US3921023A - Method for joining carbon electrodes and product thereof - Google Patents

Abstract

Method of joining carbon electrodes each provided with a protective metal layer such as aluminum by depositing a layer of second metal in molten state on the surface of the protective coatings on both electrodes in the area of the joint, and on the joint itself after the electrodes have been joined and fastened. The layer of second metal, which consists of aluminum or high aluminum alloy, is preferably applied by spraying. The thusformed electrode column is also a part of the invention.

Description

United States Patent 91 Valchev et al.

[4 Nov. 18, 1975 METHOD FOR JOINING CARBON ELECTRODES AND PRODUCT THEREOF Inventors: Alexander Yordanov Valchev; Velyu Todorov Velev, both of Sofia, Bulgaria Assignee: DMZ LENNIN, Pernik, Bulgaria Filed: Feb. 5, 1973 Appl. No.: 329,815

Related US. Application Data Continuation-impart of Ser. Nos. 56,697, July 20, 1970, and Ser. No. 106,781, Jan. 15, 1971.

[301 Foreign Application Priority Data July 21, 1969 Bulgaria 12705 US. Cl. 313/357; 13/18; 29/4721; 117/105.2; 313/355 Int. Cl. HOlJ 1/00; H01] 17/04; 11011 19/00 Field of Search 313/357, 355, 356; 13/18; 29/D1G. 39, 472.7, 460; 117/105.2, 93.1

[56] References Cited UNITED STATES PATENTS 2,005,897 6/1935 Knowles 29/DIG. 39

2,412,081 12/1946 Droll 29/D1G. 39 3,442,786 5/1969 Clukey et a1 117/105 X 3,476,586 11/1969 Valtchev et a1 13/18 X 3.553.010 l/l971 Rubisch 13/18 3,633,063 1/1972 Ando 313/357 FOREIGN PATENTS OR APPLICATIONS 251.646 4/1926 United Kingdom 13/18 Primary E.ram1'nerSaxfield Chatmon, Jr.

[5 7] ABSTRACT 3 Claims, 4 Drawing Figures 1 I :7 19 l; 1 .1 p i.

ZO-A

METHOD FOR JOINING CARBON ELECTRODES AND PRODUCT THEREOF This application is a continuation-in-part of application Ser. No. 56,697 filed July 20, 1970, and of application Ser. No. 106,781 filed Jan. 15, 1971.

The present invention refers to a method for joining carbon electrodes provided withprotective metal coatings, and to the joined electrodes resulting from such method.

The carbon electrodes for electric furnaces represent cylindrical bodies with a length of 1500 to 2400 mm. Machined in the faces of each electrode are nipple sockets.

The electrode columns of the electric furnaces are considerably longer than the commercially produced electrodes. Because of this, and with a view to a more rational and economical usage of the electrodes, each electrode column consists of several electrodes which are butt connected between successive pairs of electrodes by means of nipples. The electrode columns are exposed to frequent high mechanical loads, chiefly as a result of slips in the furnace charge during the melting process.

The weak spots of the electrode columns are the areas of the nipple joints; this is due to the following three factors:

a. It is true that the nipples are produced of highquality graphite, which has two to three times the strength of the electrode graphite; but because of their considerably smaller cross-section, the mechanical strength of the nipples is about three times lower, as compared with the mechanical strength of the whole electrode. The conditions in the critical section in the bottom of the nipple sockets in the electrodes are similar.

b. Because of the contact resistance between the face surfaces of the joined electrodes, about 50 percent of the electric current passes through the nipple at a high current density 30 to 50 Amp/cm? However, practice has shown that if an electric current of high density passes through a graphite part for a long period of time, cracks and other defects appear in the part. As a result, the strength of the nipple is lowered and occasionally fractures may form. This effect can be called fatigue.

c. The electric current concentration through the nipple causes its being heated more highly than the body of the electrode at the nipple'socket. It has been shown that in normallyelectrically loaded electrodes (l Amp/cm the temperature difference between nipple and socket is 400C; in higher loaded electrodes this difference will be considerably greater. The thermal expansion of the nipple can cause the formation of cracks in the nipple seating holes or sockets, i.e. it can reduce the mechanical strength of the joint many times over. On account of the above-mentioned three factors, the failures of the electrode columns due to mechanical overloading always occur in the nipple joints. The breakage may occur in the nipple, in the nipple sockets, or simultaneously in the nipple and the nipple sockets. A mechanical weakening of the electrode as a result of the nipple joints is inevitable. However, it has been established that in highly current-loaded electrodes the breakages due to a high current density in the nipple rise sharply.

It has been suggested in the past to increase the electric conductivity and the mechanical strength of the carbon electrodes by embracing them in metal jackets. In order to form an electric bridge between two adjacent electrodes or between two adjacent metal jackets, respectively, it has been suggested to use a metal sleeve which makes perfect electrical contact with both adjacent metal jackets. However, such electrodes have not found any application in practice, since in the conditions of electric-arc steelmaking furnaces, cheap and accessible metals (steel in this particular case) will melt and flow at the very moment when the electrode enters the furnace space. Moreover, it is obvious that a metal sleeve cannot provide a reliable electric contact between two adjacent metal jackets. If the sleeve is of steel, (as this would be the case in steelmaking arc furnaces), then at the first heating to a temperature higher than 800C the sleeve will be loosened, causing a cessation of the pressure of the sleeve on the surface of the steel jackets. The ensuing increased contact resistance will inevitably result in the formation of electric sparks and electric arcs, which will cause the melting of the sleeve.

It has also been suggested in the case of joining carbon electrodes for film projectors to use metal sleeves, which should improve the electric contact between the two adjacent electrodes. It is possible that at the comparatively mild temperature conditions in the projectors such sleeves could be useful.

The first high-quality protective coating in steelmaking practice is the one disclosed in VALCHEV et al., U.S. Pat. No. 3,348,929. At the present this coating is widely used in electric steelmaking furnaces throughout Europe. A further improvement of this coating is disclosed in VALCHEV et al., U.S. Pat. No. 3,476,586.

The protective coating in accordance with such VALCHEV et al. patents, which is used in industry, consists of an aluminum alloy with an aluminum content of over 65% (usually about this alloy being fixed to the surface of the carbon electrode by electric arc treatment. The coating is of high electric conductivity about 0.07 Ohms mm /m. Despite the comparatively small thickness of the coating (about 0.8 mm), its electrical conductivity is usually equal to the electric conductivity of the graphite electrode itself. In other words, an electrode with the aforementioned protective coating features an electric conductivity which is twice that of the uncoated electrode.

It could be expected that electrodes with the aforementioned protective coating can be used in the case of a increase of the current density. In practice, the current density can be increased by 50%. The reason for the lower allowable current density is due to the fact that exactly in the nipple joint the electric connection between the coating of both adjacent electrodes is interrupted. The electrically conductive coating actually diverts the flow of the increased part of the current through the confronting surfaces of the electrodes (65% for example), but in any event the nipple is electrically highly loaded.

During the operation of the electrodes in the electric furnaces, the aforementioned coating is always melted (melting point about 600C). For this reason and because of the stated reasons, the use of metal sleeves for forming an electric bridge between the coating of two adjacent electrodes is impossible. The use of layers of comparatively high-melting metals, such as steel, copper and nickel, which are deposited in the area of the nipple joint by spraying in a molten state, is likewise not efficient, since the high coefficient of linear expansion 3 of such layers and the presence of a molten aluminum coating result as a rule in a rejection of the deposited high-melting layer.

It is a general object of the present invention to provide a reliable electric bridge between the aforedescribed coating of two adjacent electrodes provided with protective coatings of the type above described. This is achieved, according to the present invention. by using the properties of an aluminum layer or a layer of an alloy of high aluminum content (aluminum content higher than 60%), this layer being deposited upon coated electrodes by spraying in a molten state. Such a layer has the following properties:

a. a high electric conductivity 0.100 to 0.12 Ohms mm-/m;

b. when heating at a temperature higher than the melting point of the sprayed-on layer (i.e. higher than 660C), the layer does not flow, but remains on the surface of the electrodes;

c. the described layer, which is deposited by spraying in a molten state, is oxidized considerably faster than the coating, which is electric-arc treated; however,

such oxidation proceeds slowly enough that by its use the high electric loading of the nipple is delayed by 12 hours.

The invention is effected in the following way: After joining and fastening the coated electrodes, a layer of aluminum or an alloy of high aluminum content is deposited on the protective metal surface of both electrodes in the area of the joint by spraying in a molten state (metallizing). The width of this band is usually about 15 mm (7 to 8 mm on each electrode). The thickness of the deposited layer is usually from 0.5 to 1.0 mm, but it can be greater.

The invention will be more readily understood upon consideration of the accompanying drawings, illustrating the practical application of the method in accordance with the invention, in which:

FIG. 1 is a fragmentary semi-exploded view, partially in elevation and partially in vertical section through the confronting ends of two carbon electrodes with a protective coating of high aluminum content, which are about to be joined;

FIG. 2 is a fragmentary view in elevation of the electrodes of FIG. 1 after having been joined;

FIG. 3 is a view similar to FIG. 2, but showing the spraying of molten aluminum or of an alloy of high aluminum content upon the joined electrodes at the joint; and

FIG. 4 is a view partially in elevation and partially in vertical section through the finished joint between electrodes.

In FIG. 1 there is shown two similar carbon electrodes l0 and 11 each provided with a protective coating 20, disposed in alignment and about to be joined. The confronting ends of electrodes 10 and 11 are provided with frusto-conical threaded sockets l2 and 14, respectively, a graphite nipple 15 of double-ended frusto-conical shape being shown screwed into the socket 12 in electrode 10. After electrode 11 is screwed onto the nipple 1 5, the joined electrode pair shown in FIG. 2 results.

After the joining and fastening of the electrodes to produce the product of FIG. 2, a metal layer 19 of aluminum or of an alloy of high aluminum content is applied to the surface of both electrodes and in the area of the joint, as well as on the joint itself. This is carried out by spraying aluminum or an alloy of high aluminum Rli 4 content 17 in a molten state from a metallizing gun 16. The width W (FIGS. 3 and 4) of the deposited metal strip 19 is about 15 mm (7 to 8 mm for one electrode).

Depending on the production conditions and the current practice. the joining of the electrodes takes place in the electric furnace or on a special stand outside the electric furnace. In the first case, all joining operations are carried out over the roof of the electric furnace, without taking the electrode columns out of the electrode holders. In the second case, the columns to be joined are taken out of the electrode holders and carried to a special stand, where the joining takes place. In this case the lifted short electrode column is usually replaced by another previously joined electrode column, whereas the joining of the short electrode column takes place after its cooling. However, in some steelmaking plants the joining on a stand outside the furnace is carried out at once and the same hot column is returned to the furnace.

The method according to the present invention can be applied successfully with both techniques of joining the electrodes: Over the roof of the electric arc furnace and on a stand outside the electric arc furnace. This is possible for electrodes with the protective coating, because even for electrodes of 600 mm diameter, the time needed for depositing the metals is only two to three minutes.

When using electrodes joined according to the above-described method, the electric current density through the nipple is initially only one-half that of conventional joints between electrodes.

In the course of oxidation of the metal layer 19, the current through the nipple increases gradually, and after its total oxidation (after the second or the third heat) the current is naturally distributed in a known way. However, at this time the joint in question has already been lowered deep into the furnace. In this way the appearance of fatigue of the nipple is delayed by about 12 hours, and the joint is electrically relieved during the hardest working conditions when the joint is close to the electrode holder. Moreover, the greatest temperature difference between nipple and nipple socket is obtained in the upper part of the electrode columns. The diverting of a considerable part of the current through the layer of aluminum or an alloy of high aluminum content 19 reduces many fold the temperature difference between nipple and nipple socket, and hence the probability of crack formation.

By the use of the above-described method of joining electrodes with a protective coating, an additional favorable effect is obtained, namely, that this additional metal coating protects the joint from premature oxidation, whereby the strength of the joint is slightly improved.

The above-described invention is intended for furnaces with electrically highly-loaded electrodes, for ultra high-power furnaces in particular. Its application provides the possibility for a further increase of the current.

The following example illustrates the application of the invention:

EXAMPLE A l00ton electric arc furnace uses graphite electrodes of 450 mm diameter with a protective coating of high aluminum content. The joining of the electrodes takes place over the roof of the furnace. After joining and fastening the new electrode section, the clamping jaw of the electrode holder is loosened and an oxy-acetylene metal spraying gun with a capacity of 2.5 Kg aluminum per hour is directed at the joint. The spraying gun is operated, and the electrode column starts to rotate simultaneously around its axis at a speed of l revolution per minute. The required quantity of aluminum is deposited in three minutes, then the metal spraying gun is stopped, the electrode column is lowered, and is clamped at the required position in the electrode holder.

Although the invention is illustrated and described with reference to a plurality of preferred embodiments thereof, it is to be expressly understood that it is in no way limited to the disclosure of such a plurality of embodiments, but is capable of numerous modifications within the scope of the appended claims.

What is claimed is:

l. A method of joining cylindrical carbon electrodes for are furnaces comprising the steps of applying protective metal coating layer with an aluminum content higher than 65% to each of the electrodes to be joined, joining and fastening two such electrodes aligned endto-end, and within an arc furnace depositing a layer of a second metal with an aluminum content higher than 60% by spraying in a molten stateon the coating of both joined electrodes and on the joint between them in such a way that the deposited layer of said second metal forms an electrically conductive bridge between the protective coating layers on both aligned electrodes.

2. A method according to claim 1, wherein in the area of the joint the deposited layer of said second metal has a width of about 15 mm and a thickness of about 1 mm.

3. A column of electrodes for an arc furnace, comprising aligned abutting joined cylindrical carbon electrodes, each electrode having a protective metal coating with an aluminum content higher than 65%. and a layer of a second metal with an aluminum content higher than 60%, which second layer has been deposited by spraying in a molten state in the area of the joint between the adjacent electrodes, such layer of second metal having a width of about 15 mm and a thickness of aboutlmm.

Claims (3)

1. A METHOD OF JOINING CYLINDRICAL CARBON ELECTRODES FOR ARC FURNACES COMPRISING THE STEPS OF APPLYING PROTECTIVE METAL COATING LAYER WITH AN ALUMINUM CONTENT HIGHER THAN 65% TO EACH OF THE ELECTRODES TO BE JOINED, JOINING AND FASTENING TWO SUCH ELECTRODES ALIGNED END-TO-END, AND WITHIN AN ARC FURNACE DEPOSITING A LAYER OF A SECOND METAL WITH AN ALUMINUM CONTENT HIGHER THAN 60% BY SPRAYING IN A MOLTEN STATE ON THE COATING OF BOTH JOINED ELECTRODES AND ON THE JOINT BETWEEN THEM IN SUCH A WAY THAT THE DEPOSITED LAYER OF SAID SECOND METAL FORMS EN ELECTRICALLY CONDUCTIVE BRIDGE BETWEEN THE PROTECTIVE COATING LAYERS ON BOTH ALIGNED ELECTRODES.

2. A method according to claim 1, wherein in the area of the joint the deposited layer of said second metal has a width of about 15 mm and a thickness of about 1 mm.

3. A column of electrodes for an arc furnace, comprising aligned abutting joined cylindrical carbon electrodes, each electrode having a protective metal coating with an aluminum content higher than 65%, and a layer of a second metal with an aluminum content higher than 60%, which second layer has been deposited by spraying in a molten state in the area of the joint between the adjacent electrodes, such layer of second metal having a width of about 15 mm and a thickness of about 1 mm.

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