US2478225A - Induction melting of palladium and palladium alloys - Google Patents

Description

Tatented Aug. 9, 1949 INDUCTION MELTING F PALLADIUM PALLADIUM ALLOYS Ralph Hall Atkinson, Westfield, N. J assignorto The International Nickel Company, Inc., ,New York, N. .Y., a corporation of Delaware No Drawing. Application December '6, 1947, Serial No. 790,238

.14 Claims.

The present invention relates to a method for induction melting and casting palladium and alloys containing appreciable amounts of palladium, and, more particularly, to the production of clean, non-porous, dense castings made of palladium and its alloys which can be satisfactorily wrought and annealed without deterioration of physical properties.

The prior art has strongly favored torch melting over the induction melting practice for the production of palladium and palladium alloy castings. This is due to difficulties encountered in properly deoxidizing these metals during induction melting. 'The results of induction meltme have been the production of castings which were generally unsatisfactory due to porosity, low ductility, high scalping losses, etc. Prior art attempts to overcome these difiiculties have included the use of certain gaseous and/or solid deoxidizers. In the use of solid treating agents, e. g., metallic aluminum and/or calcium boride, it was found to be almost impossible to avoid adding an excess in order to insure sound metal. This usual1y resulted in a deterioration of the physical properties of the cast and subsequently wrought products, such as lowered ductility, high base hardness in the fully annealed state, poor surface condition, etc. The prior art also proposed the use of hydrogen or carbonaceous atmospheres. Hydrogen atmospheres, howeveiyreactwlth the cruciblejlining 'and tend to produce porosity, lower ductility and generally poorer physical properties. 'Carbona'ceous gases, e. g., city gas, were also proposed but their use increased the gassiness of the melt and resulted in poor-quality, unsound ingots which became blistered upon annealing after cold working.

For the aforesaid reasons, the prior art practice has favored torch melting with oxygen-city gas or with oxygen and hydrogen flames. 'By closely controlling the proportions of the combustion mixture, it was possible, under certain conditions, to obtain satisfactory results although these results were not predictable. It Was felt, however, that the induction melting technique offered many obvious advantages over the present torch melting practice provided the aforesaid difficulties could be overcome. Many attempts have been made to improve the technique for the induction melting of'p'alladium and palladium-containing alloys but'none, insofar as I am aware, was entirely successful in producing consistently satisfactory results.

I have discovered a method of induction melting palladium and palladium-containing alloys whereby satisfactorily sound, dense and substantially gasefree castings possessing good physical properties are consistently produced. The castings produced by the method of the present invention can be subsequently cold worked and fully annealed to the minimum possible hardness Without any blistering and without any noticeable loss of quality, ductility, or deterioration of surface appearance, such as dull color.

It is an object Qf'thepresent invention to provide a'method of induction melting palladium and palladium alloys whereby satisfactory prodnets are consistently produced.

The present invention has as an object the production of clean, sound palladium and palladium-containing alloys free from contaminating, reduced refractory metals.

It is another object of this invention to provide a method of induction melting palladium and palladium-containing" alloys whereby satisfactory products, free from porosity and having h gh ductility and good surface quality, are obtained.

It is still another object of this invention to provide a method of producing induction melted palladium and palladium alloys which, when solidified, arelsound, properlydeoxidized and degassed, and which have an improved as cast surface that requires only a minimum amount of scalping.

Furthermore, the present "invention contemplates a method of producing sound, clean, induction melted castings of palladium and palladium-containing alloys, which can be successfully cold worked'by virtue of inherently good ductility and lack of porosity.

Another object provided by this invention is to produce induction melted castings of palladium and palladium -al1cys,=which upon annealing after cold working, will revert to a condition of maximum ductility without the development of surface blisters or porosity.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following-description:

In general, the present invention provides a method-of controlled induction melting of palladium and paladium-containing alloyswhereby the ingots,'upon casting, are substantially free from reduced refractory metals and possess a soundnessand freedom-from harmful inclusions, which in turn-promote'maximum ductility surface quality-uponsubsequent processing. The method also provides castings of palladium, palladium-base alloys and palladium-containing a1- loys, such as the dental alloy type, to which only a minimum amount of surface scalping and/or top cropping is necessary in order to prepare the ingots for further processing. The control of the induction melting operation is accomplished by first melting down the palladium or palladium alloy in air and holding the molten metal exposed to air for a length of time sufficient to oxidize all the impurities contained therein; then applying to the surface of said molten a protective atmosphere containing, as essential constituents, varying amounts of hydrogen or carbon monoxide, substantially all of the balance being a gas or gases from the group consisting of argon, helium, neon, krypton and xenon, preferably argon and/or helium, and most preferably argon.

In the creation of the protective atmosphere over the surface of the molten metal, theinert gas, e. g., argon and/or helium, is preferably applied continuously whereas the deoxidizing gas, 1. e., hydrogen or carbon monoxide, may be either continuously applied or preferably intermittently injected into the atmosphere. Another method of carrying out the invention is to apply the inert gas and the reducing gas successively or alternately for short periods of time until the metal is deoxidized. This method is advantageous when using a hood to confine the gases over the melt. When the amount of metal to be deoxidized is small, it is not necessary to use an atmosphere-confining hood. When the amount of metal to be treated is relatively large, however, some form of gas-confining hood is necessary. The simultaneous use of hydrogen and carbon monoxide in the protective atmosphere is to be avoided since, at the temperatures involved, these gases tend to react and the products of this reaction will cause porosity in the cast metal under certain conditions. Upon completion of the melting and deoxidation stages, the protective atmosphere can then be removed and the molten metal cast in a manner well known to those skilled in the art. It is essential that the molten metal be cast with reasonable promptness, for example, by beginning to cast within about five seconds, after completion of the deoxidation and the removal of the protective atmosphere.

The amount of hydrogen or carbon monoxide to be allowed to be present at any one time in the protective atmosphere will vary considerably depending on the type and amount of alloy be ing melted and deoxidized, the nature of the refractory crucible being used and the method of application of the deoxidizing gas. For instance, if the hydrogen or carbon monoxide is applied in a continuous stream throughout the entire deoxidation stage, the amount of either of these gases to be permitted to be present in the protective atmosphere at any one time should be limited to a maximum of about 50% by volume, and preferably limited to about by volume in order to properly deoxidize the molten metal and at the same time avoid excessive reaction of the deoxidizing gas with the refractory crucible and the incorporation of harmful refractory elements in the molten metal. If, on the other hand, the deoxidizing gas is intermittently injected into the protective atmosphere, it is permissible in certain cases, to allow the amount of said gas to reach about 100% by volume of the atmosphere for only very short periods of time, for example, not more than about five seconds after vigorous gassing ceases. Such short periods of time are suflicient, where the intermittent in- 4 jection method is used to permit proper deoxidation without harmful reaction with the refractory crucible.

In carrying this invention into practice, it is most preferred that argon be used as the inert component of the atmosphere because of its extremely low solubility in, and its low rate of diffusion through, palladium and palladium-containin alloys. Hydrogen is preferred as the deoxidizing constituent of the protective atmosphere by virtue of its more potent effect relative to carbon monoxide, and also because the point of complete degassing and deoxidation can be most clearly observed. As hereinbefore stated, it is preferred that the treatment be accomplished with the hydrogen gas intermittently in- .iected into the argon atmosphere. It is believed that the intermittent injections of hydrogen, which in turn produce fluctuations in the partial pressure of the hydrogen, promote a more rapid deoxidation and degassing than is the case where the hydrogen is continually applied at a constant partial pressure; since, in the former case, each time the partial pressure of hydrogen drops to a minimum, the hydrogen dissolved in the molten metal at the higher partial pressures leaves the melt and sweeps other dissolved gases along with it. The point of complete deoxidation is clearly marked by the absence of gassing when the last hydrogen injection is given.

Magnesia and magnesia cement crucibles or crucible linings are preferred for induction melting palladium and its alloys in reducing atmospheres. Palladium and its alloys melted in alumina, alundum, zircon (zirconium silicate), Zirconia (zirconium oxide), and fireclay crucibles in reducing atmospheres exhibit less ductility when compared with the same metal melted in magnesia and magnesia cement crucibles. Where a powerful deoxidizer such as hydrogen is used, it appears that the hydrogen tends to reduce the refractory crucible material as well as any oxides contained in the melt. Thus, if the refractory contains aluminum, silicon and/or zirconium, these elements, by reduction of their oxides, can be incorporated in the metal, thereby impairing the quality of the casting, such as lowered ductility, higher base hardness after fully annealing, embrittlement, etc. In the case of magnesia-lined crucibles, a magnesium acetate or magnesium formate binder is found to be most satisfactory. Aqueous solutions of the bonding agents are prepared by dissolving magnesium oxide in the respective acids, using the proportions given as follows:

Magnesium Acetate Solution Magnesium Formate Solution Magnesium oxidenugremsnfi Magnesium oxide ..grams 5 Glacial acetic acid c. c. 17 Formic acid (sp. gr. 1.2} ..c. c ...ll Water .lc. c l0(l Water c. c...l0D

EXAMPLE NO. ,1

A small sand crucible .oi'about 25 c. 0. capacity is lined with one-quarter inch of pure magnesia. About fifty grams of palladium metal are induction melted in the crucible in air to oxidize impurities in the palladium. After the impurities are oxidized, the air over the molten metal is displaced byargon delivered at a rate of about 25 c. e. per second. A hydrogen jet, delivering about 25 c. c. of hydrogen'per second, is then intermittently applied for a few seconds at a time, until the metal is deoxidized, as shown by the absence of gassing when the hydrogen is injected. The gas jets are turned off, the argon first and then the hydrogen, and the molten palladium is cast into :a graphite :mold in a conventional manner. As noted hereinafter, a very satisfactory product .is "obtained.

EXAMPLE .NO. 12

Using the same crucible and equipment as in Example No. 1, about 47.75 gramsoi palladium sponge are melted and oxidized in air. The air over the molten metal is then displaced by argon and 2.25 grams of ruthenium powder are added and melted into the palladium to form an alloy of about 4.5% ruthenium and about 95.5% palladium. The molten alloy is then deoxidized by applying a jet of hydrogen as in Example 1. At the point of complete deoxidation, the argon gas and then the hydrogen gas are shut off, and the palladium-ruthenium alloy is cast in the conventional manner.

EXAMPLE NC. 3

A small 25 c. 0. capacity sand'crucible was provided with one-quarter inch thick lining of a proprietary magnesia cement. About 47.75 grams of palladium sponge are melted in the crucible using the same equipment as in Examples No. 1 and No. 2. After the initial oxidation of the impurities, .a pro-mixed atmosphere comprising about 96% argon and about 4% hydrogen is applied at a rate of about 29 o. c. per second. About 2.25 grams of pure ruthenium powder are immediately added and melted in to form a palladium-ruthenium alloy. After about 5 minutes exposure to the deoxidiz ing mixture, the treated 'molten metal may be cast as in in Examples No. land No. 2. As an alternative, the power may be switched ofi, the metal allowed to solidify in. the crucible to further assist in eliminating any dissolved gases that may remain and. the metal may then be remelted in the same argon-hydrogen mixture, the same not being removed until immediately prior to casting in the conventional manner, e. g.,

in a graphite mold.

EXAMPLE NO. 4

The following example illustrates the application of the invention to a palladium alloy containin only 40% palladium: 12 grams of pure palladium, 9 grams of pure silver and 9 grams of electrolytic copper were induction melted in a small alumina crucible usingan atmosphere of argon over the metals. A hydrogen jet was applied intermlttentlya few seconds at a time until the metal was free from oxygen as shown by the absence of gassing when the hydrogen was injected. The hydrogen was then turned off, 0.02 gram of calcium b'orid'e added, the argon removed and the alloy castinto a warm copper mold. A good, sound ingot :wasobtai ned.

Good ingots were obtained bythe procedures described in the aforementioned examples. Annealed wrought strips made from these ingots were generally softer and denser than metal treated in a pure hydrogen or carbon monoxide atmosphere devoid of argon and/or helium. By comparison, the overall quality of the metal was at least equivalent in every respect to, and in many cases superior to, the best metal produced by the conventional torch melting technique.

The improved results obtained by the present invention are further illustrated by the data contained in Tables No. land No. 2 as given hereinafter. These data were obtained upon a series of palladium-base alloys containing about 4.5% ruthenium. These alloys were induction melted under a variety of atmospheres indicated in Table No. 1 as deoxidizing treatment. The gassiness of the various ingots cast therefrom was observed and is recorded in Table No. 1. The ingots were then scalped where necessary, alternately cold rolled in steps of 58% reduc-- tion and annealed, to produce strips 0.01 inch thick. After the final annealing, observations were made on the color of the strip surfaces and the amount of blistering, and the data recorded in Table No. 2. Density and hardness measurements were made and are indicative of the (le gree of porosity and the degree of purity of the metal. Density (grams per cubic centimeter) and hardness as fully annealed '(Vickers hardness with a 5 kilogram load) for each strip are recorded in Table No.2.

2% Zn and He 0.1% Cam and A 10 H2 0% good ingot.

100% CO gassy,;low-density ingot.-

CO and N2 gassy ingot with round top. 10% H2 and N z gassy ingot with blown top. H; and He= sound ingot.

City Gas and AL-.. gassy ingot. 4% H2 and 967 A sounlgl ingot.

H2 and A Do.

a Carbon monoxide displaced by nitrogen gas after the refining treatment. 7

Continuous application of thepremixed atmosphere during'the refining cycle.

a Three or four brief-applications of'tbe firstencmed gasintoithe second-named gas atmosphere.

TABLE' No; 2

Cold roll d and annealed 4.5% mthemopal'ladium alloys Strip Surface Density Hardness lidrelt ful1ly1Anfullylg an 0. nee e: nea e gggg Color per c.-c. VHN, clp't considerable. 11.618:

11.982- 86.0 11.951, 92.0 115933 8017 0.. 115906. I891 considerable. 12. 0117 90. 9 none. '12. 018 8623 considerable. .12. 01 i- 1 N 0 accurate reading possible-*due'to excessive-blistering.

The use of the method of inductionmelting palladium, palladium-base alloys. and-palladiumcontaining alloys as contemplated by the present invention produces castings which are consistently sound and clean and which possess uniformly good physical properties and overall excellent quality including good ductility, freedom from blistering after cold working and annealing, excellent surface appearance, etc. The present invention is applicable, in general, to the induction melting and deoxidation of palladium and alloys containing palladium in amounts as low as about 30%, such as the dental alloy type, and is particularly applicable to palladium-base alloys, i. e., alloys containing about 50% or more palladium and the balance substantially one or more other elements, such as platinum, rhodium, iridium, osmium, ruthenium, gold, silver, manganese, tungsten, iron, nickel, cobalt and copper. The present method of induction melting and deoxidizing palladium-containing alloys is especially beneficial to treating pure palladium, palladium-ruthenium alloys and other high-palladium content alloys because hydrogen dissolved in palladium reduces the usual refractories rather rapidly at the melting points of these alloys (2700-2900 F.).

Although the present invention has been described in conjunction with certain preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such variations and modifications are to be considered within the purview of the application and the scope of the appended claims.

I claim:

1. A method for the induction melting of palladium and palladium-containing alloys to produce sound, clean castings possessing good physical properties and capable of being wrought and annealed satisfactorily which comprises induction melting a metal from the group consisting of palladium and palladium-containing alloys, applying to the surface of said molten metal an argon atmosphere, intermittently injecting into said atmosphere a small but effective amount up to about 10% by volume of hydrogen to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

2. A method for the induction melting of palladium and palladium-containing alloys to produce sound, clean castings possessing good physical properties and capable of being wrought and annealed satisfactorily which comprises induction melting a metal from the group consisting of palladium and palladium-containing alloys, applying to the surface of said molten metal an atmosphere containing a small but effective amount up to about 50% hydrogen by volume and the balance essentially argon to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

3. A method for the induction melting of palladium and palladium-containing alloys to produce sound, clean castings possessing good physical properties and capable of being wrought and annealed satisfactorily which comprises induction melting a metal from the group consisting of palladium and palladium-containing alloys, applying to the surface of said molten metal an argon atmosphere, intermittently injecting into said atmosphere a small but effective amount up to an amount approaching about 100% by volume of hydrogen to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

4. A method for the induction melting of palladium and palladium-containing alloys to produce sound, clean castings possessing good physi cal properties and capable of being wrought and annealed satisfactorily which comprises induction melting a metal from the group consisting of palladium and palladium-containing alloys, subjecting said molten metal to an oxidizing treatment, applying to the surface of said molten metal an atmosphere containing a small but effective amount up to about 50% by volume of one gas from the group consisting of hydrogen and carbon monoxide and the balance essentially gas from the group consisting of argon and helium to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.-

5. A method for the induction melting of palladium and palladium-containing alloys to produce sound, clean castings possessing good physical properties and capable of being wrought and annealed satisfactorily which comprises induction melting a metal from the group consisting of palladium and palladium-containing alloys, applying to the surface of said molten metal an atmosphere containing a small but effective amount up to about 50% by volume of one gas from the group consisting of hydrogen and carbon monoxide and the balance essentially gas from the group consisting of argon and helium to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

6. A method for the induction melting of palladium and palladium-containing alloys to produce sound, clean castings possessing good physical properties and capable of being wrought and annealed satisfactorily which comprises induction melting a metal from the group consisting of palladium and palladium-containing alloys, subjecting said molten metal to an oxidizing treatment, applying to the surface of said molten metal an atmosphere containing essentially gas from the group consisting of argon and helium, intermittently injecting into said atmosphere a small but effective amount up to an amount approaching about by volume of one gas from the group consisting of hydrogen and carbon monoxide to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

7. A method for the induction melting of palladium and palladium-containing alloys to produce sound, clean castings possessing good physical properties and capable of being wrought and annealed satisfactorily which comprises induction melting a metal from the group consisting of palladium and. palladium-containing alloys, applying to the surface of said molten metal an atmosphere containing essentially gas from the group consisting of argon and helium, intermittently injecting into said atmosphere a small but effective amount up to an amount approaching about 100% by volume of one gas from the group consisting of hydrogen and carbon monoxide to refine said molten metal, and thereafter promptly casting said molten metal whereby 9 sound, clean castings possessing good physical properties are produced.

8. In the method for induction melting, refining and casting palladium and palladium-containing alloys, the improvement which comprises applying to the surface of molten, inductionmelted metal from th group consisting of palladium and palladium-containing alloys an argon atmosphere, intermittently injecting into said atmosphere a small but effective amount upon to about 10% by volume of hydrogen to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

9. In the method for induction melting, refining and casting palladium and palladium-containing alloys, the improvement which comprises subjecting molten, induction-melted metal from the group consisting of palladium and palladiumcontaining alloys to an oxidizing treatment, applying to the surface of said molten metal an argon atmosphere, intermittently injecting into said atmosphere a small but effective amount up to an amount approaching about 100% by volume of hydrogen to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

10. In the method for induction melting, refining and casting palladium and palladiumcontaining alloys, the improvement which comprises applying to the surface of molten, induction-melted from the group consisting of palladium and palladium-containing alloys an argon atmosphere, intermittently injecting into said atmosphere a small but effective amount up to an amount approaching about 100% by volume of hydrogen to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

11. In the method for induction melting, refining and casting palladium and palladiumcontaining alloys, the improvement which comprises induction melting metal from the group consisting of palladium and palladium-containing alloys in the presence of an atmosphere containing a small but effective amount up to about 10% hydrogen and the balance essentially argon to refine said metal whereby sound, clean castings possessing good physical properties can be produced therefrom.

12. In the method for induction melting, refining and casting palladium and palladiumcontaining alloys, the improvement which comprises induction melting metal from the group consisting of palladium and palladium-containing alloys in the presence of an atmosphere containing a small but effective amount up to about 10% hydrogen and the balance essentially argon to refine said metal and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

13. In the method for induction melting, refining and casting palladium and palladiumcontaining alloys, the improvement which comprises applying to the surface of molten, induction-melted metal from the group consisting of palladium and palladimn-containing alloys an atmosphere containing essentially gas from the group consisting of argon and helium, intermittently injecting into said atmosphere a small but eifective amount up to about 10% by volume of carbon monoxide to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

14. In the method for induction melting, refining and casting palladium and palladiumcontaining alloys, the improvement which comprises applying to the surface of molten, induction-melted metal from the group consisting of palladium and palladium-containing alloys an atmosphere containing essentially gas from the group consisting of argon and helium, intermittently injecting into the said atmosphere a small but effective amount up to an amount approaching about by volume of carbon monoxide to refine said molten metal, and thereafter promptly casting said molten metal whereby sound, clean castings possessing good physical properties are produced.

RALPH HALL ATKINSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,518,083 Lohmann Dec. 2, 1924 1,752,474 Anderson Apr. 1, 1930 1,845,694 Wood Feb. 16, 1932 OTHER REFERENCES Hoke, Refining Precious Metal Wastes, published 1940 by Metallurgical Company, New York, N. Y., pages 300 through 304.

Steel, Removing Dissolved Gases, Oct. 14, 1946, pages 110, 122, 125, 126 and 128.

ertificat e of lorrecti oii r Patent No. 2,478,225 August 9; 1949 RALPH" BALL ATKINSON It is hereby certified that error-sf eppear in the printed specification of the above numbered patent requiring correction; gs follows:

Column 9, line 11, for the word hpon read up lines and 34, after inductionmelted insert metal; 1.

and that the said Letters Patent should be read with? these, corrections therein that the same may conform to the record; of the case in the Patent Ofiice.

Signed and sealed this 7th day February, A. D: 195 0.

THOMAS} E MURPHY,

Assistant Uommz'ssz'mr of Patents.