US3189953A - Electron-beam furnace with magnetically guided beam - Google Patents

Description

SRCH EGSM June 22, 1965 H. R. SMITH, JR

ELECTRON-BEAM FURNACE WITH MAGNETICALLY GUIDED BEAM Filed May 27, 1960 2 Sheets-Sheet l -BBK June 22, 1965 H. R. SMITH, JR 3,189,953

ELECTRON-BEAM FURNAGE WITH MAGNETICALLY GUIDED BEAM Filed May 27, 1960 2 Sheets-Sheet 2 INVENTOR. Hai/f 2 'M/ 7H, J/'z W Pal/Ma'cam United States Patent O 3,189,953 ELECTRN-BEAM FURNACE WHTH MAGNETICALLY GUHDED BEAM Hugh R. Smith, lr., Piedmont, Calif., assigner to Stautfer `Chemical Company, New York, NX., a corporation of Delaware lliiled May 27, 1960, Ser. No. 32,215 2 Claims. (Cl. 22--57.2)

This invention relates to electron-beam furnaces for heating materials by electron bombardment in a high vacuum, and particularly for melting materials and casting ingots therefrom, with resulting purification, degasification to an exceptionally high degree, and other benefits.

It is known that materials processed in electron-beam furnaces have exceptional and often highly advantageous properties because of the purification and exceptionally high degree of outgasing achieved in such furnaces. Furnaces heretofore described handle a wide variety of materials satisfactorily in a technical sense, and at a reasonable cost as regards relatively expensive materials that are otherwise hard to process, but the economic extension of the process to other uses has made necessary continuing increases in furnace size and in the duration of uninterrupted furnace runs. These objectives are not easily attained. In particular, increases in furnace size cause progressively increasing problems in the focussing and control of the higher-current electron beam, particularly in the presence of higher rates of evolution of gaseous matter from the melting material. The duration of uninterrupted runs in the furnace may be limited by instability of the electron beam, and particularly by changes in the electrode configuration, emission properties, and the like, due to the buildup of deposited material on the electrodes by condensation of evolved gaseous matter, which may include both volatile impurities and vapors of the material being processed, the collection of splattered molten material, and other causes. The present invention provides an improved electron-beam furnace capable of larger-scale operation for longer uninterrupted periods of time.

Briefly stated, electron-beam melting-and-casting furnaces include, within a continuously evacuated tank, a container for the molten material, which most commonly has the form of an annular, water-cooled casting mold open at its top and bottom ends. Solidilied material may be progressively withdrawn through the bottom end of the mold to form a cast ingot of progressively increasing length. An electron gun directs a beam of electrons downward into the open top end of the mold to bombard the material therein and maintain a molten pool of material atop the solidifying ingot. Preferably, melt stock is progressively fed in, eg., from one side of the beam7 so that the melt stock is continually melted off as it advances into the electron beam. The so-melted material falls into the open top of the mold for continually replenishing the molten material in the pool.

According to the present invention, an annular electromagnet winding, which extends coaxially around the annular mold, provides a magnetic field having flux lines converging into the mold through its open top. The electron gun, preferably annular in shape also, is spaced a relatively great distance, compared to the mold diameter, above the open top of the mold, and the electrodes of the gun are shaped and aligned to launch the electrons on paths parallel to the magnetic liux lines. Because of its remoteness, the gun suffers relatively7 little from the deleterious effects of gaseous matter evolved by the melting and molten material. The magnetic field focuses and guides the electrons from the gun into the open top end of the mold, even though gaseous matter evolved Patented .lune 22, 1965 ice from melting and molten material may provide a copious supply of ions within the space traversed by the beam. Additionally, the electro-magnetic forces vigorously stir the molten material in the pool, thereby aiding the escape of volatile impurities and providing other benelicial effects.

The foregoing and other aspects of the invention may be understood better from the following illustrative description and the accompanying drawings.

FIG. l of the drawings is a highly schematic vertical section of an improved electron-beam furnace.

FIG. 2 is a fragmentary schematic, vertical section of the same furnace, drawn to a somewhat larger scale, showing typical magnetic flux lines and electron trajectories.

Referring to the drawings, an annular copper mold 1, with its axis vertical, has open upper and lower ends and is provided with a water jacket 2 through which water or other coolant is continually circulated by conventional means (not shown), whereby the mold is cooled to solidify molten material therein. Other parts of the furnace may also be water-cooled, as desired, such being accomplished by obvious means requiring no description. The solidified material may be progressively withdrawn through the open bottom end of mold 1 to form a cast ingot 3 of progressively increasing length, which may be cut off from time to time as desired. Progressive withdrawal of the cast ingot is accomplished, for example, by means of rollers 4 driven by an electric motor 5.

An annular electromagnet winding 6 extends coaxially around mold 1, as shown, and has a vertical axis concentric with the open top of the mold. Preferably, this winding is protected by an inner sheath 7 of insulation and an outer sheath 8 of metal. Wires 9 and 10 connect the winding to a D.C. power supply 11 in series with a rheostat 12, whereby the winding is supplied with energizing direct current of adjustable magnitude. The soenergized magnet produces a magnetic field, of adjustable strength, having a vertical axis of symmetry and having magnetic flux lines that converge into the open top end of mold 1 as represented by broken lines 13, FIG. 2.

An annular electron gun is vertically aligned above the open top end of mold 1, and preferably is spaced therefrom by a large distance relative to the diameter of the mold for the purpose of reducing the deleterious effects of evolving gaseous matter upon the gun. In its preferred form, the electron gun comprises an annular thermionic cathode 14, most commonly made from a horizontal loop of tungsten wire, connected through leads 15 and 16 and a transformer 17 to an A.C. supply 18, which supplies alternating current through wire 14 for heating the same to thermionic-emission temperature. An accelerating electrode 19 isl closely spaced be low cathode 14, and a focusing electrode 20 is closely `spaced above cathode 14, as shown. Electrical connections are provided for maintaining accelerating electrode 19 at substantially the same electric potential as mold 1, preferably ground potential. This is indicated schematically in the drawing by the ground-connection symbols at 21 and 22. Cathode 14 and focusing electrode 20 are maintained at substantial negative potentials, commonly 5,000 to 15,000 volts, relative to the accelerating electrode. This is accomplished, for example, by means of connection 23 between electrode 20 and lead 16 and connection 24 between lead 16 and the negative terminal of a high-voltage D.C. supply 25.

The overall design of the electron gun may be similar to that described in the copending patent application of Charles Hanks, Serial Number 818,306, tiled June 5, 1959, now Patent 2,994,801, issued Aug. 8, 1961, and assigned to the same `assignee as the present application.

In the present furnace employing a magnetic field to guide the electron beam, the cathode, accelerating electrode, and focussing electrode are shaped and aligned to direct electrons downwardly and inwardly parallel to the converging magnetic lines of force, forming a hollow, cone-like, converging beam, and thereafter the magnetic field plays a significant part in focusing and guiding the electron beam into the open top end of mold 1.

A horizontal feed trough 26 extends inwardly above mold 1 and has a discharge end 26 adjacent to the open top end 0f the mold. This feed trough (and thereby its contents) is electrically grounded as indicated by the conventional symbols at 27. Melt stock 28 in any convenient form, eg., rods, bars, blocks of compacted powder, etc., is fed through the trough 26 into one side of the electron beam entering mold 1. Feed mechanism is symbolized by the rollers 29 driven by the electric motor 30. As the melt stock emerges from the discharge end of the feed trough it is bombarded and melted away by the electron beam. The so-melted material falls into the open top end of mold 1 for continually replenishing a pool 3 of molten material, which rests on top of cast ingot 3 and is supported within a skull or depression which forms automatically in the solidifying material.

The magnetic guidance which this invention provides has several significant advantages that appreciably improve the operation of the furnace and permit operation on a larger and more economical scale for longer peirods of time without interruption. In large-scale operation, large amounts of gaseous matter may be evolved, usually at an irregular rate, especially from the melting material at the inner end of the melt stock and to some extent from the molten pool 3. Much of this gaesous matter condenses readily upon the first relavely cool surface that it touches, and in some cases even the hot cathode 14 may be a suiciently cool surface for this to occur. Any substantial buildup of deposited materials upon the gun electrodes will soon lead to faulty gun operation, and therefore the more remote location of the gun facilitated by the magnetic guidance greatly increases life of the `gun by removing it from the regions where gaseous matter is most concentrated.

Furthermore, the evolved gaseous matter becomes ionized and may form plasma which would serially distort any electric potential fields depended upon for focusing. With the present furnace, the electrons are accelerated to substantially full velocity within a short distance of the cathode, i.e., approximately the distance from cathode 14 to electrode 19, and thereafter the trajectories are controlled chiefly by the magnetic field, even though the electron paths may pass through highly conductive plasma regions which could otherwise divert the electrons away from the desired trajectories. In particular, without magnetic guidance gases evolved from melting material tend to divert electrons away from pool 3 to melt stock 28, so that the melting plane and rate of melting of the melt stock are not readily controllable, which leads to erratic operation. Furthermore, such diversion may cause the development of a cool spot in that portion of the molten pool below and closest to the discharge end of the feed trough, which under certain circumstances can cause these portions of the pool to freeze over, cause poor ingot quality, and may lead to further operating dithculties.

With magnetic guidance as herein described, evolved gases, ionized matter, and even plasma have little effect upon the electron trajectories. The electron beam remains well-defined and sharply focussed into the open top end of mold 1. The melt stock is cut off at a precisely located and well-defined plane at the edge of the electron beam, and hence the melting rate can be controlled accurately merely by controlling the rate .at which the melt stock is advanced toward the beam.

Even apart from the considerations discussed above, magnetic guidance as herein disclosed makes possible larger-scale operations with larger furnaces operating with higher electron-beam currents, and also makes possible a more remote location of the electron gun with consequent longer life, permiting longer periods of uninterrupted operation, because of the effectiveness of the magnetic field in counteracting the defocusing effects of electronic space charge. The electron gun preferably is so designed that the electrons leaving any selected segment of the gun are directed along substantially parallel paths. However, the electrons repel one another, and hence each segment of the conelike beam tends to spread out in thickness at increasing distances from the gun, as is illustrated in FIG, 2 wherein the shaded areas between lines 31 and 32 and between lines 33 and 34 represent the space filled by the beam.

Spreading of the beam segments, however, requires that certain electrons move in directions that are not exactly parallel to the magnetic field, and the resulting electromagnetic forces bend the electron paths in such a way that the electrons tend to spiral, corkscrewlike, around magnetic flux lines. Hence, the electron paths are kept everywhere substantially parallel to the magnetic field and must converge with the field. This limits the spreading of the beam and the stronger the magnetic eld the tighter the spirals and the more limited the spreading. By adjusting the rheostat 12, and thereby adjusting the magnetic field strength, the total cross-sectional area of the beam entering the open top end of mold 1 can be adjusted so that the beam just covers the molten pool for supplying heat thereto in an almost ideal manner. Even a moderate magnetic field can guide into the open top end of the mold a much larger electron current than would otherwise be permitled by beam spreading due to electronic space charge.

The magnetic field provides still another advantage; that is, the electromagnetic forces, caused by electric currents within pool 3 moving nonparallel to the field, produce vigorous stirring of the molten pool. This stirring is extremely beneficial in bringing impurities to the surface of the pool, from which they can readily evaporate.

lt is essential that the space between the electron gun and the open top of mold 1 be maintained at a high vacuum, preferably one micron of mercury absolute pressure or less, and this requires continuous evacuation by high-capacity pumps to remove the gaseous matter, which may be copiously and erratically evolved during largescale operations. For this purpose, the mold, the electron gun, and associated parts are included within a vacuum tank 35 connected through a large-area duct 36 to highcapaeity vacuum pumps 37. Appropriate air locks (not shown) can be provided, as desired, for introduction of melt stock, the removal of ingots, the replacement and manipulation of electron guns, and the like. Evacuation is also aided by condensation of vapors upon the vacuum tank walls and other cool surfaces.

It will be understood that the specific embodiment illustrated is merely one example of how this invention may be carried out, and that numerous changes and modifications are possible without departing from the inventive principles herein disclosed.

What is claimed is:

ll. An electron-beam furnace for melting material and casting ingots therefrom in a high vacuum, comprising an annular, metal mold having a vertical axis and having open top and bottom ends, means for continuously cooling said mold to solidify molten material therein, means for progressively withdrawing the solidified material through the bottom end of the mold as an ingot of progressively increasing length, an annular electromaguet winding extending coaxially around said mold, a directcurrent power supply connected to supply energizing direct current to said winding, the so-energized winding providing a magnetic eld having a vertical axis of symmetry and having flux lines converging into said mold through its open top end, an annular electron gun vertically aligned above the top end of the mold and spaced therefrom by a large distance relative to the diameter of the mold, said gun comprising a thermionic cathode consisting of a horizontal loop of wire and connections for supplying heating current therethrough, an accelerating electrode closely spaced below said cathode, a focusing electrode closely spaoed above said cathode, means for maintaining said accelerating electrode at substantially the same electric potential as said mold, and means for maintaining said cathode and focusing electrode at substantial negative potentials relative to the accelerating electrode, said focusing electrode and cathode and accelerating electrode being shaped and aligned to form a hollow, conelike, electron beam directing downwardly and converging inwardly, being everywhere substantially parallel to said converging magnetic eld, .a vacuum tank enclosing said electron gun and said mold and the space therebetween, means for continuously evacuating said tank to a high vacuum, a substantially horizontal feed trough extending inwardly toward and having a discharge end closely adjacent to the open top end of said mold, means for maintaining said trough at substantially the same electric potential as said mold, and means for feeding material to be melted through said feed trough into one side of the electron beam entering the mold, whereby the material emerging from the discharge end of the feed trough is melted away by the electron beam, the so-melted material falls into the open top end of the mold and there solidiiies atop the ingot progressively forming therein, the electron beam entering the mold bombarding and heating the material therein to maintain a molten pool atop the solidiiied ingot, the magnetic eld guiding the electrons from the gun into the mold even though gaseous matter evolved from melting and molten material may provide a copious supply of ions within the space between the electron `gun and the mold.

2. An electron-beam furnace comprising a container for molten material, said container having an open top, means providing a magnetic eld having ilux lines converging into said container through its open top, an electron gun aligned to project a beam of electrons along said converging flux lines into said container, a feed trough having a discharge end adjacent to the open top of said container, means for feeding material to be melted through said trough into one side of the electron beam entering the container, whereby the material emerging from the discharge end of the feed trough is melted away by the electron beam and the so-melted material falls in the said container, a vacuum tank enclosing at least the space between said gun and said container, and means evacuating `said tank continuously, whereby electrons are guided from the gun into the container by the converging magnetic iield, even though gaseous matter evolved from melting and molten material may provide a copious supply of ions within said space, and whereby electromagnetic forces stir the molten material within the container.

References Cited bythe Examiner UNITED STATES PATENTS 1,562,825 11/27 Evreynofr" 13--11 2,040,215 5/36 Rava 75-10 2,761,002 8/56 Laird et al. 75-10 2,844,706 7/58 Lorenz 219-69 2,880,483 4/59 Hanks 22-57.2 2,932,720 4/60 Stohr 219-72 2,963,530 12/60 Hanks 22-57.2 2,968,723 1/61 Stiegerwald 219-43 FOREIGN PATENTS 252,037 5/ 26 Great Britain. 504,084 4/ 39 Great Britain.

MICHAEL V. BRINDISI, Primary Examiner. RAY K. WINDHAM, MARCUS LYONS, Exarnmars,a

Claims (1)

1. AN ELECTRON-BEAM FURNACE FOR MELTING MATERIAL AND CASTING INGOTS THEREFROM IN A HIGH VACUUM, COMPRISING AN ANNULAR, METAL MOLD HAVING A VERTICAL AXIS AND HAVING OPEN TOP AND BOTTOM ENDS, MEANS FOR CONTINUOUSLY COOLING SAID MOLD TO SOLIDIFY MOLTEN MATERIAL THEREIN, MEANS FOR PROGRESSIVELY WITHDRAWING THE SOLIDIFIED MATERIAL THROUGH THE BOTTOM END OF THE MOLD AS AN INGOT OF PROGRESSIVELY INCREASING LENGTH, AN ANNULAR ELECTROMAGNET WINDING EXTENDING COAXIALLY AROUND SAID MOLD, A DIRECTCURRENT POWER SUPPLY CONNECTED TO SUPPLY ENERGIZING DIRECT CURRENT TO SAID WINDING, THE SO-ENERGIZED WINDING PROVIDING A MAGENTIC FIELD HAVING A VERTICAL AXIS OF SYMMETRY AND HAVING FLUX LINES CONVERGING INTO SAID MOLD THROUGH ITS OPEN TOP END, AN ANNULAR ELECTRON GUN VERTICALLY ALIGNED ABOVE THE TOP END OF THE MOLD AND SPACED THEREFROM BY A LARGE DISTANCE RELATIVE TO THE DIAMETER OF THE MOLD, SAID GUN COMPRISING A THERMIONIC CATHODE CONSISTING OF A HORIZONTAL LOOP OF WIRE AND CONNECTIONS FOR SUPPLYING HEATING CURRENT THERETHROUGH, AN ECCELERATING ELECTRODE CLOSELY SPACED BELOW SAID CATHODE, A FOCUSING ELECTRODE CLOSELY SPACED ABOVE SAID CATHODE, MEANS FOR MAINTAINING SAID ACCELERATING ELECTRODE AT SUBSTANTIALLY THE SAME ELECTRIC POTENTIAL AS SAID MOLD, AND MEANS FOR MAINTAINING SAID CATHODE AND FOCUSING ELECTRODE AT SUBSTANTIAL NEGATIVE POTENTIALS RELATIVE TO THE ACCELERTING ELECTRODE, SAID FOCUSING ELECTRODE AND CATHODE AND ACCELERATING ELECTRODE BEING SHAPED AND ALIGNED TO FORM A HOLLOW, CONELIKE, ELECTRON BEAM DIRECTING DOWNWARDLY AND CONVERGING INWARDLY, BEING EVERYWHERE SUBSTANTIALLY PARALLEL TO SAID CONVERGING MAGNETIC FIELD, A VACUUM TANK ENCLOSING SAID ELECTRON GUN AND SAID MOLD AND THE SPACE THEREBETWEEN, MEANS FOR CONTINUOUSLY EVACUATING SAID TANK TO A HIGH VACUUM, A SUBSTANTIALLY HORIZONTAL FEED TROUGH EXTENDING INWARDLY TOWARD AND HAVING A DISCHARGE END CLOSELY ADJACENT TO THE OPEN TOP END OF SAID MOLD, MEANS FOR MAINTAINING SAID TROUGH AT SUBSTANTIALLY THE SAME ELECTRIC POTENTIAL AS SAID MOLD, AND MEANS FOR FEEDING MATERIAL TO BE MELTED THROUGH SAID FEED TROUGH INTO ONE SIDE OF THE ELECTRON BEAM ENTERING THE MOLD, WHEREBY THE MATERIAL EMERGING FROM THE DISCHARGE END OF THE FEED TROUGH IS MELTED AWAY BY THE ELECTRON BEAM, THE SO-MELTED MATERIAL FALLS INTO THE OPEN TOP END OF THE MOLD AND THERE SOLIDIFIES ATOP THE INGOT PROGRESSIVELY FORMING THEREIN, THE ELECTRON BEAM ENTERING THE MOLD BOMBARDING AND HEATING THE MATERIAL THEREIN TO MAINTAIN A MOLTEN POOL ATOP THE SOLIDIFIED INGOT, THE MAGNETIC FIELD GUIDING THE ELECTRONS FROM THE GUN INTO THE MOLD EVEN THROUGH GASEOUS MATTER EVOLVED FROM MELTING AND MOLTEN MATERIAL MAY PROVIDE A COPIOUS SUPPLY OF IONS WITHIN THE SPACE BETWEEN THE ELECTRON GUN AND THE MOLD.

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