Furnace Apparatus Utilizing A Resultant Magnetic Field Or Fields Produced By Mutual Interaction Of At Least Two Independently Generated Magnetic Fields And Methods Of Operating An Electric Arc Furnace

Abstract

In a furnace having an electric arc extending between an electrode and material to be melted which is composed at least partially of conductive material, a resultant magnetic field having a desired field configuration is produced by mutual interaction of two or more separately and independently generated magnetic fields and the resultant magnetic field is utilized to improve furnace operation in a number of ways. In some embodiments, two fields are generated, one by a field coil in the tip of the electrode and one by a coil at or near the wall of the furnace, which latter coil may be a solenoid having a length to diameter ratio equal to or greater than one or a concentrated coil having a length to diameter ratio substantially less than one, and the resultant field has a configuration which may increase or improve arc-moving forces on the arc provided to reduce erosion of material from the electrode by arc action thereon, or may improve focusing of the arc between electrode and melt, or may improve control of a diffused arc, or may improve stirring of the melt, or may control the portion of the surface of the melt to which the arc strikes, or may prevent arc flares to the wall of the furnace, or may prevent glow discharges, or may increase feed rate, or may improve grain structure in an ingot produced, or may include any combination of the aforementioned and other improvements. In another embodiment, three magnetic fields are separately generated, one by an electric tip field coil, one by a solenoid and one by a concentrated coil adjacent the solenoid and having an axially adjustable position thereon. In a further embodiment, two magnetic fields are separately generated by two field generating means external to the electrode and under some conditions no interacting magnetic field is generated within the electrode. In other embodiments, two electrodes are mounted in the furnace each with an arc extending therefrom to the melt, and three electrodes are mounted in the furnace each with an arc extending therefrom to the melt. New and improved processes and methods of furnace operation are also described.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the copending application Ser. No. 291,466, filed concurrently by R.A. Akels, A.R. Vaia, F.A. Azinger and C.B. Wolf, and Ser. No. 291,470, filed concurrently by Frank J. Kolano, both of which are assigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates to improvements in the performance of electric arc melting apparatus, in which an arc takes place from an electrode to a melt, and is concerned with improving performance in a large number of ways by generating two magnetic fields within the furnace at least one of which extends at least partially through the melt and the portion of the volume of the furnace which includes the electrode tip, the separately generated magnetic fields interacting with each other to provide a resultant magnetic field which gives improved performance.

The invention is concerned with arc movement on the electrode to reduce erosion, arc focusing, control of the area of the pool on which the arc impinges, stirring the melt, increasing the feed rate, preventing arc flare, preventing glow discharges, providing increased heating efficiency, providing for better grain structure and greater homogeneity in an ingot produced, and improvement in operation in a diffused-arc mode.

2. Discussion of the Prior Art

It is known to use an electrode with means only in the tip or adjacent the tip for setting up a magnetic field transverse to the arc current path which exerts a force on the arc which causes the arc to move substantially continuously in generally repetitive paths around the arcing surface of the electrode; such an electrode is shown in U.S. Pat. No. 3,369,067, issued to S. M. DeCorso for "Nonconsumable Annular Fluid-Cooled Electrode for Arc Furnaces," and U.S. Pat. No. 3,385,987, issued to C. B. Wolf et al. for "Electrode for an Arc Furnace Having a Fluid-Cooled Arcing Surface and a Continuously Moving Arc Thereon," both of said patents being assigned to the assignee of the instant invention.

It is also known in the art to utilize only an external field generating means to generate a magnetic field within a furnace, with a portion of the field extending through the melt and a portion of the field occupying at least part of the remaining volume of the furnace and existing in the area of a carboniferous or consumable electrode usually axially mounted in the furnace, and in the area between electrode and melt. U.S. Pat. No. 2,727,937, issued to Boyer constitutes part of the prior art.

It is also old in the art to use a single magnetic field generating means to interact with an arc between a rotating electrode and the melt, such that the field reacts with the arc to cause it to move in a direction opposite the direction of rotation as shown in U.S. Pat. No. 3,597,519, issued to G. A. Kemeny and R. A. Akers entitled "Magnetic-Field Rotating-Electrode Electric Arc Furnace Apparatus and Methods."

The applicant has used several years ago a combination in an arc furnace in which two magnetic field producing means are used to control an arc. The specific magnetic field producing means used by the applicant in the past comprised using a relatively short electrode tip magnet having 3,200 ampere turns of magnetomotive force available and a solenoid having 5,040 ampere turns of bucking or opposing magnetomotive force available, the magnetic fields produced by these forces being specifically arranged to cancel at one point along the centerline of the electric arc furnace in which this apparatus was being used. The relationship between the sources of magnetic fields or the generators of magnetomotive force were such that the net or resultant megnetic field strengths at the electrode tip was provided primarily by the electrode tip field coil rather than the solenoid even though the magnetomotive force generated by the electrode tip magnet was relatively smaller than the magnetomotive force of the solenoid. This generated a balanced resultant magnetic field near the electrode surface most conducive to faster arc rotation and less surface wear or erosion on the electrode. In the latter balanced field systems the field strength of the solenoid is more nearly equal to the field strength of the tip coil than in embodiments of disclosed inventions described hereinafter.

SUMMARY OF THE INVENTION

In electric arc furnace apparatus of the type in which an arc takes place between an electrode and a melt, two magnetic fields are separately generated within at least a portion of the volume of the furnace to produce a resultant magnetic field configuration which accomplishes certain desirable results with respect to the electrode, the melt, and the associated arc, and accomplishes certain other desirable results with respect to the melt and the feed rate of material to the melt. In some embodiments, two magnetic fields are generated, one by a solenoid adjacent the wall of the furnace, the field extending at least a substantial distance below the level of the melt and extending at least a substantial distance above the position of the electrode tip, and the other magnetic field is generated by a compact and relatively short field coil, the winding of which is composed of many layers, at least one of the two above-mentioned field coils being slidable so that its axial position with respect to the melt and/or the other field coil is adjustable, to provide a wide variety of desired resultant magnetic field configurations. In other embodiments, the resultant magnetic field is produced by a coil in the electrode tip interacting with an externally generated field produced either by a solenoid or a compact coil or both. The tip field and the net external field may be in polarity adding or polarity opposition at the axis of the electrode tip; in the latter case, in accordance with the relative strengths of the two fields, the magnetic field lines may extend substantially uniformly around the entire arcing surface of the electrode and, where the arc is in a restricted mode, greatly enhance the ability of the magnetic field to continuously move the arc in repetitive paths around the arcing surface, and while the arc is in a diffused mode, to move the diffused discharge over the arcing surface. If the fields are in the magnetic polarity opposition mode, as the relative strength of the field generated in the tip is increased with respect to the strength of an externally generated field, the magnetic field around the arcing surface of the tip may include some field lines which extend to and into the adjacent surface of the melt; under some conditions, the central portion of the melt may rotate in one direction, while the remainder of the melt is caused to rotate in the opposite direction as a result of the externally produced magnetic field while the fields are in opposition, thereby providing better stirring of the melt.

Where the polarity of the magnetic field generated in the tip is such that it is not in opposition to the externally produced magnetic field but is in polarity adding, a resultant magnetic field is produced adjacent the tip which may have a component which exerts a relatively small rotating force on the arc and additionally effectively focuses the arc between the electrode and the melt and provides control of the arc so that it does not flare outwardly to the wall of the furnace or slant between electrode and melt; stirring of the melt may be enhanced.

Further summarizing the invention, including magnetic fields selectively in polarity adding or in opposition, in a number of areas which include positioning, controlling, focusing, or moving an arc between an electrode and a melt, my invention permits a greatly increased feed rate of material to the melt, produces increased heating efficiency in the melt, gives better stirring, gives improved grain structure to ingots, and provides for arc control to reduce erosion. Ingots can be formed from magnetic materials with acceptable grain structure even though portions of the ingot are cooled during the ingot-forming process below the Curie temperature.

It has been found by experiment that my apparatus can, under certain conditions, including the use of a vacuum furnace, substantially reduce the pressure at which the arc goes into glow, and also permit a greater arc length. With fields in opposition and operating with an arc length of one inch, it was found that while arcing to graphite the arc went into glow discharge in the 0.700-0.800 torr pressure range.

When the fields were changed to magnetic polarity adding, it was found that the pressure in the furnace could be reduced to 0.25 millitorr before glow discharge occurred, and at 0.06 torr the arc length could be increased to three inches.

In some embodiments, the two magnetic fields are produced by a solenoid adjacent the furnace wall or the mold or crucible and a concentrated coil with a magnetic yoke adjacent the solenoid and having positions axially adjustable with respect to the solenoid so that the fields mutually interact as desired and a resultant field having a desired configuration is produced. In other embodiments, one magnetic field is generated by a field coil in the electrode near the arcing surface, preferably in the tip of the electrode, and the other magnetic field is produced by either a solenoid adjacent the furnace wall or the mold or crucible or a concentrated relatively short field coil having a number of wound electrically conducting layers insulated from each other.

Where the magnetic field generated by means within the tip is in polarity opposition to the magnetic field generated by the solenoid or another coil adjacent the furnace wall, and where the tip is generally annular and generally U-shaped in cross-section with the electrode field coil disposed within the tip, the normal magnetic field which would be generated by the coil in the tip alone is shaped by mutual interaction with the additional magnetic field in the same volume of space whereby the resultant magnetic field adjacent the arcing surface is more uniform in direction in that it more closely follows the contour of the arcing surface and its capability in rotating the arc while its in a constricted mode is greatly increased. If the relative strengths and magnetic polarities of the tip magnetic field and external magnetic field are adjusted to accomplish this specific result, the magnetic field extending over substantially all portions of the arcing surface of the tip may be nearly of substantially uniform strength as modified by the .phi. = BA equation; as the strength of the tip field is increased with respect to that of the solenoid field, the field strength between the tip and the surface of the melt is increased with the magnetic field lines extending between the electrode and the melt focusing the arc on the melt which tends to rotate the center of the melt in the opposite direction from that in which the arc rotates; the remainder of the melt outside of the influence of the tip field may rotate in the same direction as the arc rotates, giving greatly improved mixing. The above-described rotational effect is produced only while the fields are in opposition. The tip field and external field are in some other applications or during certain portions of a melting operation caused to be of such polarity that they add to each other; the shape of the resultant magnetic field within the furnace and adjacent the electrode provides for focusing the arc on the arcing surface but with less moving force on the arc and is especially suitable for use under certain conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to the perferred embodiment exemplary of the invention shown in the accompanying drawings, in which;

FIG. 1 shows a partial sectional elevation of a typical magnetic field pattern produced by a prior art electrode in which there is a magnetic field coil within a tip which is generally annular in shape and generally U-shaped in cross-section;

FIG. 2 shows the magnetic field configuration produced within a furnace by a concentrated coil near or adjacent the furnace wall;

FIG. 3 shows the resultant magnetic field configuration where a tip field and an externally produced field produced by a compact coil outside the furnace wall are in magnetic polarity opposition within the furnace;

FIG. 4 shows a resultant magnetic field configuration where the externally produced magnetic field and the tip field are in polarity adding and the externally produced magnetic field is generated by a compact field coil disposed near the wall of the furnace;

FIG. 4A illustrates simplified electrical circuit diagrams for reversing the magnetic polarity of the tip field, that of the solenoid or concentrated coil field, and those of two fields with respect to each other;

FIG. 5 shows the resultant magnetic field configuration where the magnetic field generated in the electrode tip and an externally produced solenoid field are in polarity opposition;

FIG. 6 shows an additional resultant magnetic field configuration where the tip field and the externally produced solenoid field are in polarity opposition, but where the strength of the tip field relative to that of the solenoid field has been increased over the relative strengths illustrated in FIG. 5;

FIG. 7 also illustrates the resultant magnetic field where the tip field and the externally produced solenoid field are in polarity opposition, and the strength of the tip field has been further additionally substantially increased relative to the strength of the solenoid field;

FIG. 8 is an illustration of the resultant magnetic field where the tip field generated within the electrode and the externally produced solenoid field are adding at the center line of the furnace;

FIG. 9 is a schematic electrical circuit diagram for automatically reducing the strength of the tip field a certain time after the arc starts, and for other purposes;

FIG. 10 shows both a solenoid and an axially adjustable compact field coil adjacent the solenoid for setting up two separately generated external magnetic fields which interact with each other, and one or both of which further interact with the magnetic field generated by a coil in the tip of the electrode;

FIG. 11 shows furnace apparatus in which both a solenoid field which extends above the electrode tip and preferably to the bottom of the melt is used in addition to another magnetic field generated by a concentrated coil having its position axially adjustable along the length of the solenoid to provide a resultant magnetic field within the melt, which gives improved furnace performance in a number of ways, the arc being illustrated as diffused;

FIG. 12 shows a diffused arc which would extend from a fluid cooled tip having no field coil therein to a melt;

FIG. 13 illustrates the effect of the important formula .phi. = BA;

FIG. 14 is a view of a further embodiment of my invention employing a mold with a retractable bottom, and showing other novel features;

FIG. 15 illustrates a cusp magnetic field which may be produced in the apparatus of FIG. 14 under certain conditions under the control of an operator of the apparatus;

FIG. 16 illustrates a cross-sectional view of furnace apparatus utilizing two electrodes;

FIG. 17 illustrates a resultant magnetic field configuration which may exist in a plane passing through the two electrodes in the two electrode apparatus of FIG. 16, or which may exist in a plane passing through any two electrodes in the three electrode apparatus of FIG. 18; and

FIG. 18 illustrates a cross-sectional view of an embodiment of the invention in a furnace employing three electrodes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings and FIG. 1 in particular, an electrode generally designated 14 has a supporting column 15 and a tip 16 secured to the supporting column 15 and forming an arcing surface 17. The electrode 14 is shown oriented or disposed with reference to what may be a generally cylindrical furnace wall or a mold 19. Only one half of the cross section in elevation of the electrode 14 is shown for simplicity of illustration, it being understood that the other half of the electrode (not shown) is generally symmetrical with that shown. The longitudinal axis of the electrode is shown by the dashed line 14a which may also be the centerline of the furnace. It will be understood that the supporting column 15 is comprised at least partially of electrically conductive material; is adapted to be connected to a terminal of one polarity of a source of electrical potential to produce and sustain an arc 96 between the electrode tip 16 at the arcing surface 17 and the surface of opposite polarity 97. The electrode tip may conveniently be generally U-shaped in cross section as shown in elevation, with an annular space therein in which is disposed a magnetic field coil 18 for setting up or producing the magnetic field shown. It will further be understood that the electrode tip may have a passageway therein, not shown, which may be generally U-shaped in cross section extending around the entire perimeter of the tip through which cooling fluid may pass to remove heat flux from the arcing surface 17 and tip 16, and it will be further understood that the supporting column 15 may have fluid passageways therein opening into the passageways in the tip 16 for bringing cooling fluid to the tip 16 and conducting fluid from the tip 16, these fluid passageways in the supporting column not being shown for convenience of illustration. It will be understood that the passageway in the electrode 14 for the flow of cooling fluid may comprise two semicircular parallel paths as taught for example in U.S. Pat. No. 3,316,444, issued on Apr. 25, 1967 to R. M. Mentz entitled "Arc Heater for Use with Three Phase Alternating Current Source and Chamber and Electrode Structure," and assigned to the assignee of the instant invention. Portions of the aforedescribed electrode 14 may be considered as part of the prior art; an electrode similar to that described in U.S. Pat. No. 3,369,067, issued Feb. 13, 1968 to S. M. DeCorso for "Non-Consumable Annular Fluid Cooled Electrode for Arc Furnaces," or an electrode similar to that described in U.S. Pat. No. 3,369,068, issued to P. F. Kienast for "Steam Bleeding in Metal Electrode Arc Furnace," or an electrode similar to that described in U.S. Pat. No. 3,476,861, issued to Charles B. Wolf for "Insulating Non-Consumable Arc Electrode," the above patents being assigned to the assignee of the instant invention, may be employed as the electrode shown in FIG. 1, all of the electrodes in the referenced patents having a magnetic field coil in the tip or close to the arcing surface, and a fluid passageway in the tip for conducting heat flux away from the arcing surface.

In FIG. 1, flux lines of the magnetic field previously alluded to and set up by the electrode tip field coil 18 are shown at 71 to 79, inclusive, and follow the classical pattern of a magnetic field set up by a solenoid having a length which is small relative to its diameter; the electromagnetic field shown is exemplary only, since it represents a field produced by a noncritical excitation which may be varied at the will of a user of the apparatus.

It is noted that all of the lines 71 to 79 extend generally away from the field coil 18 toward the wall 19 of the furnace or mold.

The lines 80 to 94, inclusive, extending toward the longitudinal axis of the electrode by their relative spacing indicate the portion of the total magnetomotive force generated which is required to set up a magnetic field in the axial area between adjacent lines; as would be expected, the lines are closely spaced within the coil indicating that a considerable portion of the total magnetomotive force is required to force the field through the interior of the solenoid, the lines becoming progressively more distant from an adjacent line on each side thereof as the axial distance from the coil increases.

An arc is shown at 96 between the arcing surface 17 and the upper surface of the melt 97, it being understood that the melt may be comprised or composed at least partially of electrically conductive material and may form a surface of polarity opposite that of the tip; it is seen that several of the magnetic field lines traverse the path of arc 96, lines 71, 72 and 73, for example, and while the magnetic field lines do not follow the contour of the arcing surface and are not completely perpendicular to the arc path 96, by Fleming's rule relating current direction, magnetic field direction and conductor or arc movement direction, components of force are exerted on the arc 96 which cause it to move substantially continuously in an annular path around the electrode and between the electrode 14 and the melt 97. To some extent under assumed conditions, the arc 96 would tend to follow the magnetic field lines and to be focused thereby so that the arc 96 may be thought of as slanting somewhat outwardly from the longitudinal axis 14a.

Table I entitled "Magnetic Field Values for Electrode Field Coil," which follows, shows calculations of the magnitude of the magnetic field vector in gausses, and the angle of the field vector measured from the axis 14a in degrees, for 10 axial positions of 9 inches to 18 inches, inclusive, at a field radial position of 0.1 inch (the electrode axis 14a represents 0.0 inch), 10 similar axial positions at a field radial position of 1.0 inches, ten similar axial positions at a field radial position of 2.0 inches, ten similar axial positions at a field radial position of 3.0 inches, ten similar axial positions at a field radial position of 4.0 inches, 10 similar axial positions at a field radial position of 5.0 inches, ten similar axial positions at a field radial position of 6.0 inches, and 10 similar axial positions at a field radial position of 7.0 inches.

The Table I shows that the electrode tip field coil excitation force was 8,000 ampere turns, that its axial location or extension was substantially 16.6 to 17.8 inches, and that its radial position or extension from the axis of the electrode was substantially 1.325 to 1.675 inches, which gives an indication of its length and inside and outside diameters.

The zero inch axial position may correspond to the bottom of the furnace or mold, or may be a selected point distant from the field coil. In Table I, all of the repetitive radial position field values made at an axial position of 17.0 inches would represent measurements made in a plane passing through the field coil 18 and extending perpendicular to the longitudinal axis of the electrode.

The values of the magnitude of the magnetic field vector and the angle of the magnetic field vector to the axis are those of a magnetic field which can be employed in my invention; they will become more meaningful hereinafter where additional figures of the drawings are discussed which disclose a resultant field pattern produced by the mutual interaction of two or more independently generated magnetic fields, but it is to be understood that my invention is not limited to the use of a field having the precise values shown in Table I or those of any of the other tables to follow.

TABLE I

MAGNETIC FIELD VALUES FOR ELECTRODE FIELD COIL

Electrode Field Coil Excitation (-) 8000 amp. turns Electrode Field Coil Location Axial -- 16.6 to 17.8 inches Radial -- 1.325 to 1.675 inches

Field Points

Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (gauss) (Degrees) 9 0.1 7.9 178.98 10 0.1 11.5 178.84 11 0.1 17.8 178.67 12 0.1 29.3 178.45 13 0.1 53.0 178.14 14 0.1 108.3 177.74 15 0.1 257.8 177.30 16 0.1 665.7 177.51 17 0.1 1161.8 179.52 18 0.1 906.3 181.86

9 1.0 7.7 169.80 10 1.0 11.2 168.48 11 1.0 17.0 166.79 12 1.0 27.6 164.56 13 1.0 48.8 161.54 14 1.0 96.5 157.40 15 1.0 222.1 152.02 16 1.0 642.2 149.08 17 1.0 1441.3 175.34 18 1.0 1057.0 204.59

9 2.0 7.1 159.80 10 2.0 10.1 157.22 11 2.0 15.0 153.94 12 2.0 23.4 149.62 13 2.0 38.7 143.75 14 2.0 69.0 135.33 15 2.0 134.7 122.17 16 2.0 295.5 95.21 17 2.0 370.7 15.03 18 2.0 400.4 291.17

9 3.0 6.3 150.14 10 3.0 8.7 146.44 11 3.0 12.4 141.76 12 3.0 18.2 135.67 13 3.0 27.5 127.43 14 3.0 42.6 115.69 15 3.0 65.6 97.45 16 3.0 91.4 65.78 17 3.0 99.9 12.48 18 3.0 97.5 312.98

9 4.0 5.4 140.97 10 4.0 7.2 136.32 11 4.0 9.7 130.51 12 4.0 13.4 123.05 13 4.0 18.4 113.18 14 4.0 25.1 99.57 15 4.0 32.6 79.87 16 4.0 37.9 50.43 17 4.0 39.2 9.15 18 4.0 38.8 324.81

9 5.0 4.5 132.37 10 5.0 5.8 126.96 11 5.0 7.4 120.29 12 5.0 9.6 111.90 13 5.0 12.2 101.06 14 5.0 15.1 86.71 15 5.0 17.6 67.22 16 5.0 19.1 40.73 17 5.0 19.4 7.18 18 5.0 19.3 332.01

9 6.0 3.7 124.39 10 6.0 4.5 118.40 11 6.0 5.6 111.13 12 6.0 6.8 102.15 13 6.0 8.2 90.81 14 6.0 9.5 76.43 15 6.0 10.4 57.80 16 6.0 10.9 34.10 17 6.0 11.0 5.91 18 6.0 10.9 336.77

9 7.0 3.0 117.03 10 7.0 3.5 110.63 11 7.0 4.2 102.96 12 7.0 4.9 93.66 13 7.0 5.6 82.25 14 7.0 6.2 68.10 15 7.0 6.6 50.58 16 7.0 6.8 29.30 17 7.0 6.8 5.02 18 7.0 6.8 340.15

As previously stated, my invention is concerned primarily with the mutual interaction of two separately produced magnetic fields to produce a unique and novel resultant magnetic field configuration in the volume of the furnace near the electrode, or at the arcing surface of the electrode, or in the space between the electrode and the melt, or within the melt, or to control the arc in one or more fashions simultaneously, or to reduce electrode tip erosion, or to produce one or more simultaneous stirring motions in the melt, or to provide improved grain structure in an ingot formed in the furnace, or to increase the feed rate, or to provide preferential alloy formation, or other new and desirable results to be described hereinafter.

Referring now to FIG. 2, to assist in understanding the invention there is shown the magnetic field generated by a concentrated magnetic field coil 26 disposed outside of the furnace along or adjacent to the outside wall thereof, the inside wall of the furnace may be indicated by the line 23, the concentrated field coil 26 having an axial location substantially the same as the axial location of the field coil 22 within the electrode 20, it being understood, however, that for purposes of explanation, the electrode field coil 22 is not energized in FIG. 2 and sets up no magnetic field of its own.

The wall 23 of the furnace is shown as a defining line for convenience.

The line 21 represents the centerline of the furnace which in this case coincides with the longitudinal axis of the electrode 20 but is not limited to so coinciding. Lines of the magnetic field of coil 26 are shown, for example, at 25. It is understood that in both FIGS. 1 and 2, the wall of the furnace may be comprised of a substantially nonferromagnetic material such as copper, or copper with a nonferromagnetic stainless steel outer lining or of any material not substantially altering the magnetic fields unless proper adjustments are made.

In FIG. 2, the axially spaced lines 24 represent ampere turn lines showing the portion of the total magnetomotive force of coil 26 required to generate the magnetic field between any of the adjacent lines 24.

I have employed a special computer to calculate the magnitude of the field vector in the unit of the gauss and the angle of the field vector with respect to the longitudinal axis measured in degrees at 80 radially spaced and axially spaced positions within the volume of the furnace, or more precisely, that portion of the volume of the furnace of most interest and including the portion thereof adjacent the arcing surface of electrode 20 and extending for a number of inches beneath the arcing surface, and these measurements are set forth in Table II which follows entitled "Magnetic Field Values for Concentrated Furnace Coil". Table II shows that the furnace coil excitation was +7,000 ampere turns, that the furnace coil location in an axial direction is extended from substantially 15.5 to 17.5 inches from the plane of the bottom of the furnace, and in a radial direction from substantially 13.5 to 14.5 inches from the center line 20a. The magnetic field is measured at numerous radial positions from the axis 21 at 0.1 inch, 1.0 inch, 2.0 inches, 3.0 inches, 4.0 inches 5.0 inches, 6.0 inches and 7.0 inches in each of 10 planes extending substantially perpendicular to the axis having axial positions of 9 inches through 18 inches in 1 inch increments, respectively. It is to be noted that the axial position of the concentrated coil 26 is from 15.5 to 17.5 inches from the bottom plane of the furnace, that the field measurements at axial positions of 15 inches, 14 inches, 13 inches, 12 inches, 11 inches, 10 inches, and 9 inches are all axially below the concentrated coil, that the distance below the coil increases as the axial position in inches decreases, and that the zero axial position may or may not be shown in the portion of the field illustrated since the magnetic field at zero axial inches is of no significant interest.

It is further to be noted that the furnace coil location in axial and radial inches corresponds to current filaments which lie at substantially the center of the outermost, innermost, uppermost and lowermost turns of coil 26 and that the dimensions of the coil 26 may be slightly greater than would be inferred from the dimensions appearing in the table.

It is further to be noted that a horizontal plane at substantially 17.0 axial inches above the bottom of the furnace would pass through both coils 26 and 22.

The field is exemplary only as one produced by a certain coil excitation which may be varied at will by a user of the apparatus.

TABLE II

MAGNETIC FIELD VALUES FOR CONCENTRATED FURNACE COIL

Furnace Coil Excitation (+) 7000 amp. Turns Furnace Coil Location Axial -- 15.5 to 17.5 inches Radial -- 13.5 to 14.5 inches

Field Points

Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (Gauss) (Degrees) 9 0.1 84.7 359.75 10 0.1 92.2 359.77 11 0.1 99.6 359.79 12 0.1 106.5 359.82 13 0.1 112.6 359.86 14 0.1 117.6 359.90 15 0.1 121.1 359.94 16 0.1 122.9 359.98 17 0.1 123.0 0.02 18 0.1 121.2 0.06

9 1.0 84.8 357.45 10 1.0 92.3 357.67 11 1.0 99.8 357.92 12 1.0 106.8 358.22 13 1.0 113.0 358.57 14 1.0 118.0 358.95 15 1.0 121.6 359.36 16 1.0 123.4 359.78 17 1.0 123.4 0.02 18 1.0 121.6 0.06

9 2.0 84.9 354.86 10 2.0 92.7 355.28 11 2.0 100.3 355.79 12 2.0 107.5 356.40 13 2.0 113.9 357.09 14 2.0 119.2 357.86 15 2.0 122.9 358.69 16 2.0 124.8 359.56 17 2.0 124.8 0.44 18 2.0 122.9 1.31

9 3.0 85.2 352.16 10 3.0 93.2 352.79 11 3.0 101.2 353.56 12 3.0 108.8 354.47 13 3.0 115.6 355.52 14 3.0 121.2 356.70 15 3.0 125.2 357.98 16 3.0 127.3 359.32 17 3.0 127.3 0.68 18 3.0 125.2 2.02

9 4.0 85.5 349.31 10 4.0 94.0 350.13 11 4.0 102.4 351.15 12 4.0 110.6 352.38 13 4.0 118.0 353.81 14 4.0 124.2 355.43 15 4.0 128.6 357.19 16 4.0 131.0 359.05 17 4.0 131.0 0.95 18 4.0 128.6 2.81

9 5.0 85.9 346.24 10 5.0 94.8 347.24 11 5.0 104.0 348.51 12 5.0 113.0 350.06 13 5.0 121.2 351.89 14 5.0 128.2 353.98 15 5.0 133.3 356.30 16 5.0 136.0 358.75 17 5.0 136.0 1.25 18 5.0 133.3 3.70

9 6.0 86.2 342.89 10 6.0 95.8 344.04 11 6.0 105.8 345.53 12 6.0 115.9 347.40 13 6.0 125.4 349.66 14 6.0 133.6 352.30 15 6.0 139.7 355.24 16 6.0 142.9 358.39 17 6.0 142.9 1.61 18 6.0 139.7 4.76

9 7.0 86.3 339.18 10 7.0 96.8 340.43 11 7.0 108.0 342.12 12 7.0 119.5 344.30 13 7.0 130.7 347.01 14 7.0 140.6 350.25 15 7.0 148.2 359.94 16 7.0 152.3 357.94 17 7.0 152.3 2.06 18 7.0 148.2 6.06

Particular reference is made now to FIG. 3, showing an embodiment of my invention in which both the concentrated coil 26' external to the furnace and the coil 22' within the electrode tip are energized by direct current, the polarity of the two magnetic fields being such that the two fields are in opposition to each other on the centerline or electrode axis 20b, within the space of the furnace and within the melt, it being assumed that the melt may be comprised of nonmagnetic or diamagnetic material or that if composed of magnetic material, the temperature of the material is above the Curie point.

The word "centerline" is used merely for convenience. The axis of the fields should be substantially parallel, but may depart radially from each other by the order of 20 percent of the mold diameter without adjustment of the two fields. Concentrated coil 26' at or near furnace wall 23 has a coil excitation of value (+) 4,000 ampere turns, while the field coil 22' in the electrode tip 20 has an excitation of (-) 8,000 ampere turns. It should be understood that the excitation values stated with respect to the field pattern shown in FIG. 3 and in other magnetic field patterns of other figures, while corresponding to the computed data of the respective tables, are not limiting, are not critical except under certain conditions, and that in most if not all cases, the relative excitations of the coils producing the magnetic fields may be varied with respect to each other within limits so long as a resultant magnetic field configuration is produced which accomplishes the objective which a user of the furnace apparatus desires to accomplish. Coil 26' has a variable resistor 26a in series therewith, symbolizing means for adjusting the value of the energizing current. Coil 22' in the electrode tip has variable resistor 22a in series therewith symbolizing means for adjusting the value of the energizing current. Variable resistor 20d symbolizes means for adjusting the arc current.

With further reference to FIG. 3, lines 101 to 108, inclusive, and line 117 represent magnetic field lines; lines 109, 110, 111, 112, 116 and 119 are ampere turn lines.

The field of FIG. 3 satisfies Maxwell's equations; the gradient may be observed, and the divergence is zero.

Horizontal planes passing through each of the points e of the field line 102 could be used to define a portion of the furnace volume where mutual interaction of the two fields is greatest as indicated by the radius of curvature of field line 101 between points c--c.

In FIG. 3, with reference to the following Table III entitled "Magnetic Field Values for Concentrated Furnace Coil and Electrode Field Coil Canceling on the Centerline," zero axial inches may correspond to the bottom of the mold having the wall 23, and assuming a 2 inch arc length the arcing zone would be located approximately 13.8 to 15.8 axial inches above zero axial inches.

TABLE III

MAGNETIC FIELD VALUES FOR CONCENTRATED FURNACE COIL AND ELECTRODE FIELD COIL CANCELING ON THE CENTERLINE

Furnace Coil Excitation (+) 4000 amp. turn Furnace Coil Location Axial -- 15.5 to 17.5 inches Radial -- 13.5 to 14.5 inches Electrode Field Coil Excitation (-) 8000 amp. turn Electrode Field Coil Location Axial -- 16.6 to 17.8 inches Radial -- 1.325 to 1.675 inches

Field Points

Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (gauss) (degrees) 9 0.1 40.5 359.90 10 0.1 41.2 0.03 11 0.1 39.2 0.30 12 0.1 31.6 1.10 13 0.1 11.5 7.75 14 0.1 41.2 174.22 15 0.1 188.7 176.33 16 0.1 595.6 177.21 17 0.1 1092.0 179.50 18 0.1 837.1 182.00

9 1.0 40.8 359.89 10 1.0 41.8 0.11 11 1.0 40.4 2.58 12 1.0 34.7 9.06 13 1.0 22.9 37.29 14 1.0 41.8 122.11 15 1.0 163.5 140.77 16 1.0 582.7 145.54 17 1.0 1371.0 175.09 18 1.0 993.8 206.21

9 2.0 41.7 357.41 10 2.0 43.4 359.43 11 2.0 43.7 3.14 12 2.0 41.9 10.97 13 2.0 39.1 30.11 14 2.0 49.7 67.57 15 2.0 112.4 90.75 16 2.0 297.1 81.39 17 2.0 440.0 13.19 18 2.0 429.4 300.02

9 3.0 42.9 355.33 10 3.0 45.6 357.66 11 3.0 47.7 1.42 12 3.0 49.3 7.82 13 3.0 51.9 18.76 14 3.0 61.2 34.19 15 3.0 88.8 44.80 16 3.0 137.7 36.80 17 3.0 171.8 7.51 18 3.0 154.2 333.50

9 4.0 44.2 352.64 10 4.0 47.9 354.93 11 4.0 51.5 358.22 12 4.0 55.4 2.93 13 4.0 60.6 9.22 14 4.0 69.2 16.05 15 4.0 84.1 19.77 16 4.0 102.8 15.78 17 4.0 113.8 3.77 18 4.0 106.8 349.87

9 5.0 45.4 349.40 10 5.0 49.9 351.52 11 5.0 54.8 354.32 12 5.0 60.0 357.85 13 5.0 66.3 1.92 14 5.0 74.1 5.74 15 5.0 83.6 7.80 16 5.0 92.8 6.65 17 5.0 97.0 2.43 18 5.0 93.1 357.47

9 6.0 46.4 345.71 10 6.0 51.7 347.64 11 6.0 57.4 350.08 12 6.0 63.7 353.00 13 6.0 70.5 356.21 14 6.0 77.9 359.25 15 6.0 85.1 1.49 16 6.0 90.7 2.40 17 6.0 92.6 2.12 18 6.0 89.6 1.48

9 7.0 47.2 341.60 10 7.0 53.1 343.36 11 7.0 59.6 345.59 12 7.0 66.8 348.29 13 7.0 74.4 351.34 14 7.0 81.9 354.51 15 7.0 88.5 357.53 16 7.0 92.9 0.12 17 7.0 93.8 2.27 18 7.0 90.8 4.18

In Table III it will be noted that magnetic field is not a scalar quantity which can be fully described by a number defining intensity in gausses; it is also essential to know the angle to the axis; the rate of change of the vector field and its divergence can be ascertained from a careful study of Table III.

The divergence of the vector field may be thought of as a scalar quantity; utilizing the operator "nabla" or del, the gradient within the vector field may be generally obtained.

With the two fields in polarity opposition as shown in FIG. 3, an art extending axially between the electrode tip and the melt must pass through magnetic field lines exemplified by line 117, which magnetic field lines exert a strong force on the arc and cause the arc to rotate substantially continuously in generally repetitive paths around the closed track formed by the annular arcing surface of the electrode.

A field configuration such as that shown in FIG. 3 is especially useful when starting up the furnace because a strong rotating force is exerted on the arc, which may be expected to usually start up in a restricted mode, to cause it to rotate rapidly and thereby avoid substantial erosion of material from the arcing surface. It will be understood that at furnace start up the material within the furnace is not necessarily in a molten state; the surface may be chunky, irregular, and tend to have stubs to which the arc would attach, tending to prevent rotation of the arc. It will be understood that during a metal treating process, considerations of arc movement and arc control vary from time to time during the various stages of the process. At start up and for some minutes thereafter arc rotation on the electrode is one of the, if not the, most important considerations except perhaps where the arc is diffused. As the material initially in the furnace becomes molten and assumes a liquid state considerations of focusing the arc over a certain portion or portions of the surface of the melt to obtain maximum heating efficiency, and considerations of the stirring of the melt by a magnetic field and by the motion of the arc become important to produce a homogeneous melt and a desired grain structure. Where materials are added to the melt to produce alloys, other considerations enter, as will become more clearly apparent hereinafter, and the rate of feed material and the position of material feeds also become important after the initial start up phase is completed.

It will be understood that all molds and furnace vessels of all embodiments may if desired have one or more insulating materials placed axially in extending wall portions or strips to assist in preventing the occurrence of eddy currents in the wall of the mold.

Particular reference is made now to FIG. 4 which shows a resultant magnetic field when the field of the concentrated coil 26" and the electrode tip coil 22" are in magnetic polarity adding on the same axis or on parallel axes where the electrode axis could be slightly displaced from the coil 26" axis. It is generally understood for all the configurations shown that the electrode is not necessarily required to be concentric with or at its axis be parallel to the external coil(s). Reasonably, substantial variations produce rather minor effects on accomplishing the purpose of a particular configuration. The magnetic field pattern shown in FIG. 4 is produced by coil excitations in which that of the concentrated furnace coil 26" is (+) 4,000 ampere turns and the tip field excitation is 8,000 ampere turns. The two (+) signs in parenthesis indicate that the two magnetic fields are in magnetic polarity adding. Precise computed magnetic field strengths giving the magnitude of the field vector in gausses and the angle of the field vector to the axis in degrees at 80 points are shown in Table IV following, entitled "Mangetic Field Values for Concentrated Furnace Coil and Electrode Field Coil Adding on the Centerline."

The phrase "on the centerline" is not intended to be limiting or critical, it is merely a convenient descriptive device, the true meaning of which is elaborated herein.

TABLE IV

MAGNETIC FIELD VALUES FOR CONCENTRATED FURNACE COIL AND ELECTRODE FIELD COIL ADDING ON THE CENTERLINE

Furnace Coil Excitation (+) 4,000 amp. turns Furnace Coil Location Axial -- 15.5 to 17.5 inches Radial -- 13.5 to 14.5 inches Electrode Field Coil Excitation -- (+) 8,000 amp. turns Electrode Field Coil Location Axial -- 16.6 to 17.8 inches Radial -- 1.325 to 1.675 inches Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (gauss) (degrees) 9 0.1 56.3 359.64 10 0.1 64.2 359.60 11 0.1 74.7 359.53 12 0.1 90.2 359.38 13 0.1 117.3 359.08 14 0.1 175.4 358.57 15 0.1 327.0 357.86 16 0.1 736.0 357.74 17 0.1 1232.1 359.55 18 0.1 975.5 1.73

9 1.0 56.1 356.41 10 1.0 63.8 356.07 11 1.0 73.8 355.37 12 1.0 88.1 353.97 13 1.0 112.2 351.24 14 1.0 161.1 346.24 15 1.0 285.6 338.44 16 1.0 703.8 332.02 17 1.0 1511.6 355.56 18 1.0 1120.8 23.15

9 2.0 55.4 352.95 10 2.0 62.7 352.41 11 2.0 71.5 351.31 12 2.0 83.0 349.10 13 2.0 99.8 344.77 14 2.0 127.8 336.45 15 2.0 183.0 320.83 16 2.0 310.8 288.42 17 2.0 302.4 199.17 18 2.0 382.3 101.22

9 3.0 54.6 349.68 10 3.0 61.2 349.16 11 3.0 68.7 348.10 12 3.0 77.2 345.98 13 3.0 86.9 341.89 14 3.0 97.3 334.18 15 3.0 104.8 319.80 16 3.0 91.3 292.71 17 3.0 32.3 219.90 18 3.0 74.0 86.08

9 4.0 53.7 346.57 10 4.0 59.8 346.30 11 4.0 66.2 345.66 12 4.0 72.6 344.35 13 4.0 78.2 341.93 14 4.0 80.9 337.88 15 4.0 76.5 332.22 16 4.0 59.1 329.01 17 4.0 36.4 352.10 18 4.0 49.1 319.36

9 5.0 52.9 343.53 10 5.0 58.7 343.60 11 5.0 64.6 343.59 12 5.0 70.1 343.38 13 5.0 74.2 342.93 14 5.0 75.6 342.45 15 5.0 72.4 342.97 16 5.0 64.8 347.41 17 5.0 58.5 359.29 18 5.0 60.6 13.32

9 6.0 52.2 340.38 10 6.0 58.0 340.83 11 6.0 63.9 341.45 12 6.0 69.4 342.26 13 6.0 73.7 343.40 14 6.0 76.0 345.16 15 6.0 75.6 348.20 16 6.0 73.1 353.40 17 6.0 70.7 0.95 18 6.0 70.3 8.95

9 7.0 51.6 336.97 10 7.0 57.6 337.74 11 7.0 63.9 338.87 12 7.0 70.1 340.49 13 7.0 75.4 342.74 14 7.0 79.3 345.84 15 7.0 81.2 350.03 16 7.0 81.3 355.45 17 7.0 80.2 1.80 18 7.0 78.6 8.23

In FIG. 4 rheostat 26a in one of the leads to the concentrated field coil 26" symbolizes means for adjusting the strength of the magnetic field produced by coil 26", and rheostat 22a in one of the leads to the field coil 22" in the electrode tip symbolizes means for adjusting the strength of the magnetic field produced by coil 22". Variable resistor 20d symbolizes means for adjusting the value of the arc current.

The fields adding arrangement of FIG. 4 as well as that of FIG. 8 shown hereinafter, is especially valuable for vacuum furnace operation, where the arc may have a tendency to go into glow, with electrons having a tendency to pass from the electrode to the wall of the vessel or mold, through a substantial volume therebetween, rather than pass from electrode to melt. It has been found in practice that in some other magnetic field configurations the arc would glow at pressures in the order of 0.700 to 0.800 torr; but with the tip and external fields adding, the furnace apparatus could be operated to 0.25 millitorr without glow discharge, and to 0.06 torr an arc length of three inches could be obtained, whereas the standard arc length theretofore was one inch.

It was further found that arcing to graphite with tip field only went into glow at about the pressure range of 0.700 to 0.800 torr.

Particular reference is made now to FIG. 4A, illustrating one possible switching circuit arrangement for reversing the polarities of the fields produced by coils 26' and 26", and solenoid generated fields hereinafter to be described, and also reversing the magnetic polarity of the magnetic fields produced by the electrode tip coils 22' and 22" and other tip magnetic field producing coils hereinafter to be described, and also a circuit for reversing the magnetic polarity of the field generated in the tip with respect to that of the externally generated field, so that these fields may be selectively adding or selectively opposing in accordance with the wishes of a user of the apparatus. In FIG. 4A, sources of direct current potential are shown at 65 and 66, electrically connected by polarity reversing switches 67 and 68 respectively to magnetic field coils 69 and 70 respectively, coil 69 symbolizing either a solenoid or a concentrated coil, and coil 70 symbolizing a magnetic field generating coil in the tip of an electrode. The two switches may be ganged or may be separately manipulated. Coils 69 and 70 may also symbolize the solenoid and concentrated coil of FIGS. 10 and 11, hereinafter to be described. A similar circuit may be employed with the tip field coil 351 of FIG. 10.

With further reference to the operation of an electric arc furnace employing a field coil in the electrode and an externally produced magnetic field, and having means for selectively causing the two fields to add "on the centerline" of the furnace or for causing the two fields to be in magnetic polarity opposition, the advantages, procedures, and adjustments will be described with reference to a typical melting operation.

During the start-up portion of the melt cycle, the electrode may be brought into contact with a striker placed in the bottom of the mold containing the material to be melted. The striker generally consists of chunks of scrap and other material usually of approximately 30 percent density by volume. Very often when "striking" an intense electric arc forms between the tip and the solid material, or several arcs form, causing damage to the tip if the arcs are not caused to move rapidly over the arcing surface by the magnetic field. In prior art furnace processes and methods, this problem is more severe since most of the initially melted material flows away from the melt surface leaving appendages of solid material protruding above the melt which cause local arcing. Severe damage and even punctures or burn throughs of the arcing surface of the electrode have been the result of these conditions.

In the embodiments of my invention which employ a concentrated external field coil, to wit, FIGS. 2, 3 and 4, the magnetic field configuration illustrated in FIG. 3 overcomes the problems discussed above and prevents damage to the electrode. It will be noted from FIG. 3 that a magnetic field line 117 substantially encircles the arcing surface of the electrode in the view shown partially in cross section, it being understood that lines similar to the line 117 extend in a similar manner around the entire periphery of the annular tip. The magnetic field illustrated by line 117 and other magnetic field lines, not shown for convenience of illustration, must be traversed by an arc between the electrode and the melt, and the magnetic field lies in such a direction with respect to the path of the arc current that a force is exerted on the arc, a strong force which causes the arc to rotate rapidly around the electrode and to move in substantially repetitive annular paths between the electrodes and the melt, preventing damage to the electrode and substantially reducing the possibility of burn through. Similar remarks apply to FIGS. 5, 6 and 7.

Referring again to FIG. 3, the magnetic field configuration of FIG. 3 is vastly superior at start up to a magnetic field produced in a furnace merely by a solenoid or compact coil mounted near the furnace wall, either inside or outside of the furnace. Where only a solenoid or compact coil field is used and there is no field generated in the electrode tip, damage to the electrode occurs relatively frequently and can only be avoided by very careful start up procedures. On the other hand, in my invention as exemplified by the field configuration of FIG. 3, for example, a strong arc-rotating force is produced by the magnetic field which follows the contour of the arcing surface and exerts a strong rotating force on the arc without tending to concentrate the arc on the inside diameter or outside diameter of the electrode. During these conditions, the arc struck to melt 50 is generally substantially perpendicular to the electrode tip surface and to the magnetic field lines if the combination of fields is similar to that shown in FIG. 3. The arc is, therefore, caused to move generally circumferentially around the electrode, more evenly distributing the heat to the electrode and, therefore, preventing damage and reducing erosion of tip material.

Summarizing, it is seen that the effect of the two fields in opposition in FIG. 3, that is the effect of the field from the concentrated remotely situated coil on the field generated by the electrode field coil is to shape the resultant magnetic field so that it is more nearly parallel to the electrode tip or arcing surface, or more precisely, that the magnetic field lines follow paths similar to the contour of the electrode surface, to restrict the arc fringing effect.

After a large molten pool is formed, pool rotation becomes an important factor in the operation of the furnace. Pool rotation is governed by the direction and strength of the field or fields, the area of the surface through which field lines extending from the tip pass, the depth below the source of the pool to which magnetic field lines extend, the arc current level, the arc current direction, and distribution within the melt, and the temperature and fluidity of the pool. The direction of pool rotation may be reversed by utilizing the polarity-reversing circuit of FIG. 4A. With further reference to the phenomena of pool rotation, where the tip field and external field are in polarity opposition, within the projected area of the tip field, the force may create a pool center which rotates in one direction and a pool perimeter area which rotates in the opposite direction, as has been verified by experiments.

It will be understood that components of the resultant magnetic field exert forces on current paths extending from the arc spot site on the surface of the pool to the wall of conductive material of the mold which cause stirring or a swirling motion. The mixing may be at least, in part, proportional to the magnitude of the component of current perpendicular to the magnetic field vector. On the other hand, the center of the pool may be more greatly under the influence of the magnetic field generated in the electrode tip, so that the center of the pool may swirl in one direction while the perimeter portion of the pool outside of the influence of the tip field may swirl in the opposite direction.

It has also been experimentally observed that diffused arc modes can occur at atmospheric pressure, with the added external field.

The operating conditions described just heretofore generally relate to operation of the furnace at atmospheric pressure or at reduced pressure.

When the pressure in the furnace is reduced, and all embodiments of my invention include vacuum furnace operation at any value of reduced pressure, the arc discharge is more likely to follow magnetic field lines rather than take the shortest path to the electrode. If there is a high magnetic field strength near the electrode inside diameter (FIG. 1); the arc generally emanates from the arcing surface area between the mean and inside diameters of the tip, because of the higher magnetic field strength in that region, instead of from all the arcing surface area. This greatly increases the local heat flux concentration in that area which in general tends to decrease tip life. The greater the tip field strength the greater the arc concentration near the inside diameter.

Also, if the external field has a high value, the excitation of the external field coil or solenoid must be reduced in strength a reasonable time after the pool has been formed and rendered completely molten in order to reduce the rotational forces on the pool. If the external field is not reduced, the resultant vortex formed becomes so great that the arc is not stable, and it causes erratic operating conditions generally. Great vortex and resulting arc instability are avoided in my invention, as shown in FIGS. 3 and 4, by adjusting the resistance value of 26a. It will be understood, as previously pointed out, that the magnetic field plots of FIGS. 3 and 4 represent the resultant magnetic field produced by mutual interaction of the concentrated external field coil and the tip field coil which have certain dimensions given heretofore, certain positions with respect to each other given heretofore, and certain relative excitations in ampere turns. As previously stated, the fields of FIGS. 3 and 4 are exemplary, and adjustment of one or both of the generated magnetic field strengths is contemplated to accomplish whatever purpose or purposes are desirable during a particular phase or stage of the complete melting and metal treating or ingot forming operation.

With the two magnetic fields in polarity adding, when the excitation of the external coil is substantially reduced relative to the tip coil excitation, the arc tends to follow lines of the magnetic field extending between tip and melt, which has a substantial component directly to the mold wall. This causes the arc to go into glow at much lower arc voltages than would otherwise occur, and also the current component to the mold wall tends to weld material to the collar and prevent feed material from reducing the pool. By carefully adjusting the strengths of the two fields with respect to each other, the disadvantages described in the last two sentences hereinabove may be avoided or substantially reduced.

By decreasing the strength of the tip field with respect to the external field, the fringing component of magnetic field, that is, the number of magnetic lines of flux which leave the tip and reach or fringe toward the mold wall, is reduced (see FIGS. 5 to 7), there is less current conduction to the mold, and the arc voltage can be substantially increased from, for example, 40 volts to, for example, 50 volts, increasing the power input and improving the melting conditions. Generally speaking this results in an improved feed rate, which may reach 15 pounds per minute as compared to 4 pounds per minute, which is a typical feed rate in prior art furnaces and processes. As previously stated, when "starting up" a beginning furnace operation, it is desirable in some instances, to operate with a strong resultant magnetic field at the electrode to prevent the arc from flashing to the side of the electrode while striking the arc, with resulting severe damage and even failure of the electrode.

As will be seen more clearly hereinafter, the electrode should not be operated, except under unusual circumstances, without any magnetic field present because any small disturbing or perturbing condition (for example, stray fields from the leads) will cause the arc to wander to the side of the electrode causing damage to either the electrode or the mold.

With the fields in polarity adding as shown in FIG. 4, an additional focusing effect is provided, complementing or adding to the focusing effect of the externally generated field. With the field arrangement of FIG. 4 as contrasted with that of FIG. 3, for any given value of the component of the magnetic field external to the electrode contributing to the stirring of the melt, a stronger arc-focusing field between tip and melt is obtained under the tip. It will also be noted by comparing the field of FIG. 3 with that of FIG. 4 that the arc 27 to melt 28 of FIG. 4, strongly tends to follow magnetic field lines, particularly magnetic field lines extending in directions similar to the lines designated 55 and 56, so that the arc acts over a larger area of the pool, that is, an area of the surface of the pool larger in diameter than the mean diameter of the electrode.

In FIG. 4, additional magnetic field lines are represented by lines 57 to 63; the lines 64 to 68 inclusive represent ampere turn lines.

Additionally summarizing, if there is a problem with erosion of material from the arcing surface by arc action thereon, a resultant field configuration with the fields in polarity opposition on the centerline such as that shown in FIG. 3 can be profitably maintained throughout an entire "run," that is from the time an arc is started to the material initially placed in the furnace, until the formation of an ingot is completed by continually adding material to be melted and maintaining on top a molten pool as the bottom portion of the ingot cools and hardens, material being added until the ingot attains the desired length.

Where there is no substantial erosion problem of material from the arcing surface, as for example when melting stainless steel, the resultant magnetic field configuration of FIG. 4 (tip and external fields adding) may be desirable throughout the entire run, except perhaps for a few minutes during and after furnace start up, at which start up the arc may be in a restricted mode and later go into a diffused mode or when "stubs" occur in the feed material comprised of heterogeneous scrap, one or more pieces of which might be expected to project from the pool, or when the material being melted emits nonmetallic vapors which may deposit on the electrode.

As will be understood by those skilled in the art, certain materials when melted to form an ingot give off vapors which tend to form a deposit on the relatively cool arcing surface of the electrode. On such occasions, it may be desirable to use a field-opposing resultant field which may exert a strong moving and rotating force on the arc, the relative field strengths of the independently generated magnetic fields being adjusted as desired.

In FIGS. 11 and 12, hereinafter to be described, symbolic feed means are shown; it is to be clearly understood that all embodiments of the invention including those of FIGS. 2-8, 10-12 and FIGS. 14-18 may include feed means for forming an ingot, not shown merely for convenience of illustration, and that in many cases a feed rate substantially in excess of those presently obtainable is permitted.

In FIGS. 11 and 12, hereinafter to be described, vacuum pump means are shown for reducing the pressure in the furnace and maintaining it at a desired reduced pressure, or for producing other controlled atmospheric conditions in the furnace; it is to be understood that all embodiments of the invention including those of FIGS. 2-8, 10-12 and FIGS. 14-18 may include vacuum pump means usable at the will of the operator for maintaining a reduced pressure or controlled atmosphere in the furnace, the vacuum pump not being shown in FIGS. 2-8 and 14-18 merely for convenience of illustration.

At reduced pressure, for example a pressure in the order of 20 mm of mercury, the arc may frequently go into a diffused mode of operation, and the entire discharge may rotate under the influence of a resultant field similar to that shown in FIG. 3.

All embodiments of the invention, illustrated in FIGS. 1-12 and 14-18 may include means, not shown for convenience of illustration, for continually adjusting the position of the electrode or electrodes as the level of the melt changes as additional material is added to form the ingot.

For a better understanding of the invention, particular reference is made to FIG. 13, representing a plane in a symmetrical magnetic field through a generally cylindrical volume of space.

FIG. 13 may be understood with reference to the table hereinbelow, closely defining one field of FIG. 5, hereinafter described, in which line 172 very nearly shows the uniformity of a field in a solenoid.

Radius (Inches) Gauss (Axial) Gauss (Radial) 0.1 55.83 0.08 1.0 56.16 0.77 2.0 57.06 1.27 3.0 58.26 1.39 4.0 59.48 1.16 5.0 60.58 0.73 6.0 61.51 -0.24 7.0 62.29 -0.23

The field described above is very nearly uniform, varying only 11.6 percent in magnitude from the value on the axis between the axis and one inch from the coil.

With further reference to FIG. 5 it will be understood that the lines 138, 221, 222, 223, 224, 225, 226 and 227 represent magnetic field lines. The spacing between the lines represent volumes of constant magnetic flux; therefore, the amount of flux represented by the space between lines 221 and 222 is the same as that between 222 and 223, and so forth. This is not true, however, within line 221 or outside of line 227 due to the method of computing the fields. The relationship between the flux density given in the above table and the flux is represented by the equation .phi. = BA, where .phi. represents flux, B is the flux density and A is the area. FIG. 13 represents a plot of the spacing of the lines for a flux density shown for a portion of FIG. 5 if B is constant, recalling that A is a function of the square of the radius for a circle. Therefore, relating to FIG. 5, it is understood that the gradient of the magnetic field is represented by the changing in spacing of the lines, as modified by the .phi. = BA equation.

It will be understood that the various magnetic field patterns of the various figures may represent resultant field patterns in which the laws governing flux density, as seen in FIG. 13, are modified with respect to a plane passing perpendicular to the axis of some particular furnace at some particular axial position. FIG. 13 is merely typical and exemplary.

As previously stated, while a strong field in the electrode tip region is desirable at start up of the furnace to exert a strong moving or rotating force on the arc, after start up, the electrode tip field may be reduced and the externally produced field may be substantially relied upon to maintain the arc between the electrode and the surface of the melt, to rotate the melt, and to perform other desirable functions.

Particular reference is made to FIG. 9, where a circuit for automatically accomplishing reduction of the tip field at a predetermined time interval after start up, and for automatically maintaining the tip field or restoring it to a pre-existing value under certain conditions is shown. In FIG. 9, the electrode is generally designated 301 and is shown as having a field coil 302 in the tip, electrode 301 being connected by way of lead 303, relay energizing winding 304, switch 305 and lead 303 to a terminal of one polarity of a source of potential, not shown for convenience of illustration, the terminal of other polarity of the source of potential being, for convenience of illustration shown at ground potential and constituting the melt 307 in the furnace shown in fragmentary form, the melt being composed at least partially of conductive material and being connected to ground 308. Arc 309 is shown taking place between electrode 301 and melt 307.

For convenience of illustration, the field coil 302 has one terminal connected to ground 308 and the other terminal thereof connected by way of lead 310 to one contact 311 of a pair of normally open relay contacts of the relay 312, the other contact 313 of the pair being connected by way of lead 314 to a source of potential 315 having the other terminal thereof connected to ground 308, terminals 311 and 313 having variable resistor 316 connected thereacross. It is seen that when the relay 312 is energized, and the substantially no-resistance circuit is closed between contacts 311 and 313, the full potential of source 315 is applied across the field coil 302 in the electrode tip, and a strong magnetic field component is set up which exerts a rotating or moving force on the arc 309. When relay 312 is deenergized and the circuit through contacts 311-313 is open, the potential applied to coil 302 is reduced by the voltage drop in resistor 316 and the ampere turns of excitation of coil 302 is reduced in accordance with the ratio of the value of resistor 316 to total resistance in that circuit, thereby reducing the strength of the electrode tip field with reference to the strength of an externally produced magnetic field within the mold or furnace produced by a solenoid or concentrated coil.

The relay 318 energized from the aforementioned winding 304 has a pair of normally open contacts 319 and 320, contact 319 being connected to the aforementioned lead 314, contact 320 being connected by way of the winding 322 of a relay generally designated 323 to lead 324 which is connected to a source of potential for energizing the solenoid, not shown. The relay generally designated 323 has a pair of normally closed contacts 325 and 326, contact 325 being connected to lead 314, contact 326 being connected by way of winding 327 of still an additional relay generally designated 328 to the aforementioned lead 324; relay 323 has an additional pair of normally open contacts 329 and 330, contact 329 being connected to lead 314, contact 330 being connected to one contact 331 of a pair of normally open contacts of relay 328, the other contact of the normally open pair being 332, contact 332 being connected to one contact 333 of a pair of normally open contacts including contact 334 of a time delay relay generally designated 335, contact 334 being connected to lead 314. The winding 336 of time delay relay 335 has one terminal thereof connected to contact 332 and the other terminal thereof connected to the aforementioned lead 324.

The aforementioned time delay relay 328 has an additional pair of normally open contacts 339 and 340, contact 339 being connected to lead 314, contact 340 being connected to one contact 341 of an additional set of normally open contacts on time delay relay 335, the other contact of the normally open pair of contacts, that is, the contact 342 associated with 341 being connected to lead 314. Contact 341 is also connected to one terminal of the winding of the relay 312, the other terminal of the winding of relay 312 being connected to lead 324 and thence to the previously described source of potential, not shown.

In the operation of the circuit of FIG. 9, the contacts 319 and 320 of relay generally designated 318 close when arc current flows in lead 303. As aforementioned, relay 323 which is energized when arc current begins to flow has normally closed contacts 325-326 which energize the winding 327 of time delay relay 328 on loss of arc current, and has normally open contacts 329-330 which closes in the circuit of time delay relay 335 with arc current. The time delay relay 328 has a preset delay time coming on; this requires that arc current is off for a preset time before the field is actuated. It also has a delay, for example one minute, going off to insure that time delay relay 335 is actuated before the time delay relay 328 goes off. The normally open contacts of relay 328 prevent time delay relay 335 from being actuated by relay 318 until the arc current has been off a preset length of time (for example one minute) before time delay relay 335 recycles. Normally open contacts 339-340 of time delay relay 328 supply current to the winding of relay 312 which increases the current to the electrode field coil 302 to full strength after the arc current has been off a preset length of time.

With reference to time delay relay 335, this relay stays on a preset or predetermined length of time after being actuated; and therefore sets the length of time the field is on at full strength after the arc 309 is initiated. Normally open contacts of time delay relay 335 are provided to permit the relay to hold itself in, such that if the arc goes out momentarily while the field is on, the field will not be completely shut-off, that is the field of tip coil 302. The normally open contact to relay 312 holds the field on at full strength for the preset length of time after the arc is initiated. As aforementioned the relay 312 actuates the field coil current control, switching the excitation of field coil 302 from full excitation to a value of less than full excitation in accordance with the instant value of variable resistor 316 under the control of the operator of the apparatus.

Particular reference is now made to FIG. 5, wherein an electrode generally designated 20 has a magnetic field coil 130 in the tip 131 thereof with leads for energizing the coil including a variable resistor 124 in one of the leads 123, variable resistor 124 providing means for adjusting the current in the coil and adjusting the excitation thereof. It will be understood that the tip has passageways therein for the flow of cooling fluid for conducting heat flux from the arcing surface of the tip, these passageways not being shown for convenience of illustration, and that the supporting column of electrode 20 has fluid inlet and fluid outlet passageways for conducting fluid to and from the passageway in the tip. The electrode 20 is generally axially positioned within a mold generally designated 133. Mold 133 may be cylindrical in shape, and may be composed of nonmagnetic material such as copper, or composed of copper with a nonmagnetic stainless steel outer lining. Adjacent the mold on the outside thereof is a solenoid 132 having one lead 125 connected at the lower end thereof, and the lead 126 with variable resistor 127 in series therein connected to the upper end thereof, variable resistor 127 symbolizing means for adjusting the strength of the magnetic field generated by the solenoid 132. It is to be understood that the magnetic field configuration generated by the solenoid is symmetrical, only one-half of the solenoid, one-half of the mold, and one-half of the electrode being shown for simplicity of illustration. Lines 134 to 137 inclusive are ampere turn lines; lines 221-227 are magnetic field lines. Lines 171 to 178 inclusive are additional ampere turn lines representing the portion of the total magnetomotive force required to produce that much flux in that area in ampere turns per inch. Other ampere turn lines are shown at 194. If desired the space between the lines 171-178 could be expressed in percentage of the total magnetomotive force. Mold 133 is illustrated as containing a melt with an upper surface 122 which is somewhat bowl shaped indicating the effect of swirling or stirring motion of the melt produced by the magnetic field or fields under certain conditions. Arc 121 takes place from the tip 131 to the surface 122 of the melt. Magnetic field lines which are shown at 138 as well as the previously mentioned lines 221 to 227 inclusive give an indication of the flux density and the direction of the flux. It is noted that the magnetic field may be thought of as curvilinear, that field line 138 follows very closely the contour of the arcing surface of the tip 131 and may be thought of as substantially parallel thereto as described previously in the background. The unique field pattern of FIG. 5 was produced with a tip field coil 130 composed of eight turns and a solenoid 132 composed of 36 turns with the solenoid field coil being energized at 140 amperes per inch or an energization of (+) 5,040 ampere turns and the tip field coil energization being (-) 3,200 ampere turns. The relative excitations of the two magnetic field coils indicate the magnetic field at the tip is in polarity opposition to the magnetic field generated by the solenoid.

The portion of the volume of the furnace designated "T" may be thought of as the portion where the resultant field is most useful.

It will be noted that arc 121 passes, and must pass, through a strong magnetic field exemplified by field line 138 as it extends from the surface of the melt to the arcing surface of the tip, a strong moving and rotating force is exerted on the arc which causes it to move at a rapid rate around the arcing surface of the electrode.

Variable resistor 129 in lead 128 which brings current for the arc 121 symbolizes means for adjusting the value of the arc current.

Table V hereinbelow entitled "Magnetic Field Values for Furnace Solenoid and Electrode Field Coil Canceling on the Centerline" gives values of magnetic field calculations at 88 radially positioned and axially positioned points in the portion of the volume of the mold of interest, that is, adjacent the electrode and immediately beneath the electrode. In the Table, zero axial inches is an arbitrary location and would be outside of the portion of the magnetic field shown, but at zero radial inches would lie on the axial centerline 165 of the electrode and the solenoid 132, which is generally the same as that of the mold.

TABLE V

MAGNETIC FIELD VALUES FOR FURNACE SOLENOID AND ELECTRODE FIELD COIL CANCELING ON THE CENTERLINE

Mold Solenoid Excitation (+) 5040 amp. turn Mold Location Axial -- 12.4 to 47.4 inches Radial -- 8.25 inches Electrode Field Coil Excitation (-) 3200 amp. turn Electrode Field Location Axial -- 30.3 to 31.4 inches Radial -- 1.34 to 1.69 inches

Field Points

Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (gauss) (degrees) 22 0.1 57.1 0 23 0.1 56.9 0.03 24 0.1 55.8 0.08 25 0.1 53.3 0.19 26 0.1 47.9 0.46 27 0.1 35.9 1.43 28 0.1 6.6 20.73 29 0.1 77.2 175.00 30 0.1 282.5 177.45 31 0.1 414.7 180.41 32 0.1 215.5 183.20

22 1.0 57.2 359.93 23 1.0 57.1 0.26 24 1.0 56.2 0.79 25 1.0 54.0 1.78 26 1.0 49.5 4.12 27 1.0 40.4 11.40 28 1.0 26.5 49.64 29 1.0 73.0 125.71 30 1.0 320.6 146.96 31 1.0 544.7 184.07 32 1.0 217.0 219.12

22 2.0 57.6 359.81 23 2.0 57.6 0.40 24 2.0 57.1 1.28 25 2.0 55.7 2.82 26 2.0 53.1 5.97 27 2.0 49.2 13.43 28 2.0 47.0 32.51 29 2.0 69.3 63.04 30 2.0 181.3 60.22 31 2.0 231.7 351.73 32 2.0 135.4 292.06

22 3.0 58.2 359.61 23 3.0 58.4 0.35 24 3.0 58.3 1.37 25 3.0 57.7 2.95 26 3.0 56.9 5.66 27 3.0 56.3 10.51 28 3.0 58.6 18.41 29 3.0 70.0 25.87 30 3.0 93.8 19.90 31 3.0 105.3 356.54 32 3.0 86.7 336.51

22 4.0 59.0 359.33 23 4.0 59.3 0.13 24 4.0 59.5 1.12 25 4.0 59.5 2.46 26 4.0 59.7 4.38 27 4.0 60.5 7.03 28 4.0 63.1 10.01 29 4.0 68.9 11.27 30 4.0 76.4 7.51 31 4.0 79.4 358.84 32 4.0 79.3 351.09

22 5.0 59.7 359.01 23 5.0 60.2 359.81 24 5.0 60.6 0.69 25 5.0 61.0 1.74 26 5.0 61.5 3.00 27 5.0 62.6 4.37 28 5.0 64.5 5.45 29 5.0 67.4 5.37 30 5.0 70.3 3.29 31 5.0 71.3 359.63 32 5.0 69.4 356.25

22 6.0 60.6 358.70 23 6.0 61.1 359.45 24 6.0 61.5 0.22 25 6.0 62.0 1.03 26 6.0 62.6 1.85 27 6.0 63.6 2.59 28 6.0 64.8 3.01 29 6.0 66.4 2.75 30 6.0 67.6 1.64 31 6.0 68.0 359.94 32 6.0 67.2 358.35

22 7.0 61.3 358.42 23 7.0 61.8 359.12 24 7.0 62.3 359.79 25 7.0 62.8 0.42 26 7.0 63.4 1.00 27 7.0 64.1 1.45 28 7.0 64.9 1.65 29 7.0 65.7 1.48 30 7.0 66.4 0.91 31 7.0 66.5 0.09 32 7.0 66.1 359.32

While mounting of the electrode including the magnetic field coil therein in coaxial position with respect to the coil, whether compact or solenoid, generating the external magnetic field, is convenient and preferable, my invention is not limited to precise coaxial mounting. Some departure from coaxial may be allowed within limits which still provide the desired interaction of the two fields with each other with a resultant field configuration which accomplishes the object of a user of the apparatus.

Particular reference is made now to FIG. 6 which shows the resultant magnetic field, which differs from the magnetic field shown in FIG. 5, when the magnetic field coil 130' in the tip and the solenoid 132' have the same relative positions with respect to each other but their relative excitations vis-a-vis each other are changed, so that the magnetic field pattern of FIG. 6 results where the solenoid excitation is still generally (+) 5,040 ampere turns, but the electrode field coil excitation is increased to (-) 8,000 ampere turns from (-) 3,200 ampere turns as shown in FIG. 5.

In FIG. 6 ampere turn lines are shown at 151-153, and 181-183, with other ampere turn lines related to the magnetomotive force being shown at 142. Field coil 130' has energizing leads 123a and 123 with variable resistor 124 in lead 123 symbolizing means for adjusting the excitation of the field coil 130' in the tip. Lead 128 with variable resistor 129 brings arc current to the electrode, variable resistor 129 symbolizing means for varying the value of the arc current. Magnetic field lines are shown at 139, 140, 141, 143, 144, 145, 146, 147 and 148. It will be understood that a mold, not shown for convenience of illustration, forms part of the furnace, that the solenoid 132' is mounted adjacent the exterior wall of the mold with variable resistor 127 for adjusting the excitation of the solenoid, and that lead 125 is connected to the terminal of opposite polarity of the solenoid power supply. In FIG. 6 an arc 188 is shown between the arcing surface or tip 131' and upper surface 189 of a melt in the furnace.

Computed values of the magnitude of the field vector in gausses and the angle of field vector relative to the axis 165 in degrees are shown at 88 points in Table VI hereinbelow, entitled "Other Magnetic Field Values for Furnace Solenoid and Electrode Field Coil Canceling on the Centerline."

TABLE VI

OTHER MAGNETIC FIELD VALUES FOR FURNACE SOLENOID AND ELECTRODE FIELD COIL CANCELING ON THE CENTERLINE

Mold Solenoid Excitation (+) 5040 amp. turn Mold Location Axial -- 12.4 to 47.4 inches Radial -- 8.25 inches Electrode Field Coil Excitation (-) 8000 amp. turn Electrode Field Location Axial -- 30.3 to 31.4 inches Radial -- 1.34 to 1.69 inches

Field Points

Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (gauss) (degrees) 22 0.1 53.3 0.06 23 0.1 51.5 0.14 24 0.1 47.8 0.30 25 0.1 40.7 0.68 26 0.1 26.6 2.14 27 0.1 45.0 149.92 28 0.1 78.7 175.72 29 0.1 286.9 176.64 30 0.1 800.5 177.75 31 0.1 1130.9 180.38 32 0.1 632.5 182.72

22 1.0 53.5 0.58 23 1.0 51.9 1.36 24 1.0 48.6 2.81 25 {1.0 42.5 6.16 26 1.0 31.5 16.90 27 1.0 20.8 75.38 28 1.0 71.9 135.33 29 1.0 249.4 143.56 30 1.0 882.1 150.30 31 1.0 1455.8 183.81 32 1.0 618.3 213.63

22 2.0 54.3 0.98 23 2.0 53.1 2.31 24 2.0 50.8 4.61 25 2.0 46.9 9.28 26 2.0 41.2 20.36 27 2.0 38.7 48.35 28 2.0 63.6 85.47 29 2.0 155.3 95.84 30 2.0 414.6 71.61 31 2.0 486.2 350.11 32 2.0 315.7 276.00

22 3.0 55.4 1.10 23 3.0 54.8 2.67 24 3.0 53.6 5.13 25 3.0 51.8 9.40 26 3.0 50.1 17.19 27 3.0 51.6 30.56 28 3.0 64.6 46.19 29 3.0 99.2 50.52 30 3.0 149.1 32.35 31 3.0 169.0 354.54 32 3.0 135.9 320.29

22 4.0 56.7 0.95 23 4.0 56.6 2.51 24 4.0 56.3 4.70 25 4.0 55.9 7.95 26 4.0 56.3 12.73 27 4.0 59.1 19.01 28 4.0 67.0 24.60 29 4.0 81.6 24.52 30 4.0 97.9 14.76 31 4.0 103.8 357.63 32 4.0 93.7 341.79

22 5.0 58.1 0.60 23 5.0 58.3 2.02 24 5.0 58.5 3.79 25 5.0 58.9 6.07 26 {5.0 60.1 8.86 27 5.0 62.7 11.77 28 5.0 67.6 13.58 29 5.0 74.6 12.42 30 5.0 81.1 7.13 31 5.0 83.2 358.97 32 5.0 79.4 351.32

22 6.0 59.3 0.16 23 6.0 59.8 1.38 24 6.0 60.3 2.77 25 6.0 61.0 4.32 26 6.0 62.3 5.94 27 6.0 64.3 7.30 28 6.0 67.3 7.79 29 6.0 70.9 6.69 30 6.0 73.9 3.73 31 6.0 74.8 359.58 32 6.0 73.0 355.64

22 7.0 60.5 359.70 23 7.0 61.0 0.75 24 7.0 61.6 1.82 25 7.0 62.4 2.89 26 7.0 63.5 3.86 27 7.0 65.0 4.53 28 7.0 66.9 4.60 29 7.0 68.8 3.80 30 7.0 70.3 2.11 31 7.0 70.7 359.88 32 7.0 69.9 357.75

The relative excitations of FIG. 6 which represent an increase of the strength of the tip field relative to that of the external field, compared to the relative strengths of FIG. 5, may be permissible and especially useful after the arc has started, and after the original particulate or solid material has been heated or melted to form a melt and stable conditions of operation have been attained. It is noted that the arc 188 extends in a somewhat sloping fashion between the electrode tip and the surface 189 of the melt which is different from that shown in FIG. 5, indicating that the area of the surface of the melt covered by the arc may be greater with the relative field excitations of FIG. 6 than with the relative field excitations of FIG. 5 which is of great advantage in heating the entire melt uniformly.

Radially extending zone T'-T' may indicate a resultant magnetic field area of particular interest.

Particular reference is made now to FIG. 7, where the strength of the field produced in the tip has been further increased with respect to that of the field produced by solenoid 132". In FIG. 7, the magnetic field configuration is produced by an electrode tip field coil excitation of (-) 8,000 ampere turns as in FIG. 6, but the solenoid 132" excitation is (+) 1,512 ampere turns. This arrangement is also different from the one shown in FIG. 5. It shows a way to increase relative magnetic field strength at the tip by reducing the magnetic field generated by the solenoid. Ampere turn lines are shown at 185 with other ampere turn lines being shown at 162, 163, 164, 166 and 159. Magnetic field lines are shown at 154, 155, 156, 156A, 157, 158, 168, 169, and 170, the magnetic field lines indicating the orientation of the magnetic flux, the density of the lines indicating the strength of the field. The upper surface 192 of the melt is shown as being bowl-shaped showing the result of the vortex effect of the level of the melt. Rheostat 127 symbolizes means for adjusting the strength of the solenoid field, rheostat 129 symbolizes means for adjusting the value of the arc current, and it is understood that a rheostat, not shown for convenience, or other suitable means, is connected in the leads to the tip field coil 130" for varying the adjustment of the field coil excitation, this rheostat not being shown because of the complexity of the magnetic field lines and ampere turn lines adjacent or near the electrode. Arc 191 is shown taking place to the surface 192 of the melt; it is noted that the arc slants at a considerably greater angle than the slant of the arc in FIGS. 5 and 6, indicating that the area of the melt surface circumscribed by the arc is even greater with the relative magnetic field excitations of FIG. 7 because the strength of the component of the magnetic field generated by the tip has been increased and also its relative strength is increased. It being understood that the tip field and the solenoid field are also in magnetic polarity opposition in FIG. 7, as well as in FIGS. 5 and 6, with the relative excitations different in each of the three figures. In FIG. 6, when the arc is in a diffused mode of operation, the area of the melt surface in direct contact with the arc is greater than it is in FIG. 5.

In FIG. 7 it is noted that the arc 191 at the point of attachment to the electrode tip 131" is not outside of the mean diameter of the tip. This follows from the change in the resultant field pattern at the tip. Where the strength of the tip field relative to the external magnetic field is greater than that shown in FIG. 5, there is a tendency for field lines to concentrate the arc spot on the electrode between the mean diameter and the inside diameter of the annular electrode tip. The region T"-T" is of special importance in this embodiment.

With further reference to FIG. 5, it is to be noted that the magnetic field following the contour of the arcing surface is especially valuable where the arc is produced by alternating current and the force on the arc varies as the current in the arc varies during each alteration of the alternating current with the result that the area of the tracks of an alternating current arc on the arcing surface may be wider than the track of an arc produced by direct current.

Field strength and direction are given at numerous points in Table VII which follows.

TABLE VII

FURTHER MAGNETIC FIELD VALUES FOR FURNACE SOLENOID AND ELECTRODE FIELD COIL CANCELING ON THE CENTERLINE

Mold Solenoid Excitation (+) 1512 amp. turns Mold Solenoid Location Axial -- 12.4 to 47.4 inches Radial -- 8.25 inches Electrode Field Coil Excitation (-) 8000 amp. turns Electrode Field Coil Location Axial --30.3 to 31.4 inches Radial -- 1.34 to 1.69 inches

Field Points

Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (gauss) (degrees) 22 0.1 11.5 0.45 23 0.1 9.1 0.98 24 0.1 5.0 3.15 25 0.1 2.6 168.75 26 0.1 17.0 176.59 27 0.1 47.7 177.28 28 0.1 122.5 177.25 29 0.1 330.8 177.08 30 0.1 844.4 177.87 31 0.1 1174.9 180.36 32 0.1 676.3 182.55

22 1.0 11.8 4.28 23 1.0 9.6 8.95 24 1.0 6.3 24.49 25 1.0 4.8 101.75 26 1.0 16.3 145.31 27 1.0 43.5 152.25 28 1.0 107.7 151.96 29 1.0 286.0 148.80 30 1.0 920.6 151.65 31 1.0 1499.7 183.70 32 1.0 655.3 211.51

22 2.0 12.5 7.31 23 2.0 11.0 14.11 24 2.0 8.9 30.31 25 2.0 8.4 69.44 26 2.0 15.4 108.61 27 2.0 34.3 121.83 28 2.0 74.5 121.48 29 2.0 165.7 111.16 30 2.0 402.8 77.56 31 2.0 443.0 349.13 32 2.0 314.3 268.02

22 3.0 13.5 8.65 23 3.0 12.6 15.33 24 3.0 11.6 27.70 25 3.0 11.8 49.34 26 3.0 15.7 74.53 27 3.0 26.5 88.73 28 3.0 46.8 89.12 29 3.0 79.0 76.08 30 3.0 114.3 44.26 31 3.0 125.2 352.57 32 3.0 106.0 304.84

22 4.0 14.6 8.61 23 4.0 14.2 14.20 24 4.0 13.9 22.82 25 4.0 14.5 35.22 26 4.0 17.0 49.19 27 4.0 22.9 58.76 28 4.0 22.7 59.14 29 4.0 45.3 48.51 30 4.0 56.3 26.28 31 4.0 59.7 355.76 32 4.0 53.8 326.69

22 5.0 15.6 7.71 23 5.0 15.6 12.05 24 5.0 15.7 17.87 25 5.0 16.4 25.10 26 5.0 18.3 32.49 27 5.0 21.7 37.38 28 5.0 26.9 36.91 29 5.0 32.8 29.53 30 5.0 37.5 15.54 31 5.0 38.9 357.59 32 5.0 36.4 340.28

22 6.0 16.5 6.42 23 6.0 16.6 9.68 24 6.0 17.0 13.57 25 6.0 17.7 17.85 26 6.0 19.1 21.72 27 6.0 21.3 23.87 28 6.0 24.2 22.87 29 6.0 27.3 17.91 30 6.0 29.6 9.31 31 6.0 30.3 358.65 32 6.0 29.1 348.32

22 7.0 17.3 5.05 23 7.0 17.5 7.47 24 7.0 17.9 10.09 25 7.0 18.6 12.66 26 7.0 19.6 14.70 27 7.0 21.1 15.52 28 7.0 22.8 14.42 29 7.0 24.5 11.08 30 7.0 25.8 5.72 31 7.0 26.1 359.27 32 7.0 25.4 352.99

Particular reference is made now to FIG. 8 showing a resultant magnetic field configuration where the two fields are adding on the centerline and the solenoid excitation is (+) 5,040 ampere turns and that of the electrode field coil is (+) 3,200 ampere turns. In FIG. 8 it will be understood that the mold of the furnace is not shown for convenience of illustration, but is near or adjacent the solenoid 132'". The solenoid 132'" has means symbolized by the rheostat 127 for adjusting the excitation thereof and the tip field coil 130'" has means symbolized by the rheostat 124 for adjusting the current in field coil 130'" and rheostat 129 symoblizes means for adjusting the current in the arc 214 shown taking place from the electrode tip 131'" to the surface 213 of the melt. In FIG. 8, the surface of the melt is shown as being generally level although the invention is not limited thereto. Ampere turn lines are shown at 201, 202, 203, 212 and 204 through 210, inclusive, and magnetic field lines are shown at 289, 290, 291, 292, 293, 294, 295, 296, 297 and 298, the magnetic field lines indicating the flux density and the orientation of the flux.

In FIG. 8 zero axial inches, that is, the point corresponding to zero axial inches in table of data hereinafter, is not in view. It is noted in FIG. 8 that the arc 214 extending between arcing surface 131'" and the surface 213 of the melt follows closely the magnetic field lines such as line 291 indicating a strong arc focusing effect by the resultant magnetic field when the two fields are adding on the centerline.

Table VIII following, entitled "Magnetic Field for Furnace Solenoid and Electrode Field Coil Adding on the Centerline of the Electrode" shows the magnitude of the field vector in gausses and the angle of the field vector to the axis in degrees at 88 selected axial and radial positions adjacent the electrode and in the volume of the furnace adjacent the electrode area immediately underneath the electrode.

TABLE VIII

MAGNETIC FIELD FOR FURNACE SOLENOID AND ELECTRODE FIELD COIL ADDING ON THE CENTERLINE

Mold Solenoid Excitation (+) 5040 amp. turns Mold Solenoid Location Axial 12.4 to 47.4 inches Radial 8.25 inches Electrode Field Coil (+) 3200 amp. turns Electrode Field Location Axial 30.3 to 31.4 inches Radial 1.34 to 1.69 inches

Field Points

Axial Radial Magnitude of Angle of Field Position Position Field Vector Vector to Axis (inches) (inches) (gauss) (degrees) 22 0.1 62.2 359.92 23 0.1 64.1 359.91 24 0.1 66.6 359.88 25 0.1 70.2 259.81 26 0.1 76.4 359.68 27 0.1 89.0 359.41 28 0.1 119.2 358.87 29 0.1 202.6 358.10 30 0.1 408.1 358.24 31 0.1 540.2 0.32 32 0.1 340.6 2.02

22 1.0 62.2 359.18 23 1.0 64.0 359.08 24 1.0 66.3 358.81 25 1.0 69.6 358.22 26 1.0 75.1 357.01 27 1.0 85.8 354.49 28 1.0 110.1 349.36 29 1.0 178.4 340.58 30 1.0 431.5 336.10 31 1.0 670.1 3.32 32 1.0 324.1 25.03

22 2.0 62.1 358.44 23 2.0 63.7 358.28 24 2.0 65.7 357.84 25 2.0 68.2 356.91 26 2.0 72.0 355.03 27 2.0 78.2 351.23 28 2.0 89.6 343.40 29 2.0 112.8 326.73 30 2.0 161.4 282.81 31 2.0 108.9 162.07 32 2.0 146.3 59.34

22 3.0 62.0 357.82 23 3.0 63.4 357.67 24 3.0 64.8 357.21 25 3.0 66.5 356.25 26 3.0 68.5 354.43 27 3.0 70.8 351.05 28 3.0 72.7 344.85 29 3.0 70.1 333.94 30 3.0 49.5 319.86 31 3.0 21.9 17.62 32 3.0 58.0 37.26

22 4.0 62.0 357.35 23 4.0 63.1 357.28 24 4.0 64.1 356.93 25 4.0 65.1 356.18 26 4.0 65.9 354.86 27 4.0 66.2 352.73 28 4.0 64.9 349.73 29 4.0 60.3 346.82 30 4.0 51.5 348.87 31 4.0 46.9 2.41 32 4.0 54.0 13.07

22 5.0 62.0 357.03 23 5.0 62.9 357.07 24 5.0 63.7 356.89 25 5.0 64.1 356.44 26 5.0 64.4 355.70 27 5.0 64.0 354.69 28 5.0 62.6 353.71 29 5.0 59.9 353.63 30 5.0 56.6 355.96 31 5.0 55.3 0.93 32 5.0 57.3 5.38

22 6.0 62.2 356.84 23 6.0 62.9 357.00 24 6.0 63.4 356.99 25 6.0 63.6 356.82 26 6.0 63.6 356.52 27 6.0 63.2 356.20 28 6.0 62.2 356.10 29 6.0 60.8 356.63 30 6.0 358.18 59.5 31 6.0 59.0 0.55 32 6.0 59.7 2.77

22 7.0 62.5 356.76 23 7.0 63.0 357.02 24 7.0 63.3 357.15 25 7.0 63.5 357.19 26 7.0 63.4 357.18 27 7.0 63.1 357.22 28 7.0 62.5 357.45 29 7.0 61.8 358.03 30 7.0 61.2 359.07 31 7.0 60.9 0.41 32 7.0 61.2 1.71

Particular reference is made now to FIG. 10, showing an additional embodiment of my invention in which three separately generated magnetic fields are produced for controlling the arc and stirring of the melt in a furnace in a manner to give improved efficiency of operation. In FIG. 10, an electrode generally designated 348 having an electrode tip 349 with fluid passageway 350 therein for cooling has a field coil generally designated 351 for setting up a magnetic field with at least a strong field component extending radially in all directions across the annular arcing surface of the electrode. The mold generally designated 353 is fluid cooled by the fluid passageway 354 having an inlet or outlet 355, it being understood that near the upper portion of the mold there is an additional passageway forming either an outlet or an inlet for cooling fluid to the passageway 354. It will be understood that the interior of the mold 353 or the chamber 352 is evacuated if desired to a predetermined reduced pressure by means, not shown for convenience of illustration, or that the atmosphere can be controlled by supplying a gas, for example, an inert, oxidizing, or reducing gas to the chamber. The mold 353 may be composed of nonmagnetic material and may have adjacent the outside wall thereof a solenoid 357 which it is understood is connected to a source of potential by way of means symbolized by rheostat 365 in lead 366 for adjusting the energization of the solenoid (the power return lead not being shown). The solenoid it is noted, extends above and below the axial position of the electrode tip 349. It is noted that the arc 359 may be a diffused arc which may occur for example when the pressure in chamber 352 is reduced to a predetermined value, for example pressure of 20 mm of mercury, and which also may occur while only a solenoid field and tip coil field are present even at atmospheric chamber pressure. A relatively solidified portion of the ingot is shown at 360 and a relatively fluid portion of the ingot is shown at 361. Axially slidable on the solenoid is a concentrated coil 362 with a magnetic yoke 364, the energization of the concentrated coil being adjustable by means symbolized by rheostat 371 in lead 369, leads 369 and 370 being connected to a source of energizing potential, not shown.

A vacuum pump 358 of any suitable design is shown communicating by conduit 356 with the chamber 352 inside the mold 353 for reducing the pressure therein to a predetermined level although in some instances the chamber may be pressurized above atmospheric pressure for special purposes. Variable resistor 373 in lead 372 symbolizes means for adjusting the value of the current in arc 359. As aforementioned, variable resistor 365 in lead 366 symbolizes means for adjusting the excitation of the solenoid 357. Variable resistor 376 in lead 375 symbolizes means for adjusting the excitation of the electrode tip field coil 351. For convenience of illustration, leads 369 and 370 for energizing the compact coil 362 are shown as passing through bores or follow tubes 367 and 368 of very small cross section in the magnetic yoke 364, the bores being kept at a very small diameter to reduce as little as possible the low reluctance path formed by the yoke for magnetic field lines generated by coil 362, but it should be understood that other means for bringing leads to the coil 362 could be employed. Leads 369 and 370 are electrically insulated from the material of yoke 364. As previously stated, variable resistor 371 in lead 369 symbolizes means for varying the excitation of the compact coil 362 disposed within the yoke.

It is to be noted that the arc 359 in FIG. 10 is in a diffused mode of operation. Such a diffused mode of operation is generally seen at reduced pressures; the effect of the influence of the magnetic field of coil 351 is to rotate or move the entire diffused arc around the arcing surface of the electrode, the arcing surface 380 being shown as generally flat with a central aperture 381 therein in accordance with the annular construction of the electrode tip 349. It will be understood that when the arc goes into a diffused mode of operation, that the erosion rate of material from the arcing surface 380 may be reduced by one or two orders of magnitude since the power of the electric arc is not concentrated in an intensely hot arc spot upon the arcing surface which spot causes an instantaneous temperature rise at the side of the arc spot which may cause the material of the electrode arc tip 349 to melt and vaporize. As stated in connection with previously described embodiments, even though the material at the site of the arc spot may melt and rise in a fraction of a millisecond to a very high temperature, the arc in previously described embodiments moves at such a sufficiently rapid rate that the dwell time of the arc spot at any point on the surface is very short. The melted material to a very large extent is not sublimated or evaporated but begins to cool immediately in a fraction of a millisecond after the arc spot leaves the site. In a diffused mode of arc operation, several or many arc spots are formed or a diffused mode of attachment exists which results in a lower erosion rate.

The magnetic field configuration of FIG. 10 provides for effective control of the arc and for control of material mixing in the molten pool 361.

It is well known in the art of vacuum arc melting that it is desirable to control the mixing in a molten pool, especially at the liquidus-solid innerface to control the proportion of alloy ingredients and reduce the grain size. Control is also desirable at the top surface of a molten pool in melting with a nonconsumable electrode in order to get adequate consumption of the material being fed into the melt as it contacts the melt, and to minimize the thickness of the collar on the ingot and at the mold wall. In the prior art, the magnetic field which interacts with the electrical current flow to create the stirring force is generally produced by a solenoidal coil wrapped or surrounding a substantial portion of the length of the mold, making the field nearly uniform in orientation and strength throughout the entire mold. The mixing is proportional to the magnitude of the component of current perpendicular to the magnetic field vector.

The embodiments of my invention described heretofore in which an electrode tip field is used in conjunction with an externally produced mold field, whether by a solenoid or a concentrated coil, provide for improved operation over prior art practices employing an external field as the exclusive field generating means in the furnace. The previously described embodiments permit increasing the rate of mixing, and in fact independently provide a sufficient field for good mixing under some conditions.

The apparatus shown in FIG. 10 has among other advantages, provision for the control of the regions of mixing by using the combination of a movable external field produced by the axially slidable compact coil and yoke, the field of the coil in the electrode tip, and a biasing solenoidal field, all three of which interact within the confines of the furnace mold. To improve the concentration of the various magnetic fields, the movable coil 362 as aforementioned is provided with a yoke 364 made of permeable material to provide a low reluctance path and focuses the magnetic field produced by the coil 362. In the practice of my invention according to the apparatus of FIG. 10, the movable field coil is either moved simultaneously with the electrode at a proper orientation thereto as the position of the electrode is changed to accommodate changes in the level of the melt to maintain a desired arc length, or moved independently of the electrode in accordance with different periods or stages in the melting operation, for example, stages resulting in variations in the depth of the molten pool 361 to, for example, maintain a strong field while, for example, melting reactive metals such as titanium in the axial portion of the mold where the liquidus-solid interface is present and consideration of grain structure are predominant, and maintain weak field, or produce field cancellation while melting ferrous materials.

Summarizing the advantages of the apparatus of FIG. 10, mixing is controllable by zones. Therefore, the mixing of the material on the surface of the pool can be different from that of the base; finer grain structure in ingot formation is obtained because of movement of the solidus-liquid interface. The invention includes a magnetic field produced by solenoid 357 in polarity adding to that of coil 362 and in polarity opposition to that of coil 351; the invention also includes producing a magnetic field by solenoid 357 in polarity opposition to that produced by coil 362 but in polarity adding to that produced by coil 351; it also includes a field produced by solenoid 357 adding to both those produced by coils 351 and 362 or opposing both those produced by coils 351 and 362. Likewise, the invention includes producing a tip field by coil 351 adding to either one of the field produced by coils 357 and 362 while opposing the other, adding to both, or opposing both.

A further advantage offered by the apparatus of FIG. 10 is that the externally generated magnetic field or fields are employed in some applications to focus the arc between the electrode and the melt. The coil in the tip is used in some applications to improve the magnetic field shape around the outside of the electrode and adjacent the arcing surface. Also compact coil 362 may be moved into a position opposite tip coil 351 to generate a local increased field strength around tip 349 and electrode 348 to further prevent arcing and reduce the possibility of damage.

Particular reference is made now to FIG. 11, in which the upper portion of the furnace is shown at 385, having a cover 386 with a disc 387 composed of electrically insulating material having an aperture therein through which the electrode 348' passes. Fluid inlets and outlets of the supporting column portion of electrode 348' are shown at 388 and 389, and tube 379 it will be understood passes through the electrode supporting column and communicates with the central aperture of the tip, such as 381 of FIG. 10. Tube 379 may be used for feeding material such as an inert gas into the furnace through the central aperture through the electrode.

It will be understood that the electrode 348' contains a passageway in the tip for the flow of cooling fluid for conducting heat flux from the tip, the passageway in the tip not being shown for convenience of illustration, the passageway in the tip communicating with fluid inlet and outlet 388 and 389. In FIG. 11, the material containing portion of the mold is designated 353, having a cylindrical passageway 354 therein and therearound for the flow of cooling fluid, with the fluid outlet 382, and the fluid inlet 355. Adjacent the outside wall of the mold is solenoid 357' with variable resistor 371 in series with one of the energizing leads, symbolizing means for adjusting the excitation of solenoid 357'. Variable resistor 373 in lead 372 connected to the supporting column of electrode 348' symbolizes means for adjusting the value of the current in arc 359'. Vacuum pump 358 is connected by conduit 356 with the upper portion of the vacuum chamber 392 for reducing the pressure therein to a predetermined value.

It will be understood that in the apparatus of FIG. 11, as well as in the apparatus shown in FIGS. 10 and 12, suitable means, not shown for convenience of illustration, may be provided for sensing the pressure in the chambers 352, 392, 392' or inside the furnace, the sensing means being operatively connected to the pump 358 for maintaining the pressure at a desired value. In all of the embodiments of FIGS. 10, 11, and 12 it will be understood that, if desired, the vacuum pump 358 may run continuously or intermittently to remove gasified impurities from the melt, that is, impurities which are gaseous at a temperature-pressure combination below the temperature-pressure combination at which the material of the melt becomes gasified. In FIG. 11, the invention includes an electrode, having either a magnetic field coil in the tip or not having a magnetic field coil in the tip, it being stated hereinbefore that where the electrode operates in a vacuum furnace at reduced pressure, the arc may assume a diffused mode of operation such as that shown at 359', and an electrode tip field may in some cases be dispensed with since the diffused arc spreads over substantially the entire area of the arcing surface with a resulting marked decrease in erosion of material from the arcing surface, and the two fields generated by solenoid 357' and concentrated coil 362' may make a third field unnecessary. The fields of coils 362' and solenoid 357' may be in magnetic polarity adding or in magnetic polarity opposition. Where magnetic field coil 362' is axially positioned in the region of the arc, the coil with the low reluctance path provided by yoke 364' results in a very intense magnetic field in the portion of the furnace adjacent the electrode tip, between the tip and the melt, and in the upper portion of the melt. The effect of this field is to concentrate the arc between the electrode and the melt, and to increase the stirring action of the magnetic field on the melt. Where the solenoid field of solenoid 357' is in polarity adding to the field of coil 362', additional stirring effect and additional concentrating effect is obtained, but it will be understood that the fields of coil 362' and solenoid 357' may be in magnetic polarity opposition, resulting in a magnetic field configuration in the portion of the mold adjacent the tip, and the region of the arc, and adjacent the upper surface of the melt, which increases control of the arc to provide desirable conditions of operation. In particular, a strong solenoid field produced by solenoid 357' sets up magnetic field lines extending through the furnace generally parallel to the axis of the electrode; these magnetic field lines impose an obstacle to the arc striking from the electrode tip to the wall of the mold. Coil 362' may also be placed as shown such that its greatest affect is on the solidification front to preferentially increase or decrease the pool rotation as desired for various materials.

Particular reference is made to FIG. 12. The structure in FIG. 12 is similar to that in FIG. 11 except that the solenoid 357 has been dispensed with, and the dimensions of magnetic field coil 362" and yoke 364" have been altered so that the field coil and yoke are slidable along the outside wall of the fluid cooled mold 353" . The invention disclosed in FIG. 12 may include an electrode 348" having a magnetic field coil, not shown, in the tip, or alternately an electrode having no magnetic field coil in the tip.

For completeness, both the furnace apparatus of FIG. 11 and that of FIG. 12 is shown as having a bin or hopper 396 for feed material, the bin 396 having conduit 398 passing through an aperture 397 in the top of the furnace. It will be understood that means, not shown, are provided for maintaining the chamber in the furnace hermetically sealed, that many radially spaced and/or peripherally spaced feed points may be provided if desired.

Further summarizing the operation of the apparatus of FIGS. 10, 11 and 12 and paying particular attention to FIG. 12, and assuming that electrode 348" has a magnetic field coil in the tip, one primary purpose of the concentrated field coil 362" and the yoke 364" is to control the magnetic field to shape the field in the area of the arc also positioning the coil 362" near the solidification zone in the mold mixing is controlled at the soldification front The position of the yoke coil and its relative value of excitation are used to control the grain structure and to provide an improved grain structure in the mold and particularly the solid portion thereof indicated at 360" , FIG. 12.

It will be understood that the concentrated coils 362' and 362" , FIGS. 11 and 12, have means, not shown, for adjusting the excitations thereof.

It will be understood that under some conditions of operation, the current will be high through the portion of the melt which is indicated by the fluid portion 361" , and the current will be relatively low through the cold portion which has contracted away from the mold, this contraction not being shown for convenience of illustration. In such a case, a very strong field is desirable from the coil-yoke combination to effect stirring and prevent alloy concentrations near the sides of the furnace, where alloy materials are added. A strong magnetic field under such conditions also reduces grain size.

Particular reference is made again to FIG. 11. It is noted that the solenoid 357' extends around the outside of the furnace substantially the entire length of the mold. As previously stated, the concentrated coil 362' with its yoke 364' is slidable along the outside of the solenoid in an axial direction to adjust its axial position. If the field of coil 362' adds to the solenoid field, it produces a strong axial field for focusing the arc 359' , and also produces good stirring at the top of the melt.

Particular reference is made to FIG. 14 where a further embodiment of my invention is shown. The furnace generally designated 429 is adapted to be evacuated if desired to a selected reduced pressure or also it could be raised to an elevated pressure above atmospheric by way of conduit 439 and valve 440 connected to a vacuum pump or gas supply, not shown. An electrode generally designated 432 extends through the top of the furnace and is insulated therefrom as shown. Feed bin or hopper 431 symbolizes means for feeding material to the melt; it is understood that multiple hoppers may be used in all embodiments. Rheostat 447 in lead 448 connected to electrode 432 symbolizes means for adjusting the current and/or power of arc 438 extending between the arcing surface of the electrode and the upper surface of molten pool 434 having a cooled ingot portion 435 formed in fluid-cooled melting crucible 430. Retractable bottom 436 connected to retracting arm 437 is lowered as the length of the ingot increases to maintain a desired arc length. Mounted external to the melting crucible, which it is understood may be composed of non-ferromagnetic material, are two field coils 441 and 442 with rheostats 443 and 444 in the energizing leads thereto, respectively, for adjusting the excitation of the coils and thereby adjusting the strengths of the magnetic fields generated thereby. The positions of the coils with respect to each other and along the length of the melting crucible are adjustable and maintainable by means, not shown for convenience of illustration. A field coil 433 in the electrode near the arcing surface generates a magnetic field, and means, not shown for convenience of illustration, are provided for adjusting the strength of the magnetic field generated by coil 433. Coils 441 and 442 may be excited in magnetic polarity adding or magnetic polarity opposition. Coil 433 may be in magnetic polarity adding or magnetic polarity opposition to one or both of the fields produced by coils 441 and 442, and a resultant field produced by mutual interaction of the separately generated fields.

When coils 441 and 442 are in magnetic polarity adding, the result is a field within crucible 430 wherein the lines of the field extend axially through the crucible and through the molten pool. Within the molten pool, because of radially extending current filaments from the arc site on the surface of the pool to the electrically conductive wall of the crucible, a strong stirring action is exerted on the material of the pool with benefits similar to those heretofore described in connection with other embodiments of my invention.

The magnetic field from coils 441 and 442 also extends through the crucible above the level of the melt, extending to the arcing surface of the electrode and in the space between the arcing surface or tip portion 428 of the electrode and the wall of crucible 430. Depending upon whether the magnetic field of electrode tip coil 433 is in magnetic polarity adding or magnetic polarity opposition with the field produced by coils 441 and 442, arc 438 is focused between electrode and melt, arc 438 is rotated, the area of the surface of the pool upon which the arc impinges is controlled, glow discharge is inhibited, striking of the arc from electrode to the wall of the mold or crucible is inhibited, movement of the arc out of its desired position as a result of conductive vapors given off from the melt is inhibited, and all of the other functions or operations described hereinabove with respect to other embodiments of the apparatus are obtainable.

When coils 441 and 442 are so energized that their fields are in magnetic polarity opposition, the apparatus of FIG. 14 offers unique advantages. A cusp field extends across the entire crucible, and depending on the axial location of the "break" between coils 441 and 442, this cusp field may extend across and through the molten pool. The positions of the coils may be so located that the cusp field is present at any selected portion of the solidus-liquidus interface between pool and cooled ingot portion. The shape of the molten pool may be changed from the semi-oval shape shown to one in which cooling occurs near the walls of the crucible and the molten pool has a more nearly uniform diameter almost to the bottom thereof. This offers advantages, including more nearly uniform dispersal of material fed to the melt around the periphery of the melt, better grain structure, a more nearly homogeneous ingot, and others.

Particular reference is made to FIG. 15 where a cusp field such as may be produced by coils 441 and 442 is shown. The arrows represent the direction of magnetic flux. Arrows m and n indicate the presence of a field component substantially perpendicular to the axis 165A of the crucible.

An electrode 451 with magnetic field coil 452 is shown. It is seen that notwithstanding the magnetic polarity opposition of coils 441 and 442, in the region of the electrode, the field of coil 441 extends generally axially through the crucible. It may mutually interact in manners previously described with the field of coil 452 in the electrode, depending upon whether the field of coil 452 is in magnetic polarity adding or magnetic polarity opposition with that of the external coil 441.

In FIG. 15, the shaping of the magnetic fields could be made different by increasing the distance between coils 441 and 442 or otherwise.

The cusp field offers stirring of the melt with the axially extending portion of the field, and control of the stirring of the melt throughout, especially at the solidification front.

Particular reference is made to FIG. 16 showing an embodiment of my invention in which two electrodes spaced from each other a sufficient distance so that the magnetic fields generated by coils, not shown, in the tips of the electrodes do not substantially interact with each other. A mold generally designated 455 has inner and outer wall portions 456 and 457 which may be composed of non-ferromagnetic, diamagnetic, or ferromagnetic or other material with a fluid passageway 458 between the wall portions. A magnetic field coil 459 encloses the mold over at least a substantial portion of the axial length of the mold. Extending at least a portion of the axial length of the field coil, and, if desired, extending into the mold, are two electrodes generally designated 461 and 462 which may be similar to each other, having supporting columns conveniently including coaxial cylindrical portions 463 and 465 forming fluid passageways 464 and 466 which bring fluid to and from an electrode tip which includes a magnetic field coil; the tip may have a construction similar to that of the electrode shown in FIG. 1. Leads to the field coil may be brought through the axially extending passageway 464. Passageway 467 may be an opening through the electrode for the admission of gas or material into the furnace. It will be understood that the field coil of each electrode has means such as a rheostat in the electrical connection thereto for adjusting the excitation of the tip coil, and that each electrode has means, not shown, for adjusting the value of the arc current. Further, it will be understood that polarity reversing means, either for the tip field coils or the mold coil 459, or for both, is provided so that the tip fields may selectively be in magnetic polarity opposition to the field of mold coil 459 or in polarity adding with the field of the mold coil.

Particular reference is made to FIG. 17, where the tip field coils 471 and 472 of electrodes 461 and 462, respectively, are shown. They have fields shown for illustrative purposes in magnetic polarity opposition to the mold field generated by coil 459, and generally speaking, the interactions of the tip fields with the axially extending lines of the field generated by coil 459 resulting in resultant fields adjacent the electrodes resembling the resultant fields of FIG. 6 and others when the fields are plotted by selecting two cylindrical zones for analysis and showing a two dimensional plot of the three dimensional field. Line 476 represents the tangent point of the two cylinders of study around electrode 461 and 462. It is seen that electrodes 461 and 462 are sufficiently spaced from each other that their fields do not substantially interact or distort each other; that is evidenced by field lines 475, 476 and 477 of the field generated by mold coil 459; these three field lines extend axially through the mold without any substantial bending or curvature or distortion. Arcs 464 and 465 extend from the electrodes to the upper surface of the melt shown at 463.

Particular reference is made now to FIG. 18 where an embodiment of my invention employing three electrodes is shown, the three electrodes being generally designated 481, 482, and 483. A circular or cylindrical mold shown at 485 may be composed of non-ferromagnetic material, with inner and outer wall portions 486 and 487, respectively, having fluid passageway 488 therebetween, and a magnetic field coil 489 encloses the mold over at least a portion of its axial length, the three electrodes extending at least an axial distance into field coil 489 and if desired extending an axial distance into the mold. It will be understood that where a mold with a retractable bottom is employed, the level of the melt can be maintained close to or at the upper end of the mold; since the electrodes are maintained an arc's length away from the surface of the molten pool, in some applications the electrodes need not extend into the mold. Each of the electrodes 481, 482 and 483 includes a preferably fluid cooled tip having a magnetic field coil therein, the coils not being shown for convenience of illustration in FIG. 18. The spacing between electrodes is sufficient so that no substantial interactions between tip fields need occur; the tip fields by mutual interaction with the axially extending field lines of the field generated by coil 489 produce near the electrodes resultant fields which may be similar to those shown in FIGS. 3-8, depending on the relative excitations of the tip field coils and whether or not the tip fields are in magnetic polarity adding or magnetic polarity opposition to the field generated by coil 489.

Certain terms are used in the specification and claims appended hereto, particularly in claiming the apparatus of the figures which show a resultant magnetic field, or magnetic field configuration within the furnace, resulting from the mutual interaction of two separately generated magnetic fields which are in magnetic polarity opposition or in magnetic polarity adding as the case may be, and where one field has a certain strength relative to the strength of the other field. The term "radius of curvature" is used in its ordinary geometrical or mathematical sense, but it should be understood that where the radius of curvature between selected points on a magnetic field line shown in one of the magnetic field plots is designated, that the selected points are arbitrary, and that the claims are not limited by the selected points, the selected points being chosen because of their positions in the area near the electrode, between the electrode and the melt, or including at least the upper portion of the melt.

The term "interaction zone" as employed in the specification and claims indicates the portion of the volume of the mold wherein the resultant field may in some cases most markedly vary from a field pattern which would be set up by the field coil in the tip alone, or by the external magnetic field coil alone, whether concentrated or solenoidal in shape. The designation of a certain portion of the volume of the furnace as an interaction zone is not limiting, and where the zone is enclosed in dashed lines in the figures it will be understood that the choice is arbitrary, that the zone is not two dimensional but extends around the entire furnace.

The term "divergence" is used in its classical and generally accepted sense, for example, as employed in a work by Skilling entitled "Fundamentals of Electric Waves," John Wiley and Sons, Inc., 1942.

The term "gradient" is employed in its generally accepted meaning such as employed in the aforementioned work by Skilling and in many authoritative works on magnetic fields and magnetic circuits.

The term "vector field" is employed in a conventional manner.

The term "nabla" is used in its conventional sense.

It will be understood that in all the tables appearing herein the strength of the magnetic field vector and its angle with respect to the axis at points other than the numerous points for which the strength has been accurately given, may be computed from formulae well known to those skilled in the art.

Since the resultant fields of some of the figures have radial symmetry, where desired, a user of the apparatus may think of the information contained in the tables in terms of polar coordinates. The term "polar coordinate" is used in its ordinary sense such as that employed in the aforementioned work of Skilling.

The term "scalar product" is used in its ordinary and conventional sense to indicate the product obtained by multiplication of two vector quantities in a certain manner and is defined to be the product of their lengths by the cosine of the angle between them.

In the specification and claims, the words "normal" or "normally" when used with reference to the position of a magnetic field line or line of force refer to the position which the magnetic field line or line of force would have in the absence of a second magnetic field which by mutual interaction "distorts" the field and produces a resultant field with field components which result from the mutual interaction.

When one or more magnetic field lines shown in a drawing are expressly designated or referred to in a claim, it will be understood that the position of the line or lines are exemplary, and that they may be moved within reasonable limits from the exact position shown in the magnetic field plot by adjusting the relative excitations of the two magnetic field producing coils without departing from the spirit and scope of the invention, it having been stated on numerous occasions hereinbefore that the resultant magnetic field plots of some of the figures are illustrative of resultant magnetic fields having certain properties suitable for accomplishing the purposes of the invention, but that the invention is not limited to resultant magnetic fields having the exact configuration shown. My invention also includes methods and processes for operating an electric arc furnace as well as the structure shown, the methods and processes including the steps of generating two fields in magnetic polarity opposition or generating two fields in polarity adding, adjusting the relative magnitudes of the first and second fields to obtain desirable control features relating to arc movement on the electrode, focusing of the arc between the electrode and the melt, stirring the melt, increasing the feed rate, improving the grain structure, and other desirable objectives. Further methods and processes include producing three magnetic fields, and varying the intensities and locations as well as the relative magnetic polarities to accomplish certain results.

Whereas the invention has been largely illustrated with reference to furnace apparatus for forming an ingot, my invention includes apparatus for producing a resultant magnetic field by mutual interaction of two or more separately generated magnetic fields in furnace apparatus for heating material in a vessel where an arc takes place between an electrode and at least partially conductive material. My invention includes application in a skull furnace where material heated in a ladle is poured therefrom into a mold.

Whereas the invention has been described with reference to a cylindrical mold, which may be the most convenient shape, my invention includes vessels or molds having other than cylindrical shapes, the substantial parallel alignment of the magnetic poles of the separately generated magnetic fields which mutually interact to produce a resultant field being maintained.

Whereas the invention has been described and illustrated with the electrode and field coils in concentric and coaxial alignment, it should be understood that the electrode and its field coil need not be on the centerline of the furnace or mold and may also be located at an angle to the vertical axis of the furnace, mold or crucible. Minor deviations in alignment will produce some changes in the magnetic field flux configurations but these need not be sufficient to deleteriously affect melting performance of tip material erosion.

It should also be understood that some coils and electrodes are given different designations such as 132, 132', 132" , etc. These may be and often are the same field coils at different values of excitation.

Whereas field coils have been shown in general with each having a separate power supply with means for adjusting the current level, my invention includes putting different field coils in series arrangement which is particularly attractive when more than one electrode is used in a single furnace. Furthermore, my invention includes putting different field coils in series arrangement and varying the current in individual field coils by shunting them with a suitable resistor. Furthermore, even for a single electrode and particularly for the case where such a single electrode may employ three or more field coils, my invention includes the method of adjusting relative field coil current levels by putting series connected coils in parallel or vice versa.

Whereas field coils have been shown as having only electrical connections, my invention includes field coils which may be fluid cooled thus requiring fluid cooling connections.

Whereas all embodiments of my invention show separate field coil power supplies, my invention does not preclude the possibility of connecting one or more coils in series with the arc current.

In general, when my invention describes shaping the magnetic field particularly in the area of the tip, it is understood that what is meant is that the magnitude and direction of the resultant vector field is more favorable and, therefore, has a more desirable "shape."

It is also understood in all applications described that the arc current may be DC or AC and that one terminal of the arc current power supply is connected to the electrode with the opposite connected to the melt which consists of at least partially conducting material.

All of the teachings herein with respect to the embodiment of FIG. 14 which shows a mold with a retractable bottom may be applied to the other embodiments.

Claims

I claim as my invention:

1. An electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, a substantially nonconsumable electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field with a portion of said first magnetic field existing in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, an axially disposed concentrated electrical coil mounted in a predetermined position with respect to the furnace vessel for generating an additional magnetic field within at least a portion of the volume of the vessel, the electrode being so mounted with respect to the means for generating an additional magnetic field that the magnetic poles of the first magnetic field lie in a line substantially parallel to a line passing through the additional magnetic field, the first magnetic field and the additional magnetic field being capable of being selectively in polarity adding or in polarity opposition, both the first magnetic field and the additional magnetic field being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel at at least one predetermined point in that volume which is significantly closer to said arcing surface than to both said furnace wall and said melt producing a resultant magnetic field which will, depending upon the relative strength and polarity of said first magnetic field over those of said additional magnetic fields, have a strength and orientation in said volume which is either substantially transverse to the arc and especially adapted to rotate the arc or which is substantially parallel to the arc and especially adapted to focus the arc and to stir the melt.

2. Electric arc furnace apparatus according to claim 1 in which the arc extends substantially parallel to the axis of the vessel in a path between the electrode and the material to be melted, in which the two fields are in polarity opposition for at least a portion of the time of operation along lines parallel to said vessel axis and the resultant field has a strong component extending radially across the arcing surface, said component being generally transverse to the arc path and exerting a force on the arc which causes the arc to move substantially continuously around a closed track formed by the arcing surface, the movement of the arc reducing the rate of erosion of material from the arcing surface as a result of arc action thereon.

3. Electric arc furnace apparatus according to claim 2 including means for reversing the relative polarity of one magnetic field with respect to the other after at least a portion of the material to be melted has been reduced to a molten condition whereby the two magnetic fields are then in polarity adding, and means for adjusting the strength of one magnetic field relative to the strength of the other field whereby a resultant field is produced with a strong component extending between said electrode and said melt and at least to some depth within the melt, said last named component focusing the arc between said electrode and said melt and by interaction with current filaments in the melt extending from the site of the arc spot thereon assisting in stirring the melt.

4. Electric arc furnace apparatus according to claim 2 including means for adjusting the strength of at least one magnetic field relative to the other whereby the relative strengths of the two magnetic fields are so adjusted with respect to each other that the resultant field extends radially across the arcing surface and substantially follows the contour of the arcing surface and is substantially parallel to the arcing surface around substantially the entire outside contour thereof.

5. An electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field, a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, said furnace having a wall comprised of non-ferromagnetic material, an elongated solenoid mounted adjacent the wall of the furnace for generating an additional magnetic field within at least a portion of the volume of the vessel, the solenoid extending axially upward to a position at least above the highest axial position of the electrode tip, the solenoid extending axially downward to a position below the lowest level of the melt formed by the melting of the material initially placed in the furnace, the electrode being so mounted with respect to the solenoid that the magnetic poles of the first magnetic field lie in a line substantially parallel to a line passing through the magnetic poles of the additional magnetic field, the first magnetic field and the additional magnetic field being selectively in polarity adding or in polarity opposition, both the first magnetic field and the additional magnetic field being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel at at least one predetermined point in that volume which is significantly closer to said arcing surface than to both said furnace wall or said melt producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance, said first magnetic field being substantially stronger than said additional magnetic field produced by said solenoid.

6. Electric arc furnace apparatus according to claim 1 in which the wall of the furnace comprises non-ferromagnetic material and in which said compact coil is mounted at a predetermined axial position adjacent the wall of the furnace.

7. Electric arc furnace apparatus according to claim 6 in which said compact coil is mounted at an axial position adjacent the wall of the furnace substantially corresponding to the axial position of the tip of the electrode within the furnace.

8. Electric arc furnace apparatus according to claim 6 in which the axial position of said compact coil is adjustable.

9. Electric arc furnace apparatus according to claim 7 in which said magnetic field coil in the electrode and said compact coil are so energized with respect to each other that their magnetic fields are in magnetic polarity opposition along the axis of the electrode and along the centerline of the furnace.

10. An electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field, a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, said furnace having a wall comprised of non-ferromagnetic material, an elongated solenoid mounted adjacent the wall of the furnace for generating an additional magnetic field within at least a portion of the volume of the vessel, the solenoid extending axially upward to a position at least above the highest axial position of the electrode tip, the solenoid extending axially downward to a position below the lowest level of the melt formed by the melting of the material initially placed in the furnace, the electrode being so mounted with respect to the solenoid that the magnetic poles of the first magnetic field lie in a line substantially parallel to a line passing through the magnetic poles of the additional magnetic field, the first magnetic field and the additional magnetic field being selectively in polarity adding or in polarity opposition, both the first magnetic field and the additional magnetic field being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel at at least one predetermined point in that volume which is significantly closer to said arcing surface than to both said furnace wall or said melt producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance, said first magnetic field being substantially stronger than said additional magnetic field produced by said solenoid, a compact field coil mounted adjacent the solenoid and having an axially adjustable position thereon, a magnetic yoke enclosing the ends and the outside of the compact coil and providing a low reluctance path, and means for energizing the compact coil to produce a third magnetic field in at least a portion of the volume of the furnace, said resultant magnetic field configuration in the furnace being produced by the mutual interaction of at least one said field with any other said field.

11. Electric arc furnace apparatus according to claim 10 in which the axial position of the compact coil and yoke is adjusted with respect to the level of the melt whereby a strong component of the resultant field exerts a stirring force on the melt.

12. Electric arc furnace apparatus according to claim 11 and including in addition means for substantially continually feeding material into the furnace to form an ingot, the stirring force produced by said strong component of the resultant field permitting an increased feed rate with more rapid heating and dispersion of the fed material, improved grain structure and a more nearly homogeneous ingot.

13. Electric arc furnace apparatus according to claim 10 including means for reversing the relative polarity of one magnetic field with respect to the other and means for adjusting the strength of one magnetic field relative to the other, the resultant magnetic field produced by mutual interaction of the field produced by the coil in the electrode and the field produced by the solenoid is shaped by adjustment of the relative strengths of said two fields and the selection of their relative magnetic polarities to have at least one strong component effective for rotating and focusing the arc, the relative axial position of the compact coil and yoke selected to provide a magnetic field for stirring the melt.

14. An electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field, a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, said furnace having a wall comprised of non-ferromagnetic material, an elongated solenoid mounted adjacent the wall of the furnace for generating an additional magnetic field within at least a portion of the volume of the vessel, the solenoid extending axially upward to a position at least above the highest axial position of the electrode tip, the solenoid extending axially downward to a position below the lowest level of the melt formed by the melting of the material initially placed in the furnace, the electrode being so mounted with respect to the solenoid that the magnetic poles of the first magnetic field lie in a line substantially parallel to a line passing through the magnetic poles of the additional magnetic field, the first magnetic field and the additional magnetic field being selectively in polarity adding or in polarity opposition, both the first magnetic field and the additional magnetic field being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel at at least one predetermined point in that volume which is significantly closer to said arcing surface than to both said furnace wall or said melt producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance, said additional magnetic field produced by said solenoid being substantially stronger than said first magnetic field.

15. An electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field, a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, said furnace having a wall comprised of non-ferromagnetic material, an elongated solenoid mounted adjacent the wall of the furnace for generating an additional magnetic field within at least a portion of the volume of the vessel, the solenoid extending axially upward to a position at least above the highest axial position of the electrode tip, the solenoid extending axially downward to a position below the lowest level of the melt formed by the melting of the material initially placed in the furnace, the electrode being so mounted with respect to the solenoid that the magnetic poles of the first magnetic field lie in a line substantially parallel to a line passing through the magnetic poles of the additional magnetic field, the first magnetic field and the additional magnetic field being selectively in polarity adding or in polarity opposition, both the first magnetic field and the additional magnetic field being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel at at least one predetermined point in that volume which is significantly closer to said arcing surface than to both said furnace wall or said melt producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance, said first magnetic field being substantially stronger than said additional magnetic field produced by said solenoid, a compact field coil mounted adjacent the solenoid and having an axially adjustable position thereon, a magnetic yoke enclosing the ends and the outside of the compact coil and providing a low reluctance path, and means for energizing the compact coil to produce a third magnetic field in at least a portion of the volume of the furnace, said resultant magnetic field configuration in the furnace being produced by the mutual interaction of at least one said field with any other said field.

16. Electric arc furnace apparatus according to claim 15 in which the axial position of the compact coil and yoke is adjusted with respect to the level of the melt whereby a strong component of the resultant field exerts a stirring force on the melt.

17. Electric arc furnace apparatus according to claim 16 including in addition means for substantially continually feeding material into the furnace to form an ingot, the stirring force produced by said strong component of the resultant field permitting an increased feed rate with more rapid heating and dispersion of the fed material, improved grain structure and a more nearly homogeneous ingot.

18. Electric arc furnace apparatus according to claim 15 in which the resultant magnetic field produced by mutual interaction of the field produced by the coil in the electrode and the field produced by the solenoid is shaped by adjustment of the relative strengths of said two fields and the selection of their relative magnetic polarities to have at least one strong component effective for rotating and focusing the arc, the relative axial position of the compact coil and yoke being selected to provide a magnetic field for stirring the melt.

19. Electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, first and second means mounted in predetermined position with respect to the furnace vessel for generating a first additional magnetic field and a second additional magnetic field respectively within the vessel, the first and first and second additional magnetic fields by mutual interaction in the vessel producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance.

20. The combination as claimed in claim 19 wherein said furnace vessel comprises ferromagnetic material.

21. The combination as claimed in claim 19 wherein said furance vessel comprises nonmagnetic material.

22. Electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field, a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, a solenoid and compact coil mounted in a predetermined position with respect to the furnace vessel for generating a first additional magnetic field and a second additional magnetic field respectively within at least a portion of the volume of the vessel, the first magnetic field and the first and second additional magnetic fields being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and second additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance.

23. Electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, first and second means mounted in a predetermined position with respect to the furnace vessel for generating a first additional magnetic field and a second additional magnetic field respectively within at least a portion of the volume of the vessel, the first magnetic field and the first and second additional magnetic fields being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and second additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance, said furnace vessel comprising ferromagnetic material, said first and said second additional magnetic fields being in polarity opposition with respect to each other.

24. Electric arc furnace apparatus comprising a furnace vessel adapted to receive at least partially electrically conductive material to be heated to form a melt, an electrode, means for mounting the electrode in predetermined position with respect to the vessel, the electrode including means forming an arcing surface and means within the electrode mounted near the arcing surface for generating a first magnetic field a portion of which exists in the space external to the electrode and near the arcing surface, the electrode and the material to be melted being adapted to be electrically connected to terminals of opposite polarity of a source of electrical potential to produce and sustain an electric arc therebetween, first and second means mounted in a predetermined position with respect to the furnace vessel for generating a first additional magnetic field and a second additional magnetic field respectively within at least a portion of the volume of the vessel, the first magnetic field and the first and second additional magnetic fields being generated in at least that portion of the volume of the vessel which includes the area near the arcing surface and at least a portion of the space between the arcing surface and the material to be melted, the first and second additional magnetic fields by mutual interaction in said last named portion of the volume of the vessel producing a resultant magnetic field having a configuration especially suited to control the arc in a manner to give improved furnace performance, said furnace vessel comprising ferromagnetic material, said first and said second additional magnetic fields being in polarity addition with respect to each other.

25. Electric arc furnace apparatus including in combination, a mold having a substantially cylindrical wall adapted to receive at least partially conductive material to be melted, a substantially nonconsumable electrode mounted in the mold and extending axially therein, the electrode and the material to be melted being adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, a first magnetic field generating means mounted outside of the wall of the furnace and means for energizing said first magnetic field generating means, a second magnetic field generating means mounted outside of the wall of the furnace and means for energizing said second magnetic field generating means, said first and second magnetic field generating means interacting within said furnace to give improved furnace performance.

26. Electric arc furnace apparatus according to claim 25 including means for substantially continually feeding additional material to the melt therein to form an ingot.

27. Electric arc furnace apparatus according to claim 25 including means for evacuating the furnace to a preselected pressure therein.

28. Electric arc furnace apparatus including in combination, a mold having a substantially cylindrical wall composed of diamagnetic material and adapted to receive at least partially conductive material to be melted, an electrode mounted in the mold and extending axially therein, the electrode and the material to be melted being adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, an elongated solenoid mounted adjacent the wall of the furnace, means for energizing the solenoid, a concentrated magnetic field coil mounted adjacent the solenoid and having an axially adjustable position therealong, a magnetic yoke enclosing the ends and at least a partion of the outside of the concentrated coil and forming a low reluctance path, and means for energizing the concentrated coil, the axial position of the concentrated coil and yoke being selected whereby a resultant magnetic field in at least a portion of the volume of the furnace resulting from the mutual interaction of the magnetic field generated by the solenoid and the magnetic field generated by the concentrated coil has a predetermined field configuration useful in improving the operation of the furnace.

29. The method of operating an electric arc furnace including the steps of producing an electric arc between an electrode and conductive material to be melted to form a melt, energizing a tip field coil in said electrode tip to produce a first magnetic field, generating a second magnetic field having a characteristic of the magnetic field of a compact coil within at least a portion of the volume of the furnace which magnetic field lines extending substantially axially through the furnace between the electrode tip and the surface of the melt, interacting the first and second magnetic fields at at least one predetermined point significantly closer to said arcing surface than to both said furnace wall or said melt to produce a resultant magnetic field adjacent the arcing surface and between the arcing surface and the surface of the melt which will, depending upon the relative strength and polarity of said first magnetic field and said second magnetic field, have a strength and orientation which may be alternately substantially transverse to said arc and especially suited to rotate the arc or which may be substantially parallel to the arc and especially suited to focus the arc and stir the melt.

30. A method of operating an electric arc furnace having a wall and adapted to receive conductive material to be melted to form a melt which comprises producing an electric arc between the material and an electrode spaced from the surface thereof, generating a first magnetic field by means in the electrode, generating a second magnetic field by means external to the electrode, the two magnetic fields interacting in a field interaction zone adjacent the electrode with a resulting distortion of both magnetic field patterns at at least one predetermined point significantly closer to said arcing surface than to both said furnace wall or said material and the production of a resultant magnetic field which will, depending upon the relative strength and polarity of said first magnetic field and said second magnetic field have a strength and orientation which may be alternately substantially transverse to said arc and especially suited to rotate the arc or which may be substantially parallel to the arc and especially suited to focus the arc and stir the material, the relative strength of said second magnetic field being substantially different than the relative strength of said first magnetic field in said field interaction zone.

31. A method according to claim 30 including the step of moving the arc around a closed arc track on said arcing surface substantially continuously.

32. A method according to claim 31 in which said first and second fields are in polarity adding and the strength of the first magnetic field generated in the electrode is substantially larger relative to the strength of the second magnetic field whereby the resultant magnetic field includes magnetic field lines which extend between the electrode and the surface of the melt at an oblique angle with respect to the axis of the electrode around the entire periphery of the electrode, said lastnamed magnetic field lines tending to focus the arc between the electrode and the melt.

33. A method according to claim 31 in which said field interaction zone adjacent the electrode is produced by generating the magnetic field in the tip and the externally produced magnetic field in magnetic polarity opposition with respect to each other along an axis substantially coinciding with an axis of the electrode.

34. A method according to claim 30 in which the furnace is generally cylindrical with a wall composed of diamagnetic material and the externally produced magnetic field is produced by a solenoid adjacent at least a portion of said furnace wall.

35. A method according to claim 31 in which the externally produced magnetic field is produced by a solenoid adjacent at least a portion of said wall.

36. A method according to claim 30 including the additional step of continually feeding additional material to the melt and adjusting the position of the electrode as the level of the melt rises to maintain a substantially constant arc length between the electrode and the melt, and in which said externally produced magnetic field is additionally characterized as having magnetic field lines which would normally extend axially through the furnace to a position above any poistion attained by the electrode and to a position below any level reached by the level of the melt in the absence of field distortion by the separately generated magnetic field in said electrode.

37. A method according to claim 31 including the additional step of continually feeding material to the melt in the furnace and repeatedly adjusting the position of the electrode as the level of the melt rises to maintain a substantially constant arc length between the electrode and the melt, and in which the externally produced magnetic field is additionally characterized as normally, in the absence of a distorting magnetic field produced in the electrode having lines which extend substantially axially through the furnace to a position above the highest position reached by the electrode and to a position below the lowest position attained by the surface of the melt.

38. A method according to claim 30 in which the field produced in the electrode and the externally produced field are in adding magnetic polarity.

39. The method according to claim 30 in which the field produced in the electrode and the externally produced field are in opposing magnetic polarity.

40. The method of operating an electric arc furnace having a furnace wall, adapted to receive at least partially conductive material to be melted which comprises producing an electric arc between the electrode and the material to be melted to reduce the material to a molten state, generating a first magnetic field within the tip, generating a second magnetic field within the furnace by independent field generating means disposed adjacent the wall of the furnace, the first and second magnetic fields interacting to produce a resultant field in the furnace in the volume thereof encompassing the electrode tip and extending to the surface of the melt, the resultant magnetic field controlling the arc to provide improved furnace operation, and utilizing a concentrated field coil with a yoke of magnetic material therearound, a portion thereof, disposed external to the second field generating means for independently creating within the furnace a third magnetic field in the portion of the volume of the furnace occupied by the melt, said third magnetic field being selectively in magnetic polarity opposition or magnetic polarity adding with respect to the second generated magnetic field, the additional resultant magnetic field pattern produced by interaction of the second and third magnetic fields providing improved furnace operation.

41. A method according to claim 40 in which the concentrated coil and the magnetic yoke therearound are additionally characterized as having their axial positions adjustable along the length of the second magnetic field producing means to control the furnace operation in at least one aspect to increase the stirring of the melt and increase the rate dispersion of additional material fed to the melt while in a relatively upper position along said axial length of the second magnetic field producing means, and while in a relatively lower axial position to have less effect on stirring the melt but an increased effect on the grain structure of an ingot formed in the furnace.

42. A method according to claim 40 including the additional step of evacuating the furnace to a sufficiently low pressure wherein a diffused arc discharge occurs between the electrode and the melt.

43. The method of operating an electric furnace according to claim 40 in which said furnace wall comprises primarily dielectric material.

44. The method of operating an electric arc furnace adapted to receive at least partially conductive material to be melted and adapted to have a controlled atmosphere therein comprising the steps of producing an arc between an electrode and the melt while maintaining the pressure within the furnace at a sufficiently low value whereby the arc goes into a diffused mode of operation, utilizing a solenoid adjacent the wall of the furnace and extending substantially from the bottom thereof to a position higher than that reached by the electrode as the position of the electrode is adjusted as the level of the melt increases to maintain a substantially constant arc length between the electrode and the melt, the magnetic field lines of the solenoid normally extending in a generally axial direction through the volume of the furnace and adjacent the electrode and between the electrode and the melt, said lines assisting in preventing an arc from striking from the electrode to the wall of the furnace and assisting in focusing the arc between the electrode and the melt, said lines tending to produce movement within the melt resulting in increased heating efficiency and more rapid dispersion of material in the melt thereby increasing the feed rate of material to the melt, and utilizing a compact magnetic field coil having a magnetic yoke therearound with a position adjustable axially along the length of the solenoid, moving the concentrated coil to an axial position adjacent the bottom of the melt to improve the grain structure of an ingot formed by the solidified portion of the material to be melted and improve the homogeneity of the ingot, and moving the concentrated coil to a position axially nearer to the surface of the melt, the concentrated field coil in said last-named position assisting in controlling the arc and assisting in improving the heating efficiency of the furnace.

45. A method according to claim 61 in which the magnetic field produced by the solenoid and the magnetic field produced by the concentrated coil are in magnetic polarity opposition.

46. A method according to claim 44 wherein the magnetic field produced by the solenoid and the magnetic field produced by the concentrated coil are in magnetic polarity adding.

47. The method according to claim 45 including the additional step of fluid cooling the arcing surface of the electrode to assist in reducing the rate of the erosion of material therefrom.

48. The method of operating an electric arc furnace to form an ingot composed of at least partially electrically conductive material which includes the steps of placing an initial amount of material in a mold, mounting an electrode within the furnace in spaced position with respect to the material, the electrode having a tip forming an arcing surface and a magnetic field coil therein close to the arcing surface, energizing the field coil to generate a first magnetic field, energizing an elongated solenoid mounted near the wall of the mold to generate a second magnetic field with magnetic field lines normally extending axially through the furnace, creating an arc between the electrode and the material, adjusting the energization of the solenoid whereby the strength of the magnetic field of the solenoid is relatively great compared to that of the tip field coil and a resultant magnetic field is produced by mutual interaction of the two magnetic fields which has a strong vertical component and maintaining said adjustment while the pool is shallow to exert large rotational forces on the material of the pool, and thereafter after the pool has become deeper with the fields in polarity adding increasing the strength of the tip-generated field with respect to the solenoid-generated field to decrease the vertical field component of the resultant field.