US3790691A

US3790691A - Method for electroslag remelting of consumable electrodes

Info

Publication number: US3790691A
Application number: US00243885A
Authority: US
Inventors: J Emelyanenko; V Baglai; N Podola; J Latash; O Bondarenko; B Medovar; I Pentegov; B Paton; V Lebedev
Original assignee: Institut Elektrosvarki Imeni E O Patona Akademii Nauk Ukrainskoi Ssr
Current assignee: Institut Elektrosvarki Imeni E O Patona Akademii Nauk Ukrainskoi Ssr
Priority date: 1969-06-16
Filing date: 1972-04-13
Publication date: 1974-02-05
Anticipated expiration: 1991-02-05
Also published as: BE749887A; GB1278169A; AT301776B; SE356198B; US3665081A; DE2018065A1; JPS5031977B1; DE2018065B2; FR2049951A5

Abstract

The invention relates to the field of electrometallurgy, and in particular to a method for electroslag remelting. The method is characterized in that it utilizes a transformer power supply means, which can be three-phase or which can consist of several single-phase transformers. The method uses secondary windings of the transformer power supply and incorporates center tappings of those secondaries interconnected to provide for an equality of the line speeds of melting the electrodes and for a uniform temperature field within the slag bath. The method can be utilized in the process of producing unique hollow or solid ingots and slabs of high-grade steels and alloys having a weight of 200 tons and higher.

Description

United States Patent 1191 Paton et al.

METHOD FOR ELECTROSLAG REMELTING OF CONSUMABLE ELECTRODES Inventors: Boris Evgenievich Paton; Vladimir Konstantinovich Lebedev; Boris Izrailevich Medovar; Igor Vladirnirovich Pentegov; Jury Vadimovich Latash; Oleg Petrovich Bondarenko; Vitaly Mikhailovich Baglai; Nikolai Vasilievich Podola; July Georgievich Emelyanenko, all of Kiev, U.S.S.R.

Assignee: Institut Elektrosvarki Im. E.O.

Patona, Kiev, U.S.S.R.

Filed: Apr. 13, 1972 Appl. No.: 243,885

Related U.S. Application Data Division of Ser'. No. 26,733, April 8, 1970, Pat. No. 3,665,081.

Foreign Application Priority Data June 16, 1969 U.S.S.R 1336305 U.S. c1. 13/9, 13/34 1m. 01. 1105b 3/60 Primary Examiner-Roy N. Enval], Jr. Attorney, Agent, or Firm-William A. Strauch et al.

[5 7] ABSTRACT The invention relates to the field of electrometallurgy, and in particular to a method for electroslag remelting. The method is characterized in that it utilizes a transformer power supply means, which can be threephase or which can consist of several single-phase transformers. The method uses secondary windings of the transformer power supply and incorporates center tappings of those secondaries interconnected to provide for an equality of the line-speeds of melting the electrodes and for a uniform temperature field within the slag bath. The method can be utilized in the process of producing unique hollow or solid ingots and slabs of high-grade steels and alloys having a weight of 200 tons and higher.

13 Claims, 9 Drawing Figures PATENTED FEB 5W4 snaznurs FIG. 2

METHOD FOR ELECTROSLAG REMELTING OF CONSUMABLE ELECTRODES CROSS-REFERENCE TO RELATED APPLICATION This application is a division of application Ser. No. 26,733, filed Apr. 8, 1970, now U.S. Pat. No. 3,665,081.

BACKGROUND OF THE INVENTION The present invention relates to the field of electrometallurgy and, more particularly, to a method for the electroslag remelting of consumable electrodes. The method can be utilized with apparatus to produce unique hollow or solid ingots and slabs having a weight of up to 200 tons and higher.

Apparatus for electroslag remelting consumable electrodes are known in the art. For example, a bifilar (two-electrode) apparatus comprising a mold, an electrode holder with electrodes fixedly secured therein with respect to each other, which electrodes are insulated one from another and are fed from one singlephase transformer is known (cf. the USSR Authors Certificates No. 2198, cl. 188 5/56 and No. 2594, cl. 188 5/56 which correspond to French Pat. No. 1,545,017). As explained in the corresponding French patent and in an earlier filed Great Britain Pat. No. 1,117,202 at page 3, lines 95-106, a bifilar arrangement of parts is accomplished by the parallel and close spacing of the current conductors. This arrangement results in electrical compensation of the magnetic fields produced by high current AC. power, decreases the inductive reactance, and considerably increases the power factor (Cos (i Also known in the art are three-electrode electroslag remelting furnaces fed either from one three-phase transformer or from several single-phase transformers (cf. the book Electroslag Remelting by B.I. Medovar et al., p. 21, Metallurgizdat Publishers, Moscow, 1963).

Such apparatus make it possible to produce highquality metal in ingots having weights of to tons. In this case the pan or bottom plate of the electroslag remelting furnace mold must be connected either to the neutral point (in a three-phase furnace) or to the center tapping of the secondary winding of the transformer (in a single-phase furnace) in order to provide an equality of the line speeds of melting for electrodes having different melting temperatures, different cross sections or a change in the geometry of their disposition in the slag bath. The lower end portions of the electrodes are immersed in the slag bath by a common crosspiece and the equality of line speeds of melting is particularly important at a low depth of the bath which is sometimes required by the process conditions. The single phase can be supplemented by an equalizing reactor or choke connected in parallel to the transformer in the bifilar furnaces. The provision for equality of the line speeds of melting allows for the trouble-free operation of the apparatus.

When the weight of the ingot to be cast is increased a to 100 tons, perference is given to multielectrode circuits, for example those containing four-electrodes, sixelectrodes and higher. Such apparatus having input power of higher than 10,000 KVA are to be fed not from a single three-phase transformer but from several single-phase transformers, which are inserted into the three-phase mains with their primary windings and are located symmetrically relative to the geometrical center of the furnace in order to obtain the shortest possible dimensions and a full symmetry for the power network of the apparatus. For lower ingot tonnage apparatus, one three-phase transformer can be used.

The remelting of four, six and more consumable electrodes, secured in a common crosspiece of such an apparatus necessitates a provision for maintaining equality in the line speeds of melting of the consumable electrodes, because in the process of casting the electrodes having a weight of 20 to 50 tons, and higher, they will inevitably be affected by pores and cavities, i.e., there will bedifferences in the electrodes sections. Furthermore, the geometry of disposition of the electrodes relative to the symmetry axis of the mold may be disturbed, and this will cause an unbalanced melting condition for the electrodes or even the endportion of some of the electrodes can melt sufficiently so they can become located on the surface of the slag bath and/or the immersion of the end portion of other electrodes into the slag bath to a position resulting in a short circuit with the face of the underlying metal pool. This leads to emergency operating conditions of the apparatus.

The known methods of elimination of the unbalance in the line speeds of melting of electrodes having different cross-sections in high-power electroslag remelting apparatus, for example by means of the connection of the pan or bottom plate of the apparatus with the neutral point of the star formed by the secondary windings of three transformers, are not suitable for large tonnage furnaces since in this case the neutral wire has a considerable inductive resistance due to the fact that such a circuit cannot provide for a fully compensated noninductively charged current conductor. Therefore, the effectiveness of the neutral wire is very low and all the undesirable phenomena mentioned above are quite possible to appear in the process of melting.

An object of the present invention is to eliminate the above-mentioned disadvantages.

BRIEF SUMMARY OF THE INVENTION The principal object of the invention is to provide a method for electroslag remelting of an even number of, not less thanfour, groups of consumable electrodes which makes possible the production of high-quality metal ingots weighting tons and higher and to completely eliminate the effect of the above-mentioned unbalanced conditions in melting the consumable electrodes, independent of the reasons of their appearance as well as to eliminate emergency operating conditions.

In order to provide a fully compensated noninductive current-supply line to the electrodes, each of the busbar conductors connecting the center tapping of one secondary winding of the transformers with the center tappings of the remaining secondary windings is preferably placed in bifilar arrangement with respect to the busbar conductors connecting the terminals of this secondary winding to the groups of electrodes. The junction point or connection of all the center tappings of the secondary windings is preferably positioned on the symmetry axis of the electrode holder.

To provide for an increase in the efficiency of stabilization of the line speeds of melting of the electrodes, the groups of electrodes are preferably located at the apices of a regular polygon so that each two groups of electrodes connected to the terminals of any one seeondary winding of the transformer is located either at two adjacent apices of the polygon formed by the arrangement of the electrodes.

In order to increase the coefficient of filling the mold and to reduce the height of the apparatus, it may be equipped with a metal blank mounted in the central part of the electrode holder and electrically insulated therefrom, one end of the blank being connected to the junction of the busbar conductors from the center tappings of the secondary windings of the transformer means. By this manner of electrical connection the metal blank is connected to the power supply through a normally neutral center tapping circuit rather than being directly connected to the normal power circuit of the secondary windings as in the case of the electrodes. The metal blank may be composed of metal which differs from that of the consumable electrodes by a chemical composition when additional alloying of the ingot is desired.

Furthermore, the method may utilize an equallizing reactor or choke connected in parallel to each secondary winding of the transformer, the center tapping of this reactor being connected with the center tappings of the other equalizing reactors connected to the remaining secondary windings of the transformer.

The method of electroslag remelting in accordance with the present invention successively solves all the problems above-mentioned Further objects and disadvantages of the invention will be apparent from the following detailed description of some specific embodiments of the invention, reference being made to the accompanying drawings, in which:

FIG. 1 is an electric circuit diagram of an apparatus, operable in accord with the method of this invention, for remelting of four consumable electrodes (defined as four groups of electrodes), in which the center tappings of the secondary windings of the two transformers are interconnected through a busbar conductor;

FIG. 2 is an electric circuit diagram of the apparatus, for remelting four consumable electrodes, in which the interconnected center tappings are taken from two equalizing reactors provided in parallel with each of the two transformers;

FIG. 3 is an electric circuit diagram of the apparatus, for casting hollow ingots from 16 groups of electrodes, in which the center tappings of the secondary windings of the transformers (eight) are interconnected in the center of the electrode holder;

FIG. 4 is an electric circuit diagram of the apparatus, for remelting six consumable electrodes (six groups), in which the secondary windings of the transformers are connected with two electrodes disposed at two adjacent apices of a hexagon;

FIG. 5 is a vector diagram of voltages between the electrodes in the circuit shown in FIG. 4;

FIG. 6 is an electric circuit diagram of the apparatus for remelting six consumable electrodes, in which the secondary windings of the transformers are connected with two electrodes disposed at diametrically opposite apices of a hexagon;

FIG. 7 is a vector diagram of voltages between the electrodes of the apparatus shown in FIG. 6;

FIG. 8 (on the sheet with FIG. 3) is an electric circuit diagram of the apparatus for remelting six consumable electrodes, in which the center tappings of the equalizing reactors are connected with a metal blank; and

FIG. 9 is a furnace and electrical diagram of the apparatus shown in FIG. 1.

DETAILED DESCRIPTION Referring to the circuit diagram of the apparatus illustrated in FIG. 1, four groups of consumable electrodes I, II, III and IV are secured in a fixed position relative to one another in a common electrode holder (not shown) and are subjected to electroslag remelting. Each electrode group consists of a single electrode. The consumable electrodes are connected in pairs to the two terminals of each of the secondary windings of the two

transformers

5, 6. The electrodes I and II are respectively connected to the

terminals

1 and 2 of the secondary winding of the transformer 6, while the electrodes III and IV are connected to the terminals 3 and 4 of the secondary winding of the transformer 5. The center tapping 0 0 of the secondary windings of the

transformers

5 and 6 are interconnected by a busbar conductor 7 which is insulated from the mold 8 and from the other members of the construction of the apparatus. In this case the primary windings of the

transformer

5 and 6 may be connected to any of the phases of a three-phase main. The connection of the center tapping 0,, 0 of the secondary windings of the

transformers

5, 6 through the common busbar conductor 7 provides for a required electric coupling between all the electrode groups I, II, III, IV and ensures active selfcontrol of the line speeds of melting the consumable electrodes.

When it is technologically difficult to tap out the center point of the secondary windings of the transformers, equalizing reactors or chokes may be used for this purpose. As shown in FIG. 2, in this case, the groups of electrodes I, II, III and IV are also connected in pairs to the terminals of the secondary windings of the

transformers

5 and 6. The electrodes I and II are connected to the

terminals

1 and 2, while the electrodes III and IV are connected to the terminals 3 and 4, and equalizing rectors or chokes 9 and 10 are connected in parallel to the secondary windings of the

transformers

5 and 6, respectively. The center tappings 0 and 0 of the equalizing

reactors

9 and 10 are interconnected by a busbar conductor 7 which is also insulated from the members of the construction of the apparatus.

When operating an apparatus with a large number of groups of electrodes, the use of the shortest electrical connectors for the power network becomes a stringent requirement both for increasing technical and economical characteristics of the apparatus as well as for providing for its working capacity on heavy currents, it is necessary to provide for a fully compensated noninductive current conductor supplying power to the consumable electrodes. This object is attained in a general case of an electric supply circuit for the apparatus shown in FIG. 3. This apparatus is used for casting hollow ingots by remelting consumable electrodes with an even number of groups of electrodes n 2 m (where

m

2, 3, 4, 5, the groups of electrodes I, II, III, IV, V, VI, A and B being connected in pairs to the terminals of the

secondary windings

1, 2, 3, 4, 5, 6, a, b, etc. of the transformers whose number is one-half that of the n groups. The center tappings 0 0 0 0,,, etc. of the secondary windings are interconnected through the busbar conductors 7, each conductor being placed in bifilar arrangement to the busbar conductors connecting the ends of the corresponding secondary windings with two groups of electrodes.

The point of connection 1 1 of all the center tappings of the secondary windings of the transformers is located directly in the electrodes holder along the symmetry axis of the molds l2 and 13 and is electrically insulated therefrom. The secondary windings of these transformers may be connected to a single-phase or three-phase mains either with a cyclic alternation of phases in the direction of the bypass of the mold or with a phase alternation according to any preselected law.

A complete rendering of FIG. 3, as is apparent, will illustrate eight transformers with 16 groups of electrodes (I XVI), each group including two electrodes.

For increasing the efficiency of the self-stabilization of the line speeds of melting of the electrodes in the apparatus when highly powered, the groups of consumable electrodes are disposed at the apices of a regular polygon, in which case the two groups of electrodes connected to the terminals of one secondary winding of the transformer are located either at two adjacent apices or at two diametrically opposite apices of the said polygon.

All further embodiments of the invention are illustrated on the apparatus shown in FIG. 4 wherein the apparatus has six groups of consumable electrodes, I, II, III, IV, V, VI fixedly secured relative to each other and insulated from each other. The groups of electrodes are supplied with current from three

singlephase transformers

14, 15, 16 connected to threephase mains. In this case each electrode group has one electrode. When only six consumable electrodes are used in the system, the apparatus can be supplied from one three-phase transformer, having three secondary windings.

The consumable electrodes I, II, Ill, IV, V, V] are located at the apices of a regular polygon and form three pairs of electrodes I-Il, III-IV, V-VI which are respectively connected in series to the secondary windings I, 2, 3, 4, 5 6 of three single-phase transformers, each being disposed at an angle of 120 to the symmetry axis of the apparatus. The simplest structure of network having short conductors, which is very important during the operation of the apparatus on heavy currents (from 50 to 70 kA), is obtained if each of the pairs of the electrodes I-II, III-IV, V-VI is disposed at two apices of a hexagon formed by the electrodes belonging to one face of the hexagon, while the junction point or connection 11 of all the center tappings of the transformers is located on the symmetry axis of the electrode holder. In this case a minimum inductance of the entire network of the apparatus is obtained because each of the busbar conductors 7 connecting the center tappin gs of the secondary windings of the power supply transformers is placed in bifilar arrangmeent to the busbar conductors connecting the terminals of the secondary windings of the corresponding transformer to the electrodes of the apparatus.

FIG. 5 shows a vector diagram of voltages between the electrodes 1, II, III, IV, V, VI arranged in the circuit shown in FIG. 4. The connection of all the center tapping provides for the required electric coupling between the electrodes and transformers as well as for active self-control of the line speed of melting the electrodes. The center tappings of all the secondary windings have the same potential and are shown in the diagram by the point 11 which divides all line voltages U, n, urm v-w in half- When it is assumed that all of the electrodes are immersed into a slag bath at the same depth, then the voltages, for example between the electrode VI and the other electrodes, according to the voltage diagrams, are equal, therefore U U/2; U V3/2) U; w-1V W) rn-m VrF Where U is a line voltage. In this case, the equivalent resistances of the slag between the end of each electrode and the molten metal bath ar equal, therefore, the currents passing through each of the electrodes are also equal. Now suppose that the electrode VI becomes immersed into the slag deeper than the other electrodes due to a difference in a cross section thereof or due to a disturbance of the geometry of disposition of the electrodes in the slag bath or due to a different melting temperature of the electrodes. In this case, the resistance between the end of the immersed electrode VI and the molten metal bath is lower than the resistance between the ends of the other electrodes and the metal, due to the fact that each of the electrodes I, II, III, IV, VI acquires a certain current increment A J determined by the voltage and resistance between the electrode VI and each of the electrodes, which are geometrically summed up in the electrode VI. Therefore, the electrode VI is melted more rapidly than the remaining electrodes and the depth of immersion of the electrodes into the slag bath is brought into balance. If the cause of the unbalance in melting the electrodes is invariable during the whole period of melting, the depth of immersion of the electrode VI will be somewhat larger than that of the other electrodes. This difference in the depths of immersion depends on improved gain factor realized by the whole system of slag-control described herein.

The circuit of the apparatus shown in FIG. 4 features some assymetry of voltages; the electrode VI with the electrode V have a voltage Uyry U and U rr U/2, as has each of the electrodes, i.e., the voltage between the electrodes is not proportional to the distances therebetween. Therefore, in order to provide a full symmetry of the diagram of voltages between all the electrodes and to increase the effectiveness of stabilization of the line speeds of melting the consumable electrodes, each pair of the electrodes say III-VI, connected to the same

secondary windings

3, 6 of the transformers 14 may be connected at two opposite apices of the regular hexagon as shown by FIG. 6. In this case, the vector diagram of voltages between the electrodes shown in FIG. 7 completely follows the geometry of disposition of the electrodes in the slag bath, in which case the voltages between any adjacent electrodes, for example I-II, II-III, III-IV, etc. are equal and amount U/2; the voltage between the electrodes disposed at the apices of the equilateral triangles (for example A II, IV, Vl, or A I, III, V) are also equal and amount to lT/E U; the voltages between the electrodes disposed at the opposite apices of the hexagon Un-y, U U are equal and come to U, i.e., there is provided a full symmetry of voltages as the voltages between the electrodes rise in proportion to the distances therebetween, therefore, the effectiveness of self-control is higher as compared to the circuit shown in FIG. 4.

To increase the coefficient of filling of the mold and to reduce the height of the apparatus, a metal blank insulated from the members of the construction of the apparatus can be positioned in the center of the regular polygon formed by the consumable groups of electrodes l-Vl. The lower end of the metal blank is then immersed in the slag bath. The center tappings of the secondary windings of the transformers or equalizing reactors may be either connected to the said metal blank or be insulated therefrom. Shown in FIG. 8 is an electric circuit diagram of an apparatus for remelting in a cooled mold of six consumable electrodes l-Vl disposed at the apices of a regular hexagon and divided in pairs l-Il-lll-lV-VVl, each being correspondingly connected to the

terminals

1, 2, 3 4, 5, 6 of the secondary windings of three single-

phase transformers

14, 15, 16 whose primary windings are connected to three-phase mains. Connected to the terminals of each secondary winding 1 2 3, 4 5, 6 is a winding of an equalizing reactor respectively 18, 19, 20. The center tappings 0,, 0 of the reactors are interconnected with busbar conductors 7, the junction point of all the busbar conductor being connected to a consumable metal blank 17, which is located in the center of the hexagon. The lower end of the blank is immersed into the slag bath for melting by the molten slag. The operating principle of the apparatus shown in FIG. 8 is similar to that described above.

FIG. 9 shows the arrangement of the furnace and the two transformers and 6 of FIG. 1. The busbar conductor 7 is shown interconnecting center tappings 0 and 0 Electrode groups I, II, III, IV are insulated from each other by insulation 7a retained in electrode holder 71). The electrodes are melted in slag bath 8a provided in cooled mold 8 and the melted metal collects in the molten metal pool 8b and then solidifies to form a high quality ingot 8c.

Thus, the proposed apparatus makes it possible to carry out the process of electroslag remelting of any even number, but not less than four, groups of consumable electrodes fixedly secured in a common electrode holder with respect to each other and having different cross sections, different melting temperatures. or a different chemical composition. The apparatus corrects for disturbances of the geometry of disposition of the electrodes in a slag bath. Through use of the apparatus it is possible to completely eliminate the unbalance in the character of melting of the electrodes and to avoid the possibility of occurrence of the emergency operating conditions.

It is clear that in case of a great number of transformers and considerable distances between the electrodes, for example during the casting of a hollow ingot having a large diameter or a rectangular ingot-slab, it is possi ble to connect the center tappings of the transformer windings or the center tappings of the equalizing reactors into groups, each group having at least two windings. In this case there is no electric coupling of the pan or bottom plate and the ingot with the power source through the electrodes, and this significantly simplifies the construction of the apparatus particularly when casting the ingots weighing more than 100 tons and when casting the hollow ingots.

Furthermore, in the proposed apparatus, due to the presence of voltages between all the electrodes, there is provided a current through the slag between these electrodes, and this leads to creation of a uniform temperature field within the slag bath. This is especially important for casting of large shaped ingots.

We claim:

1. A method for electroslag remelting of metal wherein an even number of consumable electrodes at least equal to four depend into and are remelted by electric power in a molten slag bath in a mold to make at least one ingot from remelted electrode metal comprising: the steps of arranging the plurality of electrodes into an even number of at least four groups of consumable electrodes in which each of the groups contains at least one electrode and in which each group is insulated from one another, except for a current path through the molten slag bath; supplying electric power to the groups of electrodes from a plurality of discrete electric power supply means less in number than the total number of groups of electrodes by connecting each discrete power supply means with an electrical connection to an associated two of the groups of electrodes; and providing an equalizing current circuit between at least two of the discrete power supply means, whereby melting speeds of the plurality of electrodes deriving electrical power from those at least two discrete power supply means is equalized.

2. A method according to claim 1, wherein the discrete power supply means are transformer secondary windings each of which has its terminals electrically connected to two different groups of the electrodes and wherein the step of equalizing melting speed is accomplished by providing as the equalizing current circuit, center tapping means electrically connected to at least two of the secondary windings and by interconnecting center taps provided on said at least two center tapping means.

3. A method according to claim 2, further comprising the step of electrically interconnecting the center tapping means from all of the secondary windings by interconnecting all center taps on all center tapping means.

4. A method according to claim 3, further comprising the steps of providing an auxiliary consumable metal blank arranged relative to the electrodes and the mold so that one end of the blank can be immersed into the molten slag bath; and using the auxiliary blank as a common connection for the interconnected center taps.

5. The method as claimed in claim 4, further comprising: alloying of the ingot being made by using a said auxiliary metal blank of a different chemical composition from that of the metal of the consumable electrodes.

6. A method according to claim 2, further comprising arranging the groups of electrodes in a regular polygon configuration with the groups disposed at the apices of the polygon configuration and disposing each of the two groups of electrodes, which are connected with the terminals of their associated secondary winding of the transformer means, at two adjacent apices of the polygon configuration.

7. The method according to claim 2, further arranging the groups of electrodes at the apices of a regular polygon formed thereby so that each two groups of electrodes connected to the terminals of one secondary winding of the transformer are disposed at diametrically opposite apices of the polygon.

8. The method according to claim 2, wherein each of the center tapping means include an equalizing reactor, and the method of equalizing melting speed comprises: electrically connecting each equalizing reactor in parallel to the associated secondary winding of the transformer means; and electrically interconnecting center tappings of at least two reactors.

9. The method according to claim 8, wherein center tappings of all reactors are interconnected.

10. The method as claimed in claim 2, further comprising using conductors interconnecting all of the center tappings and disposing each interconnecting conductor from the center tapping means of each secondary winding in bifilar arrangement to the conductor connections from the terminals of the associated transformer secondary windng and the groups of electrodes powered thereby; and locating the connection junction of all the center tappings on the vertical axis of symmedirection relative to each other during remelting.

DETACH HERE BEFORE MAILING THE 'IYIED CEKHFICATE TO THE YMI'IINI UffICE 1 UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION Patent No- 3, 691 Dated February 5 1974 Inventor) Boris Evgenievich Paton et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown belowi Column 5, line 55, change "arrangmeent" to --arrangement-.--,

Column 6, line 11, change "ar" to --are-.

Column 6,

lines

1 and 2 the line voltages should read as follows:

-- I-II III-IV v-vI---.

Column 6,

lines

7 and 8, the formulas should read as follows:

" vI-I U vI-Iffi: UP vI-Iv= U; VI-III i I 2 2 2 vI-v I Column 6, line 39 the formulas should read asffollows:

--U U; and U U Column 6, line-58, the formula should read as follows: I

Column 6, line 60, the line voltage values should read as follows:

. Uri-v III- -V-I' UI-IV" Column 7, line 17-, change "2 'to --2 Column 7, line 19, change "0 to -O si a and sealed this 9th day of July 1974.

(SEAL) I "'Attest:

s MCCOY M. GIBSON, JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents Patent No.

Inventor(s) MCCOY M. GIBSON, JR. Attesting'Officer a UNITED STATES PATENT OFFICE CERTIFICATE OF. CORRECTION 3,790,691 Dated February 5, 1974 Boris Evgenievich Paton et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below Column 5, line 55, change "arrangmeent" to --arrangement-.,

Column 6, line 11, change "ar" to are.

Column 6,

lines

1 and 2 the line voltages should read as follows:

-- I-II III-IV' VVI--.

Column 6,

lines

7 and 8, the formulas should read as follows: vI-I g In-11 U; VI-IV [g U; vI -III= g J Column 6, line 39 the formulas. should read asffollows:

"-'UVI V Ugand U Column '6, line 58, the formula should read as follows:

Column 6, line 60, the line voltage values should read as follows:

Column 7, line 19, change "0 to O Signed and sealed this 9th dayof July 1974.

I (SEAL) 'Attest:

C. MARSHALL DANN Commissioner of Patents

Claims

10. The method as claimed in claim 2, further comprising using conductors interconnecting all of the center tappings and disposing each interconnecting conductor from the center tapping means of each secondary winding in bifilar arrangement to the conductor connections from the terminals of the associated transformer secondary windng and the groups of electrodes powered thereby; and locating the connection junction of all the center tappings on the vertical axis of symmetry of all electrodes to provide for a non-inductive current conductor arrangment to the electrodes.

11. The method according to claim 1, wherein all of said electrodes are maintained immovable in a vertical direction relative to each other during melting.

12. The method according to claim 1, wherein relative vertical movement is provided between the mold and the combination of all of said electrodes during melting.

13. The method according to claim 12, wherein all of said electrodes are maintained immovable in a vertical direction relative to each other during remelting.