US2798107A - Consumable electrode arc furnace construction and operation - Google Patents

Description

y 1957 E. Fl BORON El AL 2,798,107

CONSUMABLE ELECTRODE ARC FURNACE CONSTRUCTION AND OPERATION Filed. Aug. 31, 1955 4 Sheets-Sheet 1 I I l Y B 5 F Jr r'\ /|e I I '1 I 'j N M ARC .C%/ SS VOLTAGE R0 SENSER f l5 I-fl A ---lo 28' l4 VSR O POWER SUPPLY INVENTORS Wvllmm H. G 01:91; a

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CONSUMABLES ELECTRODE ARC FURNACE CONSTRUCTION AND OPERATION Filed Aug. 31 1955 4 Sheets-Sheet 2 IIIIIIII 3m l I I I I I I IIII IIIIIIIIIIIIIIIII I'IIIIIIIIIIIIII.

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ATTORNEYS E. F. BORON El AL 2,798,107

4 Sheets-Sheet 3 MAL ATTORNEYS July 2, 1957 CONSUMABLE ELECTRODE ARC FURNACE CONSTRUCTION AND OPERATION Filed Aug 31, 1955 lected operating voltage across the arc.

nie States Patent:

CONSUMABLE ELECTRODE ARC FURNACE CONSTRUCTION AND OPERATION Eugene F. Boron, Niles, and William H. Gorga, Warren,

Ohio, assignors to Mallory-Sharon Titanium Corporation, Niles, Ohio, a corporation of Ohio Application August 31, 1955, Serial No. 531,633

6 Claims. (Cl. 13-13) The invention relates to a method of and apparatus for controlling the arc in a consumable electrode arcmelting furnace for melting titanium and titanium alloys, and more particularly to a method of and apparatus for forming a sound and solid ingot from an arc-melted titanium or titanium alloy electrode despite unstable are conditions which may normally result in producing a faulty ingot.

7 arc melting. The atmosphere within the furnace may be maintained under pressure and may comprise argon gas.

Alternatively, the furnace melting chamber may be purged with argon gas and maintained under conditions of high vacuum during melting. Although pressure melting may sometimes be used, many circumstances or requirements dictate the use of vacuum melting.

Arc furnaces normally are operated to maintain a se- Usual control equipment is provided coordinated with the power supply sothat when the voltage across the arc increases, as the electrode metal burns otf, the electrode is lowered; or when the voltage across thearc is reduced, the electrode is raised in order to maintain proper arc characteristics indicated by the predetermined arc voltage present and desired to be maintained.

' The are is normally quite stable when vacuum melting pure titanium and certain titanium alloys. However, the arc has been found to be unstable, at times, when vacuum melting titanium sponge containing some chlorides, or when vacuum melting titanium-manganese alloys or titanium-aluminum-vanadium alloys.

When the arc becomes unstable, the furnace with the described usual controls operates in a completely erratic manner to continuously raise the electrode within the I water-cooled crucible and deposit a thin shell of electrode metal around the inner wall of the crucible; The resultant product formed in the crucible is a pipe,,in effect,

.rather than a solid ingot.

These ditlieulties have presented a serious problem in the art because heretofore it has been impossible to commercially satisfactorily vacuum arc-melt a titaniummanganese alloy, such asan 8% manganese alloy.

We have discovered a solution to this problem in which certain additional controls are exercised on or impressed upon the normal control and operation of a consumable velf) and is lowered intermittently as a predetermined amount of electrode is burned on. This operation and control has enabled sound ingots .to beformedwhen vacuum arcmelting materials even with the presence of an' unstable are, which unstable are otherwiseprevents the forniatiori of a sound ingot.

Accordingly, it is a general object ofthe present invention to provide a new arc-melting arrangement and pro cedure which enables sonnd ingots to. be for'me'd. by vacuum arc-melting titanium-containing material despite unstable arc characteristicsproduc'ed by suchrnat er ial which normally results in forming a faulty ingot.

Furthermore, it'is an object of the present invention to provide a new arc-melting arrangementand procedure for a consumable titanium-containing.electrode vacuum arc-melting furnace which enables titanium material containing chlorides or manganeseor aluminum-vanadium alloys or containing some other substance ormaterial which may produce an unstable 'arc to be satisfactorily vacuum arc-melted in the furnace to form a sound ingot. Furthermore, it is an object of the present invention to provide a new arc-melting arrangement and procedure which enables sound ingots to be formed by vacuum arcmelting titanium-containing material despite ditficulties heretofore encounteredin the yacuum arc-melting of such material. C H Y Also, it is an object of the present invention to provide anew arc-melting arrangement and procedure which avoids difficulties heretofore encountered in vacuum arc- I melting certain types of consumable. titanium-containing electrodes in an arc-melting furnace.

Moreover, it is an objectfofthe present invention to provide a new consumable electrode arc-melting furnace arrangement and procedure in which the eletcrode ,is intermittently lowered and permtted .toQburn off a .predetermined amount after each lowering in oder to convert electrode material into soundingot metal.

Finally, it is an object of the presentinvention .to provide a new co-nsumableelectrode are. melting arrangement and procedure which can beincorporated readily at low cost in existing ,arc furnaces. to solve the stated problems in the art, to eliminate the diflictiltiesenumerated, to generally improve, thetrnanufacture of titanium I or titaniumalloy ingots, andto obtain the foregoing v advantages and .desiderata in a simple and effective Jinanner.

These and other objectsand advantages apparent to those skilled in the art from the 'followin'gdescription and claims, may be obtained, .thestate'd results achieved,,a nd

,thefdescribed difficultiesv overcome by thev methods, steps,

operations, procedures, constructions, arrangements, combinations and sub-combinations which comprise the presentlinvention, the nature of which are set forth in the following general statements, a preferred embodiment of whichillustrative of the best mode in which applicants have contemplated applying the principles-is set forth in the following description and shownin the drawings,

and which are particularly and distin ctly pointed out and set forth in the appended claims forming part hereof.

The nature of the improvements in consumable electrode vacuum. arc-melting furnaceconstruction and arrangement of the present invention maybe stated in general terms as including a water-cooled,crucible, a con- W sumable electrode, means for supplying power to the electrode and crucible whereby an are may be struck from the end of the electrode to melt, deposit and solidify metal from the electrode in the,crucible, mechanism for raising and lowering the electrode; control means normally responsive to the arc voltage actuating said mechanism to raise the electrode as the arc voltage. decreases and to lower the electrode as the varc voltage. increases; means for actuating said control means. to prevent raising of the electrode, to initiate lowering of the electrode after an amount of electrode metal has burned off sufficiently to increase the arc voltage a predetermined amount over that amount selected to be maintained, and to continue such lowering until a predetermined arc voltage is reestablished; and means for rendering said actuating means inoperative.

The nature of the improvements in consumable electrode vacuum arc melting furnace operation of the present invention may be stated in general terms as preferably including the steps of controlling the feed of a consumable electrode in an arc melting furnace using electrode metal normally producing an unstable arc and in which the electrode is normally lowered or raised responsive respectively to an increase or decrease of arc voltage to maintain a selected are voltage, by initially striking an arc and permitting the normal feed mechanism to raise the electrode until arc voltage approaching said selected voltage is obtained; then stopping further arc-voltagechange-actuated raising of the electrode and permitting the electrode to burn off until a predetermined upper limit are voltage occurs, then lowering the electrode until a predetermined lower limit are voltage is established having a value above the normal selected are voltage, and repeating the electrode burn-E at intermittent lowering steps until the electrode has been substantially consumed.

By way of example, the improved consumable electrode arc furnace construction and operation are shown some what diagrammatically in the accompanying drawings forming part hereof, wherein:

Figure 1 is a diagrammatic view illustrating the improved furnace control construction;

Fig. 2 is a diagrammatic view illustrating generally the normal operation of a consumable arc furnace with normal control of a stable arc;

Fig. 3 is a fragmentary view similar to Fig. 2 indicating the lowering of the electrode upon occurrence of increased arc voltage;

Fig. 4 is a fragmentary view similar to Fig. 3 illustrating the raising of the electrode upon occurrence of decreased arc voltage;

Fig. 5 is a view similar to Figs. 3 and 4 illustrating the filling up of the crucible with metal deposited from the electrode while maintaining a selected predetermined arc voltage;

Fig. 6 is a view similar to Fig. 2 illustrating diagrammatically the possible nature of an unstable arc atttempting to maintain a selected predetermined arc voltage with normal control as in Figs. 2 to 5;

Fig. 7 is a view similar to Fig. 6 illustrating diagrammatically the deposit of metal in pipe-like fashion within the crucible under unstable are conditions attempting to maintain a selected predetermined voltage and operating under normal control as in Figs. 2 to 5;

Fig. 8 illustrates diagrammatically the initial striking of the arc in accordance with the improved operation of the present invention;

Fig. 9 illustrates the next stage of operation with the improved control in accordance with which the electrode is held stationary after raising from striking the arc, and after a selected predetermined arc voltage has been established by such electrode raising;

Fig. 10 is a view similar to Fig. 9 illustrating the electrode being held and burned off until a predetermined upper limit are voltage occurs;

Fig. 11 is a view similar to Fig. 10 illustrating the lowering of the electrode from the position of Fig. 10 until a predetermined lower limit are voltage is established;

Fig. 12 is a view similar to Figs. 8 to 11 illustrating the deposit of metal from the electrode in a watercooled crucible to form an ingot under conditions of an unstable arc but in accordance with the invention by maintaining an arc voltage within a selected variation range.

Similar numerals refer to similar parts throughout the various figures of the drawings.

In describing the invention below, for convenience the term titanium-containing electrode is used to refer to an electrode or to electrode material composed of titanium in the form of sponge, or scrap, or a combination thereof, in some manner formed to a condition in which the material can serve as an electrode; or the electrode may be composed of previously melted titanium-containing material ingots formed into an electrode for a second stage melting operation; and any such electrode may also contain one or more alloying elements such as manganese, vanadium and aluminum to provide a desired 'tanium alloy.

Referring more particularly to Fig. l, a copper crucible is indicated at 1 surrounded by a spaced water jacket 2 so that water 3 may be circulated through inlet 4 and utlet to and through the space between crucible 1 and jacket 2'. to maintain the crucible l cooled. A titaniumcontaining electrode 5 is mounted for movement up and down within crucible 1 and may be suspended by cable leans 6 and a holder 7. The cable means may lead to a drum 8 driven by reversing motor 9 so that the cable 6 may be raised or lowered.

'Direct current power supply is connected through line it) to the crucible at 11., and through line 12 to the electrode at 13 for supplying power for the are A maintained between the lower end of electrode 5 and a bath of molten metal 14 deposited in the crucible 1 by arc melting.

Usual arc voltage meter equipment 15 is connected 'with lines 10 and 12 to sense the arc voltage existing at all times, and the arc voltage senser 15 communicates through lines 16 and 17 to usual normal control equipment 18 actuated by changes in the arc voltage and connected through lines 19 and 20 with motor 3 to actuate the motor for raising the electrode 5 when the voltage of are A decreases below a predetermined amount, and for lowering the electrode 5 when the arc voltage increases above a predetermined amount.

In Fig. l the normal stable arc is indicated generally at A, and the furnace may be operated either under pressure or vacuum. Argon atmosphere is maintained within the furnace chamber 21 when pressure melting, and the chamber 21 may be purged with argon and exhausted when vacuum melting.

In normal operation of the furnace arrangement illustrated in Fig. 1, the electrode 5 is lowered initially to strike an are at the bottom of crucible 1. Then the electrode immediately is raised until an operating arc voltage of a selective amount is established by the spacing of the lower end of electrode 5 above the molten bath of metal 14 deposited in the crucible 1 from the melting of the consumable electrode 5. Thereafter, the predetermined operating arc voltage of are A is maintained by the described operation of the voltage senser 15, the normal control mechanism 13 and motor 9.

Assume, for instance, that the open circuit voltage of lines 16 and 12 is volts and that the operating arc voltage sought to be maintained is, say, 40 volts. This predetermined or selected operating arc voltage V5 is indicated in Fig. 2 and normally is maintained by raising and lowering the electrode as indicated by the arrow 22 in Fig. 2 to maintain a proper spacing between the lower end of electrode 5 and the bathof metal 14 deposited from the arc melting of the electrode 5.

As the electrode 5 burns off, the arc voltage increases, indicated at V1. This increase in voltage actuates the senser 15 and control mechanism 18 to operate motor 9 to lower the electrode 5 as indicated by the arrow in Fig. 3, until the arc gap is such that a 40 volt arc voltage V5 is re-established. The re-establishment of such selected voltage Vs re-establishes proper melting conditions and the proper deposit of metal in the crucible 1 proceeds.

If the arc voltage drops below 40 volts, as indicated by decreased voltage Va in Fig. 4, the motor 9 is actuated to raise the electrode 5 as indicated by the arrow in Fig. 4 until the arc voltage Vs of 40 volts again is established. Thus, as generally indicated in Fig. 5, the

furnace continues to operate, while maintaining a selected are voltage Vs, to burn off metal from the consumable electrode 5 and to deposit the same 'as molten metal 14 in crucible 1 until the desired amount of metal has been deposited to form a desired ingot when solidified.

The described normal operation of a consumable electrode arc furnace for melting titanium-containing electrodes is typical of either pressure or vacuum melting where the electrode material is such as to produce a stable arc. Normally, there is no difliculty in producing and maintaining a stable are when pressure melting. The vacuum melting of commercially pure titanium normally produces a stable are when the titanium is relatively free of chlorides. Thus a stable are is provided when vacuum melting pure titanium sponge having no chlorine therein.

However, when titanium sponge contains some chlorifle, or contains manganese or vanadium and aluminum for alloying purposes, frequenty an unstable arc results when vacuum melting. If the titanium material of electrode 5 contains both chlorine and manganese, an unstable are always results when attempting to vacuum melt the same. Since the preponderance of titanium sponge is made by a procedure which includes the formation of magnesium chloride, there usually always is some slight residual amount of magnesium chloride in such sponge, so that as a practical matter it has been impossible to vacuum melt an 8% manganese-titanium alloy without forming an unstable arc.

The exact explanation of how and why such an unstable arc occurs, and exactly how it affects the operation of the usual control means 13 is not known, although the result is known.

One possible explanation of the effect of chlorides on the are is that the same may break up to form magnesium and chlorine vapor or gas in the arc at the temperature existing in the arc. Such vapors or gases may rte-combine in the are as manganese chloride or magnesium chloride or perhaps as a titanium chloride. The exact nature of these reactions in the arc is not known but it is believed that the presence of chlorine in the are may isolate the arc to small points or zones at the tip of the electrode causing rapid overheating of the electrode at such points and the rapid expulsion of metals or gases at such points. This particularly could be so with respect to manganese which has the highest vapor pressure. Such overheating may decrease the arc voltage at these points, causing the electrode to raise rather than to lower, which normally should be the movement of the electrode as it burns off.

A possible explanation of the effect of manganese on are stability is that manganese or other alloying metals producing an unstable arc may dissociate and form a gaseous state in the are which decreases the arc voltage through the gaseous metal ions, as indicated in Fig. 6, wherein gaseous metal ions are indicated at 23 and the decreased are voltage is indicated by the symbol V5 being a decreased selected voltage. Such decreased voltage V5 actuates the control 18 to operate the motor 9 to raise the electrode 5 just as a similar operation by a decreased voltage is illustrated in Fig. 4. In addition to the foregoing, another condition exists because the instability of the are and the ionization thereof as at 23. The unstable arc deposits metal at 24 (Fig. 6) at either side of the electrode which immediately freezes thereon due to the cooling efifect of the water 3 in the cooling jacket. Meanwhile the molten metal 14 collecting within the crucible 1 below the electrode 5 is relatively hotter than the cooled solidified metal 24 deposited along the side walls of the crucible. At the hot zone the resistance is increased and is relatively greater than the resistance at the cooler zone 24. This change in resistance characteristics taken with the manganese ion gas 23 in the field of the arc continues to lower the arc voltage and continues to actuate the motor 9 to raise the electrode 5.

i The are then shorts across from the electrode 5 to the sidewalls of the crucible 1, as indicated at 25 in Fig. 7.

The decreased voltage (indicated by symbol Va of the, shorted arc 25, decreased by the metal ionization therein and the lower resistance due to the relatively cooler crucible sidewalls causes the electrode 5 to be raised and the are 25 continues to deposit metal on the sidewalls of the crucible as the electrode 5 continues to be raised, thereby forming a pipe 26, as indicated diagrammatically in Fig. 7, rather than a solid ingot.

In accordance with the present invention, these difiiculties resulting from normal operation control of an unstable are are eliminated. However, normal operation is retained and possible of use when the materials being melted produce a stable arc. The invention contemplates including a voltage sensitive relay labeled VSR and indicated at 27 in Fig. l, and which may be of a type such as disclosed in page 1 of a publication dated March 7, 1955, of General Electric Company entitled Industrial Electronics-Miscellaneous DevicesElectronic Voltage-sensitive Relay in the normal control circuit. This voltage sensitive relay 27 is actuated at lower limit voltage, say 38 volts, to render the normal control 18 inoperative. Relay 27 also is de-energized at an upper limit voltage of say 43 volts to place the normal con trol circuit 18 in operation. The lower limit voltage of 38 volts and the upper limit voltage of 43 volts may be adjusted to any desired value. The normal control circuit is adjusted to attempt to maintain a selected voltage of say 36 volts rather than volts, or at least to maintain a selected voltage below the lower limit voltage of actuation of relay 27. Voltage sensitive relay 27 is arranged to 'be cut in or cut out of the normal control circuit 18 by manual switch 28.

The operation of the new control arrangement of Fig. 1, including the voltage sensitive relay 27, will now be described with reference to Fig. l and Figs. 8 to 12.

First, switch 28 is opened manually thus momentarily permitting operation of the furnace by normal control circuit 18. Electrode 5 is lowered until it contacts the bottom of crucible 1 as indicated diagrammatically in Fig. 8 to strike an are 29. At this instant, the arc voltage is zero, indicated byv the symbol V0 which is a voltage substantially below the 36 volt setting of normal controller 18. The motor 9 thus moves to rapidly raise the electrode 5 until an arc voltage of about 36 volts is approached or established.

Now, switch 28 is manually closed which renders the normal control circuit 18 inoperative, and motor 9 is stopped and ceases to raise electrode 5, holding the same stationary as indicated in Fig. 9 with approximately the selected normal arc voltage Vs of 36 volts. With electrode 5 held stationary, as in Fig. 10, the electrode continues to burn off, the arc gap increases, and metal deposits in the bottom of crucible 1 until the arc voltage increases to the upper limit Vul of the 43 volt setting for the voltage sensitive relay 27. At this time, the relay 27 is de-energized rendering the normal control circuit 18 operative. Because of the increased voltage (4-3 volts) the normal control 18 actuates motor 9 to lower electrode 5 as indicated in Fig. 11 (and similar to the normal operation of Fig. 3) until the arc voltage is established at the 38 volt lower limit setting of relay 27, indicated in Fig. 11 by the symbol VLL. At this time, relay 27 is again actuated to render the normal control circuit 18 inopera tive and to hold the electrode 5 in lowered position. The electrode 5 then burns off until an upper limit arc voltage Vul is again reached whereupon the electrode is again lowered.

This burn-oil and intermittent lowering of the electrode 5 continues, as indicated generally in Fig. 12, until the desired amount of electrode metal has been melted and deposited in the water-cooled crucible. The arc voltage varies during the burn-off and intermittent lowering between the stated upper and lower limits, or varies within the voltage variation range of 43 to 38 volts, indicated by the symbol Vvr in Fig. 12.

The new control of the operation of a consumable arc furnace by intermittent lowering of the electrode and then permitting the electrode to burn off, results in proper melting of metal and filing up of the cavity of the watercooled crucible to form a solid sound ingot, after the arc melted metal has solidified, from titanium-containing material that normally produces an unstable are with the attendant ditficulties described.

The new control arrangement of the present invention may be used in connection with melting any titaniumcontaining material having alloying elements therein that have a melting point lower than the melting point of titanium or that cause dissociated gases in the are which decrease the resistance of the arc and actuate the normal control mechanism to raise the electrode when such raising otherwise is not called for.

The improvements of the present invention are particularly adapted for use in connection with a relatively small-diameter crucible 1 in which a relatively largediameter electrode 5 is being melted, because such conditions with an unstable are are most prone to result in the pipe-forming operation illustrated in Figs. 6 and 7.

Accordingly, the present invention provides for improved operation of a consumable titanium-containing electrode are furnace whereby titanium-containing material which normally produces an unstable arc and forms a faulty ingot under normal furnace operation control may be satisfactorily melted even under unstable are conditions to provide a solid and sound ingot.

Thus the improved arrangement of the present invention provides the described advantages, overcomes prior art difficulties that have been particularly troublesome, unables certain titanium alloys to be manufactured which heretofore have been ditficult to melt; and solves existing problems in the art, and overcomes the stated difficulties and obtains the advantages and desiderata indicated in an extremely simple manner.

In the foregoing description certain terms have been used for brevity, clearness and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such terms are utilized for descriptive purposes herein and not for the purpose of limitation and are intended to be broadly construed.

Moreover, the description of the improvements is by way of example, and the scope of the present invention is not limited to the exact details illustrated, or to the specific voltages referred to, or to the exact sizes indicated.

Having now described the features, discoveries and principles of the invention, the operation and procedures of preferred method steps thereof, the construction and operation of the improved arc furnace arrangement, and the advantageous, new and useful results obtained thereby; the new and useful methods, steps, operations, procedures, discoveries, principles, combinations, sub-combinations, construction and elements, and mechanical equivalents obvious to those skilled in the art, are set forth in the appended claims.

We claim:

1. Consumable electrode vacuum arc melting furnace construction including a crucible, a consumable titaniumcontaining electrode, means for supplying power to the electrode and crucible to establish an are between the electrode and crucible, mechanism mechanically and elec trically connected with the electrode and responsive to decrease and increase in arc voltage below or above predetermined selected are voltage and constructed to raise the electrode upon said decrease in arc voltage and to lower the electrode upon said increase in are voltage and normally operative to maintain said selected are voltage; voltage sensitive relay means electrically connected with said mechanism; switch means connected with said mechanism and relay means, said relay means being operative to control said mechanism when said switch means is closed and said relay means'being disconnected from said mechanism when said switch means is open; said relay means including means operative at voltages at and below a selected lower limit are voltage constructed to render said mechanism inoperative to raise or lower the electrode, when the relay means is connected with said mechanism; and said relay means also including eans de-energized at and above a selected upper limit are voltage constructed to render said mechanism operative to lower the electrode, when the relay means is connected with said mechanism; and said upper and lower limit relay voltages being set at values defining a voltage range higher than the selected are voltage.

2. Consumable electrode vacuum arc melting furnace construction including a crucible, a consumable titaniumcontaining electrode, means for supplying power to the electrode and crucible to establish an are between the electrode and crucible, mechanism mechanically and electrically connected with the electrode and responsive to decrease and increase in arc voltage below or above predetermined selected are voltage and constructed to raise the electrode upon said decrease in arc voltage and to lower the electrode upon said increase in arc voltage and normally operative to maintain said selected are voltage; voltage sensitive relay means electrically connected with said mechanism; switch means connected with said mechanism and relay means, said relay means being operative to control said mechanism when said switch means is closed and said relay means being disconnected from said mechanism when said switch means is open; said relay means including means operative, when the relay means is connected to said mechanism, to maintain said mechanism inoperative at any time to raise the electrode, said mechanism when maintained inoperative holding the electrode stationary while metal burn-off occurs until the are voltage increases to a predetermined upper limit are voltage higher than said selected are voltage; said relay means also including means operative, when said are voltage increases due to electrode burn-off to said upper limit are voltage, to render said mechanism operative to lower the electrode; and said relay means including means operative, when the electrode has been lowered to establish a predetermined lower limit are voltage having a value above the selected are voltage, to render said mechanism inoperative to move the electrode; whereby the electrode burns off and is intermittently lowered while operating at an arc voltage varying within the selected range defined by said upper and lower limit are voltages.

3. The method of operating a consumable electrode vacuum arc melting furnace with a titanium-containing electrode normally producing an unstable are, including the steps of controlling the feed of the consumable electrode in the arc-melting chamber normally to lower or raise the electrode responsive respectively to an increase or decrease of arc voltage to maintain a selected are voltage, initially striking an arc and permitting the normal feed control to raise the electrode until arc voltage approaching said selected voltage is obtained, then stopping said arc-voltage-change-actuated raising of the electrode and holding the electrode stationary while permitting the electrode to burn off until a predetermined upper limit are voltage occurs; then lowering the electrode until a predetermined lower limit are voltage is established having a value above said normal selected are voltage; and repeating the held electrode burn-off and lowering steps until the electrode has been substantially consumed.

4. The method of operating a consumable electrode vacuum arc melting furnace with a titanium-containing electrode normally producing. an unstable arc, including the steps of controlling the feed of the consumable electrode in the arc melting chamber to maintain a selected are voltage normally by lowering and raising the electrode responsive respectively to an increase or decrease of arc voltage from said selected are voltage; initially striking an arc in the melting chamber and permitting the normal feed control to raise the electrode until are voltage approaching said selected voltage is obtained, then stopping further raising movement of the electrode at any time and holding the electrode stationary while electrode metal burn-oif occurs, continuing the metal burn-off while holding the electrode stationary until a predetermined upper limit are voltage occurs, then lowering the electrode until a predetermined lower limit are voltage is established; and intermittently repeating the lowering step as the electrode burns off while operating at an arc voltage varying within the selected variation range defined by said upper and lower limit are voltages.

5. The method defined in claim 4 in which the selected are voltages is 36 volts and the selected variation range defined by said upper and lower limit voltages has a 5 volt range.

6. The method defined in claim 4 in which the selected are voltage is 36 volts, the upper limit voltage is 43 volts and the lower limit voltage is 38 volts.

References Cited in the file of this patent UNITED STATES PATENTS 2,427,744 Rebuffoni Sept. 23, 1947 2,456,936 Frostick Dec. 21, 1948 2,640,947 Journeaux June 2, 1953 2,717,326 Gunton Sept. 6, 1955

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