US 3087211 A
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Apnl 30, 1963 H. w. HOWE 3,087,211
ELECTRON-BEAM FURNACE WITH OPPOSED-FIELD MAGNETIC BEAM GUIDANCE Filed May 27, 1960 '2 Sheets-Sheet 1 W P 4 Xf/mm April 30, 1963 H. w. HOWE 3,087,211
ELECTRON-BEAM FURNACE WITH OPPOSED-FIELD MAGNETIC BEAM GUIDANCE '2 Sheets-Sheet 2 Filed May 27, 1960 INVENTOR. HOAMPD 4 How: BY
P424, fif/wzdukwz fiTTUF/V! 3,087,211 Patented Apr. 30, 1963 3,687,211 ELECTRON-BEAM FURNACE WITH OPPOSED- FIELD MAGNETIC BEAM GUEDANCE Howard W. Howe, Oakland, Caiifi, assignor to Stauifer Chemical Company, New York, N.Y., a corporation of Delaware Filed May 27, 1960, Ser. No. 32,216 4 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 casing ingots therefrom, with resulting purification, degasifica tion to an exceptionally high degree, and other benefits. An object of the present invention is to provide electronbeam furnaces capable of larger scale and more economical operations. Other objects and advantages will appear as the description proceeds.
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. Solidified material may be progressively withdrawn through the bottom end of the mold to form a cast ingot of progressively increasing length.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 pool of molten rnaterial atop the solidifying ingot. Melt stock is progressively fed into the beam, either horizontally from the side or vertically from above, and the melt stock is continually melted off as it advances into the electron beam. The so-melted material falls into the open top end of the mold for continually replenishing the molten material in the pool.
Generally speaking, it is more desirable to feed the melt stock in horizontally from one side of the beam during the initial processing of materials containing considerable amounts of volatile impurities, because the horizontal feed-in usually results in a more open configuration facilitating the evacuation of evolved gaseous matter. But for remelting previously cast ingots, e.g., for further purification or other improvement of ingot quality, vertical feed-in is preferred, among other reasons, because the more closed configuration, particularly the proximity of the melting surface of the melt stock to the molten pool on top of the newly cast ingot, with the two heated surfaces substantially facing each other, conserves heat and considerably reduces the electrical power consumption.
The copending applications of Hugh R. Smith, Jr., Serial No. 32,215, filed May 27, 1960, entitled Electron- Beam Furnace with Magnetically Guided Beam and of Charles W. Hanks, Serial No. 32,217, filed May 27, 1960, entitled, Electron-Beam Furnace with Double-Coil Magnetic Guidance, both assigned to the same assignee as the present application, disclose and claim improved electronbeam furnaces utilizing magnetic fields to guide the electron beam into the open top end of the casting mold.
The present invention is a further improvement, which provides further benefits and advantages, including the following: the electron beam can be introduced at a much smaller angle to the horizontal, and therefore can travel inward much farther, e.g., between the molten pool within the upper part of the casting mold and the melting bottom end of a vertical rod of melt stock closely spaced above the melting pool in coaxial alignment with the mold. This makes possible the remelting and recasting of larger-diameter ingots, with the advantageous vertical feed-in of the melt stock, and appears to overcome all of the limitations heretofore existing on the maximum diameters of ingots that could be processed satisfactorily. Also, annular electron guns of much larger diameter can now be employed. This increases the spacing between the gun and sources of evolved gaseous matter, thereby increasing gun life, and also makes possible significant increases in the total beam current in relation to the maximum intensity of the electronic current in the immediate vicinity of the electrodes of the gun.
In accordance with the present invention, two vertically spaced, annular electromagnet windings are employed, one extending around the annular mold and the other vertically spaced above the open top end of the mold. The two windings are in vertical, coaxial alignment and are energized in bucking magnetic flux relation. As a general rule, the ampere-turns energization of the lower Winding exceeds that of the upper winding. The so-energized windings produce a magnetic field having certain flux lines that converge inwardly and downwardly between the two vertically spaced windings and into the open top end of the mold. These particular flux lines extend through and are primarily generated by the lower winding. The bucking field produced by the upper winding causes the flux lines under consideration to pass between the two windings and approach the casting mold at a smaller angle to the horizontal, which can be adjusted by adjusting the relative magnitudes of the energizing currents supplied to the two windings. In a preferred adjustment, the flux lines under consideration extend conelike between the two windings at an angle of about 45 to the horizontal.
An annular electron gun directs a conelike electron beam downwardly and inwardly along the aforesaid conelike, converging magnetic flux lines. The magnetic field guides the beam into the open top end of the casting mold and maintains the desired beam pattern under adverse conditions, such as the evolution of large quantities of gaseous matter which becomes ionized and forms highly conductive plasmas in regions traversed by the beam. Preferably, melt stock is introduced downward through the hollow, conelike electron beam, and is melted off from a precisely located melting surface a small distance above the open top end of the casting mold.
The foregoing and other aspects of the invention may be understood better from the following illustrative description and the accompanying drawings.
FIG. 1 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 axi vertical, has open upper and lower ends and is provided with a water jacket 2 through which water or other coolant is continuously 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 ingot is accomplished, for example, by means of rollers 4 driven by an electric motor 5.
A first annular electromagnet winding 6 extends coaxially around mold 1, as shown, and has a vertical axis concentric with the open top end 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 1d connect winding 6 to a DC. power supply 11 in series with a rheo-stat 12, whereby the winding 6 is supplied with energizing direct current of adjustable magnitude. A second electromagnet winding 13 is vertically spaced above the first winding, in vertical, coaxial alignment with the first winding '6 and the annular mold 1. Preferably, winding 13 is protected by an inner sheath 14 of insulation and an outer sheath 15 of metal. Wires 16 and 17 connect winding 13 to D.C. power supply 11' (which in practice may be combined with supply 11, if desired) in series with a rheostat 18, whereby winding 13 is supplied with energizing direct current of adjustable magnitude.
The two windings 6 and 13 are energized in bucking magnetic flux relation and the energizing currents, supplied to the two windings are individually adjustable by means of rheostats 12 and 18. The ampere-turns energization of winding 6 is usually greater than that of winding 13, typically about two times greater. The so-energized windings produce a magnetic field having magnetic fiux lines that extend inwardly and downwardly between the two vertically spaced windings. A majority of these flux lines converge downwardly through the lower, more strongly energized winding 6, as represented 'by broken lines 19, FIG. 2. A minority of the flux lines turn upward through the upper, less strongly energized winding 13, as represented by broken lines 20.
The flux lines of chief interest are lines 19, and particularly those of lines 19 that converge into the open top end of casting mold 1. The strength of the bucking field established by winding 13 sets the angle at which lines 19 pass between the two windings, e.g., as the bucking field is made stronger, the flux lines are pushed downward, the Zeno-field surface separating lines 19 from lines 20 being a cone that approaches a horizontal plane as the ampereturns energization of winding 13 approaches that of winding 6.
An annular electron gun directs a beam of electrons along the magnetic flux lines 19 that converge into the open top end of casting mold 1. In its preferred form, the electron gun comprises an annular, thermionic cathode 21, most commonly made from a horizontal loop of tungsten wire connection through leads 22 and 23 and a transformer 24 to an alternating-current supply 25, which supplies alternating current through wire 21 for heating the same to thermionic-emission temperature. An accelerating electrode 26 is closely spaced inwardly from cathode 21, and a focusing electrode 27 is closely spaced outwardly from cathode 21, as shown. Electrical connections are provided for maintaining accelerating electrode 26 at substantially the same electric potential as mold 1, preferably ground potential. This is indicated schematically in the drawings by ground connection symbols 28 and 29. Cathode 21 and focusing electrode 27 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 30 between electrode 26 and lead 23 and by connection 31 between lead 23 and the negative terminal of a high-voltage D.C. supply 32.
The overall design of the electron gun may be similar to that described in the copending patent application of Charles W. Hanks, Serial No. 818,306, filed June 5, 1959, 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, the accelerating electrode, and the focusing electrode are shaped and aligned to direct electrons downwardly and inwardly parallel to the converging magnetic lines of force 19, forming a hollow, conelike electron 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 vertical rod 33, of material that is to be melted, is supported above mold 1 in vertical, coaxial alignment therewith, and is continually fed downwardly into the hollow, conelinke electron beam. Feed mechanism is symoblized by rollers 34 driven by an electric motor 35. The bar of melt stock is electrically grounded, e.g., through rollers 34 as symbolized by the conventional grounding symbol at 36. As the lower end of rod 33 advances into the electron beam, the melt stock is melted away and 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 that forms automatically atop the solidifying material.
An important function of the improved magnetic beam guidance which this invention provides is to prevent the electron beam from climbing up the rod 33 of melt stock, or otherwise being diverted away from the pool 3'. Heretofore, this has been a problem which limited the size of ingots that could be processed, particularly with vertically fed melt stock, it being increasingly difficult to project the beam inward between the rod of melt stock and the molten pool, while preventing substantial diversion of the beam to the melt stock and maintaining uniform heating of the whole surface of the pool, as ingot diameters are increased to obtain larger-scale and more economical operations. The problem is not only that the beam must travel inward farther between the two heated surfaces, but also, with the larger ingots greater power, and therefore more beam current, is required, and larger quantities of gaseous matter are evolved from the melting and molten material.
The present invention has been found eifective in overcoming the above difliculties, and apparently overcomes all the limitation heretofore existing on maximum ingot size. The inside edge of the magnetically guided, conelike beam remains sharp and well defined, whereby the melt stock is melted away at a precisely located, conelike surface. Hence, the melting rate is easily controlled merely by controlling the rate at which the melt stock is fed into the beam and the beam remains well focused over the entire surface of the molten pool. Further, the diameter of the electron gun can be increased greatly, which not only removes the gun from regions containing relatively high concentrations of evolved gaseous matter, but also increases the electron-emitting area of the gun so that a greater total beam current is obtained in relation to the electronic current intensity at any given point near the electrodes of the gun.
In FIG. 2, the volume occupied by the conelike electron beam is represented by the shading between lines 37 and 38 and between lines 39 and 40. It will be noted that the beam is everywhere substantially parallel to the magnetic flux lines 19 which converge into the open top end of mold 1 and guide the beam.
In FIG. 1, there is schematically shown a vacuum tank at which encloses the casting mold, the electron gun, and associated parts. Tank 41 is continuously evacuated to a high vacuum, preferably one micron of mercury absolute pressure or less, by connection of the tank through a large-area duct 42 to high-capacity vacuum pumps 43. Appropriate air locks (not shown) may be provided as desired for the introduction of melt stock, the removal of ingots, the replacement of electron guns, and the like.
It will be understood that the specific embodiment illustrated is but 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:
1. An electron-beam furnace comprising a container for molten material, said container having an open top, two vertically spaced, coaxial electromagnet windings, one of said windings extending around said container and the other being vertically spaced above the open top of said container, means for energizing said windings in bucking magnetic flux relation, the so-energized windings providing a magnetic field having converging flux lines extending between said vertically spaced windings and into the open top of said container, and electron gun aligned to project a beam of electrons in a direction extending laterally with respect to the common axis of the windings and along said converging flux lines between said windings into said container, a vacuum tank enclosing at least the space between said gun and said container, and means for evacuating said tank continuously.
2. An electron-beam furnace comprising an annular container for molten material, said container having an open top end, two vertically spaced, annular electromagnet windings disposed in vertical, coaxial alignment, one of said windings extending coaxially around said container and the other being vertically spaced above the open top end of the container, direct-current supply means connected to energize said windings in bucking magnetic flux relation, the ampere-turns energization of the lower winding exceeding the ampere-turns energization of the upper winding, the so-energized windings providing a magnetic field having converging flux lines extending downwardly and inwardly between the two vertically spaced windings into the open top end of said container, an electron gun aligned to project a beam of electrons along said converging flux lines between said windings into said container, means for feeding material to be melted downward through the upper annular winding into said beam, whereby the fed-in material is melted by the electron beam and falls into said container, a vacuum tank enclosing said container and gun and the space therebetween, and means for evacuating said tank continuously.
3. An electron-beam furnace as in claim 2, said electron gun comprising an annular, thermionic cathode in vertical, coaxial alignment with said two windings and container, an accelerating electrode closely spaced inwardly from said annular cathode, a focusing electrode spaced outwardly from said cathode, means maintaining said accelerating electrode and container at substantially equal electric potentials, and means maintaining said cathode and focusing electrode at substantial negative potentials relative to the accelerating electrode, said focusing electrode and cathode and accelerating electrode being spaced and aligned to form a hollow, conelike electron beam directed downwardly and converging inwardly, being everywhere substantially parallel to said converging magnetic flux lines.
4. An electron-beam furnace for melting rods of material and casting ingots therefrom in a high vacuum, said furnace comprising an annular, metal mold having a vertical axis and an open top end, means for continuously cooling said mold to solidify molten material therein, a first annular electromagnet winding extending coaxially around said mold below said open top end, a second annular electromagnet winding spaced vertically above the open top end of said mold, said second winding being in vertical, coaxial alignment with said first winding and mold, direct-current supply means connected to energize said first and second windings in bucking magnetic flux relation, the ampere-turns energization of said first winding exceeding the ampere-turns energization of said second winding, the so-energized windings providing a magnetic field having converging flux lines extending downwardly and inwardly between the two vertically spaced windings into the open top end of said mold, means for individually adjusting the energizing currents of said windings, an annular electron gun coaxial with said windings and vertically positioned to direct electrons between the two vertically spaced windings, 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 inwardly from said cathode, a focusing electrode spaced outwardly from 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 as 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 directed downwardly and converging inwardly, being everywhere substantially parallel to said converging magnetic flux lines extending between said windings into the open top end of the mold, a vacuum tank enclosing said electron gun and said mold and the space therebetween, means for continuously evacuating said tank to a high vacuum, means for feeding rods of melt stock downward endwise through said second annular winding toward the open top end of said mold in vertical alignment therewith and into the hollow, conelike electron beam, and means for maintaining the rod of melt stock at substantially the same electric potential as said mold, whereby the lower end of the rod of melt stock is melted away by the electron beam and the so-melted material falls into the open top end of the mold.
References Cited in the file of this patent UNITED STATES PATENTS 2,321,886 Anderson June 15, 1943 2,423,729 Ruhle July 8, 1947 2,880,483 Hanks et a1. Apr. 7, 1959