US1955726A - Method of melting copper to produce dense castings low in oxygen - Google Patents

Description

April 1934. R. s. ARCHER El AL 1,955,726

METHOD OF MELTING COBPER TO PRODUCE DENSE CASTING'S LOW IN OXYGEN Filed July-21 1953 INVENTORS.

Robert S Archer Milan A. Matush ATTORNEY.

atented Apr. 24, 1934 METHOD OF IMELTING COPPER TO PRO- DUCE DENSE CASTINGS LOW IN OXYGEN Robert S. Archer,'Whiteflsh Bay, and Milan A. Matush, Milwaukee, Wis., assignors to A. O. Smith Corporation, Milwaukee, Wis., a corporation of New York Application July 21, 1933, Serial No. 681,432

4 Claims.

This invention relates to the melting of copper and especially to the melting of copper for the purpose of bringing the metal into a molten condition which, upon pouring, produces castings or 5 ingots characterized by high density and purity.

The great bulk of the copper produced in this country is purified by electrolysis. Electrolytic or cathode copper is substantially free from copper oxide and other impurities. But though of a high degree of purity, cathode copper has neither the shape nor the physical structure which adapt it .to be drawn into wire or used in those places for which pieces of cast or wrought copper are desired. It is therefore necessary that the cathode copper be melted and cast into wire bars or other suitable shapes before it is ready for subsequent treatment or use. When melted by the methods commonly in use, copper is oxidized and forms copper oxide which dissolves in the molten copper. Copper oxide has a detrimental efiect upon the physical propertiesof the copper in which it is contained, and to minimize this effect it is necessary to treat the molten copper, before casting, in order to remove a portion of the copper oxide which is formed when the copper is melted.

One of the widely used methods for decreasing the copper oxide content in a bath of molten copper consists of poling the bath with wooden poles. The wood reduces the copper oxide forming metallic copper and gases which escape from the bath. It is usual to pole the bath until the copper oxide content has been reduced to approximately one-half of one per cent, which corresponds to approximately five one-hundredths of one per cent of oxygen. By a continuation of the poling, the copper oxide content of the bath may be still further decreased, but over-poled copper produces brittle and porous castings which are unsuitable for further treatment or use. For this reason a complete elimination of copper oxide by poling is impracticable, and poled copper retains about one-half of one per cent of copper oxide.

While this amount of copper oxide is not particularly detrimental to the electrical conductivity of the copper, it has an undesirable effect upon the physical properties. Segregationof the copper oxide in wire bars may cause mechanical weakness which results in the fracture of the bars as they are drawn into wires. Poled copper is also less ductile than oxygen free copper, hardens more rapidly when cold worked, and requires more frequent annealing. Another un-' desirable effect of the copper oxide is the embrittlement it produces when copper containing copper oxide is heated in a reducing gas atmosphere. This embrittlement precludes the use of hydrogen annealing for copper which contains copper oxide, and has also caused premature failure in copper wires or cables soldered in the reducing gas flame from a blowpipe.

In some cases the copper oxide content of a molten copper bath is decreased by the addition of metallic deoxidizers such as copper-phosphorus or copper-silicon alloys instead of by poling.

But if too little of the deoxidizer is used, the copper is not completely deoxidized; if too much, a part remains in the copper and decreases its conductivity. The use of alloy deoxidizers is impracticable or costly when a high conductivity copper is to be produced.

Although these expedients are extensively used since the resulting defects are less injurious than the defects of the untreated metal, it is desirable to provide a method of melting copper which avoids the oxidation encountered in other methods of melting and produces molten copper free from foreign deleterious constituents.

One of the objects of this invention is to provide a method of melting copper which protects the copper from oxidation while it is being melted.

Another object of the invention is to provide a simple, rapid, and economical method of melting copper which not only avoids oxidation after the copper is melted, but also eliminates dissolved or occluded gases and fluxible nonmetallic impurities such as copper oxide, or other oxides which the copper may contain as a result of its previous treatment.

A further object of the invention is'to provide for the production of copper castings of maximum soundness, cleanliness, and purity.

In accordance with the invention, we provide a flux which is non-oxidizing with respectto copper to flux the nonmetallic or fluxible materials that are to be removed. The flux is electrically heated to temperatures above the melting point of copper by its resistance to the passage of current. A conductor composed of the copper which is to be melted forms one terminal of the electrical'heating circuit. One end of this copper terminal is submerged a short distance in the melted flux to prevent arcing at the surface and is disposed to maintain a relatively thick layer of flux between the end of the electrode and the molten metal which collects below the flux and serves as the other terminal of the heating circuit.

The figure of the accompanying drawing shows a vertical section through one type of apparatus found suitable for the melting of copper to produce castings.

A steel shell 1 supports the crucible or hearth which holds the molten copper and flux. The shell is lined with refractory material 2 and this in turn carries the graphite hearth or crucible 3.

This is provided with a connection 4 to be connected to one terminal of a source of electric energy. The other terminal of the source of electric energy is connected to the upper end 5 of the copper electrode 6. Thelower end '7 of the copper electrode 6 is immersed in the fused flux 8. The copper electrode 6 may consist of one or more strips of cathode copper, bus bar that is to be remelted, or other copper objects of suitable shg fie for use as an electrode.

e copper which melts from the lower end of the copper electrode falls through the fused flux and collects in the molten pool of copper 9 atthe bottom of the hearth or crucible 3. As the copper electrode is consumed, it is fed downward by the automatic feeding mechanism schematically indicated at 10 in order that its lower end may be constantly maintained below the surface of the molten flux. The steel shell 1 may be provided with trunnions 11 for tilting the furnace and pouring the copper, or the copper may be tapped intermittently or continuously by means of a tap hole.

When copper is melted in this way the thick layer of fiux'between the submerged end of the copper electrode and the pool of molten copper constitutes a heating resistance and a cleaning agent in which the molten copper is dispersed or disseminated in small aggregates by the action of the heated flux and the electric current. Fused anhydrous borax has been found to be very suitable for use as a flux, but otherfluxes which do not oxidize copper, and which dissolve copper oxide and other metallic oxides should give satisfactory results. Since the copper is melted below the surface of the flux, it is effectively protected from the oxygen in the atmosphere, and

a-thln film of molten copper on the end of'the electrode is exposed to the cleaning action of the flux.

Examination of fiux that has been used in this method of melting copper shows that it contains numerous small copper particles which have been trapped by the solidification of the flux. As the small particles fall through the molten flux in the melting operation, they expose a large surface to the cleansing action of the flux which is consequently able to rapidly and effectively dissolve out any fluxible impurities which the copper may contain. The small drops of copper which have been purified in this way fall through the flux layer and readily unite with the 'pool of molten metal in the bottom of the hearth or crucible.

When the copper which is melted is substantially pure, as cathode copper, the action of the flux is largely to protect the pure copper from contamination by oxygen or other impurities,

but when the copper to be melted contains cop-.

per oxide, the fiux first purifies the copper and then protects it from subsequent oxidation: The

graphite crucible also exerts a powerful reducing influencewhich makes for the elimination of oxygen, but as the graphite is not exposed to the large surface of thesmall particles of copper which pass through the flux, it is probably less efiective in the elimination of copper oxide and other fluxible impurities than the molten flux. When high conductivity copper is to be produced, care must be taken that the graphite crucible and the fiux are free from impurities which might contaminate the copper. Small traces of iron, for example, produce a much greater reduction in the conductivity of oxygen-free copper such as is produced in this method of melting than is the case with copper which contains an appreciable oxygen content.

The heating current which passes through the copper electrode, the layer of flux and the pool of molten copper is regulated to control the temperature of the metal and maintain fluidity when it is poured. Under these conditions the molten copper is acted on to liberate any gases it may contain as well as to dissolve the fiuxible impurities, and irrespective of whether the copper is cathode copper substantially free from copper oxide, or bus bar copper with an appreciable content of copper oxide, there is produced a melt of copper low in oxygen and gaseous impurities,- and suitable for the production of low oxygen copper castings or ingots characterized by high density and purity.

The following is an example of the procedure followed and the result obtained in melting copper bythis method. Anhydrous fused borax was.105 placed in the bottom of a graphite crucible connected to one terminal of an alternating current source. Enough flux to provide a layer about one to two and one-half inches thick over the surface of the molten metal was found to give good results, but more or less can be used. A bar composed of cathode copper was connected to the opposite terminal of the current source and constituted the electrode which was consumed by melting. The end of the electrode was maintained about one-fourth of an inch below the surface of the fiux to prevent arcing, and current at from 15 to 25 volts and about 800 to 1400 amperes, giving a, current density of about 400 to 700 amperes per square inch of the copper electrode, was passed through the electrode, fiux and crucible. These conditions gave a melt with a temperature of approximately 2100 to 2120 F. which was sufiicient to retain all the metal in fluid 'condition until it was poured. Near the melting end of the electrode the flux was heated locally to a somewhat higher temperature but the average temperature of the flux was not much higher than that of the metal.

The electrode was lowered into the flux as it melted away in order to maintain the described immersion in the molten flux. The molten metal was removed from the submerged end of the electrode by the action-of gravity, the heated flux and the current, and was dispersed in the flux layer between the end of the electrode and the molten pool. It settled through the flux and collected rapidly in the bottom of the crucible from whence it was poured into a mold.

The surface of a sand. cast bar of this metal which had been machined to remove the outer layer was free from porosity and resembled the surface of a rolled or forged bar after machining. The density was found to be 8.85 and the electrical conductivity about 95%. This is excellent conductivity for copper castings. This method of melting consistently gave castings of unusually high density and electrical conductivity.

Samples of wires drawn from ingots cast from copper melted by this method showed values for 150 conductivity in the annealed conditions from 99.35 to 100.80%, and oxygen contents from 0.005 to 0.013%. The wire could be bent repeatedly without fracture after annealing in hydrogen; whereas ordinary commercial copper wire will scarcely stand one bend after a similar annealing treatment in hydrogen. The castings from which this wire was drawn showed only the slightest trace of copper oxide on micro-examination of etched specimens and were dense and free from blow holes.

That the excellent results obtained by this method of melting are not due to the action of the graphite crucible and the flux alone, but are dependent upon the fine dispersion of the molten copper throughout the flux which is produced by the action of the electric current, is apparent from a comparison of the results just given with those which were obtained in the following test. Some of the same lot of copper which was used as melting stock in the-above test was melted in a similar graphite crucible under a cover of the same flux in a high frequency induction furnace, and the melt was held under the flux for about an hour to give the flux time to purify the metal. An ingot was cast in an iron mold and was found to beunsound, having a high degree of porosity and a density of only 7.94 as compared to the density of 8.90 for ingots cast in iron molds from copper melted by the improved method. It is evident that the thorough action which the flux exerts on the finely dispersed particles of molten copper which are produced by the action of the electric current in the improved method of melting is much more effective in freeing the metal from gas and in producing sound ingots than the action exerted by the same flux under different melting conditions.

Commercial bus bar copper containing copper oxide as well as gases was melted in accordance with the invention. The ingots obtained when this metal was cast were generally similar to the ingots produced by melting cathode copper, and were sound, dense, and of low oxygen content. Thus this method of melting not only avoids oxidation but exerts a refining action upon metal which initially contains impurities.

.the electrodes is possible without departing from the spirit of the invention. Thus instead of using one electrode and passing the heating current through the electrode, flux, melt and out through the crucible, two electrodes could be used, the current passing from one submerged copper electrode into the flux and molten metal, and back through the flux to another electrode; or three phase power could be used in a furnace for this method by the provision'of the requisite number of electrodes.

In view of the well known difiiculty of producing sound copper castings of high electrical conductivity, this method of melting should prove to be of particular value in the copper foundry.

The equipment required is relatively simple, and even a small furnace will have a high daily capacity because the molten copper need not be retained in the furnace for a long period of time before pouring. Unlike the usual methods of melting copper there is no initial oxidation with the formation of a high copper oxide content which is later partially reduced. The heat generated in the copper electrode by the passage of the current through it is not lost but preheats the copper prior to melting. The method is one of considerable flexibility, and is admirably adapted to the production of low oxygen copper castings of high conductivity in a foundry or to the production of wire bars to be drawn into wire.

We claim:

1. The method of melting copper which comprises submerging one end of a body of the same beneath a shallow surface layer of molten borax glass, applying a voltage insuflicient to cause arcing at the end of the body, passing current through the body and a lower relatively thick layer of the borax glass to heat the same and melt the metal, and maintaining said relative position of the metal body and borax glass as the metal body is melted away.

2. The method of melting copper in a graphite container which comprises using the copper to be melted as an electrode in an electrical heating circuit, providing a molten bath of non-oxidizing flux which dissolves copper oxide in the graphite container, submerging one end of the copper electrode below the surface of the molten flux, passing an electric current through the copper electrode, the molten flux and the graphite container, collecting the fused copper in the graphite container below the flux, and feeding the copper electrode toward the flux as it melts in order to maintain the lower end of the copper electrode below the surface of the flux.

3. The method of melting copper which comprises submerging the end of a body of the same into a molten bath of non-oxidizing flux which dissolves copper oxide, disposing the submerged end of the body in the flux to provide a layer of flux thereabove which will prevent arcing and a body of flux therebelow which has sufilcient thickness and resistance to heat the flux to the pouring temperature of copper when current is passed therethrough, heating the flux to at least the pouring temperature of copper by passing current through the body and the flux while maintaining the voltage below the arcing voltage at the submerged end of the body, and collecting the molten metal from the submerged end of the copper body in a pool beneath the molten flux.

4. The method of producing sound, dense castings or ingots of substantially pure copper which comprises using the copper to be melted as an electrode in an electrical heating circuit, submerging one end of the copper electrode below the surface of a non-oxidizing flux which dissolves copper oxide, passing an electric current through the copper electrode and the flux to melt the submerged end of the copper electrode, collecting the molten copper below the flux, and pouring the molten copper into molds.

ROBERT S. ARCHER. MILAN A. MATUSH.

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