US3489841A - Arc melting furnace and method of melting - Google Patents

Description

1970 CHIN HUAN CHENG 3,489 8 ARC MELTING FURNACE AND METHOD OF MELTING Filed Feb. 26, 1968 Wa/er 01/67 59 W0 fer- Ouf/e? v Exhausf n 9 o R. 7&3 WW. H 5d WC N 1 mm W w z w H 4 m M M T. a m m W 6g m M ,qrgon ouf Wafer 1}? United States Patent 3,489,841 ARC MELTING FURNACE AND METHOD OF MELTING Chin Huan Cheng, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Continuation-impart of application Ser. No. 531,526, Mar. 3, 1966. This application Feb. 26, 1968, Ser. No. 729,842

Int. Cl. H05b 7/18 US. Cl. 13-9 10 Claims ABSTRACT OF THE DISCLOSURE In direct arc melting, thorough mixing of molten metal is had even while supported in a water-cooled metal crucible when there is employed a crucible whose axis is tilted 30 to 60 degrees 0d the vertical and the crucible and its support means are rotated slowly during arcing. A charge is completely melted and mixed in one melting operation. An improved universal electrode mount facilitates initiating and controlling the arc.

This application is a continuation-in-part of application Ser. No. 531,526, filed Mar. 3, 1966, now abandoned.

The invention relates to an improved arc melting furnace and more particularly is concerned with improvements in an apparatus and method for direct arc melting of a metal charge.

According to prior practice in making alloys by direct arc melting, one or more pieces of each component to be alloyed are charged together in a common crucible. An arc is started from an electrode directly to the contents of the crucible and the are maintained briefly until a major part of the charge is melted. The contents of the crucible generally are allowed to cool without stirring because of the likelihood of contamination. The resulting solidified button is then inverted to present the largely unmelted and unmixed side to the electrode and the arc is again started and maintained, preferably until a major portion of the charge is again melted, after which the charge is permitted to cool and solidify. This procedure is repeated at least four additional times in an attempt to achieve uniformity of composition throughout the alloy.

The foregoing method is disadvantageous because the charge becomes subject to contamination during the repeated arcing and handling in inverting the solidified charge. Also because there is no means for stirring, it is very diflicult to prepare an alloy having a truly uniform composition, the more so because the charge is seldom entirely melted and mixed. In addition, remelting presents especially grave problems in attempting to prepare an alloy within precisely knovm composition limits because of the likelihood of metal loss during arcing. To illustrate, in one out of about five or six starts, some of the metal charge is thrown out of the crucible, thus invalidating weight measurements of starting materials. This likelihood is even greater where an intermetallic compound has been formed, the initially formed button is being remelted, and the solidified compound is of a nature that it shatters on being contacted by the arc and heated rapidly. When the arc must be started four or more times to prepare a single alloy, the chances of success in preparing an alloy of uniform composition and of known composition become rather slim.

It is therefore a principal object of the present invention to provide a method and apparatus for direct arc melting which overcomes or avoids the difficulties encountered in conventional practices.

3,489,841 Patented Jan. 13, 1970 A more specific object is to provide a method of preparing alloys by direct arc melting in which melting and mixing can be completed in one operation wherein the arc needs to be started only once for most alloys.

These and other objects and advantages of the present invention will be understood by those skilled in the art upon becoming familiar with the following description and the appended drawing in which the apparatus of the invention is shown in a slightly foreshortened front elevation, partly in section.

The invention is predicated upon the discovery that on supporting the crucible of an arc melting apparatus at an angle such that the axis of the crucible lies about 30 to 60 from the vertical and on providing means for rotating the crucible, alloying components charged to such crucible and exposed to a direct arc during rotation of the crucible become readily melted and completely mixed usually in one melting operation, with a minimal amount of contamination from the crucible and the atmosphere, substantially entirely obviating the need for remelting. The present improvement in arc melting apparatus in which the crucible is tilted and rotated is useful in any arc melting operation, where thorough mixing is desired, but is particularly described here with reference to batch operations in which a charge of metal is placed in the crucible and an inert counter electrode is employed.

Referring now to the drawing the depicted embodiment of the apparatus comprises a tilted crucible indicated generally by numeral 10, an opposed electrode 11 disposed above the crucible, means, indicated generally by the numeral 12, for supporting and rotating the crucible 10, and means, indicated generally by the numeral 13, for supporting the electrode 11. Preferably the zone around the crucible 10 and electrode 11, is enclosed as by a two-part generally cylindrical shell having an upper part 14 and a lower part 15, which meet in sealing relationship by means of respective mating flanges 16 and 17, and are closed by an upper end wall 18 and a lower end wall 19.

The upper part 14 of the cylindrical shell is conveniently formed of transparent insulating material such as glass or clear polymerized material. lt can also be formed of metal provided with a sealed transparent window section, but some means must be used to electrically insulate the electrode side from the crucible side or there can be no arc. The lower part 15 is preferably made of metal, e.g., mild steel. If desired, the zone around the crucible 10 and electrode 11 may also be enclosed by a rectangular shell or other suitable shape instead of a cylindrical enclosure.

The crucible 10 is supported by a hollow tubular shaft 20 which is journalled in a flanged opening in a corner portion of the lower end wall 19. The crucible 10 is of water-cooled construction and consists of an inner shell 21 formed of a suitable metal such as copper, and providing a cup-like or hemispherical shaped concavity and a supportive, generally concentric outer shell 22. The wall of the shell 22 is spaced apart from the inner shell 21 except for the lip 24 which is sealingly attached to the lip 25 of the inner shell 21 so as to provide an enclosed hollow space 26 between the shells for the receipt and circulation of cooling water or liquid. The outer shell 22 is mounted transversely across the end 27 of tubular shaft 20. Mounting is achieved by brazing, welding or otherwise attaching a circular flange portion 28, projecting from the periphery of an opening 29 in the outer shell 22 of the crucible 10, to the open end 27 of the tubular shaft 20. The tubular shaft 20' is supported adjacent its end 27 by a portion of the lower end wall 19 together with a collar 30 and O-ring 31 closure, while its lower end 32 is rotated in and supported by a bushing-like extension 33, having an O-ring 34 as sealing means, formed as a part of a water outlet manifold 35. The manifold 35 is, in turn, supported by a bracket 36 attached to the general support means 37 for the whole apparatus.

Mounted on the tubular shaft 20 at about midway the ends thereof is a sprocket 38 adapted to be chain driven by a power mechanism (not shown). Other drive means known to those skilled in the art may be employed, if desired. The hub 39 of the sprocket 38 has integrally formed therewith a quite large outwardly extending circular or disk shaped flange portion 40, the periphery of which extends into a mercury bath 41. The mercury bath 41 is connected by a lead 42 to a source of power (not shown). If desired, carbon brushes or other sliding electrical connection may be used instead of the flange and mercury bath. However, sliding contacts have been found to cause trouble in service in carrying heavy amperages.

Cooling water or other liquid is supplied to the crucible by means of a stationary inlet pipe 43 which is disposed inside tubular shaft and extends up to about the outer shell 22 of the crucible 10 and is axially aligned with shaft 20 and is of sufficiently smaller diameter than shaft 20 to provide an appropriate annular space therebetween to carry cooling liquid to the outlet 44.

The electrode 11 is preferably liquid cooled. The supporting assembly 13, as shown, incorporates means for liquid cooling as well as means for raising or lowering, and tilting the assembly. A central feature of the assembly 13 is an elongated tube 45 which extends about vertically through an opening in the upper end wall 18 of the cylindrical shell part 14. The lower end 46 of the tube 45 is closed by a plug 47 having a mounting block 48 attached thereto as by a threadable connection. The electrode 11, usually of cylindrical shape, is held in a cylindrical or complementarily shaped cavity in the mounting block 48, as by a set-screw 49.

Cooling liquid, such as water, is introduced into the elongated tube 45 adjacent the plug 47 by an inwardly concentric delivery pipe 50. Delivery pipe 50 is fitted sealingly through an opening in the cap 51 which closes the upper end 52 of the elongated tube 45.

Cap 51 is also provided with an extending ear 53 to which a heavy duty electrical lead 54 is attached as by a threadable connector 55. Electrical lead 54 is connected to the other side of the same source of power (not shown) as electrical lead 42. Extending through aligned holes in the side of the cap 51 and the Wall of elongated tube 45 and sealingly attached to the cap 51 is a cooling liquid outlet tube 56. Manipulation of the tiltable assembly is greatly facilitated upon surrounding a portion of cooling liquid outlet tube 56 with an insulating coating 57, preferably forming a rigid handle of a polymeric plastic material.

Control of the vertical position of the electrode 11 and its support assembly is had by means of (a) an externally threaded collar 58, which is mounted on and around elongated tube 45 and fixedly positioned relative to tube 45 as by a set-screw 59 extending through the upper flanged end 60 of the collar 58 and bearing against the wall of the tube 45, and (b) a foreshortened, internally threaded sleeve member 61. The sleeve member 61 surrounds and engages a threaded portion of the collar 58. The sleeve member 61 bears rotatably against a tubereceiving member 62, which is threadably mounted in a flanged opening 63 in the upper end wall 18 of the cylindrical shell portion 14. Rotation of the sleeve member 61 upon the collar 58 advances the collar up or down and thus the elongated tube 45 attached thereto and its supported electrode 11 are moved relative to the sleeve member 61. The tube receiving member 62 is rigidly fixed, and the sleeve member 61 bears thereagainst by the force of gravity. Rotation of the sleeve member 61 raises or lowers the electrode 11, the direction of movement depending upon the direction of the threads, Moving the collar 58 away from the tube-receiving member 62 raises the trode 11. Y

The tube-receiving member 63 is provided with a smooth walled inner cavity 64 in the shape of an inverted, truncated cone. It is essential for control of the atmosphere around the electrode and the crucible that the smallest end 65 of this conical cavity makes a very snug slide fit around the elongated tube 45 sufficient to provide a seal. Rocking or tilting of the elongated tube 45, within this cavity 64 and about the closest or narrowest portion 65, is further facilitated by providing the mating surfaces of the sleeve member 61 and the tube-receiving member 62, shown as a line 66, with a radiu of curvature having as the center the distance from the surfaces to the narrowest part 65 of the cavity 64.

Safe and positive control of tilting of the assembly and convenient means for rotating the collar 61 are provided by means of a handle 67 of insulating polymerized material which is formed about or otherwise attached to a radially projecting finger portion 68 of sleeve member 61.

Means for introducing and maintaining an inert atmosphere in the enclosed zone surrounding the crucible 10 and electrode 11 are provided in the form of an inlet connection 69 and an outlet connection 70 extending through the wall of the cylindrical shell portion 15, preferably on opposite sides of the shell, or otherwise spaced apart. Desirably, outlet 70 is provided with a safety or relief valve 71. It is also usually desirable to provide the inlet and outlet with suitable valving and pipe connections to provide for evacuating the enclosed zone so as to facilitate eflicient removal of air from the enclosed zone before filling the enclosed zone with a dry inert gas.

In carrying out the method of the invention, using the apparatus shown in the drawing, the electrode 11 and its support assembly 13 and the upper portion 14 of the cylindrical shell are lifted up, parting taking place between the flanges 16 and 17, and a charge of pieces of alloying components to be melted together are placed in the cavity of crucible 10. The charge will be made up of weighed portions of components where it is desired to make an alloy of accurately known, predetermined composition.

As indicated hereinabove, the crucible should be set, during construction of the apparatus, with the concavity side up and with the rotational axis thereof at an angle of from 30 to 60 from the vertical and more preferably 45 to 55 from the vertical.

The upper part 14 of the shell is fitted into place with flanges 16 and 17 aligned and held together by the weight of the electrode support means. The enclosed zone is evacuated and filled with an inert dry gas, e.g., dry argon, dry helium, or a mixture thereof. To assure complete dryness and inertness, the enclosed zone may be again evacuated and filled with inert gas one or more additional times. The electrode 11 and its assembly are then adjusted to provide a spacing of about to inch between the very tip 72 of the electrode and the surface of the charge, indicated generally by the numeral 73. An arc is then started by application of a potential between the electrode and the charge, care being taken to avoid arcing to the crucible. The are is maintained for a short period, such elecas from about several seconds to about 10 seconds or more, depending upon the size and nature of the charge, during which time a substantial proportion of the charge becomes molten, and usually until no solids are visible. Without interrupting the arc, rotation of the crucible 10 is commenced to expose any submerged solid parts of the charge to the arc, and to bring about good mixing of the molten metal.

Because the inner shell of the crucible is water cooled, it does not assume the temperature of the molten charge. Instead, a relatively thin layer of the charge contiguous to the crucible initially remains unmelted. As the crucible begins to rotate, the molten metal advances as a rolling globule onto a different part of the crucible surface not covered by part of the Charge and a thin layer of the molten metal cools and solidifies into a thick skin or skull. As this occurs, however, the thin layer left behind by the globule of metal, with respect to the direction of rolling, is remelted by the heat of the arc and draws up and coalesces with the charge, leaving substantially no residue on the crucible surface.

Rotation of the crucible is accomplished by activating mechanical power means (not shown in the drawing) connected by a chain 74 to sprocket wheel 38 mounted on tubular shaft 20. The speed of rotation of the crucible is not sharply critical but rotational speeds of the order of about /2 to 3 revolutions per minute, and more preferably about 1 revolution per minute, has been found to be satisfactory to obtain adequate mixing.

Cooling water or other liquid supplied to both the crucible 10 and electrode support assembly 13 throughout the heating and cooling cycle protect the crucible and electrode by preventing melting thereof, e.g., where the charge melts above the melting temperature of the crucible material. Cooling of the surfaces inhibits reaction with the atmosphere in the enclosed zone and also substantially precludes alloying of the inner shell of the crucible with the molten charge.

In order to fully meet the objectives of the invention it is highly desirable that the arc is maintained for a sufficient period of time that all of the charge is melted and adequately mixed so that the composition becomes uniform. To facilitate melting of all solid pieces in the charge it is usually desirable to grasp one or both of handles 57 and 67 and tip the electrode assembly 13 slightly so that the arc is positively directed to solid parts of the charge which are not being contacted by the arc. This is done prior to or during rotation of the crucible as demanded by circumstances. Adjustment of electrode spacing during arcing is often desirable as the electrode is usually placed quite close to the charge to make initiation of the arc easier. Lengthening of the arc is sometimes desired to maneuver and direct the arc, and if necessary, to avoid contacting the electrode tip with the charge during manipulation of the electrode. Such contact must be scrupulously avoided in preparing uncontaminated alloys. Adjustment of the electrode spacing is accomplished by rotating the handle 67 attached to the sleeve member 61.

When melting and mixing of the charge are completed, usually after 15 seconds to 1 minute of operation, rotation of the crucible is stopped, and after several more seconds, during which time the trailing skull draws up and is remelted and the molten charge forms a compact globule, the power to the electrode and crucible is cut off and arcing is stopped. The charge is allowed to cool and solidify in the protected atmosphere of the enclosed zone. The solidified charge obtained is then recovered as a button which is simply lifted out of the crucible after opening up the apparatus.

Occasionally it is found that two components to be alloyed together have such widely separated melting temperatures that good melting and mixing cannot be achieved in one arcing period without either making the temperature so high or the period so long as to cause substantial loss of the more volatile of the two components. The problem is generally more pronounced where the higher melting component is a non-metal. Such a component tends to be the predominant part of the unmelted layer or skull next to the crucible and the resultant limitation of electrical conductivity limits the intensity of are obtainable. In these situations, it is often better to keep the initial arcing period short, and to turn over the button initially formed and repeat the melting operation one, or perhaps two, times. In this manner the higher melt ing component is presented, somewhat preferentially, to the arc and sufficient mixing is obtained to form an alloy with substantially lower melting temperature and better electrical conductivity than the higher melting component. Initial mixing is, however, usually sufficient to avoid formation of the shattering type intermetallic compounds.

Power application during maintenance of the arc is usually in the range of 8 to about 50 kilowatts or more depending upon the size of the crucible and the quantity of metal in the charge. In smaller models of the apparatus using a crucible having a hemispherical diameter of about 3 inches, a power application at the level of about 12 kilowatts is found adequate to make a uniform melt weighing up to 40 grams and in a time period as short as 30 seconds. Using a crucible large enough to accommodate melts weighing 1 to 3 pounds, the power requirement runs to about 50 kilowatts or more.

The nature of the metal forming the lining of the crucible is an important factor relative to initiation of the arc. The lining should be constructed of a metal with very good electrical conductivity as well as heat conductivity. The most desiable metal to use, considering all factors, such as cost, and ease of construction is copper. Silver is good but more expensive. Stainless steel is useable, though less desirable. Platinum type metals are excellent, but the most expensive.

In making high purity alloys, that is, those substantially free of contamination, it is important to minimize arcing time. During the heating period by arcing, there is a tendency for the charge to evaporate, usually having the etfect of changing composition of the charge. Also, if cooling is not adequately provided for, the electrode may overheat during too long arcing. The extent of evaporation is usually directly proportional to arcing time and the careful operator will discontinue the operation at the first possible moment that adequate melting and mixing are obtained.

The electrode may be made of any appropriate metal which is reasonably refractory, such as tungsten or platinum. Tungsten is less expensive and appears to be the metal of choice in most operations. The electrode must be of an appropriate diameter, depending upon the size of the operation. To illustrate, a rod-shaped electrode one-half inch in diameter is used for melting charges weighing about 15 to 40 grams disposed in a three inch diameter crucible. The amount of power applied even on this scale of operation is usually too much for an electrode, of, say, three-eighths inch diameter since it normally would get extremely hot. With larger power applications, electrodes of three-quarter inch to one inch diameter or more are used.

EXAMPLES Using the apparatus and method of the invention a number of alloys were made in about 45 seconds each using an application of 40 volts of direct current and about to 200 amperes. Binary alloys were made in about 10 to 20 gram quantities using the requisite amounts of each metal to form the following intermetallic compounds of aluminum:

Total weight, Weight, Formula grams Component percent Test No CBAl; 17 Ge 58 2 MnAlr 13 Mn 25 3.. AlBi 10 B 45 4 AlSb 21 Sb 82 5 AlTi 13 T1 37 6 FeAlg 15 Fe 40 Balance aluminum.

alloys, each was remelted several times as a precautionary measure.

An additional series of binary alloys was made by using the following metals and the method and apparatus described for the above series of alloys:

First Weight, Second Weight, component grams component grams Be 2.7 Ti 7.3 Be 2 Ni 13 B 3.6 Fe 11.4 Be 2 c0 13 11 Bo 4.5 Cu 10.5

In each case a uniform melt was obtained in one are melting operation.

Another series of binary alloys was made up in about to 40 gram quantities using various proportions of the following metals, and the method and apparatus employed for the foregoing examples.

Test No.: Binary alloy 12 Titanium-vanadium. l3 Beryllium-aluminum. l4 Iron-rhenium. 15 Vanadium-chromium.

In each case a uniform melt was visibly obtained in one melting operation. This was allowed to solidify into a single, uncontaminated compact button.

The alloys prepared in the foregoing Tests 1 to 15 were free from contamination introduced in conventional operations because initial alloying was more complete and because repeated handling and remelting were avoided using the present method and apparatus. The alloys prepared in Tests No. 7 to 15 were particularly free of contamination because melting was carried out only once.

Using the apparatus of the invention and the melting technique of the foregoing examples, about a gram charge of an austenitic stainless steel was completely and successfully melted down into a compact button in one arcing period without discernible contamination.

The method and apparatus of the invention having been thus fully described various modifications thereof will at once be apparent to those skilled in the art and the scope of the invention is to be considered limited only by the claims hereafter appended.

I claim:

1. The improved direct arc melting furnace which comprises:

a hollow crucible having an inner shell and an outer shell said inner shell being of metal and formed to provide a generally cup-shaped concavity;

means for cooling said hollow crucible;

means for supporting the crucible concavity side up and at an angle such that the axis of the crucible forms an angle with the vertical in the range of to 60 degrees;

means for rotating the crucible support means whereby the crucible is rotated about its axis;

an inert electrode;

support means for positioning said electrode opposite the crucible concavity;

a power source connected to the electrode and to the crucible whereby to create a different polarity in each and to initiate and maintain an electrical arc therebetween;

and means for providing an inert atmosphere around the electrode and the crucible.

2. The apparatus as in claim 1 in which the axis of the crucible is tilted from about to degrees from the vertical.

3. The apparatus as in claim 1 in which the support means for the electrode provides for raising, lowering and tilting the electrode while maintaining an inert atmosphere around the electrode and the crucible and while an are is being maintained between the electrode and the crucible.

4. The apparatus as in claim 1 in which the electrode is water cooled.

5. The apparatus as in claim 1 in which the inner shell of the crucible is formed of copper.

6. The improved method of melting a charge of metal with a direct are which comprises:

providing a hollow water-cooled metal crucible and an opposed complementary chemically inert electrode;

supporting the crucible so that the axis is tilted at an angle of about 30 to degrees from the vertical;

placing said charge of metal in the crucible;

initiating an arc between the electrode and the charge in the crucible by the application of electrical power therebetween;

maintaining said are until a substantial part of the charge has melted;

without interrupting said are, rotating said crucible with its axis tilted at said angle until melting and mixing of the charge are substantially complete;

stopping rotation of the crucible;

shutting off the application of electrical power whereby the arc ceases;

and maintaining an inert atmosphere around the electrode and the crucible substantially throughout melting and solidification of the charge of metal.

7. The method as in claim 6 in which the arc melting process is completed in one continuous arcing period lasting less than about one minute.

8. The universal electrode mount which comprises:

an elongated generally cylindrical support shaft having an upper end and a lower end and electrode attaching means at said lower end;

a base support plate having a support shaft-receiving opening formed therethrough and an upstanding circular flange internally threaded and formed around the periphery of said opening, said support shaft extending slideably through said opening and being supported with the lower end below said plate and the upper end above said plate;

a support shaft-receiving member having the general shape of a short cylindrical sleeve and having an opening formed therethrough in the shape of a smooth walled, inverted, truncated cone, the support shaft-receiving member having a finger-engaging surface formed about the upper end of its external cylindrical surface, said member being externally threaded about its lower end, said lower end being threadably attached to the said upstanding circular flange, and the upper end of said member being provided with a convexly shaped end surface having as the radius of curvature the distance from said surface to the transverse plane of the level of greatest constriction in the opening formed therethrough said member, and said support shaft extending slideably through said support-shaft-receiving member;

an internally threaded foreshortened sleeve member having an upper end and a lower end, said lower end having an end face formed with a concave surface complementary to and resting against the convex surface of said support shaft-receiving member in substantial alignment therewith, and said sleeve member being provided with means for manually rotating said sleeve member on its cylindrical axis, and said support shaft extending slideably through said sleeve member;

and a hollow cylindrical collar having an upper end and a lower end, said support shaft extending slideably through said collar, said upper end having setscrew means for fixedly positioning said collar relative to said support shaft, said lower end being externally threaded and adapted to threadably engage the internal threads of said sleeve member;

. 9 10 whereby said apparatus is adapted to raise or lower 10. The improvement in arc melting furnace as in the support shaft relative to said base support plate claim 9 in which the crucible is supported at such an upon rotating said sleeve member about its axis. angle that the axis thereof forms an angle with the verti- 9. In a direct arc melting furnace having a water-cooled cal in the range of 45 to 55 degrees. crucible with a metallic inner shell defining a concavity; a 5 complementary electrode; support means for positioning References Cited the electrode opposite the crucible concavity; a power UNITED STATES PATENTS source connected to the electrode and to the crucible whereby to create a different polarity in each and t0 ini- 11711878 3/1965 BUFke 13-40 tiate and maintain an electrical arc therebetween; and 10 1,833,347 11/1931 AVIS X means for providing an inert atmosphere around the elec trade and the crucible, the improvement which comprises: BERNARD GILHEANY Primary Exammer means for supporting the crucible concavity side up H, B, GILSON, A i t E i and at such an angle that the axis of the crucible forms an angle with the vertical in the range of 30 15 US. Cl. X.R. to 60 degrees; and 13l0, 31 means for rotating the crucible support means whereby the crucible is rotated about its axis.

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