and was substantially free of both surface and internal defects.

While various specific embodiments of the present invention have been illustrated and described herein, it is not intended to limit the invention to any of the details herein shown, but only as set forth in the appended claims.

What is claimed is:

1. A process for casting a material selected from the group consisting of copper and copper-containing alloys comprising preparing a borax-silica casting slag consisting of between about 20 and about 30 percent by weight silica and between about 80 and about 70 percent by weight borax, and up to an insubstantial amount of impurities which are inert to the material to be cast and which do not substantially increase the freezing point of the bo-raX-silica slag, said slag having a freezing point below the freezing point of the material to be cast; forming a molten pool of said casting slag on the bottom of a casting form; forming a shell of said casting slag on the inner walls of said casting form; and pouring a molten mass of the material to be cast into said casting form and through said pool of casting slag at a rate such that only the inner surface of said shell is melted.

2. A process for casting a material selected from the group consisting of copper and copper-containing alloys comprising preparing a borax-silica casting slag containing about 72 percent by weight boraX and about 28 percent by weight silica; forming a molten pool of said casting slag on the bottom of a casting form; forming a shell of said casting slag on the inner walls of said casting form; and pouring a molten mass of the material to 'be cast into said casting form and through said pool of casting slag at a rate such that only the inner surface of said shell is melted.

3. A process for casting a material selected from the group consisting of copper and copper-containing alloys comprising preparing a bath of molten borax-silica casting slag consist-ing of between about 20 and about 30 percent by weight silica, between about 80 and about percent by weight borax, and up to an insubstantial amount of impurities which are inert to the material to be cast and which do not substantially increase the freezing point of the boraxasilica slag, said slag having a freezing point below the freezing point of the material to be cast; intimately contacting the inner walls of a casting form with said molten casting slag; chilling said molten casting slag in contact with said inner walls of said casting form to produce a shell of casting slag reinforced by said casting form; maintaining the inner walls of said shell at a temperature in excess of the outer walls thereof; and pouring a molten mass of the material to be cast through a molten layer of said casting slag and into said shell at a rate such that only the inner surface of said shell is melted.

4. The process of claim 3 wherein said borax-silica casting slag contains about 72 percent by weight borax and about 28 percent by weight silica.

5. A process for casting a material selected from the group consisting of copper and copper-containing alloys comprising forming in a casting mold a 'body of molten boraX-silica casting slag consisting of between about 20 and about 30 percent by weight silica, between about 80 and about 70 percent by Weight 'horax, and up to an insubstantial amount of impurities which are inert to the material to be cast and which do not substantially increase the freezing point of the boraX-silica slag, said slag having a freezing point below the freezing point of the material to be cast; allowing said molten casting slag to stand in said mold until a thin shell of frozen casting slag is formed on the inner surface of the casting mold; and pouring a molten mass of the material to be cast into said casting mold and through the body of molten casting slag therein so that said molten casting slag rises on the pool of molten material being cast at a rate sufiicient to form a thin shell of frozen casting slag on successive zones of the inner surface of said mold.

References Cited by the Examiner UNITED STATES PATENTS 1,955,726 4/1934 Archer et a1 75-76 2,493,394 l/19SO Dunn et al. 22-200 X 2,535,536 12/1950 Hagmaier.

2,631,344 3/1953 Kennedy 2220O MARCUS U. LYONS, Primary Examiner.

DAVID L. RECK, MICHAEL V. BRINDISI, WILLIAM J. STEPHENSON, Examiners.

Claims (1)

1. A PROCESS FOR CASTING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF COPPER AND COPPER-CONTAINING ALLOYS COMPRISING PREPARING A BORAX-SILICA CASTING SLAG CONSISTING OF BETWEEN ABOUT 20 AND ABOUT 30 PERCENT BY WEIGHT SILICA AND BETWEEN ABOUT 80 AND ABOUT 70 PERCENT BY WEIGHT BORAX, AND UP TO AN INSUBSTANTIAL AMOUNT OF IMPURITIES WHICH ARE INERT TO THE MATERIAL TO BE CAST AND WHICH DO NOT SUBSTANTIALLY INCREASE THE FREEZING POINT OF THE BORAX-SILICA SLAG, AND SLAG HAVING A FREEZING POINT BELOW THE FREEZING POINT OF THE MATERIAL TO BE CAST; FORMING A MOLTEN POOL OF SAID CASTING SLAG ON THE BOTTOM OF A CASTING FORM; FORMING A SHELL OF SAID CASTING SLAG ON THE INNER WALLS OF SAID CASTING FORM; AND POURING A MOLTEN MASS OF THE MATERIAL TO BE CAST INTO SAID CASTING FORM AND THROUGH SAID POOL OF CASTING SLAG AT A RATE SUCH THAT ONLY THE INNER SURFACE OF SAID SHELL IS MELTED.