US2955333A

US2955333A - Electric arc furnaces

Info

Publication number: US2955333A
Application number: US727339A
Authority: US
Inventors: Berry Robert Langley Page; Busby Arthur Donald; Badger Douglas Victor
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1957-04-11
Filing date: 1958-04-09
Publication date: 1960-10-11
Anticipated expiration: 1977-10-11

Description

Oct. 11, 1960 R. L. P. BERRY ET AL 2,955,333 ELECTRIC ARC FURNACES Filed April 9, 1958 23 O I 7 l //VVE/V T 0R5. Robe/f Langley Pa eBerry, A rfhur Dona/0 Bu gy Doug/as Vii forBadger, 5y w W A T TORNEYS.

,'mould o-f a semi-continuous casting machine.

United States Patent 2,955,333 ELE'C'IRI C ARC" FURNACES Filed Apr. 9, 1953, s... No. 727,339 7 Claims priority, application Great Britain Apr. 11, 1957 I 2 Claims. (Cl. 22-573) This invention relates to casting furnaces and more particularly to electric arc furnaces for the production of ingots of highly reactive metals such as for example, titanium, with reference to which the invention will be hereinafter described. In'one of the techniques now in common practice ,for the production of ingots of titanium, an electric arc ,furnace is operated with the use of a consumable eleo Itrode compacted or otherwise fabricated from the metal in sponge or powder form in which it is obtained from its compounds, the electric are being struck between the consumable electrode and a water cooled copper hearth. It is also common practice to produce the final ingot by the method of double melting, thatis, the ingot produced in the first instance by melting of the consumable electrode is itself made the electrode in a subsequent arc melting process from which the final ingot is derived. Since it has been found necessary to provide a clear- I ance of at least one inch between the outer surface of .the electrode and the inner surface of the vessel in which the molten metal is collected, it will be appreciated that starting with an electrode 8 inches in diameter, the diameter of the ingot finally produced by the double melting technique will be a minimum of 12 inches. The present invention is particularly concerned with the .problem of casting titanium bar of small diameter, that is,

of dimension much closer product, for example, rolledbar or rod.

- At present, small diameter titanium bar is produced by machining a large sized ingot, forging down or rollfing the product of the machining operation and then .finally. forging or rolling tothe appropriate size. It will be appreciated that these operations involve considerable wastage of material, the initial machining operation alone being accompanied by up to loss in ingot weight, and involve considerable costs in forging or rolling. q

The present invention is concerned with the production of small diameter bar for subsequent processing to a wrought product, for example, rolled bar or rod, directly from a comparatively large diameter consumable ..electrode and there is employed for this purpose a semiacontinu'ous casting furnace, that is, a furnace in which the ingot is built up in and continuously withdrawn at a suitable rate from a mould which is open at both ends. According to the present invention, there is provided a method of semi-continuously casting metals in which an arc is struck between a consumable electrode of the metal to be melted and a water-cooled annular metal hearth, and the molten metal so formed passes through an aperture in the hearth into a water-cooled The invention further comprises a semi-continuous casting furnace adapted to operate with a consumable electrode having a cross-section of a given size or within a given range of sizes, said furnace comprising a hearth having a cross-section which tapers towards an outlet Birmingham, and

to that of the final wrought ingot cylindrical chamber Patented Oct. 11, 1960 the inlet without making contact with the walls of the chamber in thevicinity of said inlet;

In carrying out the method of the present invention in a furnace designed in accordance therewith, a consumable electrode of titanium which is itself the product of a previous melting operation, is disposed in a cylindrical melting chamber maintained at a pressure below 1 mm. of mercury and is supported therein by means of a titanium stub, welded at one end to the top of the electrode and clamped at its other end in a copper block mounted upon the lower end of a verticallydisposed electrode ram. The electrode ram enters the through a vacuum seal, and at its upper end, which is outside the chamber, is connected by means of a coupling, to actuating means. The actuating means comprises a vertical rack coupled to the ram and actuated by a driving pinion operated by driving gear for the purpose of downward vertical movement of the electrode ram and therefore of the electrode itself during the are melting operation.

The bottom of the cylindrical melting chamber is formed by a water-cooled copper hearth of inverted frusto-conical shape, terminating at its lower end in an outlet which constitutes the inlet orifice of a vertical mould disposed below the hearth. The mould is of smaller diameter than that of the electrode, while the outlet from the hearth is of still smaller diameter, the relative cross-sectional dimensions of the'mouldand the outlet from the hearth being so chosen that molten metal flowing through the outlet will fall directly into the inner zone of the mould, that is to say, without making contact wit-h the walls of the chamber in the vicinity of the outlet from the hearth. The mould is surrounded by cooling coils or a cooling jacket so that solid metal is continuously built up from the pool into a long ingot which may be retracted from the mould by means of a vertically operating retraction ram screwed into the bottom end of the ingot.

To operate the furnace, the retraction ram is screwed or plugged into a base plug and the ram is actuated to raise the plug toward the upper end of the mould. In the starting position of the plug its upper end .is substantially below the outlet from the hearth.

The consumable electrode is then moved into position by means of its associated ram and an arc is struck between the electrode and the hearth or between the electrode and a skull of titanium formed on the hearth. As the electrode is consumed, molten metal flows through the outlet of the hearth which thus acts as an inwardly directed lip over which the metal drops into the pool without making contact with the uppermost portions of the walls of the mould. During the melting operation the ingot is slowly and continuously retracted tomaintain a gap between the surface of the pool and the outlet from the hearth.

By reason of the design of the hearth outlet in relation to the dimensions of the mould and to the method of operating the furnace, no problem is presented by the tendency for metal to solidify around the outlet from the hearth. In contrast, with arrangements in which the pool is allowed to build up in the hearth, the formation of an annular plug of metal around the outlet, this being somewhat remote from the sourceofheat, can result .in a tearing of the ingot during retraction and also in damage to the hearth.

One embodiment of the invention is accompanying drawing. Referring to illustrated in the the drawing, an

internal diameter than shaped lower end -.heavy gauge copper and is provided with water connections and with baffles to ensure :eflicient cooling. An

electrode 5, of considerably larger diameter than the di- 4 is attached to an electrode guide ameter of the aperture rod 6 which passes through the lid 7 of the chamber 1 in a vacuum seal 8. By means of the guide 'rodfi, the electrode may be raised or lowered in the chamber 1.

- Because of the relatively large-diameter of the chamber :1

water cooling is not required for the chamber, but a radiation shield 19 is provided to preventoverheatingof the chamber. An electromagnet 10 around the hearth 2 controls the arc struck between the electrode 5 and the :hearth 2.

To the lower .side of the hearth 2.-is attached a water- 'cooled metal mould 11, open at both ends and of larger the aperture 4. Below the mould llis an elongate chamber of larger internal diameter than that of the mould and this chamber is referred to as the ingot chamber 12. An observation window 13 and a vacuum connection 14 are provided in the upper, detachable portion of the ingot chamber 12. At the lower end :of the ingot chamber 12 is a sealing plate 15 through which passes, in a vacuum seal 16, the retraction rod 17. At diametrically opposite points on the lower part of the ingot chamber 12 are two ports 18 both sealed with caps.

The furnace can be evacuated, and if required filled with inert gas, through the connections 14 and .19.

The electrode 6 is connected to the negative pole and the hearth 2 to the positive pole of a suitable direct current supply.

In operation, the lid 7 is removed and the guide rod 6 raised above the chamber 1. The electrode 5 is fitted to the guide rod 6 in a known manner and lowered into the chamber 1 and the lid 7 replaced.

With the sealing plate 15 and the retraction rod 17 withdrawn, a base plug .20 made from the same kind of metal as that being melted is attached to the rod 17 by means of a threaded hole in the lower end portion of the plug engaging with the screw 21 on the end portion of the rod. The plug also has a hole 22 extending from one side to the other along a diameter and so positioned that when the plug is at its lowest position the hole 22 is in alignment with the ports .18. .In the upper end portion of the plug is a dovetail recess 23 extending from one side to .the other and having a slight taper from one end to the other. The sealing plate 15 is replaced and the rod raised to move the plug 20 into the mould 11 to a position close to the lower part of the hearth 2.

The furnace is evacuated through the connections 14 and 19 and the electrode 5 lowered to the hearth 2 and an arc struck between the lower end of the electrode and the upper surface 3 of the hearth. The electromagnet 10 causes the arc to rotate around the lower end of the electrode 5 and metal 'is thus evenly melted from the electrode the .end of which assumes a conical shape. The electrode 5 can, of course, be provided with a conically before placing it in the furnace.

The molten metal drops off the lower end of the electrode 5 into the mould 11, but some metal may also fall onto'the upper surface 3 of the hearth 2 and then enter the mould 11 by dropping otf the lower edge of the aperture 4. For this purpose the aperture 4 is of smaller diameter than that of the mould 11 .in order to prevent molten metal from running down the side of the mould, where, because of the .eifect of water cooling, the metal would solidify to Lform a skull. An ingot produced in such "amannerwould have a very poorsurface.

The molten metal entering the mould 11 has only a small degree of super'heat and quickly solidifies and builds up a mass of solid metal conforming to the cross section of the mould 11 and keyed on to the plug 20 by the dovetail recess 23. The level of the solidfied metal is kept contant by retracting the rod "17 and with it, the solidified metal. When the electrode 5 is completely melted or when the ingot 24 is of the required size, the arc is broken. Inspection of the progress of melting may becarried out by means 0f the-observation window 13 and a s'imilar'observation window (not shown) in the chamber 1. The procedure for removing the ingot 24 is first to break the vacuum within the furnace, bring the 'hole 22 into alignment with the .ports 18 from which the caps are removed, insert a bar through the ports 18 and the hole 22, open the sealing plate 15 and unscrew the rod 17 from the plug 20. The purpose of inserting the bar into 'the hole 22 is to prevent the ingot rotating when the rod 17 is unscrewed. The ingot may then be pulled out of the furnace and the plug 20 may be separated from the ingot by moving the plug in a direction transverse to the axis of the ingot so that the dovetail is disengaged.

In a second embodiment of the invention the mould 11 is provided on its internal surface with a lining of a material having a lower thermal conductivity than the metal from which the mould is made. For example, a graphite lining is used with a copper mould. The lining reduces the rate of cooling of the molten metal and so tends to prevent the formation of laps in the surface.

The method can be applied to various sizes of electrode and of ingot. The electrode is always larger than the ingot produced, for example, a 12 inch diameter electrode would be used to produce a 6 inch diameter ingot and the diameter of the aperture in the hearth in such a case would be about 4 inches. The method is useful for producing ingot suitable for rolling to rod.

We claim:

1. A semi-continuous casting furnace adapted to operate with a consumable electrode having 'a cross-section of a given size or within a given range of sizes, said furnace comprising a hearth having a cross-section which tapers towards an outlet at the bottom thereof, said hearth being constructed to operate as an electrode, means including sidewalls defining a mould chamber below said outlet and in communication therewith, said outlet constituting an inlet orifice for said mould chamber which produces an ingot of smaller cross-section than that of the consumable electrode, the cross-sectional area of said inlet orifice being smaller than the cross-sectional area of the mould chamber so that molten metal will flow through the inlet without making contact with said side walls of said chamber in the vicinity of said inlet orifice.

2. A semi-continuous casting furnace as claimed in claim 1, in which said side walls of said mould are lined with a material having a lower thermal conductivity than that of the ingot mould. material.

References Cited in the file of this patent UNITED STATES PATENTS