Abstract:
A method for controlling electromagnetic flux concentration in a discharge guide tube for a metal refining apparatus is provided. The discharge guide tube comprises a base plate, an extension, a central orifice that extends through the extension from a source of liquid metal to an outlet in the discharge guide tube for directing a stream of metal therethrough, and an interior discharge guide tube flux concentration configuration; an induction heater system that generates an electromagnetic field in the discharge guide tube, the induction heater system being disposed on the extension with a gap defined therebetween, the induction heater system and the discharge guide tube being capable of relative vertical movement and subsequent positions with respect to each other with the gap being essentially constant. The method for providing electromagnetic flux concentration comprises providing current to the induction heater system; generating an electromagnetic field resulting from the step of providing current; and directing an electromagnetic flux to the central orifice at locations defined by the interior discharge guide tube. The step of generating an electromagnetic flux also generates heat and the step of generating heat provides a control of the flow of the stream of metal in its liquidus condition. The electromagnetic field is applied at a substantially constant level regardless of the relative vertical movement and subsequent positions between the induction heater system and the discharge guide tube.

Description:
The government may have rights in this invention pursuant to Contract No. 1457-96-01288 awarded by DARPA. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to methods of providing and controlling electromagnetic flux in dispensing refined metal. In particular, the invention relates to providing, controlling, and concentrating electromagnetic flux during dispensing of refined metal using a copper guide tube. 
     Electroslag refining is an exemplary metal refining process that is used to melt and refine a wide range of alloys, including but not limited to superalloys, for removing various impurities therefrom. U.S. Pat. No. 5,160,532, issued to Benz et al., discloses an electroslag refining apparatus that is assigned to the Assignee of the present invention, General Electric. Other ESR structure are set forth in several U.S. patents issued to the Assignee of the present invention, General Electric, including U.S. Pat. Nos. 5,310,165; 5,325,906; 5,332,197; 5,348,666; 5,366,206; 5,472,097; 5,649,992; 5,649,993, 5,683,653, 5,769,151; 5,809,057; and 5,810,066, the contents of each are incorporated herein. 
     In general, an electroslag refining apparatus comprises an ingot being connected to a power supply, for example one of an alternating or direct current power supply. The ingots comprise unrefined alloys that may include various defects or impurities, which are desired to be removed during the refining process to enhance its metallurgical properties, including, but are not limited to, oxide cleanliness, grain size and microstructure. The ingot forms a consumable electrode that is suitably suspended in a water-cooled crucible, which contains a slag corresponding with the alloy being refined. The slag is heated by passing an electrical current from the consumable electrode through the slag into the crucible. The slag is maintained at a suitable high temperature for melting the lower end of the consumable electrode into an ingot melt. As the consumable electrode melts, a refining action takes place with oxide inclusions in the ingot melt being exposed to the liquid slag and dissolved therein. Refined liquid melt of the ingot melt falls through the slag by gravity, which may be augmented or diminished by electromagnetic forces or other means. The liquid refined melt is collected in a liquid melt pool at the bottom of the crucible. The slag, therefore, effectively removes various impurities from the melt to effect the refining thereof. The refined melt may be extracted from the crucible by an induction-heated, segmented, water-cooled copper guide tube. The refined melt extracted from the crucible thus provides a liquid metal source for various solidification processes including, but not limited to, powder atomization, spray deposition, spray forming, investment casting, melt-spinning, nucleated-casting, strip casting, and slab casting. 
     In the above-described electroslag refining apparatus, the crucible can be formed of copper, and is typically water-cooled to form a solid slag and/or metal skull on its surface. The solid slag or metal skull bounds the liquid slag and prevents damage to the crucible itself. The bottom of the crucible typically includes a water-cooled, cold hearth, which can be formed of copper, against which a solid skull of the refined melt forms for maintaining the purity of the collected melt at the bottom of the crucible. A discharge guide tube assembly below the hearth can also be formed of copper. The discharge guide tube assembly is often segmented and water-cooled and allows the formation of a solid skull of the refined melt for maintaining the purity of the melt as it is extracted from the crucible. The skulls can prevent contamination of the ingot melt from contact with the parent material of the crucible. 
     The electroslag refining apparatus also may include a plurality of water-cooled induction heating electrical conduits that surround the discharge guide tube. The conduits inductively heat the melt and the discharge guide tube can control the discharge flow rate through the discharge guide tube. Accordingly, the thickness of the skull formed around the discharge orifice may be controlled and suitably matched with melting rates of the consumable electrode for obtaining a substantially steady state production through the discharge guide tube. 
     The discharge guide tube and cold hearth of some electroslag refining apparatuses are generally structurally complex, and generally comprise a plurality of fingers or segments, which are surrounded by the induction heating electrical conduits. These induction heating electrical conduits are often single piece units that are typically provided with a set configuration to conform with the configuration of the discharge guide tube. The configuration is provided to define a gap between the induction heating electrical conduits and the discharge guide tube. This configuration is suitable for heating the melt in and about the discharge guide tube in electroslag refining applications. However, if one or both of the induction heating electrical conduits and discharge guide tubes are moved with respect to one another, the gap therebetween changes due to the single-piece configuration of the induction heating electrical conduits. Therefore, the heating of the melt in and about the discharge guide tube in electroslag refining applications may be influenced, often detrimentally. 
     The above-described electrical conduits generate, an electromagnetic field, and an associated electromagnetic flux within the discharge guide tube, generating heat in the liquid metal stream flowing therethrough. The intensity of the generated electromagnetic field and resulting electromagnetic flux is typically related to the heating capability of the guide tube apparatus. As the electromagnetic flux intensities increase, the heating capability within the discharge guide tube increases. A high field intensity and electromagnetic flux, and resultant high heating capability in a guide tube apparatus, is often desirable for creating an initial stream of metal, melting any undesired solid metal within the electroslag refining apparatus, and superheating the stream flowing through the discharge guide tube. The electromagnetic flux intensity in the electroslag refining apparatus can be enhanced by providing at least one of a high applied voltage in the electrical conduits and an increased number of induction heating elements disposed about the electroslag refining apparatus. 
     Further, the configuration and structure of the electroslag refining apparatus can limit a number of induction heating elements. Further, the current amount is limited by the configuration and structure of the induction heating elements and an availability of electrical energy. Thus, the electroslag refining apparatus may be limited in its the capability to control or enhance the electromagnetic flux and thus the flow in the discharge guide tube and the heating of the metal flowing therethrough. 
     Accordingly, a need exists to provide a method for controlling electromagnetic flux concentration in dispensing refined metal, for example, but not limited to, dispensing refined metal from electroslag refining apparatuses. 
     SUMMARY OF THE INVENTION 
     In an aspect of the invention, method for controlling electromagnetic flux concentration in a discharge guide tube for a metal refining apparatus is provided. The discharge guide tube comprises a base plate, an extension, a central orifice that extends through the extension from a source of metal to an outlet in the discharge guide tube for directing a stream of metal therethrough, and an interior discharge guide tube flux concentration configuration; an induction heater system that generates an electromagnetic field in the discharge guide tube, the induction heater system being disposed on the extension with a gap defined therebetween, the induction heater system and the discharge guide tube being capable of relative vertical movement and subsequent positions with respect to each other with the gap being essentially constant. The method for controlling electromagnetic flux concentration comprises providing current to the induction heater system; generating an electromagnetic field resulting from the step of providing current; and directing an electromagnetic flux to the central orifice at locations defined by the interior discharge guide tube flux concentration configuration in response to the generating an electromagnetic field. The step of generating an electromagnetic flux also generates heat and the step of generating heat provides a control of the flow of the stream of metal in its liquidus condition. The electromagnetic field is applied at a substantially constant level regardless of the relative vertical movement and subsequent positions between the induction heater system and the discharge guide tube. 
     These and other aspects, advantages and salient features of the invention will become apparent from the following detailed description, which, when taken in conjunction with the annexed drawings, where like parts are designated by like reference characters throughout the drawings, disclose embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of an electroslag refining apparatus for a method, as embodied by the invention; 
     FIG. 2 is a schematic illustration of a discharge guide tube for a method, as embodied by the invention; 
     FIG. 3 is a bottom illustration of a discharge guide tube for a method, as embodied by the invention; 
     FIG. 4 is a top, partial section illustration of a discharge guide tube of FIG. 2 taken along line  3 — 3 ; 
     FIG. 5 is a schematic, partial section illustration of a discharge guide tube for a method, as embodied by the invention; 
     FIG. 6 is a schematic, partial section detailed illustration of a discharge guide tube of FIG. 1 for a method, as embodied by the invention; 
     FIG. 7 is a schematic, partial section detailed illustration of another discharge guide tube for a method, as embodied by the invention; and 
     FIG. 8 is a graph of field intensity versus cold-walled induction guide tube axial tube distance. 
    
    
     DESCRIPTION OF THE INVENTION 
     A method for controlling and concentrating flux during dispensing from a metal refining apparatus and process comprises disposing an induction heater assembly on a discharge guide tube with a gap defined therebetween. The metal refining apparatus and process comprise any appropriate metal refining apparatus and process, such as, but not limited to, an electroslag refining apparatuses and processes, vacuum-induction metal apparatuses and processes, induction melt apparatuses and processes, electron beam cold hearth apparatuses and processes, plasma arc apparatuses and processes, vacuum arc remelting apparatuses and processes, and others. The following description will refer to electroslag refining apparatuses and processes as exemplary refining apparatuses and processes in which refined metal is dispensed therefrom. This refining apparatus and process is merely for description purposes, and other metal refining apparatuses and processes are within the scope of the invention. 
     The discharge guide tube comprises an interior discharge guide tube flux concentration configuration concentrating electromagnetic flux and consequent heating of a stream of refined metal flowing through a central orifice in the discharge guide tube. The method, as embodied by the invention, will be described hereinafter with respect to a configuration of an electroslag refining apparatus  10  and discharge guide tube  22 . 
     An electroslag refining apparatus  10  for conducting the method, as embodied by the invention, is schematically illustrated in FIGS. 1 and 2. The electroslag refining apparatus  10  comprises a cylindrical crucible  12  in which an ingot  14  (also known in the art as a “consumable electrode”), which is to undergo electroslag refining, is suspended. A feed device  16  is provided for feeding the consumable electrode  14  into the crucible  12  at a suitable feed rate, as known in the art. The feed device  16  includes, but is not limited to, a suitable drive motor and transmission  16   a,  that rotate a screw  16   b  that in turn lowers, or translates downwardly, a support bar  16   c  that is fixedly joined at one end to the top of the consumable electrode  14 . Although the illustrated configuration of the electroslag refining apparatus  10  illustrates a consumable electrode  14  as the source of metal to be electroslag refined, the scope of the invention comprises other suitable sources, such as but not limited to a powder source or a liquid metal source. 
     The consumable electrode  14  comprises a suitable alloy to be electroslag refined, in which the alloy comprises nickel-, iron-, iron-nickel-, or cobalt-based alloy or superalloy. A slag  18  is provided inside the crucible  12 . The slag  18  comprises any suitable composition for refining the consumable electrode  14 . A heater device  20  is provided for melting the tip of the consumable electrode  14  as the consumable electrode  14  is fed into the crucible  12 . The heater device  20  includes a suitable electrical current power supply  20   a  that is electrically connected to the consumable electrode  14 . The heater supply  20   a  is connected to the consumable electrode  14  through the supporting bar  16   c,  for example by an electrical lead  20   b.  Electrical current is carried through the consumable electrode  14 , and through the liquid slag  18  to the crucible  12 . Therefore, the slag  18  is resistively heated to a temperature that is suitable to melt the bottom end of the consumable electrode  14 . Although the illustrated configuration of the electroslag refining apparatus  10  illustrates a consumable electrode  14  as the source of electrical current, the scope of the invention comprises other suitable sources, such as but not limited to an unconsumed electrode. 
     A copper discharge melt guide  22  is removably attached to a bottom  12   a  of the crucible  12 . The discharge melt guide  22  comprises a central orifice  32  that includes a configuration to concentrate electromagnetic flux in the central orifice  32 , so as to more efficiently heat metal therein, as described hereinafter. The discharge melt guide  22  encloses the bottom of the crucible  12 . An electrical path can be provided between the power supply  20   a  by an electrical lead  20   c.  The slag  18  is heated by the power supply  20   a,  and this heating causes a bottom tip of the consumable electrode  14  to be correspondingly heated and melted. Liquid refined melt of molten metal, or simply liquid refined melt  14   a  from the consumable electrode  14  is formed. The liquid refined melt  14   a  falls through the slag  18  and collects in a liquid metal pool or reservoir  24 , which is disposed at the bottom of the crucible  12 . 
     The electroslag refining apparatus  10  includes a cooling system  26  that cools the crucible  12  during operation of the electroslag refining apparatus  10 . The cooling system  26  can comprise a coolant supply  26   a  that is effective for pumping a coolant  26   c,  such as, but not limited to water, through a cooperating cooling jacket  26   b  that is disposed around the crucible  12 . The crucible  12  and cooling jacket  26   b  may be an integral assembly. Alternatively, the crucible  12  and cooling jacket  26   b  may be discrete components connected together in thermal cooperation. The cooling jacket  26   b  includes suitable channels or conduits that extend therethrough, and coolant  26   c  is circulated for removing heat from the crucible  12  during operation of the electroslag refining apparatus  10 . 
     A solid slag skull  18 a can form inside the crucible  12  around the liquid slag  18 . The solid slag skull  18 a can isolate the crucible  12  from the liquid slag  18  and the metal that can fall through the electroslag refining apparatus  10 . Electroslag refining of the consumable electrode  14  occurs as the metal liquid refined melt  14   a  are exposed to the slag  18 . The slag  18  dissolves inclusions, such as but not limited to oxide inclusions and nitrogen inclusions, from the liquid refined melt  14   a.    
     The crucible  12 , which typically comprises copper, is isolated from the refining process by the solid slag skull  18   a.  Therefore, the crucible  12  does not contaminate the ingot melt  14   a.  The refined melt collects in the reservoir  24  at the bottom of the crucible  12 . The ingot skull  14   b  isolates the refined melt from the crucible  12  and prevents contamination of the melt by the crucible  12 . During operation, the liquid slag  18  floats atop the pool of refined melt that is collected above the discharge melt guide  22 . 
     The discharge melt guide  22  is illustrated in FIG. 2, is configured to generate, electromagnetic flux intensity and concentration of heating when compared to conventional melt guides, such as in U.S. Pat. No. 5,809,057, to Benz, which is assigned to the Assignee of the instant invention and fully incorporated herein. It has been determined that the amount of induction heating in the discharge guide tube is generally proportional to the square of the applied electromagnetic field, and that with the interior discharge guide tube flux concentration configuration, as described hereinafter, provides electromagnetic flux in a substantially constant levels with changes in coil position. 
     FIG. 2 provides an enlarged view of the discharge melt guide  22  enclosing the bottom  12   a  of the crucible  12 . In FIG. 2, the crucible  12  comprises a solid cylindrical member, with its bottom  12   a  formed as an annular radial flange. The coolant jacket  26   b  comprises a double-walled cylinder that surrounds the crucible  12 . The coolant jacket  26   b  is hollow for receiving the coolant  26   c  for cooling the crucible  12 . 
     The discharge melt guide  22  comprises a substantially flat base plate  28 . The base plate  28  can be formed from an appropriate heat and electrically conductive material, such as but not limited to, copper. The base plate  28  comprises a circular disk that is complementary to the configuration of the crucible  12 , and comprises an upper perimeter  28   a  with a diametrical portion that engages the crucible bottom  12   a.  Thus, the discharge guide tube  22  can form a sealed attachment to the crucible  12 . Also, fasteners  30 , for example a plurality of circumferentially spaced apart bolts and cooperating nuts, can removably attach and seal the base plate  28  to the crucible bottom  12   a.  In FIG. 2, the fasteners  30  extend through apertures in the perimeter of the base plate  28  that are aligned with apertures disposed in a suitable annular flange around the base of the coolant jacket  26   b.  A gasket, or other such sealing element, may be provided between the base plate  28  and the crucible bottom  12   a  to be compressed therebetween upon assembly of the fasteners  30  that secure the base plate  28  to the bottom of crucible  12 . 
     The base plate  28  of the discharge guide tube  22  comprises an upper surface  28   b,  which together with the crucible  12 , defines the reservoir  24  for receiving and pooling liquid refined melt  14   a.  The base plate  28  also comprises an external or lower surface  28   c  that is spaced below the upper surface  28   b.  In FIG. 2, both surfaces  28   b  and  28   c  are illustrated as substantially flat and parallel to each other. This illustrated configuration is merely exemplary and is not intended to limit the invention in any way. The scope of the invention includes other configurations of these features, such as not limited to, concave, convex, arcuate, and combinations thereof, with or without flat and parallel features. 
     The base plate  28  also comprises includes a central orifice  32  (also known in the art as a “discharge guide tube orifice”) formed in an extension  29  that extends to an outlet  100 . The central orifice  32  extends generally vertically through the base plate  28  between its upper and lower surfaces,  28   b  and  28   c  respectively. The central orifice  32  allows refined melt  14 a to be drained from the reservoir  24 , for example, and in no way limiting of the invention, by at least one of gravity flow, pressure-induced flow, and vacuum-induced flow on the bottom of the refined melt  14   a.  The discharge melt guide  22  comprises a central orifice  32  that includes an interior discharge guide tube flux concentration configuration to enhance heating, by concentrating electromagnetic flux in the central orifice  32 , as described hereinafter. 
     The base plate  28  comprises at least one slot  34  formed therein, as illustrated in FIG.  3 . Each slot  34  can extend vertically through the base plate  28 . Each slot  34  directs the electromagnetic flux in the electroslag refining apparatus, as described hereinafter. The base plate  28  may alternatively comprise a plurality of spaced apart slots  34 , for example slots that are equiangularly, circumferentially spaced from each other, and that extend radially outwardly from the orifice  32  toward the perimeter of the base plate  28 . 
     A plurality of equiangularly, circumferentially spaced slots  34  are illustrated in FIG.  4 . In FIG. 4, four slots  34  are illustrated as being disposed about 90 degrees apart from each other, however, this configuration is merely exemplary of the slot configurations within the scope of the invention. The scope of the invention comprises any suitable number of slots  34  in the base plate  28 . The slots  34  may also be formed using electrodischarge machining (EDM). Alternatively, other known metallurgical processes may be used to form the slots  34 , and are within the scope of the invention. The slots  34  can be gas filled, or filled with an electrical insulation  36 , such as, but not limited to, an epoxy polymer. 
     The slots  34  define a plurality of arcuate segments or fingers  28   d.  The illustrated configuration (FIG. 4) shows four fingers  28   d  (alternatively referred to as “segments”) defined between the slots  34 . The cooling system  40  for the base plate  28  can comprise channels  40   a  that extend inside each of the fingers  28   d  for circulating a coolant  26   c  therethrough. The cooling system  40  can be provided with its own source of coolant. Alternatively, the cooling system  40  may be disposed in parallel with the cooling supply  26   a,  and use the coolant that cools the crucible  12 . This configuration is merely exemplary of the scope of the invention and is not intended to limit the invention in any manner. Each finger  28   d  can comprise an approximate 90-degree corner. The cooling channels  40   a  in the discharge guide tube  22  may be formed by drilling cylindrical holes radially inwardly from the outer perimeter of the base plate  28 . Adjacent channels  40   a  may be disposed therein to converge radially inwardly and intersect near the central orifice  32 , thus providing supply and return paths for coolant flow. 
     In the illustrated exemplary embodiment of FIGS. 2 and 4, the cooling system  40  also includes a pair of coolant manifolds  40   b  and  40   c,  which may be integrally formed with the base plate  28 . Alternatively, the manifolds  40   b  and  40   c  may be attached around the perimeter of the base plate  28 , with the supply manifold  40   b  being disposed in communication with a channel  40   a  for supplying the coolant. The return manifold  40   c  may be disposed in communication with channels  40   a  for receiving return coolant. The manifolds  40   b  and  40   c  can be connected to the coolant supply  26   a  for circulating the coolant. 
     The discharge melt guide  22  comprises an induction heater system  38 . The induction heater system  38  is disposed proximate and below the base plate lower surface  28   c  for induction heating the refined melt  14   a.  The term “proximate” means near or close to, and is used with a meaning as understood by those of skill in the art with its conventional meaning. The electromagnetic flux generated by the induction heater system  38  can be transmitted to those and other elements of the electroslag refining apparatus  10 , resulting in heating as desired . The induction heater system  38  may take any conventional configuration including, but not limited to, an annular or spiraling induction heater system configuration. 
     The coils of the induction heater system  38  can be disposed coaxially about the central orifice  32  and can extend radially over the slots  34 . As illustrated in FIG. 1, the induction heater system  38  can include one or more suitable power supplies  38   p  that provide electrical current that is sufficient for induction heating. The induction heating coils can comprise hollow coils that circulate a suitable coolant, such as, but not limited to, water. 
     The induction heater system  38  can comprise primary coils  38   a,  which are generally co-axial with the central orifice  32  along the extension  29 , and secondary coils  38   b  that are generally disposed on surface  28   c  of the base plate  28 . The induction heater system  38  is disposed on the extension  29  of the discharge melt guide  22  to define a gap therebetween. The disposition of the induction heater system  38  on the extension  29  allows movement of induction heater system  38  in the direction of arrow  500  (FIG. 6) with the gap remaining constant. The constant nature of the gap allows generated heat, and electromagnetic flux from the induction heater system  38  to be substantially unaffected by movement of the induction heater system  38  in the direction of arrow  500 . The term “substantially” is used with its normal meaning as understood by those of skill in the art. 
     The primary coils  38   a  and secondary coils  38   b  of the induction heater system  38  may define independent primary and secondary coils,  38   a  and  38   b,  respectively. Alternatively, the primary coils  38   a  and secondary coils  38   b  may be connected to create an integral coil structure for the induction heater system  38 . The primary coils  38   a  and secondary coils  38   b  may take any appropriate coil configuration and structure, such as, but not limited to, water-cooled, current-carrying conduit coils. For example, the primary coils  38   a  and secondary coils  38   b  may be overlapped at at least one portion, such as but not limited to, coiled onto themselves. The illustrated configurations are merely exemplary of the structures within the scope of the invention, and are not intended to limit the invention in any manner. 
     The primary coil  38   a  of the induction heater device  38   p  can be disposed adjacent to and surrounding the central orifice  32  for heating the refined melt  14   a  that is discharged therethrough, as embodied by the invention. The primary coil  38   a  may also control a thickness of the corresponding skull  14   b  that is disposed proximate the central orifice  32 . The secondary coils  38   b  can be spaced radially outwardly from the primary coil  38   a,  and may radially overlap (not illustrated). The secondary coil  38   b  can comprise a sufficient number of turns for sufficiently heating the refined melt  14   a  around and in the central orifice  32 . The secondary coils  38   b  can also control a thickness of the ingot skull  14   b,  as described above. 
     The central orifice  32  of the discharge guide tube (also known as a “discharge guide tube”)  22  comprises an internal configuration with a profile that concentrates heat, electromagnetic field, and electromagnetic flux (hereinafter “heat and electromagnetic flux” since the electromagnetic flux is produced by the electromagnetic field) to the stream of liquid melt that flows therethrough. As illustrated in FIG.  2  and in detail in FIGS. 5 and 6, the central orifice  32  comprises an angled and stepped profile about a central longitudinal axis  138  of the central orifice  32 . The stepped and angled profile can be formed by an inclined (first) central orifice ramp portion  130  that extends from a first central orifice portion  131  to a reduced diameter central orifice portion  132 , which defines a constriction  133 , and then to another (second) inclined central orifice ramp portion  134 , which in turn leads to a further (second) central orifice portion  135 . The diameter of the second central orifice portion  132  is less that the diameter of the first central orifice portion  131 . In other words, the ramp portion leads to a constriction  133  in the central orifice  32  at which point the flux concentration in the central orifice  32  is higher than at other portions of the central orifice  32 , such as at the first central orifice portion  131 . 
     The configuration of the central orifice  32  at which the constriction  133  is formed can comprise any appropriate configuration that can produce the concentrations of electromagnetic flux, as embodied by the invention. The angles for the inclined central orifice ramp portion  130  and  134  can comprise any angles that form the constriction  133 . The lengths of the inclined central orifice ramp portions  130  and  134  can be equal. Alternatively, lengths of the inclined central orifice ramp portions  130  and  134  need not be equal. Further, the length of the second central orifice portion  132  may vary. Also, the positioning of the second central orifice portion  132 , and thus the constriction, in the central orifice  32  may vary. For example, the positioning of the second central orifice portion  132  and the constriction in the central orifice  32  may equi-spaced in the central orifice  32 , be disposed closer to the electroslag refining apparatus  10  than an exit from the central orifice  32 , or be disposed closer to the exit from the central orifice  32  than to the electroslag refining apparatus  10 . Accordingly, the positioning and configuration of the features of the central orifice  32  to produce the heat and electromagnetic flux can vary, as long as the intended purpose of the invention is accomplished. 
     The central orifice  32 , including the ramp portion  130  that extends from a first central orifice portion  131  to a second central orifice portion  132 , may be formed by any metallurgical machining suitable process. The scope of the invention includes forming the central orifice  32  by a machining processes, such as, but not limited to, at least one of drilling, lathe turning, and electrodischarge machining (EDM). If drilling is used to form the central orifice  32 , a drilling process provides a passage, which has a diameter that generally equal to that of the constriction  133 , and larger diameters at the first central orifice portion  131  may be provided by further drilling. Thus, the central orifice  32  is formed to be generally perpendicular to the upper and lower surfaces,  28   b  and  28   c  respectively, of the base plate  28 . Therefore, the liquid refined melt  14   a  can flow straight downwardly by gravity from the central orifice  32  during electroslag refining operations. The above-described drilling is merely exemplary of formation processes within the scope of the invention, and other formation processes can be used herein. 
     The induction heating system  38  is disposed along the lower surface  28   c  of the base plate  28  in a generally concentric configuration to be disposed around the central orifice  32 . The coils of the induction heating system  38  are also disposed over the radial extent of the slots  34  for transmitting electromagnetic energy into the melt  14   a.  The heat and electromagnetic flux are transmitted to the constriction  133  in the central orifice  32  in amounts sufficient to maintain the stream of refined liquid melt  14   a  that is flowing through the central orifice  32  in a liquidus state for continued flow through the central orifice  32 , as embodied by the invention. Further, the heat and electromagnetic flux can control the flow of refined liquid melt through the central orifice  32 , as embodied by the invention. For example, if higher heat and electromagnetic flux is applied to the electroslag refining apparatus  10 , more refined liquid melt  14   a  will remain in a liquidus state, and less liquid refined melt  14   a  will solidify into the skull  14   b.  Further, some of the skull  14   b  proximate the central orifice  32  will melt, by the application of the heat and electromagnetic flux, so restrictions around the central orifice  32  will be reduced. Thus, flow through the central orifice  32  will be facilitated, and less skull  14   b  will be formed. 
     Conversely, the flow through the central orifice  32  can be lessened by reducing the amount of heat and electromagnetic flux that is applied to the electroslag refining apparatus  10 , as embodied by the invention. With less electromagnetic field generating less heat, there will be less concentrated electromagnetic flux at the constriction  133  in the central orifice  32 . Less electromagnetic flux at the constriction  133  will slow flow of the liquid refined melt  14   a  through the central orifice  32 . Thus, an amount of time that the liquid refined liquid melt  14   a  is in contact with the skull  14   b  increases, and with more time in contact with the skull  14   b,  the refined liquid melt  14   a  may solidify against the skull  14   b.  Accordingly, more skull  14   b  may form around the area at the central orifice  32  and restrict flow into and through the central orifice  32 , thereby controlling the flow in the electroslag refining apparatus  10 . The heat melts the skull may reach a limit that is bounded by the generated electromagnetic flux. At a point in the generation of the heat, the electromagnetic flux may cause a field constriction in the central orifice  32  that may limit flow. The configuration of the central orifice  32  and the interior discharge guide tube flux concentration configuration defines the electromagnetic flux amount at which point the electromagnetic flux will cause restriction in the central orifice  32 . Thus, further control on the flow through the central orifice  32  can be realized. 
     For example, and in no way limiting the invention, the flow of the refined melt  14   a  through the central orifice  32  may also be controlled, so as to be approximately equal to the melt rate of metal from the consumable electrode  14 . Accordingly, a generally steady-state flow in the electroslag refining apparatus  10  can be provided. The induction heating of the liquid refined melt  14   a  through the slots  34 , and cooling of the base plate  28  around the central orifice  32 , can be provide a balanced flow relationship during startup and steady-state operations of the electroslag refining apparatus  10 . 
     FIG. 7 is a part-sectional illustration of another central orifice  232  within the scope of the invention. In FIG. 7, similar reference characters that are used in the above figures are used to reference like features. The central orifice  232  is formed in a base plate  128  of a discharge guide tube  22  and comprises an internal configuration with a profile that concentrates electromagnetic flux and heat, to the stream of liquid melt that flows therethrough. The central orifice  232  comprises an angled and stepped profile about a central longitudinal axis  138  of the central orifice  232 . 
     The base plate  128  comprises and is formed in a similar manner as the above-described base plate  28 , however, the base plate  128  includes an extension  129  that defines a base plate chamber  150 . The base plate chamber  150  is bounded by an upper wall  151  and side walls  152 . The central orifice  232  extends from the electroslag refining apparatus  10  to the wall  151  of the base plate  128 . The stepped and angled profile of the central orifice  232  can be formed by an inclined central orifice ramp portion  230  that extends from a surface  128 ′ of the base plate  128  to a constriction  233  in the central orifice  232 . The constriction  233  may comprise a single constriction portion  235  (solid lines in FIG. 7) that extends over the entire central orifice  232  length and may terminate at the wall  151 . Thus, the stream  114   a  flows into the chamber  150 . Alternatively, the constriction  233  may comprise a constriction portion  235  that extends partially down the central orifice  232  length, and widens into an inclined portion  234  (broken lines in FIG. 7) that in turn terminates at the wall  151 . 
     The configuration of the central orifice  232  at which the constriction  233  is formed can comprise any appropriate configuration that can produce the concentrations of electromagnetic field. The angles for the inclined central orifice ramp portion  230  and  234  can comprise any angles that form the constriction  233 . The lengths of the inclined central orifice ramp portions  230  and  234  can be equal. Alternatively, lengths of the inclined central orifice ramp portions  230  and  234  need not be equal. Further, the length of the constriction portion  235  may vary. Also, the positioning of the constriction portion  235  in the central orifice  232  may vary. For example, the positioning of the constriction portion  235  may be equi-spaced in the central orifice  232 , be disposed closer to the electroslag refining apparatus  10  than an exit from the central orifice  232 , or be disposed closer to the exit from the central orifice  232  than to the electroslag refining apparatus  10 . Accordingly, the positioning and configuration of the features of the central orifice  232  to produce, electromagnetic flux and heating can vary, as long as the intended purpose of the invention is accomplished. 
     The discharge guide tube  22  may be used in cooperation with an electroslag refining apparatus  10 , as illustrated in FIG.  1 . The discharge guide tube  22  can be used in conjunction with any subsequent processing of the stream of liquid refined melt  14   a  that is discharged from the electroslag refining apparatus  10 . For example, and in no way limiting of the invention, atomizing structure may be provided for injecting a suitable atomizing gas from a gas supply to atomize the stream  114   a  of liquid refined melt  14   a  discharged from the central orifice  32 , as embodied by the invention. Alternatively, the discharge guide tube  22  may be used in cooperation with device as set forth in U.S. Pat. No. 5,381,847 to Ashok et al. 
     FIG. 8 is a graph of field intensity versus guide tube axial distance for a discharge melt guide, as embodied by the invention. The discharge guide tube that is graphed in FIG. 8 comprises a four-turn induction heater coil structure. The coils carry about 87 mA at about 143,000 Hz. The generated electromagnetic field along the axis  138  is measured with a search coil in order to determine the field in the central orifice  32 . In the figure, alternating current magnetic fields are measured for a simple coil, a straight un-constricted discharge guide tube configuration as set forth in U.S. Pat. No. 5,809,057, and a discharge guide tube configuration, as embodied by the invention. As illustrated in FIG. 8, the discharge guide tube configuration provides a clear enhancement of the generated electromagnetic field. 
     While various embodiments are described herein, it will be appreciated from the specification that various combinations of elements, variations or improvements therein may be made by those skilled in the art, and are within the scope of the invention.