US2721364A - Melting and casting apparatus - Google Patents

Description

Oct. 25, 1955 w. J. TRETHEWAY ET AL 2,721,364

MELTING AND CASTING APPARATUS 2 Sheets-Sheet 1 Filed Oct. 12, 1951 44 J wk Oct. 25, 1955 w TRETHEWAY ET AL 2,721,364

MELTING AND CASTING APPARATUS Filed Oct. 12, 1951. 2 Sheets-Sheet 2 FIG. 3 1

United States Patent MELTING AND CASTING APPARATUS William J. Tretheway, Laguna Beach, Calif., and Rollin J. Kennard, Butte, and Earl L. Buker, Great Falls, Mont., assignors to The Anaconda Company, a corporation of Montana Application October 12, 1951, Serial No. 251,024

2 Claims. (CI. 22-79) This invention relates to melting and casting cadmium metal, and more particularly is concerned with the provision of improved apparatus for melting metallic cadmium and delivering it to a mold.

A difficulty commonly encountered in casting cadmium metal is in preventing it from oxidizing, especially after removal from the melting furnace and during delivery to a casting apparatus or mold. Unless this difficulty is overcome and oxidation of the molten cadmium is substantially prevented, castings made therefrom are likely to contain dross inclusions. Another difiiculty encountered in producing dense castings of cadmium is in delivering the molten metal to the casting apparatus or mold rapidly and at a controlled temperature. Unless cadmium is cast at the optimum temperature, the castings are apt to be objectionably porous, as well as undesirably rough at their surfaces.

In the past, withdrawal of cadmium from a melting furnace and pouring into molds has been primarily a hand operation, and as such has not, as a rule, satisfactorily met the aforementioned requirements of avoiding oxidation and casting at the optimum temperature. It is the primary object of this invention to provide improved apparatus for preparing cadmium for casting and delivering it to a casting apparatus or mold mechanically, so that such requirements are met.

Apparatus according to the invention for preparing and delivering molten cadmium to a casting apparatus or mold comprises a furnace, including a melting pot, and means for heating the melting pot evenly and to quite accurately controlled temperatures. Advantageously the heating means of the furnace comprises a plurality of gas burners adapted to supply heat to the pot by an encircling flame, an exhaust fan being employed to exhaust the combustion gases from the bottom of the furnace, thereby to heat the bottom of the pot evenly.

One or more delivery pipes is removably secured to the bottom of the pot and extends therefrom to a point closely adjacent the entrance or mouth of a mold. The

delivery pipe or pipes are advantageously well insulated substantially throughout their length. Electrical heating means are provided to maintain the molten metal in the pipes, after it has been withdrawn from the pot and until it is discharged into a mold, at an optimum casting temperature. Such electrical heating means advantageously comprises a step-down transformer supplied with current from a convenient power source. The terminals of the low voltage secondary of the transformer are electrically connected by suitable conductors to the extreme opposite ends of the delivery pipe whereby the delivery pipe and the metal therein may be supplied with a low voltage heating current of adequate amperage. It is particularly advantageous, in order to control the temperature of the molten metal in the delivery pipe most accurately and economically, for the conductors bywhich the low voltage secondary is connected to the delivery pipe to be of metal of good electrical conductivity throughout a substantial part of their length, but for the end portions of such conductors, which are connected to the ends of the pipe, to be of metal of high electrical resistance and poor thermal conductivity, e. g. stainless steel. Where a number of delivery pipes are all connected to the same pot, a number of transformers are employed, one terminal of the low voltage secondary of each being electrically connected to the outer end of a delivery pipe. The other terminals of the low voltage secondaries of the transformers are electrically connected together and to the melting pot. By these means the molten metal may at all times be maintained at the desired temperature until it is actually discharged into a mold.

Another very important feature of the invention resides in the provision of means at the outer end of each delivery pipe whereby the delivery pipes are always maintained substantially full of cadmium metal. Such means comprises a gooseneck section including a vertically upstanding portion, mounted on each delivery pipe at its outer end. The main run of the delivery pipe is substantially horizontal, and the vertically upstanding portion prevents partial emptying of the delivery pipe when the molten metal is shut off by a valve located, say, near the melting pot. The outer portion of the gooseneck section is removable from the delivery pipe at a point adjacent the highest point of the vertically upstanding portion thereof. The removable portion is advantageously formed with a spout which serves to convey the molten metal to a point closely adjacent the mold face; and because it is removable it may be readily and easily kept clean and free of dross.

For a more detailed understanding of the method and apparatus of our invention, reference may be made to the accompanying drawings in which Fig. 1 is a plan view of the new cadmium melting furnace and its combustion gas exhaust system;

Fig. 2 is a sectional elevation of the apparatus of Fig. 1 taken along line 2-2 thereof;

Fig. 3 is an elevation, partially diagrammatic and partially in phantom, illustrating the cadmium metal delivery system and the electrical heating means therefor;

Fig. 4 is an enlarged sectional view of a section of the furnace illustrating the disposition of one gas burner;

Fig. 5 is an enlarged partial section on line 55 of Fig. 1;

Fig. 6 is a sectional view of the pot end of one delivery pipe and the means for supplying electric current to the pot and the pipe; and

Fig. 7 is an enlarged perspective view showing a removable gooseneck at the discharge end of a delivery pipe for molten metal.

The gas fired cadmium melting furnace, best shown in Figs. 1 and 2, comprises a furnace shell 10 suitably lined with fire brick 11 and having an opening 12 located centrally in the bottom thereof for downdraft exhaust of the combustion gasesv A steel melting pot 13 having a peripheral rim flange 14 is hung centrally by such flange in the furnace. The bottom of the pot is provided with a sump 15 to which are removably secured delivery pipes 16 and 17 for transferring molten metal to molds.

A plurality of valve blocks 18 (see Fig. 5), one for each delivery pipe, are mounted in the bottom of the sump 15, and each is provided with a valve seat 19. A poppet-type valve closure element 2% is provided for each valve block 18, and when lowered into engagement with valve seat 19 the closure element halts the flow of molten metal from the pot 13 into and through the corresponding delivery pipe. The valve element 20 is supported at the lower-end of a rod 21 which extends upwardly within the pot 13 and is pivotally connected at its upper end to a bell crank plate 22 (Fig. 2). The plate 22 is pivotally supported on a mounting bracket 23 which is in turn supported on the top of the lined furnace shell adjacent the rim or flange 14 of the pot 13. The plate 22 is also pivotally connected to one end of a threaded rod 24, the opposite end of which is provided with a handle 25 by which it may be turned. The threaded rod 24 passes through a threaded nut 26 which is secured to the mounting bracket 23. To close a poppet valve in the sump of pot 13, in order to prevent the flow of molten metal from the pot, the threaded rod 24 is turned by its handle to pivot the bell crank plate 22 in a clockwise direction, as seen in Fig. 2. To open the poppet valve the threaded rod 24 is turned in the opposite direction. A separate poppet valve is advantageously provided to control the flow of molten metal from the pot through each separate delivery pipe; and each poppet valve advantageously is provided with an individual opening and closing apparatus of the character described.

As has been previously pointed out, cadmium metal oxidizes so readily that extreme care must be taken in heating it for casting. When a gas flame is to be employed for heating the melting pot, therefore, the flame should be very carefully controlled to provide a well distributed heating of the pot, in order to avoid development of hot spots in the molten metal. To assure even distribution of heat to the melting pot 13, a plurality of gas burners, each indicated generally by the numeral 30 (Fig. l), are provided. These gas burners are all supplied with a fuel-air mixture from a common source through an inlet nozzle 31 secured tangentially to a bustle pipe 32 to which each gas burner 30 is connected by an individual feed pipe 33. As may best be seen in Fig. 4, the feed pipe 33 for the gas burner leads into a chamber 34 provided at the apex of a conically shaped member 35. A nozzle passage 36, arranged to direct a flame substantially tangentially into the combustion area between the inner face of the furnace lining 11 and the outer surface of melting pot 13, extends angularly from the chamber 34. Auxiliary combustion air enters the nozzle passage 36 through a passage 37 which opens to the atmosphere at the furnace wall. This passage also forms a peep hole or window by which one can observe if the burner is ignited. In the furnace as described, when the burners are in operation, the flame within the combustion area of the furnace will be given a whirling motion around the pot by reason of the arrangement of burner nozzles 36. The pot will, therefore, be subjected to substantially uniform heating all about its periphery.

In order to assure even heating of the bottom as well as the sides of the melting pot 13, an exhaust fan 40, capable of withstanding high temperature gases, is provided to withdraw the combustion gases from the bottom of the furnace combustion area through the bottom opening 12 and thence through a flue 41 lined with a refractory material 42. The exhaust fan discharges the combustion gases to the atmosphere through a stack 43. Such downdrafting of the melting furnace insures that the bottom of the melting pot 13, as well as its sides, is evenly heated.

More or less conventional temperature controls (not shown) are advantageously employed to control the temperature of the metal in the melting pot 13 during melting of the cadmium, and for maintaining it at the desired temperature during a casting operation. When cadmium is melted in the above-described furnace, the exposed surface of the molten cadmium in the pot 13 is protected by a fused layer of caustic (sodium hydroxide) to prevent oxidation of the metal and formation of dross. By removing the molten metal from the bottom of the pot only, through the sump 15, none of the caustic or other material floating on the surface of the molten metal is included in it.

The particular construction of the molten metal delivery pipes 16 and 17 and of the electrical heating means therefor are illustrated in Figs. 3, 6 and 7. The delivery pipe 16, preferably made of stainless steel, is 7 threaded into a stainless steel bushing or coupling 44 which in turn is threaded into a steel plate 45 welded to the outer face of the sump 15 of the melting pot 13. The delivery pipe 16 is insulated substantially throughout its length with an insulating material 46 adapted to withstand temperatures up to, say, 1000 F. This delivery pipe is provided for supplying molten cadmium from the furnace to a ball mold casting apparatus 47 illustrated in phantom. This casting apparatus forms no part of the present invention, but an advantageous form of such apparatus is disclosed and claimed in our copending application Serial No. 246,272, filed September 12, 1951, now Patent No. 2,689,989.

Over most of its run the delivery pipe 16 is substantially horizontal, but its outer end portion is bent to form a vertically upstanding section 48 which effectively serves to maintain the delivery pipe full of molten metal at all times. The upstanding section 48 forms the inner portion of a gooseneck 49 at the end of the delivery pipe. The outer portion 50 of the gooseneck is removably attached to the inner portion, at a point adjacent the highest point of the gooseneck, by mating coupling members 51 and 52 which are secured together by screws 53. The lower end of the outer gooseneck portion 50 is formed with a spout 54 adapted to be located closely adjacent the entrance to the mold of the casting apparatus 47, whereby molten metal removed from the furnace pot for delivery to the mold of the casting apparatus is exposed to an oxidizing atmosphere for but a very brief instant prior to entry into the mold. If desired, an atmosphere of carbon dioxide or other substantially non-oxidizing gas may be maintained within the casting apparatus 47. The outer portion 50 of the gooseneck advantageously is removed at the end of each cycle of casting operations, for cleaning and removal of any dross that has accumulated therein. The upstanding inner portion 48 of the gooseneck serves to maintain the delivery pipe 16 full of molten metal at all times, whereby the interior of the delivery pipe is kept free of dross.

As shown in Fig. 3, an additional valve 55 is advantageously connected into delivery pipe 16 to control the flow of molten metal therethrough. This valve is adapted to be automatically operated, to turn the flow of molten cadmium on and ofl in proper synchronism with operation of the casting apparatus 47.

Electrical heating means for the delivery pipe 16 comprises a variable step-down transformer the terminals of the high voltage primary 61 of which are connected across power lines L1 and L2. The terminals of the loW voltage secondary 62 are electrically connected to a pair of copper pipe bus bars 63 and 64. One of these copper pipes 63 is connected to the upstanding section 48 of the delivery pipe 16, as close as possible to the end thereof, by a stainless steel connector having an outer section 65 which is welded or otherwise secured to the inner gooseneck portion 48 and having an inner section 66 of substantially greater cross-sectional area than the outer section which is clamped to the copper pipe 63. The outer section 65 of the stainless steel connector, because of its reduced cross section, is of relatively high electrical resistance and hence is heated by the electrical current flowing through it to a higher temperature than the inner section 66. Thereby it is maintained at a temperature high enough to avoid undesirable cooling of the delivery pipe. Heat insulation may be used if desired to keep the temperature of the outer section 65 adequately high. The inner section 66, at least near the copper pipe 63, is uninsulated so that radiation of heat therefrom keeps the temperature at the clamp low enough to avoid oxidation of the copper and damage to the electrical connection. Stainless steel is preferred 'for making the connector 65, 66 because it does not oxidize and scale at the high temperature to which it is subjected; and also the low heat conductivity of stainless steel minimizes the loss of heat by conduction to the (cold copper pipe bus 63.

The other copper :bus bar pipe '64 is electrically connected at its lower end to the steel plate 45 on the melting pot 13 by a connector comprising a pair of stainless steel clamps 67 and 68 joined together by a stainless steel bar 69 and a stainless steel pipe 70. The clamp .67 is secured to the copper pipe 64, while the clamp 68 is secured to the outer end of the stainless steel pipe 70. The latter is concentric with and surrounds the inner end of the delivery pipe 16 but is separated from the delivery pipe by the insulation 46. The inner end of the stainless steel pipe '78 is welded to the stainless steel bushing 44 (Fig. 6) which, as has been explained, is screw-threaded to the steel plate 45. The delivery pipe 16 and the stainless steel pipe 70 both pass through the same hole in the furnace shell without either touching it or including any part of the iron shell within a conducting loop, for otherwise its magnetic character would cause heating of the shell and loss of power thereby. The stainless steel from which sections 65 and 66 of the conductor leading to the outer end of the delivery pipe and bar 69 and pipe 70 leading to the melting pots may be made may be, for example, one consisting of 18% chromium, 8% nickel, 2% molybdenum as the stabilizer and the balance iron with the insignificant amounts of impurities. The thermal conductivity of such stainless steel at 400 C. is 0.048 calorie per square centimeter per centimeter per degree Centigrade per second.

The electrical circuit for heating the metal in delivery pipe 16 and thereby maintaining it molten, therefore, comprises the low voltage secondary 62 of the variable transformer 60, the copper bus bar pipe 63 connected to one terminal of the secondary 62, the stainless steel connector 66, 65, the delivery pipe 16 and the metal therein, the bushing 44, the stainless steel pipe 70, the stainless steel clamps 68 and 67 and their connecting bar 69, and the copper bus bar pipe 64 which is connected to the other terminal of the low voltage secondary 62 of the variable transformer.

The apparatus shown in the drawings is provided with the second delivery pipe 17, generally similar to the delivery pipe 16 described above, for delivering molten cadmium to a different set of molds than those of the casting apparatus 47. The delivery pipe 17 is advantageously composed of stainless steel and is insulated substantially throughout its length with a material 71 similar to that used for insulating the delivery pipe 16. As previously described, the delivery pipe 17 is removably secured to the sump of the melting pot 13, and at least one valve 72 is provided to control the fiow of molten metal through it. While in Fig. 3 the delivery pipe 17 is shown, for reasons of convenience, as being of somewhat lesser length than is the delivery pipe 16, in practice it may be longer than the latter. At its outer end the delivery pipe 17 is provided with a gooseneck 75 of the same character as the gooseneck 49. The delivery pipe 17, like the corresponding pipe 16, is adapted to be heated electrically. To this end, a second variable stepdown transformer 80 is provided having a high voltage primary 81 connected to the power lines L1 and L2. The low voltage secondary 82 of the transformer 80 has its terminals electrically connected to a pair of copper bus bar pipes 83 and 84. One of these pipes 83 is directly connected to the copper pipe 64 which is in turn connected to the melting pot. The other of these copper pipes 84 is electrically connected to the upstanding inner section of the gooseneck 75 by a stainless steel connecting clamp 86. The normal electrical heating circuit for the delivery pipe 17 thus is the low voltage secondary 82 of the transformer 80, the copper pipes 83 and 64, the stainless steel clamps 67 and 68 and their connecting bar 69, the stainless steel pipe 70, the melting pot 13, the delivery pipe 17 itself and the molten metal contained therein, the flexible laminated stainless steel connecting clamp 86, and the copper pipe 84.

By the above-described electrical heating means the delivery pipes and the metal contained therein may be subjected to the heating action of a low voltage current of adequate amperage to maintain the temperature of the molten cadmium in the pipes at the optimum casting temperature, with minimum loss of heat to the cold conductors of the heating circuit.

In order that either or both of the delivery pipes 16 and 17 may be employed if and when the stainless steel pipe is disconnected from the pot 13 for repair or for any other reason, an auxiliary or emergency electrical connection between the copper pipe 64 and the melting pot 13 is provided. This auxiliary electrical connection comprises a steel bar 90 welded to the flange 14 of the steel melting pot 13, a stainless steel bar 91 secured thereto by a stainless steel connecting member 92, and a stainless steel clamp 93 and connection member 94, the former being connected to the copper pipe 64 and the latter to the stainless steel bar 91.

In using the above-described apparatus for preparing cadmium metal for casting and for delivering the molten metal to a casting apparatus of the type described in our aforementioned application Serial No. 246,272, now Patent No. 2,689,989 the molten metal should be supplied to the entrance to the mold of the casting apparatus without substantial exposure to an oxidizing atmosphere (such is substantially prevented by the design of the apparatus) and at a temperature of about 700 to 800 F., preferably 760 F. A higher casting temperature results in excessive time being required for the castings to cool and solidify, thus slowing up the casting operation, and a lower casting temperature results in rough castings. To assure delivery of the metal to the mold at the correct temperature, the cadmium in the melting pot 13, after being melted, should be held at a temperature in the range from 800 to 900 F., and best at about 850 F. If the temperature of the molten metal in the pot is lower, the flow of metal through the delivery pipe may be too rapid for the electrical heating system to maintain it at the desired temperature. On the other hand, if the metal in the pot is substantially higher, it may not cool off to the correct value in sufiicient time to be delivered to the mold of the casting apparatus at the proper temperature. While in order to rapidly melt cadmium supplied to the furnace and pot, the pot may be heated to a temperature of 900 F. to 1000 F., preferably about 950 F., the temperature of the molten cadmium should therefore be decreased and maintained at about 850 F. during the casting operation.

At the conclusion of a casting cycle, the valves 55 and 72 in the delivery pipes 16 and 17 are closed, thereby shutting off the flow of metal from the melting pot into the delivery pipes; and the outer portions of the gooseneck sections 49 and are removed and cleaned. The vertically upstanding inner portions of the gooseneck sections prevent the molten metal from draining from the delivery pipes when the valves are closed, and keep the pipes full of cadmium at all times. The cadmium in the delivery pipes advantageously is kept from solidifying by maintaining the flow of heating current therethrough at all times. The cadmium in the pipes meanwhile is fully protected against oxidation except at the tiny surface exposed in the vertical gooseneck section.

It is to be noted that for the purpose of remelting the metal in the delivery pipe electrically, it is important that no cadmium has drained therefrom. Otherwise there will be substantial overheating and drossing of the cadmium in that part of the pipe from which the metal has drained.

We claim:

1. In apparatus for melting cadmium and delivering it to a mold including a melting pot and means for substantially evenly heating the melting pot, the improvement which comprises a delivery pipe secured to the bottom of the pot and extending therefrom subtsantially horizontally to the mold entrance, the delivery pipe having at its outer end a substantially vertically upstanding portion forming part of a gooseneck, a power source, a stepdown transformer the high voltage primary of which is connected to the powersource, and a pair of conductors connected respectively to the terminals of the low voltage secondary of the transformer, one of said conductors being electrically connected to the outer end of the delivery pipe at the upper end of said vertically upstanding portion thereof and the other conductor being electrically connected to said melting pot, the two conductors being made of metal of good electrical conductivity throughout substantial parts of their lengths, both of said conductors, however, having their outer end portions which are connected to said upstanding portion of the delivery pipe and the melting pot respectively made of a metal of poor thermal conductivity.

2. Apparatus according to claim 1 in which a gooseneck section is removably secured to the upper end of said vertically upstanding portion of the delivery pipe and forms the remainder of the gooseneck, the reversely curved portion of the gooseneck being a part of said gooseneck section, and in which the outer end portion of the conductor connecting the low voltage secondary of the transformer to the substantially vertically upstanding portion of the delivery pipe has two sections, the one which is adjacent the end of the delivery pipe having a relatively high electrical resistance whereby it is heated to a relatively high temperature by an electrical current flowing therethrough.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Page 8, Stainless Steels by Ernest E. Thum, The American Society for Metals, Cleveland, Ohio, second 7 edition, 1945.

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Claims (1)

1. IN APPARATUS FOR MELTING CADMIUM AND DELIVERING IT TO A MOLD INCLUDING A MELTING POT AND MEANS FOR SUBSTANTIALLY EVENLY HEATING THE MELTING POT, THE IMPROVEMENT WHICH COMPRISES A DELIVERY PIPE SECURED TO THE BOTTOM OF THE POST AND EXTENDING THEREFROM SUBSTANTIALLY HORIZONTALLY TO THE MOLD ENTRANCE, THE DELIVERY PIPE HAVING AT ITS OUTER END A SUBSTANTIALLY VERTICALLY UPSTANDING PORTION FORMING PART OF A GOOSENECK, A POWER SOURCE, A STEPDOWN TRANSFORMER THE HIGH VOLTAGE PRIMARY OF WHICH IS CONNECTED TO THE POWER SOURCE, AND A PAIR OF CONDUCTORS CONNECTED RESPECTIVELY TO THE TERMINALS OF THE LOW VOLTAGE SECONDARY OF THE TRANSFORMER, ONE OF SAID CONDUCTORS BE-

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