US 3660585 A
A frozen shell metal melting means comprising a crucible which is substantially shallow in depth compared to its relatively greater horizontal dimension and heating means disposed above said crucible adapted to melt metal therein; and means at the bottom of said crucible to permit radiation of heat therefrom in sufficient amount to maintain a frozen shell of metal in contact with the bottom of said crucible.
Description (OCR text may contain errors)
United States Patent Waldron  FROZEN SHELL METAL MELTING MEANS  Inventor: Robert D. Waldron, 5620 N. 69th Place,
Paradise Valley, Ariz. 85253  Filed: June 24, 1970  Appl. No.: 49,332
Related US. Application Data  Continuation-impart of Ser. No. 733,719, May 31,
52 u.s.c1 ..13/25,13/26,13/3l, 219/349,219/405,219/421,219/1075 s1 1m.c1. ..H05b3/66 5s FieldofSearch ..13/20,22,25,31; 75/1042; 219/405, 411, 347-349, 420, 421, 426,
 References Cited UNITED STATES PATENTS 3,343,828 9/1967 Hunt ..l3/31X [4 1 May 2, 1972 3,409,729 11/1968 Hanks et al ..13/31 2,798,108 7/1957 Poland ..l3/31 1,817,027 8/1931 Anderson 3,243,174 3/1966 Sweet 2,772,318 11/1956 Holland 2,469,412 5/1949 Roebken 2,789,152 4/1957 Ham et al 3,244,859 4/ 1966 Whiteford 3,472,941 10/1969 Floymayr 13/26 Primary Examiner-Velodymyr Y. Mayewsky Attorney-Drumm0nd, Cahill and Phillips, William H. Drummond, William C. Cahill and James H. Phillips [57 ABSTRACT A frozen shell metal melting means comprising a crucible which is substantially shallow in depth compared to its relatively greater horizontal dimension and heating means disposed above said crucible adapted to melt metal therein; and means at the bottom of said crucible to permit radiation of heat therefrom in sufficient amount to maintain a frozen shell of metal in contact with the bottom of said crucible.
14 Claims, 20 Drawing Figures Patented May 2, 1972 3,660,585
3 Sheets-Sheet l FIG. '8. 2. 2| 23 L: 23
ROBERT D. WALDRON sci IE Patented May 2, 1972 3 Sheets-Sheet 2 FIG] FIGJZ J N V [:1 N TOR.
ROBERT D. WALDRON FROZEN SHELL METAL MELTING MEANS This application is a continuation-in-part of my application Ser. No. 733,719, filed May 31, 1968 now abandoned, entitled Frozen Shell Metal Melting Means." v
Various frozen shell metal melting means and the methods have been employed for the purpose of maintaining frozen shells of metals being melted to prevent reaction or contamination of the melted metals with the material of the crucible. Many of the prior art methods have utilized conventional arc heating means wherein the metals being melted are melted in a substantially inert atmosphere with an are passing between electrodes of the metals beingmelted, and such that these metals, as the arc melts them, fall into the crucible into a central area thereof, such that superheating of the metals is excessive and may, in many instances, tend to vaporize some of the elements of the alloys of the metals being melted. Additionally, control of the frozen shell in the crucible, when are methods are employed, is oftentimes more critical, such that water cooling of the crucible is required.
It will be understood that prior art are melting methods, while they may be used to melt metals in frozen shell melting systems have several disadvantages, namely, a tendency toward the vaporization of some elements of alloys due to excessive superheating of the metal during the time that an arc is operating between an electrode of the metals being melted v and the crucible. And additionally, other problems which have to do with cooling the crucible to maintain a suitable protective shell for the molten metal to prevent it from reacting or being contaminated with the material of the crucible are well known. These problems, well known in the prior art, involve rather excessive power requirements, long cycle time, difficulties in maintaining superheating control of the melted metal, as well as maintenance and a lack of operation flexibility of the facility used to maintain frozen shell melting of various metals. p
. The present invention relates to a very simple frozen shell melting means which includes a shallow and relatively broad crucible having horizontal dimensions much greater than the depth thereof, and whereby metals may be melted in the crucible by heating means disposed thereabove, and wherein lower surface structure of the crucible radiates sufficiently to maintain a frozen shell of the molten metal; said frozen shell being in contact with the crucible and of a great horizontal area as compared to the vertical thickness thereof.
The means of the invention provides for uniform heating of metals in the crucible. These metals may be placed in the crucible in small particles or layers and various means below the crucible may be used to vary the migration of thermal energy from the bottom of the crucible accurately to maintain a frozen shell of the molten metal in contact with the crucible.
Additionally, the invention involves an overall combination of elements and method which maintains desirable control of the superheating of the metals so as to avoid vaporizing some elements of various alloys which may be heated in the crucible.
Additionally, the invention provides a means and method for rapid melting and cycling for repetitive batches of various metals, this being due in part to the proportions of the crucible and the relationship between the heating and the heat dissipation means utilized to maintain a frozen shell of the molten metal against or in contact with the crucible.
The foregoing advantages of the invention also provide for relative economy, high cycle rate and control for superheating so as to afiord maximum control of the thermal characteristics of the molten metal when used in subsequent casting of parts from the molten metal. I
Accordingly, it is an object of the invention to provide an improved means and methodfor frozen shell melting of metals which is very simple and economical to operate, efficient and which provides various advantages as aforementioned.
Further objects and advantages of the present invention may be apparent from the following specification, appended claims and accompanying drawings, of which:
FIG. 1 is a diagrammatic view showing a frozen shell melting means of the invention, wherein a heating means is suspended above a crucible generally disclosed in section and employing radiant reflectors surrounding an area between the heating means and the crucible; I
FIG. 2 is a view similar to FIG. 1, but showing superimposed radiation shields above the heating means of the invention and a movable radiation inhibitor vertically adjustably mounted in spaced relation below the crucible of the invention;
FIG. 3 is a view similar to FIG. 1, but showing a modification of a radiation inhibiting means adapted to inhibit radiation from the bottom of the crucible of the invention;
FIG. 4 is another view similar to FIG. 1, but showing a further modification of the invention, wherein a perforate radiation shield is disposed in spaced relationbelow the crucible and above a secondary movable radiation inhibiting shield;
FIG. 5 is a perspective view of a crucible of the invention and showing a generally cup-shaped configuration of the crucible and conforming cup-shaped'configuration of an induction heating coil adapted to be located below the crucible and to partially compensate for radiation of thermal energy from the bottom of the crucible;
FIG. 6 is a diagrammatic view of an incandescent resistance heating element adapted to be disposed above the crucible of the invention;
FIG. 7 is a view similar to FIG. 6, but showing a further modification of the crucible heating means of the invention;
FIG. 8 is a diagrammatic view of a coil-type incandescent resistance heating element to be used as a heating means for the crucible of the invention;
FIG. 9 is a diagrammatic view of zigzag heating element showing a modification of that as disclosed in FIG. 8;
FIG. 10 is an induction heating means of the invention employing an incandescent cone to be used for heating the curdble of the invention;
FIG. 11 is a fragmentary diagrammatic view of an incandescent plate of an induction heating means of the invention which is a modification of that shown in FIG. 10;
FIG. 12 is a view similar to FIG. 6, but showing an induction heating means relative to a corrugated sheet;
FIG. 13 is a further modification of the heating means of the invention relatedto electron beam heating of the crucible of the invention;
FIG. 14 is a diagrammatic view showing a direct induction zigzag coil to be utilized for heating the crucible of the invention;
FIG. 15 is a diagrammatic view of a double spiral direct induction coil for heating the crucible of the invention;
FIG. 16 is a diagrammatic view of the three-phase Y connected double coil operable as a direct induction coil for heating the charge in the crucible of the invention;
FIG. 17 is a diagrammatic view of a further modification of the invention disclosing a spiral configuration of a polyphase direct induction coil used for heating the charge in the crucible of the invention;
FIG. 18 is a fragmentary perspective view of rectangular in cross section conductors of the invention;
FIG. 19 is a side elevational view of molten metal being poured from the crucible of the invention through a heated radiation shield into a mold; and v a FIG. 20 is a side elevational view of the crucible of the invention being tilted and causing molten metal to be poured through a radiation shield into a mold.
THe following is a general description of the invention, together with basic considerations concerning the function of the invention, as will be hereinafter described in detail. While the following description will'refer specifically to titanium, it will be understood that the method is limited only in regard to the melting range of the metals chosen. At the low temperature end, the method becomes inefficient due to the inability to remove sufficient heat by radiation, and at the high temperature and by the lack of a suitable refractory radiator or other heat source. The method should be capable of satisfactory performance for steels, cobalt and nickel-base alloys on the low melting range to tantalum on the refractory metal end. The choice of crucible and radiator material must, of course, be determined by the specific metal and melting range selected.
Any frozen shell melting system may be characterized by three characteristic temperatures, the freezing temperature, T,, the skin temperature, T,, and the melt temperature, T,,,. For successful foundry use, T, must be sufficiently depressed below T, to afford a safety factor, while T,, must be elevated above T, to provide sufiicient superheat to allow for satisfactory pouring. Since metals are generally good thermal conductors, the presence of a temperature gradient T -T, implies a sizeable heat flow. Titanium is one of the poorer conductors so this heat flux is somewhat smaller than encountered with some other metals.
In a conventional water-cooled, copper container it is necessary to hold T, not only below the melting point of copper, but in fact below the boiling point of water. This arises since the thermal conductivity of copper is so much higher than that of titanium that practically the entire temperature difference will occur through the titanium and satisfactory cooling demands that generation of steam in the water lines be prevented. Thus, a temperature differential of some 3,000 F. is typically encountered with a massive heat flux.
ln contrast, by utilizing a container with a melting point higher than the melt such as molybdenum or tungsten, it is only necessary to depress the skin temperature by some 100-200 F. to prevent accidental contact with liquid titanium. In this method, a total temperature differential of 300-40 F. may be sufficient resulting in a much smaller heat fiux. This flux may be most conveniently removed by radiation and/or gas flow. Since the thermal conductivity of M0 is approximately four times that of pure Ti and even higher in comparison with commercial alloys, the temperature drop through a thin-walled container will be negligible and optical monitoring of the outer surface will adequately insure against melting of the frozen crust.
, The principal problems involved in development of this method involve the geometry of heat flow to obtain an essentially isothermal outer wall. According to the invention, several auxiliary aids are available to aid in this task.
According to this invention, due to the lower heat flux and power loss, the power requirements are somewhat reduced and several methods of heating seem practical. Possible Systems are induction heating, radiant heating from a resistively, inductively, or electron beam heated source, or possibly an arc image method as well as solar or gas fired furnaces. The preferred arrangement consists of a top heater with a broad shallow container of molybdenum cooled by radiation on the lower surface.
The steady state power loss due to radiation from a hot body depends on the absolute temperature, emissivity, surface area, and temperature of surrounding bodies. Thus, the radiant intensity is given by I GUT. where e emissivity and o'= 5.67 X 10' watt/m (K) At 2,000 K. a black body radiates 91 watt/cm 5 85 .wattlin For successful frozen shell melting, the melt geometry must be controlled to provide a nearly isothermal lower surface. According to the invention, the best means of achieving this is in a configuration with a uniform heat input on the upper surface with vertical heat flux and constant depth. Unfortunately, however, a horizontal temperature gradient is required to freeze the outer periphery. To minimize edge effects, a shallow pan with fiat bottom is favored, but the diameter/height ratio cannot be made too large without increasing the power requirements and lowering the pour time available before freezeup. Ratios running from 4:1 6:1 seem most desirable for capacities up to 100 lb, but may increase proportionately for larger furnaces.
The following table lists the melt capacity vs diameter for circular cylinders with a diameter/height ratio of 4: 1.
MELT CAPACITY AND POWER LOSS FOR TITANIUM The temperature l,750 K. (2,690) is about 100 F. below the melting points of most commercially important titanium alloys and probably represents an upper limit to the actual operating temperature. The column showing power loss for emissivity e 0.3 is included to estimate actual performance using clean molybdenum or tungsten crucibles.
Heat losses can further be reduced by using reflectors or radiation shields below the melt. In addition, if it is desired to lower the temperature gradient through the melt, it is possible to provide a substantial portion of the radiant heat loss by externally heating the crucible by radiation or induction sources.
While no data is directly available on the thermal conductivity of liquid titanium or its alloys, it is known that the ratio of electrical conductivities in liquid and solid phases parallels the thermal conductivities for nearly all metals. Since the electrical conductivity of titanium increases by 20-25 percent on melting, one may expect a similar increase in thermal conductivity. For pure titanium, the thermal conductivity of the solid in the vicinity of the melting point is about 0.25-0.27 watt/cmK. For commercial alloys, we may estimate the conductivity (using a modified Wiedemann-Franz law) to lie in the range 0.20 to 0.25 watt/cml(.
To determine the internal temperature gradient, we note that the radiant intensity of a body at l,750 K. with e l or e 0.3 respectively is 53.2 or 16 watt/cm giving a grad T= I/K 213 or 64 K./cm for K 0.25. The latter figure is equivalent to 292 F./in. For the case of e 0.3 and no supplemental heating, the maximum surface temperature for the previous cylindric crucibles ranges from 2,892 F. for the 4 inch size to 3 ,566 F. for the 12 inch diameter melt. The true temperatures should be somewhat lower due to the higher conductivity in the liquid phase.
The temperature drop through the tungsten or molybdenum crucible will be quite smallsince their conductivities lie in the range of 0.9 to 1 watt/cmK. and the wall thickness can probably be limited to no more than 0.1 inches. In such cases the difference would not exceed 10 F.
The heating and cooling rates for the above configurations will depend on melt depth and mean excess temperature (superheat) of the liquid phase.
The thermodynamic properties of liquid titanium are only approximately known, mainly estimated from empirical relations derived from related elements. The specific heat of liquid titanium is estimated as 8 cal/mole K. 176 joules/lb F. and the heat of fusion between 3.7 and 5 kcal/mole (146-198 k joules/lb). From these figures, it is apparent that the major contribution to the energy of cooling for moderate superheats is the latent heat of fusion. For example, a superheat of F. contributes only 9-12 percent of the energy released by freezing.
The minimum time to freezing or solidification can be calculated for the range of sizes shown in the preceeding table under the assumption of constant power loss and latent heat as the sole source of energy. The ower loss per pound varies from 1.37 to 0.46 kw/lb as one progresses from 4 to 12 inches in diameter for e 0.3. If we assume a latent heat of k watt-seconds/lb, we obtain freezing times of 128 to 380 seconds respectively.
The warmup time will depend on the excess power capacity of the furnace and the enthalpy requirement for the titanium and other structures. The increase in enthalpy of titanium from room temperature to solid phase at the melting point is 13.4 kcal/mole 530 k joules/lb. If we add the 175 k joules/lb heat of fusion we obtain a total enthalpy increase of 705 k joules/lb. We may roughly approximate heating time by assuming no radiant loss during heating and steady state loss during melting. For a furnace power capacity 50 percent in excess of the steady state power loss, we obtain heating times ranging from 514 to 1,528 seconds for the 4 to 12 inch size respectively.
The input power density required for practical melting times can be calculated for the previous examples. Thus, the upper surface has one-half the area of the bottom and sides, and under the assumption of a required power 50 percent greater than the steady state power loss, we obtain a power density of 0.308 kw/in. which is three times the radiant intensity of the lower surfaces. For radiantly heated melts, the product of source intensity, view factor, and absorptivity of the melt must equal the above figure. For disk sources with melt clearances of one-half or one diameter, the view factors are 0.38 or 0.18 with minimum source temperatures of 2,485 K. or 2,820 K. respectively assuming a surface temperature of 2,000 K.
As shown in FIG. 1 of the drawings, and in accordance with the present invention, a crucible is generally disclosed-in section. This crucible 10 is preferably of molybdenum, tungsten or the like, and capable of withstanding high temperatures as may be required in the melting of various metals. Additionally, characteristics of this crucible, as hereinbefore pointed out, include the thin structure which may not be very thick, as for example, 0. 10 inches. The thickness of the crucible may vary according to its size, however, the invention relates generally to a crucible which is quite broad in dimensions horizontally as compared to the vertical depth of the crucible. As hereinbefore pointed out, the depth of the crucible is shown and the breadth is great. The ratio of the depth to breadth may lie in a range between the ratios 1:3 to 1:8 for loads up to 100 lb. Thus, the breadth of the crucible may be three times the depth or may be as much as eight times the depth in order to provide the functional characteristics of the invention for the size range indicated.
It will be understood that while these ratios are most desirable, that they are only approximate in accordance with the present invention. 0
The method of the invention includes the previously described steps of preparing a suitable crucible having suitable dimensions and further, includes the application of heat to the crucible to attain a steady state condition of operation in which radiant heat is applied to the molten metal in the crucible and heat is radiated from the crucible to maintain said steady state operation with a suitable superheat condition of the metal so as to enable efficient pouring thereof into a mold. The method steps of the invention as hereinbefore described relate principally to the methods of obtaining steady state operation. However, increased cycle rate of melting and pouring may be attained in accordance with the method as will be hereinafter described.
The method of the invention includes the preparation of the crucible having the hereinbefore described characteristics of melting temperature and depth to breadth ratios. The method of the invention also includes the initial application of excess heat to metal in the crucible for the purpose of quickly bringing the temperature of the crucible and metal to be heated up to a degree nearly the melting temperature of the metal whereupon heat is continually applied to the metal in a steady state mode of operation after the metal is melted. The method of the operation comprises steady state operation maintaining a condition of low heat flux with a temperature differential between the molten metal and the crucible ranging between 100 C. and 500 C. whereby a frozen shell of the metal is maintained in contact with the crucible and to thereby isolate the molten metal portion of said metal from the crucible. Steady state operation of the method includes the application of sufficient radiant heat to the metal to maintain a superheated upper surface of said metal, the temperature of which equals 102 percent to 125 percent of the absolute melting temperature of the metal in the crucible. The method of the invention also comprises the tilting of the crucible and pouring molten metal from the frozen shell therein either through a radiation shield or through a heated radiation shield so as to maintain the superheat condition of the metal as it enters a mold.
In accordance with the foregoing it will be appreciated that the method of the invention comprises the increase in operating temperature of the radiant heater means of the invention during initial warming of the crucible and the molten metal. Where heater life and melting temperature permit, the use of higher than normal heater temperatures may reduce the time required to melt the charge of metal in the crucible. The said higher than normal temperatures would be temperatures which, if allowed to reach a steady state, would result in melting through the frozen shell and therefore steady state operation must be maintained as hereinbefore described.
Suppression of heat losses during the initial warming of the metal and the crucible will result in reduced time required for melting the metal in the crucible and the suppression of radiant heat losses may be accomplished by using shields such as disclosed herein for the purpose of controlling radiant energy loss from the crucible. Such radiant energy shields are disclosed in FIGS. 3 and 4 of the drawings and may also be used to control radiant energy losses to achieve a desirable temperature gradient during steady state operation.
In accordance with the present method, radiant heat may be applied to the bottom or sides of the crucible during initial warming of the metal and the crucible. For this purpose separate radiant heaters may be employed during warmup to heat the bottom and sides of the crucible to a temperature just below the melting point and thus a relatively shorter time is required to melt the metal and this increases the cycle rate of the invention.
In accordance with the method of the invention, induction heat may be utilized through the bottom or the sides of the crucible during initial warming of the crucible and metal, and such may be accomplished by means of a separate induction coil or coils below and at the sides or surrounding the crucible so as to directly couple energy for heating the charge in the crucible. Such coils may be similar to those disclosed in FIG. 5 to control .the net flow of radiation during steady state operation.
In accordance with the disclosures of FIGS. 19 and 20, the hollow cylindrical cone-shaped structures provide radiation shields or radiation heaters, or may be provided with induction coils or may consist of induction coils disposed below the region of the crucible lip in the pouring position so that the stream of molten metal is directed down the axis of such shields or heaters and thus the radiant cooling effect is limited or the metal temperature may actually be increased as the stream flows into the mold during the act of pouring. Such radiation shields or heating structures may either be stationary or articulated to move with the crucible during pouring. As shown in FIG. 19, the actual heating of the metal during pouring is especially desirable in order to insure complete fluidity of the melt as it reaches the mold.
The method of the invention provides for the ability to hold a steady state condition indefinitely and this contrasts with the consumable arc melting methods of the prior art and the present invention allows the adjustment of alloy chemistry including corrections thereto and also allows gentle stirring through induced magnetic fields to achieve a more homogeneous product. Further, the method permits repetitive small pours with incremental heat cycles without recharging the crucible. Such operation flexibility allows cycling of remelt material such as scrap with full quality control capabilities.
The method of the invention also has the advantage of permitting reloading without cooling or breaking the vacuum or atmospheric conditions surrounding the crucible. This method of the invention is in contrast to consumable arc methods since the crucible may be easily or conveniently recharged without opening seals or doors to the outside and the charge induced into the crucible may be anything from fine powder to large slabs or chunks of metal being melted. A further advantage of the method includes the ease of interchanging crucibles. This convenience of interchanging crucibles is afforded due to the absence of coolant connections to the crucible. Therefore, according to the invention, changes in sizes of crucibles may be attained without breaking the vacuum such as found in prior art methods and thus the invention permits changes in mold patterns or volume of metal being heated which saves warmup time, power and metal costs.
The crucible 10, as shown in FIG. I of the drawings, is adapted to maintain a frozen shell 12 of metal in the crucible, the metal being melted above the shell 12 by a heating means generally indicated at 14. This heating means may be suspended by suspension members 16 or may be supported above and in spaced relation with the upper portion of the crucible 10 by any suitable means.
Surrounding radiation shields 18 may be provided in order to reflect heat, as indicated by arrows 20 to the crucible, in order to prevent stray losses.
A lower surface 22 of the crucible 10 is adapted to radiate heat at a rate sufficient to maintain the frozen shell 12 in relation to the amount of heat input by the heating means 14.
The particular character of the heating means 14 is disclosed in FIGS. 5 to 17 of the drawings, which will be hereinafter described in detail.
As shown in FIG. 2 of the drawings, the invention comprises the heating means 14 above which are superimposed spaced radiation shields 23 adapted to reduce the unwanted radiation of heat upward from the heater 14.
As shown in FIGS. 1 and 2, the side portions of the crucible are designated 24 and these side portions 24 are generally arcuate and annular. The cross-section of the crucible being preferably disc-shaped, and as shown in FIG. 2 of the drawings, a radiation inhibiting shield 26 is disposed in spaced relation below the bottom 22 of the crucible, and curved portions 28 of the radiation inhibiting shield 26 are conformingly spaced with the sides 24 of the crucible, such that vertical adjustment of the radiation inhibiting shield 26 up and down relative to the bottom 22 of the crucible 10 may be accomplished to vary inhibition of radiation of energy from the bottom 22 of the crucible 10. Thus, the frozen shell 12 may be maintained in relation to the amount of heat input from the heating means 14.
Nozzles 27 may be carried by the shield 26 to direct convective gas toward the crucible for selective cooling, such as may be indicated.
An aperture plate 21 of reflective and/or insulating character having an aperture 23'may correspond to a given diameter of the crucible 12. As indicated by broken lines 25, the aperture may be smaller to correspond with a smaller diameter crucible 12. The purpose of the aperture plate is to confine the radiant energy to the area of the crucible and reduce stray power losses.
As shown in FIG. 3 of the drawings, a further modification of the invention includes a plurality of annular ring-shaped radiation inhibiting shields 32 and 33 with a disc-shaped plate 34 directly below the bottom portion 22 of the crucible. All of these members 32, 33 and 34 may be vertically adjustable relative to the bottom 22 and sides 24 of the crucible, such that independent adjustment of the rings 32 and 33 may be utilized to compensate for the relative heat flux attendant to the geometry of the bottom and sides 22 and 24, respectively.
As shown in FIG. 4 of the drawings, the invention comprises a radiation inhibiting shield 26 in spaced relation to the bottom 22 of the crucible, and a perforate radiation inhibiting shield 36 is disposed between the bottom 22 of the crucible l and the vertically movable radiation inhibiting shield 26. This structure provides for a different means in relation to the crucible 10 for controlling radiation of heat or thermal energy from the bottom 22 of the crucible 10 in order to maintain the frozen shell 12 of the molten metal in the crucible.
As shown in FIG. of the drawings, the crucible is similar to that shown in FIGS. 1 to 4, and disposed below the crucible 10 is a bowl-shaped induction coil 38 adapted to supply an amount of heat which is a fraction of that intended to be radiated from the bottom 22 of the crucible l0, and this means is a modification of the invention with respect to the control or inhibition of heat radiation migrating from the bottom 22 of the crucible 10 in order to maintain the frozen shell 12 therein.
Various modifications of the heating means 14 of the invention are shown in FIGS. 6 to 17 of the drawings.
As shown in FIG. 6, an incandescent radiant resistance heating element may be used as the heating means 14, shown in FIGS. 1, 2, 3 and 4. This incandescent radiant resistance heating element may be in the form of a corrugated sheet with deep V-grooves 40 comprising downwardly diverging surfaces In accordance with the modification of the heating means 14, as shown in FIG. 7, a ribbon 44 arranged in a zigzag pattern is provided with alternate portions which are inclined to provide for downwardly diverging surfaces 46 functioning similarly to the downwardly diverging surfaces 42 shown in FIG. 6 of the drawings. This modification, as shown in FIG. 7, is also preferably a radiant or incandescent resistance heating element.
As shown in FIG. 8, the heating means 14 may be modified to assume a coiled structural configuration with a helical form of a coil 48, this coil 48 also functioning as a radiant or incandescent resistance heating element. Likewise, the heating means 14, shown in FIG. 9 is similar to the coil 48, but is arranged in a zigzag pattern and being designated 50.
It will be understood that the heating means 14 in the particular structural arrangement, as shown in FIGS. 6 and 7, utilizing inclined or cavity surfaces may be quite efficient for a given temperature by raising the emittance of the source.
As shown in FIG. 10, the heat means 14 may be an induction heater utilizing an incandescent cone 52 having a coil 54 conformingly wound adjacent thereto.
As shown in FIG. 11, another induction heater may be utilized as the heating means 14 of the invention, and according to the modification shown in FIG. 11, a coil 56 is disposed above an annular ridged plate 58.
In the modification, as shown in FIG. 12, the heating means of the invention is an induction heating means employing a corrugated plate 60 similar to the plate disclosed in FIG. 6, and this induction heating means comprises a coil 62 disposed above the corrugated plate 60 and it provides downwardly diverging surfaces 64 comparable to those surfaces 42 disclosed in FIG. 6.
The structures disclosed in FIGS. l0, l1 and 12 being heated by a suitably shaped coil, such as the helix, the flat spiral or the zigzag coil, shown in FIGS. 10, 11 and 12, respectively, provide for efficient operation when radiation shields are employed; by using slit or multiple pieces, coupling is minimized and radiative efficiency is improved.
As shown in FIG. 13, the heating means 14 of the invention may be an electron beam heating device comprising the plate 66 which is provided with annular grooves 68 therein.
As shown in FIG. 14, a direct induction coil 70 may serve as the heating means 14 of the invention, this coil being in the form of a zigzag pattern, while the modification as shown in FIG. 15 includes a spiral-shaped coil 72 of the direct induction type.
As shown in FIG. 16, a three-phase Y connected double coil 74 is another type of induction heat device which may serve as a modification of the heating means 14 of the invention.
As shown in FIG. 17,;a coil similar to that shown in FIG. 16, is arranged in spiral form, the coil being designated 76. Accordingly, it will be appreciated that the polyphase coil shown in FIGS. 16 and 17 may operate as direct induction heaters and define specific use as a heating means 14 of the invention.
The efficiency of direct induction coils may be improved by using close spaced turns of rectangular conductors with a highly reflective metal surface to reflect surface radiation from the melt, as shown in FIG. 18.
In accordance with the disclosure of FIGS. 19 and 20, radiation shields or heated radiationshields may be utilized during pouring so that the metal is poured from the crucible through such shields and directly into a mold thereby preventing the loss of heat and attaining the superheated condition of the molten metal as itenters the mold. As shown in FIG. 19, the crucible 10, in accordance with the method, is tilted causing molten metal to pour from the frozen shell therein in a stream as indicated at 11 and this stream passes through a heated substantially conical shield designated 70 and directly into a mold 72. As shown in FIG. 20, the crucible of the invention is tilted to cause metal to pour from the frozen shell therein in a stream 11 through a radiation shield 74 which is generally conical. This shield has an upstanding shield portion 76 adapted generally to overlie the stream 11 pouring from the edge of the frozen shell of the crucible 10 and, in accordance with the disclosure of FIG. 20, the mold 78 receives metal at a desirable superheated temperature. Thus, the initial superheat temperature of the metal in the crucible 10 is not subjected to substantial radiation losses during pouring.
It will be obvious to those skilled in the art that various modifications of the present invention may be resorted to in a manner limited only by a just interpretation of the following claims.
1. In afrozen shell metal meltingmeans, the combination of: a substantially curved cup-shaped metal crucible which is shallow in depth as compared to its relatively greater horizontal dimension, the ratio of said depth, relative to said horizontal dimension, falling within the range of 1:3 to 1:8; radiant heating means disposed above said crucible and induction electrical means disposed above said radiant heating means, said radiant heating means being directed toward said crucible and having a heating capacity which will melt metal in said crucible; said crucible having an unobstructed bottom portion disposed and adapted, by means of radiation only, to radiate sufficient heat from said crucible to maintain a thin frozen shell of said metal in contact with said bottom portion.
2. The invention, as defined in claim 1, wherein: said radiant heating means is generally conical.
3. A frozen shell metal melting apparatus comprising, in combination:
a. a shallow curved metal crucible for receiving a charge of metal, which charge is to be partially melted within said crucible;
b. electrically generated heating means disposed above said charge for generally uniformly radiating heat said heat being directed onto the upper surface of said charge;
c. said crucible being constructed of heat conducting material and shaped to induce radiation of heat contained in said charge from said crucible;
d. said crucible having an essentially unobstructed bottom to permit unimpeded radiation of heat therefrom, whereby the heat radiated onto said charge may flow through said charge to said crucible and be radiated from said bottom of said crucible. 4. The apparatus of claim 3 wherein said radiating means includes a reflector to increase the amount of heat radiated onto said charge.
5. The apparatus of claim 3 wherein said radiating means includes a plurality of reflectors to increase the amount of heat radiated onto said charge.
6. The apparatus of claim 3 wherein said radiating means is generally conical. I
7. The apparatus of claim 3, further including means for shielding the heat radiating from said bottom of said crucible.
8. The apparatus of claim 6 wherein the distance between said bottom of said crucible and said shielding means is adjustable, whereby the heat radiated from said crucible is adjustable.
9. A metal melting heat flow apparatus for partially melting ametal contained within a curved metal crucible without melting the metal immediately; adjacent said crucible, said apparatus being characterized y and comprising in combina tron:
a. said crucible having a greater horizontal dimension than a vertical dimension;
b. said crucible being constructed of heat conductive material and having a bottom shaped to act as a radiant heat source;
c. electrically generated heating means for radiating heat onto the metal contained within said crucible, said radiating means being directed toward having sufficient radiant heat capacity to melt a portion of the metal contained within said crucible;
(1. said crucible being substantially free of impediments which would inhibit radiation of heat from said bottom of said crucible, whereby the heat radiated from said radiating means onto the metal is conducted through the metal to said crucible and radiated therefrom.
10. The apparatus of claim 9, further including a reflector positioned to co-operate with said radiating means for directing the radiant heat onto the metal.
11. The apparatus of claim 10, further including a heat shield disposed about said crucible for regulating the amount of heat radiated from said crucible, whereby the amount of metal melted within said crucible may be regulated by regulating the amount of heat flow through said crucible.
12. The apparatus of claim 9, wherein said radiant means is generally conical.
13. The apparatus of claim 9, wherein the melting point of said crucible is lower than the melting point of the metal.
14. The apparatus of claim 9, wherein the melting point of said crucible is higher than the melting point of the metal.