US2825641A - Method for melting refractory metals for casting purposes - Google Patents

Description

March 4, 1958 R. A. BEALL ET AL 2,825,641

METHOD FOR MELTING REFRACTORY METALS FOR CASTING PURPOSES Filed Sept. 21, 1955 fiflil- 2 Shee ts-Sheet 1 IN VEN TORS Henry L- Billie-r Rubefl A 1E1 enl .1 ohm. El E1 0351' it a METHOD FOR MELTING REFRACTORY METALS FOR CASTING PURPOSES Robert A. Beall, Albany, reg., and John 0. Borg, Torrance, and Henry L. Gilbert, Compton, Calif., assignors to the United States of America as represented by the Secretary of the Army Application September 21, 1955, Serial No. 535,765

3 Claims. (Cl. 75-10) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes Without the payment of any royalty thereon.

This invention relates to the melting of refractory metals and is more particularly directed to a method for accumulating a sufficient quantity of the molten metal to permit the casting of useful shapes therefrom.

One of the major factors which has heretofore prevented a more extensive utilization of refractory metals such as titanium or zirconium has been the unusual chemical affinity thereof for all types of materials at. elevated temperatures. This characteristic has rendered it virtually impossible to find a satisfactory crucible material which would not contaminate the molten metal. Although graphite is less susceptible to attack by molten titanium and zirconium than many other materials, the prior art still found it necessary to solidify a portion of the molten metal into a shell or layer in order to prevent direct contact between the molten metal and the graphite of the crucible. While such technique has been fairly successful in preventing these refractory metals from incurring excessive carbon embrittlement, the maintenance of the solidified shell is extremely expensive due to the precise control of the heat input and losses necessary to establish a gradual and steady thermal gradient between the melt and the walls of the crucible.

Actually, the only known method of completely avoiding crucible contamination utilizes a copper vessel having an outer jacket through which a suflicient volume of cold water is circulated to keep the metal-copper interface at a temperature low enough to prevent any reaction therebetween. However, the rapid heat transfer through the relatively cold wall of the water-cooled copper crucible severely limits the amount of molten metal which can be accumulated therein at any one time. While such limitation does not prevent the successful casting of ingots which can be built up in successive small sections, the shallow pools of molten metal normally formed in a cold-cup type of crucible are entirely inadequate for the casting of useful shapes containing more than a few pounds of metal.

It is, therefore, an object of this invention to provide a superior process for melting refractory metals such as titanium, zirconium, and the alloys thereof without incurring any appreciable contamination from the crucible in which the molten metal is contained.

Another object of this invention lies in the provision of an improved consumable electrode, electric arc process capable of melting refractory metals in sufiicient quantities to permit the casting of the molten metal into usable shapes.

A further object of this invention isto provide an im proved consumable electrode, electric arc process of melting refractory metals wherein the rate of consumption of the electrode is sufficiently rapid to permit the accumulation of considerably larger pools of molten metal than Jeretofore possible. r

ted States Pate D It is a specific object of the present invention to provide an improved process for melting titanium or zirconium electrodes in the heat of an electric arc wherein considerably greater rates of electrode consumption are obtained through the regulation of the power input to the electrodes and the vacuum in which the electric arc is maintained.

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings in which:

Fig. l is a schematic representation of the type of consumable electrode arc furnace to which the present invention is applicable;

Fig. 2 is an alternate type of furnace construction adapted for the same purpose as the furnace of Fig. 1;,

in connection with zirconium, it should be understood that the process is equally applicable to titanium and other high melting point metals of low vapor pressure as well as to the alloys thereof.

schematically illustrated in Fig. 1 is an electric arc furnace having a cold-cup type of copper vessel or crucible 12 into which molten metal is adapted to be accumulated in the form of a pool 13 through the melting of a consumable electrode 14 suspended immediately above crucible 12. Crucible 12 is preferably fabricated of copper in the form of a deep cup and is provided with a jacketed cooling chamber 15 through which a suitable cooling medium is circulated to reduce the wall temperatures thereof. Such cooling is sufiicient to solidify a layer of metal along the interior wall surfaces of crucible 12 as best indicated at 16 which serves as one of the terminals between which the electric arc is struck.

Electrode 14 may be formed from ingot sections fabricated from a plurality of platelike briquets formed of zirconium sponge which has been subjected to a compacting pressure of about 50 tons per square inch. The individual briquets are welded end-to-end to form a consumable electrode which is arc-melted into a Watercooled copper cup in an inert atmosphere of helium and argon. The resulting ingot sections are joined end-to-end with threaded nipples of the same metal to form up to nine feet of electrode 14. A rod 17 is threadably secured into the upper end of the top section of electrode 14 and projects through the top of the furnace to serve as the means by which electrode 14 is lowered to maintain the required length of the electric arc.

Crucible 12 is provided with a centrally disposed opening 18 in the bottom thereof through which the molten metal in pool 13 is arranged to be poured into a casting mold 19 by gravity flow. Opening 18 is normally blocked by an upwardly projecting head portion 20 of a hollow right angle member 21 which extends downwardly to project out of the bottom of the electric arc furnace. Member 21 is provided with a laterally projecting handle 22 by which head portion 20 is manipulated to open and close the pouring opening 18 in crucible 12. Suitable pipes 23 are contained within the interior of member 21 and are adapted to circulate water or other cooling medium therethrough in order to prevent any interruption in the layer of solidified metal at the bottom of crucible 12. Alternately, opening 18 in the bottom of the furnace can be eliminated and the molten metal poured Patented Mar. 4-, 195.8

directly into casting mold 19 by overflowing the top of crucible 12.

Casting of the molten metal may also be accomplished by providing a cold-cup type of crucible 24 having a pouring spout 25 as illustrated in Fig. 2. Then by tilt,- ing crucible 24 at the appropriate moment, the pool of molten metal 26 may be poured into a separate mold 27 suspended by appropriate means such as hanger 28 from the wall of the furnace at a proper pouring height relative to spout 25.

In order to prevent atmospheric contamination of the molten metal obtained from the melting of electrode 14, the interiorof the electric arc furnace is evacuated by suitable pumping apparatus and is thereafter filled with one of the inert gases, argon being preferred due to its ability to ionize slightly and aid in the passage of electric current. However, for the purpose of this invention, the amount of argon employed to blanket the electric arc is carefully controlled so as tofprovidea furnace pressure in the vicinity of the melting zone ranging between 20 and 40 millimeters of mercury. This requirement is based on the discovery that the rate at which consumable electrodes can be melted in an electric arc furnace varies in inverse ratio to the pressure to which the arc is subjected by the surrounding atmosphere. In fact, an intensive study of the behavior of zirconium electrodes in an electric arc furnace of the type disclosed in Figs. 1 and 2 has shown that when the interior argon pressure thereof was reduced to approximately 20 millimeters of mercury,.t'he consumption rate of the electrode increased a; a peak value approximately four times the rate normally encountered at atmospheric pressure. This critical pressure will vary with the metal, alloy addition and contaminants such as magnesium, sodiumor zinc. Such enhanced consumption rate of the electrode renders it possible to provide the abnormally deep pools of molten metal required to produce sound castings of distinct shapes. I

The aforesaid study also revealed that whenever the pressure in the vicinity of the melting zone fell below 20 millimeters of mercury, the anticipated increase in the consumption rate of the electrode frequently did not materialize but insteaddropped suddenly accompanied by a visible glow. In fact, at pressures below 20 millimeters of mercury, this condition invariably took place. This phenomena is commonly known as glow discharge and is produced the excessive ionization of the permanent gases and other volatilesliberated during the melting of'electrode 14. These volatiles consist essentially of magnesium and magnesium chloride which are invariably introduced into zirconium sponge during the commercial production thereof; Since excessive ionization detracts substantially from theintenstiy of the flow of current constituting the electric arc, the ability of the are to consume the electrodeis correspondingly diminished.

Accordingly,the attainment of a maximum rate of electrode consumption depends upon maintaining the furnace pressure as closeto 20 millimeters of mercury as possible without incurring a glow discharge; However,- the intense heat produced by the electric arc renders it virtually impossible to correctly determine the actual degree of pressure in the melting zone. Furthermore, while it: may be possible to ascertain the pressure of the argon in the upper portion of thefurnac'e, the extreme pressure differential existing between the arc melting zone and the more distant regions of the furnace preventsany accurate determination of the melting zone pressure through the interpolation ofthosej values which can be obtained in the upper end of the furnace, Since direct'measurements of the arc pressure'are, therefore, out of the question, the present inventioncontemplates the utilization of the visible or electrical nature of the discharge glow to'ascer tainits existence' and thereby inform the operator of the' furnace that the are pressuretherein had dropped'below 20 millimeters ofmercury'.

It has also been discovered that an increase in the electric power applied to the furnace can be used to dissipate the undesirable glow discharge. Thus, the technique for preventing glow discharge consists in valving or throttling the vacuum pumps to provide a maximum degree of vacuum while simultaneously varying the power supply to increase the power input to the electrodes Whenever the furnace atmosphere becomes subject to a glow discharge.

Various pools of molten zirconium were produced through the use of this technique with different combinations of furnace vacuum or pressure and power input of the furnace. A comparison of these pools shows that the volumes thereof are directly dependent on the rate of electrode consumption in accordance with the relationship indicated in Fig. 4. The electrode consumption is in turn a direct function of both the power input of the furnace and the degree of vacuum or pressure therein. As shown in Fig. 3, theoptirnum pool volume was obtainedat the highest degree of vacuum within the furnace. However, the term. 30-inch vacuum? does not indicate an exact degree ofvacuum but rather that the 'valve between the furnace and the vacuum pump was open wide. Actually,- wheh melting the electrode at a 30-inch vacuum, the gage nearthe pump oftenreads less than one millimeter of mercury pressure. Nevertheless, the leak rate of the furnace: and the ,rate of evolution of the permanent gases and c'ondensable vapors liberated from the electrode assured a mercury pressure which on the basis of experiments performed inother types. of inclosures was within a range of 20 to millimeters.

The optimum pool volume. of approximately 290 cubic inches was obtained by melting compacts of zirconium sponge to form 5 inch diameter ingots which were fastened end-to end to form an electrode weighing approximately 103 pounds. A direct current of 6000 amperes at 28jvolts was then applied to the furnace with the built-up ingot as the cathode and a solidified layer of zirconium as the anode. The resulting electric are therebetween effected the melting of the entire ingot in approximately 8 minutes at a furnace vacuum estimated at 30 inches of mercury. The rate of consumption of the cathodic ingot was, therefore, approximately 5.6 kilograms per minute from which a pool depth of 8% inches was obtained thereby providing a volume of approximately 290 cubic inches. Since only 70% of the metal melted into the cold-cup can be used to form sound castin'g's, actuallythe amount available for pouring into the mold is in the vicinity of 72 lbs. which, however, is entirely adequate for the type of castings for which zirconium'is normally employed.

In addition to providing a substantial increase in the melting rate of consumable electrodes in an arc melting furnace, the utilization of reduced pressure within the furnace also promotes electrical efficiency and stability of thea'rc itself. At atmospheric pressures, the molten metal leaves the electrode in elongated drops which often create shorts across the arc gap and then explode from the heat evolved as they carry the surge of power created by the short. These violent power surges may tear loose the flexible power conductors or at least impart considerable shake to the bus connections which may cause early deterioration of the power sources. However, at reduced pressures'the melting of the electrode occurs with boiling and bubbling which prevents the formation of the elongated drops which contribute to the creation of the power surges.

Additional pools of molten zirconium were also produced under the same conditions and by the same method but with the polarity of the current reversed so that the ingot was made the anode and the pool became the cathode; However, the reversal of polarity invariably resulted' in a lower rate of consumption of the electrode.

' This was also the case when the direct current power input was changed to an alternating current.

Although a particular embodiment of the invention has been described in detail herein, it is evident that many variations may be devised within the spirit and scope thereof and the following claims are intended to include such variations.

We claim:

1. A method for increasing the rate of melting a refractory metal taken from the group consisting of titanium and zirconium comprising the steps of melting the refractory metal as a consumable electrode in an electric arc furnace, evacuating the furnace atmosphere until the occurrence of glow discharge therein, increasing the power input to the furnace while maintaining the existing degree of vacuum therein until the glow discharge disappears, thereafter utilizing each occurrence of glow discharge as a signal to decrease the rate of furnace evacuation until the glow discharge disappears, and increasing the power input to the furnace simultaneously with the decrease in the rate of evacuation to minimize the periods of visible glow discharge.

2. A method of forming an abnormally deep pool of a superheated refractory metal taken from the group consisting of titanium and zirconium comprising the steps of melting an ingot of the metal in an electric arc furnace, evacuating the furnace atmosphere until the occurrence of glow discharge therein, increasing the power input to the furnace while maintaining the existing degree of vacuum therein until the glow discharge disappears, thereafter utilizing each occurrence of glow discharge as a signal to increase the power input to the electric arc until the glow discharge disappears, simultaneously decreasing the rate of furnace evacuation to minimize the required increase in power input, and continuously accumulating the molten metal into a circular pool wherein the depth exceeds the diameter thereof.

3. A method of casting distinct shapes of refractory metals taken from the group consisting of titanium and zirconium comprising the steps of melting a consumable electrode of the refractory metal in an inert gas atmosphere within an electric arc furnace, evacuating the furnace atmosphere until the occurrence of glow discharge therein, increasing the power input to the furnace while maintaining the existing degree of vacuum therein until the glow discharge disappears, thereafter utilizing each occurrence of visible glow discharge of the furnace atmosphere to signal the need for combining an increase in the power input and a decrease in the rate of evacuation to minimize the periods of glow discharge, varying the rates of power input and atmosphere evacuation during the melting of the consumable electrode to maintain a minimum pressure in the arc melting zone consistent with the avoidance of glow discharge, accumulating the molten metal in a noncontaminating circular crucible to form a superheated pool wherein the depth exceeds the diameter thereof, and then pouring the pool of molten metal into a casting mold disposed within the interior of the furnace.

References Cited in the file of this patent UNITED STATES PATENTS 873,958 Von Pirani Dec. 17, 1907 2,310,635 Hopkins Feb. 9, 1943 2,541,764 Herres et a1. Feb. 13, 1951 2,554,902 Godley May 29, 1951 2,659,120 Harter et al. Nov. 17, 1953 2,686,822 Evans et a1. Aug. 17, 1954 2,702,239 Gilbert et a1. Feb. 15, 1955 2,727,936 Boyer Dec. 20, 1955 2,727,937 Boyer Dec. 20, 1955 OTHER REFERENCES Transactions of the Electrochemical Society, vol. 96, No. 3, September 1949, pages 158-169.

Steel, vol. 128, issue 25, June 18, 1951; pages 77-79.

The Iron Age, Sept. 23, 1954; pages 116-119.

Claims (1)

1. A METHOD FOR INCREASING THE RATE OF MELTING A REFRACTORY METAL TAKEN FROM THE GROUP CONSISTING OF TITANIUM AND ZIRCONIUM COMPRISING THE STEPS OF MELTING THE REFRACTORY METAL AS A CONSUMABLE ELECTRODE IN AN ELECTRIC ARC FURNACE, EVACUATING THE FURNACE ATMOSPHERE UNTIL THE OCCURENCE OF GLOW DISCHARGE THEREIN, INCREASING THE POWER INPUT TO THE FURNACE WHILE MAINTAINING THTE EXISTING DEGREE OF VACUUM THEREIN UNTIL THE GLOW DISCHARGE DISAPPEARS, THEREAFTER UTILIZING EACH OCCURENCE OF GLOW DISCHARGE AS A SIGNAL TO DECREASE THE RATE OF FURNACE EVACUATION UNTIL THE GLOW DISCHARGE DISAPPEARS, AND INCREASING THE POWER INPUT TO THE FURNACE SIMULTANEOUSLY WITH THE DECREASE IN THE RATE OF EVACUATION TO MINIMIZE THE PERIODS OF VISIBLE GLOW DISCHARGE.

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