US3253076A - Apparatus for making precise castings - Google Patents

Description

ay 1966 J. P. RYNERSON, JR 3,253,075

APPARATUS FOR MAKING PRECISE CASTINGS Filed Aug. 29, 1963 INVENTOR.

JOHN P RYNERSON JR.

gawy I ATTORNEY United States Patent 3 253,076 APPARATUS FOR MAKING PRECISE CASTING S John P. Rynerson, .lr., Kokomo, Ind., assignor to Union Carbide Corporation, a corporation of New York Filed Aug. 29, 1963, Ser. No. 305,472

- 6 Claims. (Cl. 13-9) This invention relates to a novel foundry apparatus; and more specifically to a transferred plasma arc furnace especially suited for melting and casting metals by investment casting methods.

The term investment casting, also called precision casting, as usedherein refers to the many modifications of the so-called lost-wax process for producing castings that are relatively small and precise in dimensions, for example, metal denture parts, gas turbine members, and many other industrial components. Such modifications include ceramic shell molding, permanent-pattern molding, resin-bonded shell-casting, the Antioch Process, etc. Ordinariy, investment castings are relatively small and may vary in size from a few ounces to over 30 pounds. Melting and casting is usually conducted in ambient atmosphere; however, in the attainment of exceptionally high quality, the melting and casting process is often done in vacuum or within a controlled atmosphere of an inert gas.

' The compositional control during the melting and casting operations of superalloys by investment casting methods is particularly difiicult. The size and configuration of typical castings by these methods must be produced with rigid controls to high standards of'quality, for

example, stringent aircraft specifications and usually at 100 percent inspection. Further, atmosphere control is extremely critical in the melting and casting operations. Therefore, skilled artisans generally must operate within the known limitations in this unique art which cannot be compared to other metallurgical melting processes.

While composition control is critical in furnaces of this class, the primary objectives of the furnaces are to remelt and cast the prepared alloy and only to maintain the high level of compositional balance of the pre-alloyed charge metal. Skilled artisans do not expect these furnaces to be used as purification steps. The conditions that are necessary to remove harmful elements also remove necessary elements, thereby upsetting the desired composition balance. For example, in British Patent No. 866,078, in the attempt to reduce the lead-content of an alloy by the known volatilization means, there will be a concurrent loss of other elements such as carbon, nitrogen, aluminum, titanium, boron, chromium, and others that may be essential in the alloy. The contents of these elements are known to be decimated during such volatilization steps. Therefore, purification methods, such as volatilization, are not a part of melting and casting operations in furnaces ofthe class of this invention.

A typical foundry apparatus and method especially designed to produce investment castings is described by Merrick in US. Patent No. 2,125,080. The crucibles ofMerrick and other furnaces now used extensively in the art aresubstantially spherical in internal design. The

spherical interior of the crucible is particularly effective to obtain uniform heating of the charge when the focal source of heat is essentially at the center of the crucible. This feature provides the most efficient heating with the 3,253,076 Patented May '24, 1 966 .Serial No. 225,534 filed September 24,1962, the plasma arc torch may advantageously be used as a heating means for the Merrick-type furnaces. It was found that this combination effectively melts many common commercial alloys. However, there are serious drawbacks when melting most of the non-ferrous metals and the socalled super alloys. The term super alloys refers to highly alloyed compositions that basically may contain nickel, chromium, cobalt, tungsten, molybdenum, and/or iron with additional amounts of modifying elements as, for example, aluminum, columbium, titanium, zirconium, etc. for use in severe service, i.e., high tempenature wear, corrosion, cyclic loading and the like.

Control of composition and, thereby, physical and mechanical properties, is extremely difiicult for a number of the alloys and, therefore, it is necessary that these alloys be melted and cast within a controlled atmosphere of vacuum or an inert gas. Further, in some cases, vacuum melting and casting is the only acceptable method of production. This, of course, is time-consuming and costly in necessary equipment and processing steps. The use of a plasma arc torch as a heating means for all known crucible designs of precision casting furnaces retained some of the limitations inherent in other heating means as, for example, carbon arc. While the plasma arc is advantageously more efiicient, safer and less costly to operate than the other heating means, there was no significant improvement in compositional control in melting alloys that require vacuum melting and casting for best results.

The common design ofthe Merrick-type furnace contains a single opening that serves as both the charging port and the casting port; also, there are two openings for the carbon electrodes, and an additional opening for the entrance of an inert gas when necessary. These openings are normally made perpendicular to the tangent of the are in the crucible wall in order to retain as much as possible the spherical interior concept relationship of the service port to the heat source focal point in the crucible. The atmospheric conditions of such furnaces require that the number and size of openings be kept at the barest minimum. When casting the molten metal into the molds, the presently known furnaces must be completely rotated through an arc of about degrees. This operation is considerably hazardous and cumbersome. In the roll-over operation, because of the spherical design of the crucible, the molten metal is haphazardly splashed into the gate of the mold.

Stirring the molten bath is desirable for a number of advantages:

(1) Uniform alloying (2) Uniform heating (3) Avoids volatilization of desired elements Several means are used in the art to stir the molten bath during the melting step, such as a rocking mechanism or by electromagnetic induction. Mechanical rocking and electromagnetic means for stirring are expensive and complex additions to the furnace structure that may fail during operation and potentially subject personnel and equipment to injury. To avoid overheating a portion of the melt, it is necessary that the rocking mechanism operate constantly, thereby subjecting the parts to wear.

The impin-gment of the arc torch effluent against the bath affords a fair degree of stirring action; even so, the balanced spherical design of the crucible restricts the stirring to a highly localized equilibrium condition. Therefore, it is necessary to provide mechanical and/or electromagnetic means for stirring the melt in the present art.

It is the principal object of this invention to provide a more efficient melting and casting apparatus for producing metallic castings of precise dimensions.

Another object of this invention is to provide a novel crucible that promotes thorough stirring and faster melting without auxiliary equipment.

Another important object is to provide a more efficient method of melting and casting certain superalloys that now must be processed only by vacuum melting or other expensive means to obtain optimum results.

A further object is to prowide a method of melting and casting supera-lloys to obtain a high degree of composition control.

Other objects and improvements may be suggested from the following description and drawing in which the sole (figure is a vertical cross section of the apparatus of the invention with some parts shown in elevation.

The transferred arc torch 1 containing the cathode (not shown) produces an electrical-1y charged directionally stable effluent (not shown) that strikes the metal charge 2 which conducts the electrical current to the anode 3 which is water cooled and insulated by a ceramic sleeve 4. A directionally stabilized electric arc column is an electric arc column which maintains the direction implanted to it. Once direction along an axis between the non-consumable electrode and the metallic charge is established, the arc column will resist any deviation from the axis. This directional stability can be achieved by several devices as referred to hereinafter and in co-pending application Serial No. 225,534 filed September 24, 1962. The tear-shaped cavity configuration 9 is formed in crucible 8 which is backed by an insulating material 7. The tear-shaped cavity has an elongated hemispherical hearth 25 having a discharge conduit 26. One side 20 of the discharge conduit is tangent to the lower portion of the elongated hemispherical hearth and forms a straight streamlined surface therewith. Another side 22 of the discharge conduit forms a curved streamlined surface with the upper portion of the elongated hemispherical hearth. The streamlined surfaces leading from the elongated hemispherical hearth are essential. Such surfaces have no sharp corners where contaminating gases may become entrapped and also the absence of sharp corners prevents excess wear and permits molten charge to flow easily from the hearth.

The entire apparatus is supported by the steel shell 5 that forms the face 6 which supports the mold-holding device (not shown). A refractory material 11 extends the lip of the crucible and defines the casting port 10 of the furnace. The refractory material 11 defining the casting port 10 is in streamlined registry with the stream lines forming the open end of tear-shaped cavity 9. The casting port communicating with the discharge conduit is preferably positioned above a horizontal plane generally through the center of the elongated hemispherical hearth. When the port is so-p'ositioned there is very little possibility of sloshing the molten charge material out of the hearth. The are electrical power and inlet d-uit 13; and the pilot arc electrical power and discharge of cooling vwater are serviced by conduit 14. Conduit 15 provides the cooling water for the anode 3 and also serves as the positive electrical terminal; the cooling water is discharge-d from the anode by conduit 16. Components 12, 13, 14, 15 and 16 are attached to the furnace unit by a holding structure 17. The position of the electrode within the torch may be adjusted by thecontrol knob 18. Also, within the torch, there is a nontran-sferred pilot are that is used for starting the transferred are, as described in 11.8. Patent No. 2,806,124.

The apparatus rotates on the Z axis which is at the approximate center Olf the crucible as shown in the drawing. The Z axis is the center [line perpendicular to the plane of the figure. The entire apparatus as shown is mounted on a structure in a manner that permits the apparatus to be rotated over an are about degrees. The furnace may be charged with the pre-alloyed metal while the face 6 is in the horizontal position with the crucible opening 10 upward. For the casting operation, the apparatus is rotated countera'cloclcwise until the face 6 is in the horizontal position with the crucible opening 10 downward. The drawing shows the apparatus in the preferred position for the melting operation. Further, the crucible may be charged While it is in the position as shown in the drawing and any other position provided that the crucible opening 10 is above the Z axis.

This combination provides many advantageous features that overcome the problems associated with the prior art methods. Specifically, some of the advantages include the following:

(1) Components 12, 13, 14, 15 and 16 are attached to the rotating apparatus, therefore, the service lines are always in a relatively fixed position. This eliminates the cumbrous arrangement of service lines now used in the art.

(2). The metal is decanted from the crucible to the mold and may be more'uniformly controlled to obtain the optimum pour rate. This is in contrast with the spurting discharge of metal from the bunghole. type of opening in the present crucibles.

(3) The water-cooled anode 3 is insulated by a ceramic sleeve 4. This combination eliminates the failures now common due to overheating the anode area.

(4) The streamlined design of the roof of the crucible was discovered to be effective in controlling the atmosphere of the crucible. The streamlined design of the crucible together with the forcev of the issuing efiluent of the torch avoids the entrapment of vapors and air that may be in the furnace chamber, as is common in sphereshaped crucibles.

For many years, the spherical shape of the cavity of the crucible now used in the art has been advantageously used with the known heating means, i.e., direct or indirect carbon arc methods, together with mechanical rocking and/ or electro-magnetic means for stirring the molten metal. It was discovered that a device for producing directionally stable arcs of the class as described in U.S. Patents Nos. 2,806,124, 2,858,411, 2,858,412, and in ap plication Serial No. 223,484 filed September 13, 1962, now Patent No. 3,194,941 and others may be used as a heating means for melting the metal. These devices were found to be effective in melting many of the metals to be so cast. However, when melting the so-called superalloys, it was found that the control of composition was difficult. Table I-A presents a list of typical alloys that were successfully melted and cast by the process of this invention. These examples show that the novel apparatus suitable for casting a wide variety of alloys that previously required various melting practices. In other words, the apparatus of this invention may replace a variety of processing equipment now necessary to melt and cast various metals. This reduces overhead costs, and simplifies foundry operations.

not require unusual skills or materials.

The efiiuent of the arc torch provides a modicum of stirring action when used in the crucibles now known in the art; therefore, for certain metals, it is necessary to promote stirring by mechanical or electrical means.

The design of the crucible of this invention when used together with the transferred arc torch of the type as disclosed in US. Patent No. 2,806,124 provides the optimum results in completely eliminating or substantially reducing the above mentioned drawbacks in the present art. The tear-shape cavity of the crucible allows an adequate stirring action that eliminates the need for mechanical and/ or electromagnetic stirring means. It is believed that when the plasma effluent impinges the molten The refractory material 11 defining the casting port of the furnace may be composed of any suitable hightemperature refractory material that will withstand the flow of molten metal during the casting operation. Optionally, a ceramic cap (not shown) may be placed over the furnace port 10 to restrict the'gas flow from the fur-.

nace when desired.

As stated earlier, an important criterion forevaluating a melting technique is the ability to retain the fixed composition of the master charge. Chemical composition analyses of several methods have been compared as shown in Table I. The various methods that were tested are as follows:

metal,'the eccentric shape of the crucible causes an asymmetric metal flow that provides thorough mixing of the alloying elements. This mixing of the alloy during the melting and casting operation provides: (1) a faster rate of melting the entire charge, (2) a more uniformly alloyed casting, i.e, segregation is eliminated or reduced, and (3) a reduction of the thermal gradients in the charge, thereby avoiding overheating portions of the melt.-

The construction of the apparatus of this invention does The materials of construction are similar to those now known to be used in the art. The crucible may be made of any suitable refractory material such as zirconia, alumina, magnesia, silica, etc., and'may contain one or more suitable binders such as sodium silicate, feldspar, sodium aluminate, petalite and the like. Magnesia is preferred and was used in MethodIV as hereinafter described. The crucible may be constructed from a castable ceramic slurry or a rammed ceramic mix. The internal chamber may be fashioned by the use of a fugitive pattern by means somewhat similar to the lost wax process. Alternatively, the crucible may be assembled from two or more component parts. The so-assembled parts may be joined by the use of ceramic binders and/or interlocking components. It is recommended that, for optimum service life, the component parts to be joined in a manner that the parting lines are above the level of the molten metal during the melting and casting operations. The insulating material 7 in the drawing may be formed in place by a ceramic slurry or a rammed mix. The insulating material serves as a back-up material to hold the crucible in position and as a protective buffer in the event of a crucible failure. Said insulating material may be composed of any suitable ceramic material such as silica or alumina and may be bonded with one or more suitable bonders such as a soluble silicate, feldspar, sodium'aluminate, petalite, and the like. lite or sillimanite ramming mixes may be used.

The tip of the anode 3 is usually made of steel and the remaining body may be composed of copper, water cooled by known methods of construction. The ceramic sleeve 4 is preferably a preformed component that is friction fitted to the anode. The sleeve may be composed of any suitable ceramic material that provides thermal and electrical insulation.

The shell 5, facing plate 6 and holding structure 17 Commercial products, such as mul- Method I is the common induction melting method. The induction coil of the furnace provides a fair degree of metal stirring by electromagnetic means.

Method II is also an induction melting method; however, the entire operation of melting and casting is conducted in a vacuum. This is the most efficient method now used in the art. The most stringent specifications, for example, for aircraft quality products, usually require vacuum melting and casting.

Method III consists of the combination of a plasma arc heating means in a sphere-shaped crucible as further described in the co-pending U.S. Serial No. 225,794 filed September 24, 1962. A rocking mechanism and/or stirring by electrical convection means was required in these tests.

Method IV is the method of this invention featuring the plasma arc heating means and the tear-shaped crucible. No extra stirring mechanism was used. Only the asymmetrical stirring action caused by the impingement of the torch effluent on the metal in the tear-shaped crucible wa's relied on.

Method V is the prior art method that has been advantageously used for many years as further described in US. Patent No. 2,125,080. The crucible was sphere shaped, carbon arc' was'used as the heating means and a mechanical rocking mechanism was necessary'to promote stirring of the metal.

The compositional control of Method IV, this invention, as judged by small deviation from the composition of the master alloy is excellent as shown in Table I. I It is particularly significant that Method IV compares very favorably with Method II, melting and casting in vacuum, which is the most efficient and most expensive meth od now known in the art. Further, the composition control provided by Method IV may be obtained for a wide variety of products: nickel-base, cobalt-base, or iron-base alloys; regardless of whether or not the alloys heretofore 1 had to' be processed by specific methods to obtain optimum results. Thus, the method of this invention may be used advantageously to cast various known commercial alloys, and especially the superalloys.

- Test specimens were prepared from the alloys cast by Method I, Method IV, and Method V. Results of a number of tests are presented in Tables II and III. The

mechanical properties of the specimens prepared by these methods are relatively similar and meet the typical requirements in specifications for these alloys; in fact, specimens prepared by Method IV generally appear to be stronger at temperatures above about 1100 F. in the various tests.

The data in Tables I, II, and III indicate that the composition control and the mechanical properties of the specimens cast by the method of this invention are either comparable with, or superior to, those of the prior art methods. These results further show that this invention has eliminated the'need for an external stirring means to obtain optimum composition control and mechanical properties in castings produced in a melting and casting apparatus with a plasma-torch as the heating means.

A series of tests was conducted to measure the average time necessary for a complete cycle of melting and casting operations for each of the five methods described It can be seen that the rate of production has been drastically increased with the use of the plasma torch; however, it is particularly significant and unexpected that the time required for melting by Method IV is equal to the time by Method III. It was generally believed that an external mechanical stirring means and the spherical interior design of Method III were essential to obtain the minimum melting time.

Thus, it is apparent that the device of this invention provides the benefits as described in the objectives of this invention mentioned earlier: (1) The combined apparatus of this invention efficiently melts steels and nonferrous metals and casts them to precise dimensions, (2) The novel design of the crucible causes effective stirring of the metal, thereby reducing the production time and eliminating external stirring means, (3) The qualities of castings produced by the method of this invention and by vacuum melting and casting methods are comparable, (-4) Excellent control of composition of the alloy is obtained with the apparatus of this invention, (5) Further, the design of the crucible provides an atmospheric condition within the furnace chamber that is conducive to the production of high quality metal castings.

It is understood that modifications may be made within the spirit and scope of this invention. The examples and variations, suggested hereinbefore, were presently only for description and, therefore, only the limitations as indicated in the appended claims should be imposed.

TABLE I.COMPOSITIONS RESULTING FROM VAR-IOUS MELTING METHODS, WEIGHT PER- CENT ALLOY N-l (HEAT A) Method Element III ALLOY N-1 TABLE IA.TYPICAL ALLOYS CAST BY THE APPARATUS OF THIS INVENTION [Nominal Compositions 1 and Preferred Melting and Casting Methods] Mo Ta Prior Alloy Cr or Fe Co Ni or Mn Ti Al Preferred W Ob Melting Practice Nickel-Base N- 18 5 2 2 Bel. 70 6. VM VC 18.5 2.8 Bal 54 4 7 VM V0 19 4. 25 19 Bal. 3 3 VM VC 15. 5.0 Bal. 2 .3 AM 'IGC 22 10 18 1. 5 Bal. I AM AC 25 7.5 2- Bal. 10.5 AM A0 21 10 BaL' AM 160 12. 5 Bal. AM AC B21. 20. 5 AM A0 16.5 Bal. 4 3 AM AC 12 Bal. AM A 16 6 Bal. 10 15 AM IGO 1 Nominal compositions in boron, silicon, etc.

2 AM-Air Melting. AO-Air Casting. IGCInert Gas Casting. VM-Vacuum Melting. VO-Vacuum Oastmg.

weight percent, may also contain modifying elements, i.e. carbon, zirconium,

llllllllll 1 This invention.

TABLE II.-TENS1LE PROPERTIES DATA NICKEL-BASE ALLOY N-5 TABLE I. Continu ed ALLOY ALLOY 0-1 (HEAT B) 55, 000 20s. 0. 70. 0 9; egg :3,; eg "888 tit? %33 2o 47 12 25, 000 158. e 90. o 0.- .52 .46 .49 i v 10' 10' 10' Z: I Mn. .68 .69 .74 ALLOY C 1 Mo- .33 .33 .36

s .014 .012 t .013, C Bal. Bal. Bal. 10 1,500 30,000 32.9 30.0

ALLOY N-3 What is claimed is:

1. Investment casting apparatus comprising in combination (a) a closed refractory crucible the interior surface of which defines a truncated cone shaped cavity having a'-discharge conduit the entire inner surface of said truncated cone shaped cavity being continuously smooth and free from all irregularities and obstruc- :tion to flow;

(b) a casting port the walls of which are in streamlined registry with said discharge conduit and being above a horizontal plane through the center of the Temp., Method IV Method V Method I F. Test This Prior Prior Invention Art Art Ultimate, p.s.i 73, 268 67, 000 Elongation, percent 38. 3 17. 1 Ultimate, p.s.i 46, 950 43, 500 Elongation, percent 27. 24. 6 Ultimate, p.s.i- 25,250 20, 000 Elongation, percent 45. 0 35. 0

NICKEL-BASE ALLOY N-4 Ultimate, p.s.i Elongation, percent Reduction of Area, percent COBALT-BASE ALLOY' 0-1 NICKEL-BASE ALLOY N-l Ultimate, p.s.i- Yield, p.s.i

IRON-BASE, ALLOY F-l Elongation, percent Reduction of Area, percent Ultimate, p.s.i-

Ultimat p.s.i-

Elongation, percent Reduction of Area, percent Y IRON-BASE ALLOY F-l interior of said truncated cone shaped cavity;

(c) a device containing a first electrode for producing a directionally stable arc 'efliuent extending into said truncated cone shaped cavity;

(d) a second electrode associated with said crucible;

(e) and a power supply connected in circuit relation with said first and second electrodes;

2. Investment. casting apparatus comprising in com- (a) a closed refractory crucible the interior surface of I which defines a truncated cone shaped cavity having a discharge conduit, the surface of said conduit over which the molten metal flows when casting forming a substantially straight streamline with the lower surface of truncated cone shaped cavity and the opposite surface of said conduit forming a curved streamline with the upper surface of the truncated cone shaped cavity;

(b) a casting port the walls of which are in streamlined registry with said discharge conduit and being above a horizontal plane through the center of the interior of the said truncated cone shaped cavity;

() a device containing a first electrode for producing a directionally stable arc effluent extending into said truncated cone shaped cavity;

(d) a second electrode associated with said crucible;

(e) and a power supply connected in circuit relation with said first and second electrodes.

3. Investment casting apparatus comprising in combination I (a) a closed refractory crucible the interior surface of which defines a truncated cone shaped cavity having a discharge conduit the errtire inner surface of said truncated cone shaped cavity being continuously smooth and free from all irregularities and obstructions to flow;

(b) a casting port the walls of which are in streamline registry with said discharge conduit and being above a horizontal plane through the center of the interior of the said truncated cone shaped cavity;

(0) a device containing a first electrode for producing a directionally stable arc effluent extending through the roof of said crucible into said truncated cone shaped cavity;

(d) a second electrode positioned in the bottom of said truncated cone shaped cavity;

(e) and a power supply connected in circuit relation with said first and second electrodes.

4. Apparatus according to claim 3 wherein said electrode is water cooled.

5. Investment casting apparatus comprising in combination (a) a closed refractory crucible the interior surface of which defines a truncated cone shaped cavity having a discharge conduit the entire inner surface of said truncated cone shaped cavity being continuously smooth and free from all irregularities and obstructions to flow;

(b) a casting port the walls of which are in stream lined registry with said discharge conduit and being above a horizontal plane through the center of the interior of the said truncated cone shaped cavity;

(c) an arc torch containing a first electrode for producing a directionally stable arc efiiuent extending into said truncated cone shaped cavity and having a portion extending outside and beyond said crucible; electrical power, water, and gas supply conduits connected to said torch at said extended portion;

(d) a second electrode associated with said crucible and having a portion extending outside and beyond said crucible; electrical power and water supply conduits connected to said electrode through said extended portion;

(e) a power supply connected in circuit relation with said first and second electrodes; 1

(f) and a holding means fixed to said crucible and References Cited by the Examiner 7 UNITED STATES PATENTS 1,514,918 11/1924 Miller.

3,106,594 10/1963 Beasley et a1. 13-44 X 3,173,981 3/1965 Myers et a1. 139

FOREIGN PATENTS" 866,106 4/1961 Great Britain.

RICHARD M. WOOD, Primary Examiner. JOSEPH V. TRUHE, Assistant Examiner.

Claims (1)

1. INVESTMENT CASTING APPARATUS COMPRISING IN COMBINATION (A) A CLOSED REFRACTORY CRUCIBLE THE INTERIOR SURFACE OF WHICH DEFINES A TRUNCATED CONE SHAPED CAVITY HAVING A DISCHARGE CONDUIT THE ENTIRE INNER SURFACE OF SAID TRUNCATED CONE SHAPED CAVITY BEING CONTINUOUSLY SMOOTH AND FREE FROM ALL IRREGULARITIES AND OBSTRUCTION TO FLOW; (B) A CASTING PORT THE WALLS OF WHICH ARE IN STREAMLINED REGISTRY WITH SAID DISCHARGE CONDUIT AND BEING ABOVE A HORIZONTAL PLANE THROUGH THE CENTER OF THE INTERIOR OF SAID TRUNCATED CONE SHAPED CAVITY; (C) A DEVICE CONTAINING A FIRST ELECTRODE FOR PRODUCING TRUNCATED CONE SHAPED CAVITY; (D) A SECOND ELECTRODE ASSOCIATED WITH SAID CRUCIBLE; (E) AND A POWER SUPPLY CONNECTED IN CIRCUIT RELATION WITH SAID FIRST AND SECOND ELECTRODES.

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