US2392917A - Silver cladding - Google Patents

Description

Patented Jan. 15, 1946 SILVER CLADDING Edward Joseph Guinee, Roselle, N. J assignor to The H. A. Wilson Company, Newark, corporation of New Jersey N. J., a

No Drawing. Application January 26, 1944, Serial No. 519,809

Claims.

This invention is concerned with silver clad steel and particularly with steel backed silver bearings. It contemplates improvements in the welding of silver to steel and also contemplates a novel silver-clad steel product which offers numerous advantages as a bearing material, especially in aircraft engines and the like.

This application is a continuation-impart of my co-pending application Serial No. 491,971, filed June 23, 1943.

The bearings in modern internal combustion engines, particularly aircraft engines, must operate reliably under severse service conditions. In many respects silver is a satisfactory bearing material, particularly when it is employed as a relatively thin sheet or cladding on a backing of harder and stronger material such as steel. Steel backed silver bearings in which the silver is applied to the backing by casting or electrodeposition have been employed. However, such bearings are not entirely satisfactory due to a tendency for the silver to peel from the backing.

As the result of my investigations, I have developed an improved process for welding silver to steel which results in a great increase in the strength and uniformity of the bond between the silver and the steel and at the same time permits the production of a silver bearing surface having improved ductility and with an exceptionally low coemcient of friction.

In accordance with my invention, the silver is welded to the steel as follows:

A thin coating (say .0005 to .002" thick) of copper is formed on the surface of the steel by flowing molten copper thereon, the term copper being employed to describe elemental copper and copper alloys in which copper constitute 60% or more of the alloy. In other words, the coating is formed by exposing the surface to the action of a film of molten copper. Conveniently, the thin coating of copper is formed by melting an appropriate amount of the metal on the surface of the steel. The copper employed should be somewhat more than is actually required to coat the steel. The excess copper generally forms a bead on the edge of the steel, from whence it is removing by filing etc., and the use of the excess copper assures (a) adequate coverage of the steel and (b) thorough washing of the steel by the copper with resultant removal of impurities such as iron scale.

The copper-washed steel is heated at a temperature above the melting point of the copper but below the meltin point of the steel until a substantial proportion of the copper has diffused into the steel and alloyed therewith, leaving the balance of the copper on the surface. In other words the steel should be washed with the molten copper until substantial penetration occurs. A solid piece of silver is then placed on the resulting copper-coated surface to produce a coppersilver interface and the silver is welded to the steel in the absence of flux by pressing the silver against the copper coating (preferably with a force of at least pounds per square inch) and heating the assembly to a temperature in the neighborhood of but below the melting point of the silver. The heating is continued until the balance of the copper has difiused substantially completely into the silver and alloyed therewith thus substantially eliminating the copper-silver interface and producing a joint free of unalloyed copper and one in which the silver is united directly to the steel surface, the amount of copper present on the steel surface at the beginning of the silver welding step being so small that the diffusion of the copper into the silver is limited to a space immediately adjacent the final steelsilver interface, whereby substantially none of the copper penetrates completely through the piece of silver.

The copper coating should be very thin, say, 0.0005" thick to 0.002" thick. If a thicker coating i formed, it is diflicult to eliminate it completely in the silver welding step and, moreover, a thick copper film tends to diffuse completely through the silver. It has been found that the presence of the copper in the final bearing surface exerts a deleterious effect and results in a substantial reduction in bearing performance or life or both.

Elemental copper forms a satisfactory coating, I

but I prefer to form a high copper alloy on the surface, say, an alloy containing 10% nickel and 10% silver, the balance being copper.

In order to obtain a uniform film of copper on the steel, it may be desirable to employ a flux. In such case, however, it is essential to remove the flux substantially completely by a pickling operation or the like prior to applying the silver.

The copper may be applied to the steel by flowing molten copper thereover, but preferably is applied by melting a measured amount of the solid copper on the steel surface, the amount being suficient to form at least a slight bead onthe edge of the steel when the copper is melted. To facilitate distribution and to improve the character of the copper coating, it is desirable to apply the copper as a suspension of copper powder in In applying the silver to the copper coated surface, the assembly should beheated substantially above the melting point of the copper-silver eutectic (28% Cu), i. e. about 1450 1". and preferably cooled slowly to a temperature below this point. Conveniently, the assembly is heated to a temperature of around 1650' F. and is then cooled slowly over a period of, say, V4 to 2 hrs. to a temperature slightly below the melting point of the eutectic, say, to the temperature of 1400' I". when the assembly attains this temperature, further cooling may be relatively rapid.

If the silver is subjected at any stage to the action of a reducing atmosphere, and particularly to an atmosphere containing hydrogen, it should be oxygen-free, otherwise, the silver tends to blister. However, silver that is not free of oxygen can be employed if treatment is conducted in an inert or oxidizing atmosphere, although the latter is not recommended.

In the manufacture of aircraft engine bearings, and the like, I prefer to employ sliver of high purity, say, at least 999 fine, although for some purposes silver alloys may be used.

The silver employed preferably is substantially non-porous. Worked Sheets; formed from cast silver are suiliciently solid for use in the practice of my invention. In forming aircraft bearings, rolled silver sheets about 1"" thick are applied to either or both sides of the steel billets, usually about 1" thick. Following the welding operation, the resulting silver clad strips are rolled to reduce the thickness to, say, A" or even less. The rolling is conducted cold and in stages with intermediate anneals under conditions such that embrlttlement of the silver does not occur, 1. e. under such conditions any oxygen present in the silver does not have an opportunity to combine with hydrogen.

The rolled clad sheets may be formed into bearings by drawing the sheets into cup-shape or by other methods. Flanged bearings, as well as straight bushings, can be made from the rolled sheet in accordance with my vention.

The improved product of my invention comprises a silver coated steel structure, for example a steel backed silver bearing, comprising a worked silver layer welded directly to a copper-bearing steel backing, the interface between the silver and the steel being substantially free of unalioyed copper. However, the copper is alloyed with and diffused into the steel on one side of the interface and alloyed with and diflused into the silver on the other side'of the interface, the diffusion of the copper into the silver being limited to a space in the silver immediately adjacent the interface with none of the diffused copper in the balance of the silver layer. In consequence, inthe case of the steel backed silver bearing, the bearing surface of the silver is substantially free of the diffused copper.

As suggested above. the character of the bond is improved by the presence of diffused nickel along with the copper at the silver steel interface. However, care should be taken to see that the nickel, like the copper, is completely diffused.

In the case of aircraft bearings, the silver bearing surface preferably is pure, say, 999 line,

vention. By way of example, a deep drawing steel suitable for the practice of my invention has the following analysis:

Range Average Per cent Per cent 0 0. 05-0. 20 0. 18 Mn. 0. 70 Si 0. 0. 23 .65 0. 60 0. 10 0. l5 0. l2 8 0. 04 max. 0.026 P 0.04max. 0. 020

The steel surface to which the copper coating is applied should be clean and free of all oxide.

The copper coating should be very thin, of the order of 0.0005" to 0.002" thick, and preferably is formed by fusing the copper or high copper a1- loy on the steel surface.

It is essential that the copper be molten and in contact with the surface for a substantial period of time to permit some of the copper to diffuse into and alloy with the steel. Suitable copper coatings are not obtained by immersing the steel in molten copper. The copper should be flowed onto the steel and it is preferable to coat the steel by melting an excess of solid copper on the surface to be coated. The copper or copper alloy to be employed in coating may be in the form of a foil or slug, which is melted in contact with the surface and permitted to flow thereover. However, I prefer to employ the copper in the form of powder and to distribute this as a thin layer over the steel. Subsequent heating melts the copper and forms a uniform layer of the proper thickness.

Although pure copper is a satisfactory coating agent, I prefer to form a layer of a high copper alloy, especially when the coating agent is in powder form. A suitable high copper alloy can be formed on the steel by melting thereon a mixture containing 80% copper powder, 10% nickel powder and 10% silver powder, this being my preferred coating composition.

Distribution of the powder is facilitated if it is dispersed in a-suitable vehicle, for example water. Thus copper powder or the above described mixture of copper, nickel and silver powder may be suspended in water in amounts such that it may be applied with a paint brush to the steel surface, the amount of water employed being adjusted to give a material which flows readily and gives an even coating.

It is not necessary to employ a flux in applying the copper coating to the steel, if the materials are suificiently clean and if a proper atmosphere is employed. However, it may be desirable to employ a flux during the formation of the copper coating, care being taken to remove all of the flux before the silver is applied. If a flux is employed, it may be suspended along with the metal powder in the water or other vehicle. A variety of powdered fluxes may be used. I have found that a particularly good flux is made by mixing one part by weight of potassium acid fluoride with one part by weight of potassium tetraborate to form a primary mixture. One volume of this primary mixture is incorporated with one volume of borax to give the final flux mixture. One part by volume of this final flux is employed for each four parts by volume of metal powder, the two being suspended in water to give a properly flowing paint.

To take a specific example. a billet of "deep drawing steel about 1" thick is first pickled and then wire-brushed. The cleaned steel is then painted on one or both sides with the mixture of metal powder and flux, the amount 01' metal powder being calculated to produce a coating about .001" thick and leave a slight excess to form a bead. The painted billet is then heated on edge for about one-half hour at a temperature of 2000 F. to 2200 F. (say 2060 F.) in a non-oxidizing atmosphere, for example in an atmosphere of partially burned city gas but out of contact with the flame. The metal powder and the flux are melted and the resulting molten copper or copper alloy flows uniformly over the surface to be coated. The treatment also brings about an alloying of part of the molten metal with the steel, but leaves an excess of the metal on the surface plus the bead. After heating for one-half hour at the temperature indicated' above, the resulting copper coated steel billet is cooled to room temperature out of contact with oxidizing influences. The periods required for heating and cooling the billet from the treatment range are approximately of the same duration as that of treatment at temperature, so that the total time in the furnace is about an hour and a half.

After cooling to room temperature, the coated sheet may be exposed safely to the atmosphere. The bead is removed by filing.

If flux is employed, as in this instance, the coated sheet next is subjected to a thorough pickling operation to remove all traces of the flux and also any oxidation products which may have been formed. The fiux described above may be removed conveniently in a pickling solution comprising 5 lbs. of ammonium chloride dissolved in gallons of 50% hydrochloric acid. The pickling solution should be hot. At a temperature of 180 F. thorough removal of the fiux is accomplished with the particular solution in two or three minutes. At a temperature of 70 F.,

the same solution requires about twenty .minutes for proper cleaning and removal of flux.

It should be emphasized again that if flux is employed in coating the steel with the copper, it must be completely removed before the silver is applied. If no fiux is used, it may be possible to eliminate the pickling operation, although even then the pickling may be desirable in order to produce a bright copper surface for contact with the silver.

Following the pickling operation, the copper coating is given a thorough mechanical cleaning with a wire brush.

The silver cladding is applied to the copper coated steel surface by heating a silver sheet (say thick) to a temperature above the melting point of the copper-silver eutectic, but below the melting point of silver while pressing the silver firmly against the copper coating with a force of at least 160 pounds per square inch (9 tons for a 6" x 18" billet). Thus the silver and the copper coated sheet are formed into sandwiches and heated. If both sides of the steel are to be silver clad, both sides of the steel are copper coated and silver sheets are pressed against the copper coatings. Silver is applied to one side only if that form of final product is desired. The sandwiches of copper coated steel and silver are stacked in a furnace with asbestos spacer sheets between the sandwiches. Pressure is applied to the stack of sandwiches by means of weights placed on top of the stack or by means of a yoke or clamp. The stack is then heated in a protective atmosphere.

In my preferred operation, the sandwiches are heated in a muiiie in a non-oxidizing atmosphere so that they attain a temperature of about 1640 F. at the end of an hour. This temperature is above the melting point of the copper-silver eutectic (1450 F.) but below the melting point of silver. When the sandwiches have attained the temperature of 1640 F. they are held at this temperature for about 1% hrs. and then are cooled in the furnace to 1400 F., the cooling taking from 1 /2 to 2 hrs. The temperature oi 1400 F. is below the melting point of the silvercopper eutectic.

After the sandwiches have been cooled to 1400 F. they are removed from the furnace and cooled in a protective atmosphere to room temperature. For example, the sandwiches may be cooled in a steel case from which air is excluded and over which water is run. Cooling to 1200 F. is thus conducted. Thereafter the billets are cooled rapidly to room temperature by direct impact with water.

After cooling to room temperature, the sheets are removed from the cooling case and are subjected to a series of rolling and annealing treatments to reduce the thicknessof both the steel and the silver cladding. Thus a silver clad billet which is about 1" thick with a 1%" layer of silver on one or both faces may be cold rolled until the total thickness of the clad billet is about /g". In the rolling operation, it will be necessary to anneal the billet occasionally (say after every 6 to 10 passes) the annealing being carried out in a reducing atmosphere- Each anneal consumes about 3 hrs. at 1300" F. or slightly more.

In the fabrication of silver clad bearings for aircraft use, I have found that it is desirable to employ silver of high purity, although for other applications silver alloys such as sterling silver containing 7 /2% copper may be employed. In any case. however, it is necessary to restrict the diffusion of the copper into the silver by restricting the amount of copper present as a coating, so that none of the copper of the coating diffuses into the bearing surface.

In treating the silver every effort should be made to avoid blistering or embrittlement. It is desirable to test the silver employed by annealing it in the hydrogen beforehand. If the silver does not blister in this treatment, it is satisfactory for use as a cladding material for bearings. If the silver is treated in a reducing atmosphere, it should be oxygen-free, otherwise it tends to become brittle.

After cold rolling to proper thickness, the silver clad strip may be formed into bearings of appropriate shape and size by drawing or other forming operations. If the silver has been properly applied, as described above, a fiat sheet of the silver clad material may be drawn into a tubular bearing without fear of rupture of the bond between the silver and the steel.

Drawn bearings made as described above have been tested in competition with aircraft bearings made by a variety of other methods and have proved to be eminently satisfactory. not only by reason of the strength of the bond between the silver and the steel but also because of the ductility and low coefficient of friction of the bearing surface.

I claim:

1. In welding silver to steel, the improvement which comprises forming a thin coating of copper on the surface of the steel by flowing molten copper thereon, heating the steel thus coated at a temperature above the melting point of the copper but below the melting point of the steel until a substantial proportion of the copper has diffused into the steel and alloyed therewith leaving the balance of the copper'on the surface, placing a solid piece of silver on the resulting copper coated surface to produce a copper-silver interface, welding the silver to the steel in the absence of flux by pressing the silver against the copper coating and heating the assembly to a temperature in the neighborhood of but below the melting point of the silver until the balance of the copper has diffused substantially completely into the silver and alloyed therewith, thus substantially eliminating .the copper-silver interface and producing a joint substantially free of unalloyed copper and in which the silver is uniteddirectly to the steel surface, the amount of copper present on the surface of the steel at the beginning of the silver welding step being so small that the diffusion of the copper into the silver is limited to a space immediately adjacent the final steel-silver interface, whereby substantially none of the copper penetrates completely through the piece of silver.

2. In welding silver to steel, the improvement which comprises forming a thin coating of copper on the surface of the steel by exposing the surface to the action of a film or molten copper containing a small proportion of nickel and silver, heating the steel thus coated at atemperature above the melting point of the copper but below the melting point of the steel until a substantial proportion of the copper has diffused into the steel and alloyed therewith leaving the balance of the copper on the surface, placing a solid piece of silver on the resulting copper coated surface to produce a copper-silver interface, welding the silver to the steel in the ab sence of flux by pressing the silver against the copper coating and heating the assembly to a temperature in the neighborhood of but below the melting point of the silver until the balance of the copper has diffused substantially completely into the silver and alloyed therewith, thus substantially eliminating the copper-silver interface and producing a joint substantially free of unalloyed copper and nickel and in which the silver is united directly to the steel surface, the amount of copper present on the surface of the steel at the beginning of the silver welding step being so small that the diffusion of the copper into the silver is limited to a space immediately adjacent the final steel-silver interface, whereby substantially none of the copper penetrates completely through the piece of silver.

3. In welding silver to steel, the improvement which comprises forming a coating of copper ranging in thickness from .OOOSinch to .002 inch on the surface of the steelby exposing the surface to the action of afilm of molten copper, heating the steel thus coated at a temperature low the melting point of the steel until a substantial proportion of the copper has diffused into the steel and alloyed therewith leaving the balance of the copper onthe surface, placing a solid substantially non-porous piece of silver on the resulting copper coated surface to produce a copper-silver interface, welding'the silver to the steel in the absence of flux by pressing the silver against the copper coating and heating the assembly to a temperature in the neighborhood of but below the melting point of the silver until the balance of the copper has diffused substantially completely into the silver and alloyed therewith, thus substantially eliminating the coppersilver interface and producing a joint substantially free of unalloyed copper and in which the silver is united directly to the steel surface.

4. In welding silver to steel, the improvement which comprises forming a thin coating of copper on the surface of the steel by exposing the surface to the action of a film of molten copper formed by melting a mixture of copper powder and flux on the surface, heating the steel thus coated at a temperature abov the melting point of the copper but below the melting point of the steel until a substantialproportion of the copper has diffused into the steel and alloyed therewith leaving the balance of the copper on the surface, removing the flux by pickling the copper coated surface, placing a solid piece of silver on the copper coated surface to produce a copper-silver interface, welding the silver to the steel in the absence offiux by'pressing the silver against the copper coating and heating the assembly to a temperature in the neighborhood of but below the melting point of the silver until the balance of the copper has diffused substantially completely into the silver and alloyed therewith, thus substantially eliminating the coppersllver interface and producing a joint substantially free of unalloyed copper and in which the silver is united directly to the steel surface.

5. In welding silver to steel, the improvement which comprises forming a thin coating of copp r on the surface of the steel by exposing the surface to the action of a film of molten copper, heating the steel thus coated at a temperature above the melting point of the copper but below the melting point of the steel until a substantial proportion of the copper has diffused into the steel and alloyed therewith leaving the balance of the copper on the surface, placing a solid piece of oxygen-free silver on the resulting copper coated surface to produce a copper-silver interface, welding the silver to the steel in the absence of flux by pressing-the silver against the copper coating and heating the assembly in a reducing atmosphere to a temperature in the neighborhood of but below the melting point of the silver until the balance of the copper has diffused substantially completely into the silver and alloyed therewith, thus substantially eliminating the copper-silver interface and producing a joint substantially free of unalloyed copper and in which the silver is united directly to the steel surface, the amount of copper present on the surface of the steel at the beginning of the silver welding step being so small that the diffusion of the copper into the silver is limited to a space immediately adjacent the final steel-silver interface, whereby substantially non of the copper penetrates completely through the piece of silver, all heating operations in the process being conducted in a reducing atmosphere.

above the melting point of the copper but be- 6. In welding silver to steel, the improvement steel until a'substantial which comprises forming a thin coating of copper on the surface of the steel by flowing molten copper thereon, heating the steel thus coated at a temperature above the melting point of the copper but below the melting point of the steel until a substantial proportion of the copper has diffused into the steel and alloyed therewith but leaving copper on the surface. placing a solid piece of silver on the resulting copper coated surface to produce a copper-silver interface, welding the silver to the steel in the absence of this by pressing the silver against the copper coating with a force of at least 160 pounds per square inch and heating the assembly to a temperature in the neighborhood of but below the melting point of the silver until the copper on said surface has diffused substantially completely into the silver and alloyed therewith, thus substantially eliminoting the copper-silver-interface and producing a Joint substantially free of unalloyed copper and in which the silver is united directly to the steel surface, the amount of copper present on the surface of the steel at the of the silver welding step being so small that the diffusion of the copper into the silver is limited to a space immediately adjacent the final steel-silver interiace, whereby substantially none of the copper penetrates completely through the piece of silver.

7. In welding silver to steel, the improvement which comprises forming a thin coating of copper on the surface of the steel by flowing molten copper thereon, the amount of copper so employed being in excess of that required to coat the steel and diiluse thereinto, heating the steel thus coated at a temperature above the melting point of the. copperwbut below the melting point of the proportion of the 9981 has diffused into the steel and alloyed therewith but leaving a copper coating onthe surface,. placing, a solid piece or silver on the'res'ulting copper coated surface, to produce, a copper-silver interface, welding the silver'to the steel in the absence of flux by pressing the silver against the copper coating on the steel with a force of at least l60, lbs. per square inch and heating the assembly to a temperature in the neighborhood of but below the melting point of the silver until the balance of the copper has diffused substantially completely into the silver and alloyed therewith, thus substantially eliminating the copper-silver interface and producing a joint substantially free of unalloyed copper and in which the silver is uniteddirectlytothesteelsurf 8. In welding silver to steel, the improvement surface to the action of a film of molten coppernlckel alloy which amounts to more than the alloy required to coat the steel and diifuse thereinto, heating the steel thus coated at a temperature above the melting point of the copper but below the melting point of the steel until a substantial proportion of the alloy has diii'used into the steel and alloyed therewith leaving a coating of copper-nickel alloy on the surface, placing a solid piece of silver onthe resulting copper coated surface to produce a nickel-copper-silver interface, welding the silver to the steel in the absence of flux by pressing the silver a ainst the copper-nickel alloy coating and heating the assembly to a temperature above the melting point of copper-silver eutectic but below the melting point of the silver until the balance of the copper-nickel alloy has diflused substantially completely into the silver and alloyed therewith, thus substantially eliminating the copper-silver interface and producing a joint substantially free of unalloyed copper and in which the silver is united directLv to the steel surface.

9. In welding silver to steel, the improvement which comprises forming a thin coating of copper on the surface of the steel by exposing the surface to the action of a film of molten copper while heating the steel to a temperature above the melting point of the copper butbelow the melting point of the steel until a substantial proportion of the copper has diffusedinto the steel and alloyed therewith leaving a copper coating on the surfaces; placing asolid piece of silver on the resulting copper coated surface to produce a cop-- per-silver interfaca weldingthe silver to the steel'by pressing the silver against the copper coating and heating the assembly-to a temperature above the melting point of copper-silver eutectic but below the melting point of the silver until the balance of the copper has diifused substantially completely into the silver and alloyed, therewith, thus substantially eliminating the copper-silver interface and producing a joint substantially free of unalloyed copper and in which the silver is united directLv tothe steel surface. 10. In welding silver to steel 'as defined by claim 8 in which said copper-nickel alloy contains approximately copper, 10% nickel and 10% silver.

EDWARD JOSEPH GUINEE;