GB2149333A

GB2149333A - Electroslag refining method

Info

Publication number: GB2149333A
Application number: GB08427096A
Authority: GB
Inventors: Arthur H Griebel
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 1983-11-10
Filing date: 1984-10-26
Publication date: 1985-06-12
Also published as: GB8427096D0; JPH0352522B2; SE8404116D0; FR2554828A1; DE3436958A1; GB2149333B; JPS60103136A; FR2554828B1; SE8404116L

Abstract

Molten metal passes downwardly through a slag layer (5) and resolidifies as an ingot (9) thereunder. The slag is kept molten by passing electrical current through the slag layer (5) and between a first electrode (1) and a second electrode (3) which has liquid or gas cooling means as an integral part thereof. Current is passed between the electrodes through at least one electrically conductive member (7) interposed between said slag (5) and the second electrode (3). The electrically conductive member (7) has a melting temperature in excess of the bulk temperature of the slag and a portion of its surface of in contact with the slag has a temperature above the freezing temperature of the slag (5). The first electrode (1) may be a consumable electrode, as shown which is partially immersed in the slag (5) such that it gradually melts and drops of metal move downwardly therefrom. The second electrode (3) is generally the crucible (3), as shown but may be a non-consumable electrode which is partially immersed in the slag (5). <IMAGE>

Description

SPECIFICATION Metal refining process The present invention relates to a process for refining metal.

Electroslag remelting is a secondary melting or refining process. Primary production ingots, known as consumable electrodes, are re melted and allowed to resolidify under more exactly controlled conditions than can be achieved during primary melting to improve their grain structure and to remove inclusions and inclusion-forming impurities. Re-melting is achieved by resistance heating, with the electric current passing between the consumable electrode and a second electrode. The consumable electrode is partially immersed in a layer of slag, in which "Joule" heat for melting is generated. A pool of molten metal forms below the slag. The slap provides a path for the current. It also removes inclusions and inclusion-forming impurities from the melt.

The electric current is conventionally passed through the slag between the consumable electrode and the ingot. Such a procedure works very well in those instances where the ingot being formed has a cross section which is substantially the same size or larger than the cross section of the electrode(s) being melted. Such a procedure is undesirable in those instances where the cross section of the ingot being formed is smaller than the cross section of the electrode(s) being melted, such as in those instances where the ingot being formed is a bar. Ingots of smaller cross sections would necessitate frequent cutting. Cutting interrupts the current path, necessitating current removal by sliding contacts against the ingot. Sliding contacts are troublesome in that the ingots being formed are often neither smooth nor clean.

A current path which includes the ingot is also susceptible to another problem in those instances where the ingot being formed is smaller than the electrode being melted. The high current required to melt the consumable electrode could resistively heat the ingot and retard its solidification.

A need to establish an alternative current path therefore exists. Experiments have been conducted with the electric current being passed through the slag between the consumable electrode and the crucible. This resulted in instability in the operation of the furnace and damage to the crucible. It was hypothesized that a layer of slag froze adjacent to the crucible wall and that this layer of slag precipitated arcing thereacross. In these experiments the crucible was water-cooled.

The present invention provides a current path which eliminates the arching that occurs when electric current is passed through a slag between a consumable electrode and a second electrode having cooling means as an integral part thereof. It also proveds a current path which eliminates the arcing that occurs when electric current is passed through a slag between two non-consumable electrodes, at least one of which has cooling means as an integral part thereof, such as in those situations wherein molten metal is poured through a slag. The arching due to passing the electric current through the slag and between the electrodes is eliminated by passing the electric current through at least one electrically conductive member interposed between the slag and the cooled electrode. The electrically conductive member has a melting temperature in excess of the bulk temperature of the slag.For purposes of this application, bulk temperature of the slag is that temperature away from the walls of the crucible, recognizing the fact that there are temperature gradients across the slag.

Electroslag remelting processes are disclosed in many references, including United States Patents Nos. 4,108,235 and 4,145,563. Patents Nos. 4,108,235 and 4,145,563 do not disclose the current path of the present invention. The current path of Patent No. 4,108,235 is between the consumable electrode, the crucible and a mandrel used to cast hollow ingots, that for Patent No.

4,145,563 can include a crucible liner which is, however, electrically insulated from the crucible.

Processes for refining metal which is already molten are disclosed in many references. These references include West German patent No. 1,483,646. As with the references discussed in the preceding paragraph, German patent No. 1,483,646 does not disclose the current path of the present invention. It discusses a shell 1 3 of solidified slag which electrically insulates the slag from the crucible. The elctrically conductive members of the present invention preclude such insulation.

The present invention provides a process for refining metal in a crucible containing a layer of molten slag, wherein molten metal passes downwardly through said slag and resolidifies as an ingot thereunder and wherein said slag is kept molten by passing electrical current through said slag layer and between a first electrode and a second electrode, said second electrode having cooling means as an integral part thereof, including the step of passing the electrical current between said first electrode and said second electrode through at least one electrically conductive member interposed between said slag and said second electrode, said electrically conductive member having a melting temperature in excess of the bulk temperature of said slag, a portion of the surface of said electrically conductive member that is in contact with said slag having a temperture above the freezing temperature of said slag.

The present invention thus provides a pro cess for refining metal in a crucible containing a layer of molten slag, wherein molten metal passes downwardly through the slag and reso lidifies as an ingot thereunder. The slag is kept molten by passing electrical current through the slap layer and between a first electrode and a second electrode. The second electrode has liquid or gas cooling means as an integral part thereof. It is usually formed of copper or a copper alloy. Current is passed between the first electrode and the second electrode through at least one electrically con ductive member interposed between the slag and the second electrode. The first electrode may be a consumable electrode which is partially immersed in the molten slag such that it gradually melts, and drops of metal move downwardly therefrom.The second electrode is usually the crucible but may be a nonconsumable electrode which is partially immersed in the layer of molten slag. The source of current may be either direct current or alternative current, although alternating current is preferred.

The electrically conductive members have a melting temperature in excess of the bulk temperature of the slag. Their shape, thickness and conductivity are such that a portion of their surface that is in contact with the slag has a temperature above the freezing temperature of the slag. A member having a higher thermal conductivity will generally need to be thicker than a similarly shaped member having a lower thermal conductivity. Although the members generally protrude from the second electrode into the slag, they may be inlaid therein. Refractory metals such as tantalum are preferred materials from which the members may be formed. There is no critical inimum length for the members. They will usually be at least 3/16-inch (4.8 X 10-3 meters) long. Two or more members will be present in most instances.They can be interposed between the slag and the second electrode by any of those means known to those skilled in the art. Brazing is one particular means for interposing the members.

A schematic representation of elements forming a current path in accordance with the present invention is shown in the Figure.

Current is passed between consumable electrode 1 and crucible 3 through slag layer 5 and electrically conductive members 7. Also shown are ingot 9 and molten metal pool 11.

The following examples are illustrative of several aspects of the invention.

An attempt to pass current directly between a consumable electrode and a crucible, through a layer of slag, resulted in damage to the crucible. The trial was conducted under normal conditions and at normal current (1900 amperes) and voltage levels for a laboratory electroslag remelting furnace. The consumable electrode which was a nickel-base alloy was 3.5 inches (88.9 X 10-3 meters) in diameter. The resulting ingot was 1 inch (25.4 x 10-3 meters) in diameter. The inside diameter of the top of the crucible was 5.25 inches (133.4 X 10-3 meters). The crucible was inspected before the trial and was found to have no defects and only a few blemishes.

An inspection after the trial showed the crucible to be severely pitted with what appeared to be arc scars. The pits typically ranged in depth from 1/64 inch (4 X 10-4 meters) to 1/16 inch (1.6 X 10-3 meters). The trial lasted only ten minutes. If a pit were to penetrate the crucible wall, the inrush of water to the molten slag would cause an explosion. Damage to the crucible was characterized as being sufficient to prevent further trials with this current path.

The current path of the present invention was tried under much the same conditions as the trial reported in the preceding paragraph.

Four tantalum lugs, each 3/8-inch (9.5 x 10-3 meters) in length and 5/8-inch (15.9 x 10-3 meters) in diameter, were brazed into the crucible. The trial was for 6.5 minutes. The current was 2000 amperes. An inspection of the crucible revealed no damage thereto. The tantalum lugs suffered some slight erosion.

Nine additional trials were conducted with the same tantalum lugs. The same nickel-base alloy was used as the consumable electrode.

The tantalum lugs protected the crucible which suffered no damage through the trials.

Claims

1. A process for refining metal in a crucible containing a layer of molten slag, wherein molten metal passes downwardly through said slag and resolidifies as an ingot thereunder and wherein said slag is kept molten by passing electrical current through said slag layer and between a first electrode and a second electrode, said second electrode having cooling means as an integral part thereof, including the step of passing the electrical current between said first electrode and said second electrode through at least one electrically conductive member interposed between said slag and said second electrode, said electrically conductive member having a melting temperature in excess of the bulk temperture of said slag, a portion of the surface of said electrically conductive member that is in contact with said slag having a temperature above the freezing temperature of said slag.

2. The process according to claim 1, wherein said electrically conductive member protrudes into said slag.

3. The process according to claim 1 or 2, wherein said electrically conductive member is a refractory metal.

4. The process according to claim 3, wherein said refractory metal is tantalum.

5. The process according to any one of claims 1-3, wherein there are at least two electrically conductive members.

6. The process according to any one of claims 2-5, wherein said electrically conductive member is at least 3/16-inch (4.8 X 10-3 meters) long.

7. The process according to any one of the preceding claims, wherein said first electrode is a consumable electrode which is partially immersed in said layer of molten slag such that it gradually melts, and drops of metal move downwardly therefrom.

8. The process according to any one of the preceding claims, wherein said second electrode is a liquid-cooled electrode.

9. The process according to any one of claims 1-6, wherein said second electrode is the crucible.

10. The process according to any one of claims 1-6, wherein said second electrode is a non-consumable electrode which is partially immersed in said layer of molten slag.

11. The process according to claim 7, wherein the cross section of the ingot is smaller than the cross section of the consumable electrode.

12. The process according to any one of claims 1-9, wherein said second electrode is copper or a copper alloy.

1 3. A process for refining metal in a crucible containing a layer of molten slag according to claim 1 substantially as herein described with reference to the figure.