US2044742A - Composite ferrous bodies - Google Patents

Description

J1me 1936- P. A. E. ARMSTRONG ET AL COMPOSITE FERROUS BODIES Filed Nov. 27, 1934 ma 0. C I M Ha m: R- a6 1,?

INVENTORS ATTORNEYS STA COMPOSITE FERRUUS BUDIIES Percy A. E. Armstrong, Beverly llafialiifi, and Raymond R. Rogers, New York, N. K; said Rogers assignor to said Armstrong Application November 27, 1934, Serial No. 75%,976

10 Claims.

This invention. relates to the production of compound metal bodies comprising a high melting point metal and a facing or-covering of eleo trolytic iron so united thereto that separation is substantially impossible. The invention is particularly intended for use with ferrous bodies such as high carbon steels or alloy steels or other ferrous compounds but under certain circumstances may be used with other high melting point alloys such for example as certain of the chromium alloys or metallic bodies consisting primarily of type which would interfere with welding, the iron will attach itself strongly to the alloy steel and when the two are heated to a temperature in the order of about 1700 F. a difiusion between the two layers will take place producing a true weld even though the temperature employed is in the order of 1000 below the melting point of the electrolytic iron.

High melting point metals of the type here referred to, as for example stainless steels, are

characterized by thefact that they carry a thin non-metallic film or coating. This film probably is a result of the combination of the metal with oxygen, and we therefore term it an oxide, though under some circumstances other combining elements may be present.

The presence of .the non-metallic oxide film will definitely interfere with the bonding of electrolytic iron to the underbody, and even if this film is removed it reforms almost instantly-on exposure to the atmosphere. In accordance with our invention the high melting point metal is given a pickle which will remove the film and eX- pose the true metallic surface and then while such surface is still coated with the pickling liquid so as to prevent atmospheric oxidation, the

metal is submerged in a plating bath selected so' as to eliminate oxidizing conditions. When this is done, we find that the union of the electrodeposited iron on the high melting point metal is so free from any intervening oxide layer that when the metal is heated to a temperature of about 1700 F., or heated inthe usual manner oxide the bond between the two layers is extraorpreparatory to rolling or forging, a very extraordinary diffusion of the two metals into each other takes place. This diffusion seems to be of a type where the grain boundaries on the iron are the starting point, the diffusion spreading gradually 5 through the grain from all sides; and at increasing distances from the original line of demarcation, the diffusiomextends from the grain walls nearest to this line. This type of diffusion resultsin an extraordinarily close and strong weld zone forming between the bodies on simple heating and this has many of the appearances of a diffusion weld, though it takes place at temperatures substantially below the fusion zone and without the use of any pressure.

The type of diffusion obtained by our process is illustrated in the accompanying drawing, in which Fig. 1 represents a photomicrograph showing a composite member made up of electrolytic iron deposited on stainless steel in the form immediately after removal from the plating operation, and Fig. 2 represents the same product after it has been heated to a temperature of about 1700 F. for about an hour. In each figure the magnification'is 750 diameters. 25

In Fig. 1 the material at the top, with the relatively small crystal size, is a ferrous alloy containing about 18% chromium and about 8% nickel, having a surface roughened by the pickling operation. Below is the electro-deposited iron separated from the alloy by a line which, due to the acid attack between two unlike metals, apparently is quite definite but which is free from oxide. Due to the fact that there is no interposing dinarily close and. the effect is mechanically similar to a weld so that it is practically impossible to separate the electrolytic iron from the alloy either by attemping forcibly to lift the deposit or by subjecting the composite plate to repeated bending back and forth in a limited zone. Whether or not this bond involves difi'usion between the two layers is not known, but if there is any such difius'ion, it is only of a highly localized or incipient nature as compared with the type of diffusion that is apparent after heating. Fig. 2 shows the result of heating without mechanical work. The iron has undergone a marked growth of crystal size due to the soaking at an elevated temperature while-the alloy body shows relatively little change. There is, however, a noticeable intermediate zone consisting primarily of iron but with marked difiusion of the alloy extending .down between the iron crystals and permeating the crystals in such a way that in the section illustrated they resemble roots. There may be a slight migration of iron into the alloy but if so this simply results in a slight dilution of the alloy along the boundary zone and the effect is not shown in the pictures.

In order to obtain a diffusion bond such as that illustrated, it is advisable to soak the material for about an hour at a temperature which for stainless steel will be at a temperature somewhat above 1700 F., though we believe that this ac-- clerated diffusion begins to take place at substantially lower temperatures, as for example, at temperatures in the order of about 1400 F. The diffusion obtained gives a remarkably strong bond zone between the two layers and apparently accomplishes a true diffusion weld area even though the temperature used is as much as 1000 below the melting point of the iron or of any of the other principal alloying elements, and this bond is obtained quite independent of any hot working. We believe that the diffusion weld which we obtain by building up the iron layer with complete absence of oxide and then heating, is something quite new in ferrous metallurgy even though it is a fact that some types of diffusion between various metals had previously been known and recognized.

In addition to the bonding effect obtained by diffusion it may be noted that the ends of the stainless steel slabs will be embedded in the electro-deposit and this tends to reduce or eliminate the danger of separation resulting from differences in expansion, as the added increase in linear dimensions of the stainless steel simply will lock it more tightly into its iron covering, thereby further preventing any ingress of furnace gases which might interfere with the formation of the diffusion weld which otherwise takes place.

If desired, instead of plating the iron directly on the stainless steel an intervening film of some other metal such as silver, nickel, cobalt or copper may be employed, and this will not prevent the desired inter-diffusion, provided the deposits are tightly adherent and proper steps are taken to insure the absence of an oxide layer.

A particular advantage of the use of the electrolytic metal is that it permits of the inexpensive building up of relatively small plates of composite metal. Thus it is quite feasible to start with the stainless steel rolledrelatively thin and electroplate the iron backing upon it, after which the material may be brought to size with comparatively few passes in the rolls or reductions of say about for extensive working to rupture an oxide layer and form a weld is not necessary, inasmuch as the job is thoroughly welded before rolling takes place.

In carrying out this invention, the stainless steel slabs which may for example be of the standard composition containing about 18% chromium and about 8% of nickel are preferably united so that two sheets are back'to back with a separating medium between them such as a sheet of asbestos or a film of paint made up of talc in a solution of sodium silicate and if desired these inside faces may be polished. The two plates are preferably autogenously welded together to form a water-tight joint where the plates are submerged, but the plates when welded can have a vent through the extension piece which is to have the current hooked to it, thereby letting out any hydrogen that might otherwise penetrate between .are used. The exposed surfaces of the combined plates or slabs are now given a thorough cleaning with an alkali or solvent wash to remove every trace of grease. After being thoroughly rinsed the slabs are placed in a hydrochloric acid pickle, that is, a bath of relatively strong hydrochloric acid say between 35% to 37% HCl. A hot acid pickle of lower acid concentration will work equally well; however, in this case the container used for the acid bath will be a problem and preferably a room temperature acid bath is used. The inserts are kept in this bath until 5 or 6 minutes after bubbles begin to come freely from every part of their exposed surface to insure complete destruction of any surface oxide layer. Any impurities remaining on the surface should now be I removed and this may be done either by extreme agitation of the bath or by removing the slabs and rinsing and scrubbing them. If the latter procedure is followed it is very important that the slabs be returned to the pickling bath and allowed to remain for a short period until the bubbling again sets in, thereby removing the oxide that has formed during washing.

After the acid pickle has continued sufilciently far to indicate the complete destruction of the oxide coating, the slabs are removed rapidly from the acid bath directly into the plating bath without removing the hydrochloric acid, and this movement should be sufficiently rapid so that the surface will remain wet with the hydrochloric acid. Obviously it is of advantage that the acid bath is at approximately room temperature, for if the bath is heated, the slabs will also become heated and this will demand extraordinary precautions to prevent the slabs from drying as they using 40 oz. of ferrous chloride (FeC12.4HzO) plus 30 oz. of calcium chloride to the gallon of solution. Enough hydrochloric acid should be added to keep the bath well on the acid side, preferably at a pH value in the order of about 2, and this should be checked from time to time and maintained by additions of hydrochloric acid or calcium carbonate as conditions demand. The iron content of the bath may gradually increase since there is a. tendency for the anodes to dissolve chemically as well as electro-chemically. This may be remedied by removing part of the solution from time to time and adding more water and calcium chloride to maintain the composition at the desired point.

The anodes used preferably will be of relatively pure iron. However, if a somewhat less pure iron is used for the anodes and a smooth deposit is necessary they may be placed inside porous pots which will prevent the insoluble ingredients found in the anodes from fouling the main body of the solution. If such pots are used they may be made of alundum. Where the surface of the deposit can be rough then the anode can be cast iron.

The actual plating operation is preferably conducted hot. For example, the temperature may range between and 205 F. The current aoiama density should be in the order of about 60 amperes per square foot of cathode surface and the voltage which is not critical may range within quite wide limits. For example it may range from between about 0.6 to 1.0 volt where no porous pot is used up to a voltage in the order of about 2.2 volts or higher if the pots are employed. Under these conditions, it will be found that iron will be deposited at the rate of about .003 inch per .hour and if the precautions above specified are taken it will be found that the iron deposit is very firmly adherent to the stainless steel, with no sign of intervening oxide so that diffusion welding will be greatly accelerated upon heating. l Where an intervening film of silver, nickel, cobalt or copper is to be plated on the stainless steel the above procedure should be followed right through until the slabs are about to be deposited in the appropriate bath for plating the metal used. Also when the iron is to be platedover such metal it may be necessary, if there is any delay between steps, to return the plated slabs to the acid pickling bath and carry them rapidly and wet with the acid to the iron plating bath. This will eliminate any possibility of there being present an interfering oxide layer on the under coat of metal.

A procedure alternative to the above is one where the hydrochloric acid used is much weaker than that described above; for example, an acid as weak as a 3 normal solution instead of a 12 7 normal solution may be used. However, we find that it is better practice to use. an acid at least as strong as a 6 normal solution. In any of these cases it is advisable to make the metal to be pickled anodic. Thus the metal after a usual preliminary cleaning to remove grease and the like can be put in a hydrochloric acid bath of a strength of about 18% HCl and connected as an anode. The cathodes used in this case may be graphite, though other types 'of cathodes may be employed. The current density may, for ex ample, be about 60 amperes per square foot of surface, with a voltage at the generator of about 6 volts. This anodic treatment should be continueduntil the metal on being scrubbed shows a clean metallic surface. The time required will vary depending on the previous condition of the metal. Thus working with samples which appeared fairly clean to start with, this result was accomplished in about 10 minutes though ordinarily, for commercial operations, a somewhat longer time is desirable. After the metal has been given the proper length of treatment, it is scrubbed and washed with water, and returned to the pickling bath and made anodic again for a very brief interval of time; for example, an additional treatment for seconds will usually be found sufficient to eliminate any oxide formed during the scrubbing. It is then carried over to the iron plating bath with the hydrochloric acid solution covering the surface of the metal as has previously been described.

For the manufacture of composite bodies by this process where an electrolytic iron surface is the backing (as distinguished from the case where a separate backing is added as by welding or casting) it ordinarily will be advisable, though not essential, to make the iron deposit at least as thick as the slab of stainless steel and the thickness of the electrolytic iron backing may range up to any proportion desired, though ordinarily it will not be more than about ten times the thickness of the slab. This will not be unduly expensive, for the electroplating is a very inexpensive operation.

Upon the completion of the electro-deposit of desired thickness the mass will usually be me-- chanically surfaced and heated to a temperature sufiicient to drive out the occluded hydrogen and cause interdifiusion, which as stated above is very active at about 1700 F. upwards. After this the compound'metal in usual course will be subjected to hot working such as forging or rolling which may further strengthen the bond between the iron and the stainless steel and will improve the grain structure of the deposited iron, producing maximum strength and ductility. However, a valuable product will be obtained without any working or if limited cold rolling is used in place of the hot working provided the mass has been heated sumciently to drive off the hydrogen and render the electrolytic iron ductile.

While we prefer to use my process on two sheets fastened together back to back with a waterproof joint, it is also possible to use two sheets simply clamped together with a separator between them, which separator may extend out around the edges as. far as the desired plating thickness. Such separator may be made of waterproof paper or rubber, or we may use a thin plate of stainless steel having an appreciable oxide coating on its faces. When this is done the two halves can readily be separated after the plating operation is completed and then can be worked separately; or if desired they may beturned into a reverse position with the iron faces inward and the stainless steel faces outward and welded together by heat and pressure welding to form a composite body having both faces covered with stainless steel. It is also obvious that where two slabs are welded together and plated as a unit, they can be out apart and treated separately.

While this invention appears to have a primary value in connection with making composite sheets or plates, it is readily applicable for other uses. For example, valuable tubes resistant to internal corrosion can be made by rolling up thin sheets of stainless steel with the edges either crimped or welded together (or'they may be seamless either roled or as cast) and then iron can be electro-deposited on either the inside or the outside of these thin sheets to give a proper body to the tubing. For example, sheets .030 inch thick, containing about 18% chromium and about 8% nickel can be rolled into tubes or pipes having an internal diameter of about 1 inch. The edges are then welded or crimped. Following the procedure outlined above there may then be deposited an iron coating of approximately 1; inch in thickness all over the exterior. After the electroplating is finished, the pipe is preferably heated to a temperature in the order of 1700 F. or higher to cause a diffusion weld zone to form. The resulting product will be strong and will readily bend to shape and no difiiculty will arise from separation of the stainless steel and iron been applied, it is of course obvious that working of various kinds may be used. Thus after the iron has been applied electrolytically on a seamless tube and after the diffusion weld has been produced by heating, the composite product may be rolled'to finished dimensions, or if desired cold drawn or brought to size in part by hot working and in part by cold working.

While this invention has been described as primarily used with stainless steel containing about 18% chromium and about 8% nickel, it is to be understood that it is in no way limited to the materials of this particular alloy, nor do we even wish to limit it to ferrous alloys of chromium and nickel, for our invention may also be used with nickel bodies or other high alloy steels, either of the type which are known to be difficult to weld or as an aid in forging. This is particularly true with alloys which are hot short in the ingot but which are susceptible to forging after once being worked into the billet form; for example, it is known that a high speed steel of standard analysis containing about 30% carbon, 18% tungsten, 4% chromium and 1 /2% vanadium gives great difficulty in the initial forging steps, and this becomes increasingly true as the carbon is increased or as additional elements are added. With steels of this type the ingot can be electroplated under conditions adapted to lend themselves to accelerated diifusion between the electrodeposit and the body of metal, and the iron envelope will so influence and support the crystal structure on the exterior of the ingot that forging will be greatly simplified. To accomplish this there must be a close bond between the ingot and the envelope so that the envelope will serve mechanically to hold together the crystals of the ingot on the surface. A procedure particularly adapted for producing such a result with tungsten steels is as follows:

The ingot is given a thorough cleaning to remove any adhering dirt or sand and then is given a preliminary alkali anodic pickle. For this purpose the pickling bath may be one containing approximately 115 grams of sodium hydroxide and 15 grams of citric acid per liter, the balance being water. The ingot is inserted inthis bath as an anode using ordinary iron cathodes. The electrolysis may take place at room temperature with a current density of 25 amperes per square foot of surface or higher, with a voltage of about 6 volts at the generator. With this procedure, after about a minute, bubbles appear on the surface and the scale sheds off the ingot, the action seeming to be very uniform. When the ingot appears to be free of surface scale, it is removed from the bath and scrubbed and rinsed with water. It is then promptly immersed in a hydrochloric acid bath of a strength equal to about 18% HCl and kept in this bath for a few seconds and then carried over to the plating bath with its surface wet with the hydrochloric acid. It is then plated as in the other examples and after plating is subjected to the desired forging operations. During the forging operations the envelope of iron gradually scales away as a result of. heating but it may be noted that in this respect the scaling losses of the valuable alloy were greatly reduced.

Also it was found that during the early stages the iron coating tends to prevent the ingot from surface cracking and serves as a shield to protect the ingot against the injurious effect of furnace gas permeation which often adds to forging difficulties. The particuar preliminary treatment given to this steel is not claimed in the present application, as the same is the sole invention of the party Rogers.

In a similar way our invention may be employed to assist in cold drawing. Attempts to draw alloy steels frequently result in ripping of the work and the die. We have found that if the alloy steel is surrounded by an envelope of iron which is intimately united to the steel, as is the case with the iron electrolytically deposited by our process, then the resulting product can readily be drawn cold, after which the thin surface layer of iron can be removed by pickling, grinding or other convenient manner.

Another illustration of our process is the case where the high melting point metal was an alloy known as stellite having an approximate analysis of cobalt chromium 30% and tungsten 15%. This alloy has heretofore been considered substantially non-weldable. A bar of this alloy was ground square to dimensions of 4 inches by 1 inch by 1 inch. It was first given an anodic treatment in a 12 normal hydrochloric acid bath (approximately 35% 1101) with a current density of amperes per square foot using 6 volts at the generator. This treatment was continued until the piece was covered with a dark blue material which would not scrub off. The acid was washed off carefully in water and then the piece was treated anodically in a bath of sodium hydroxide and citric acid similar to that described above. This treatment was continued until bubbles arose from all parts of the surface. The piece was then washed and placed for a few seconds in a 12 normal hydrochloric acid solution without the use of electric current and then while still covered with the acid was transferred to the chloride plating bath The iron was plated on to the surface as in the other examples, and after washing the plated metal was held at a temperature of about 1750 F. for something over an hour. The iron coating was then found to be tightly adhering, as an excellent diffusion weld had taken place, so that the composite piece could be used as it was (by grinding off one face to expose the stellite) or it was available for ready welding into a larger assembly.

It will be noted that in each of the examples given the high melting point metal to be plated is carried over to the plating bath while covered with hydrochloric acid. This not only prevents atmospheric oxidation but undoubtedly develops on the face of the metal a layer of the chloride salt of the foundation metal or metals and this salt in solution probably remains held by adsorption on the surface of the metal after insertion in the plating bath. If these surmlses are correct, then when the plating bath starts there may first of all be an electrolytic redeposition of metal from the adsorbed surface layer with which metal from the plating bath would blend almost immediately. These suggestions are advanced as a possible thecry which if proven correct would tend to explain the tight adherence of the electro-deposlted layer and the ease with which interdiifusion takes place.

We recognize that in the past it has been customary to instruct those who are preparing high melting point metals for electroplating to remove the scale or oxide and one of the mediums used for this purpose has been a treatment with sulphuric acid, reliance having been placed either on the chemical action of the acid alone or in other instances this action has been accelerated by an electrolytic process. Such sulphuric acid treatment is excellently adapted to remove the heavy scale or apparent oxide but sulphuric acid used in any manner of which we have thus far learned is not adapted for our purpose, for apparently this is itself an oxidizing acid and tends to develop a non-metallic film on the surface of the layer and this film, which we believe to be an ..oxide or the prevention of the formation of oxide or the maintenance of the metal substantially free a from oxide it is intended to imply that no film is present such as that produced by the action of sulphuric acid employed in the usual manner. We do not mean to imply by this that the use of hydrochloric acid is essential to our purpose, for we have found that largely equivalent results can be obtained by treating the metal in a fused chloride bath which is also used as a plating bath.-

Also it is possible to use hydrofluoric acid to clean up the metal. No examples are given of these processes, as they both involve serious commercial drawbacks. It is also to be understood that other modifications may be made without departing from the spirit of our invention.

What we claim is:

1. The process of producing composite metal bodies which comprises preparing a mass of high melting point foundation metal so as substantially to eliminate oxide from a surface thereof, by pickling such surface in a bath comprising free hydrochloric acid, maintaining such surface wet with hydrochloric acid and thereby protecting such surface against exposure to the air or other oxidizing agent immersing such surface, while still wet, in a chloride plating bath comprising free hydrochloric acid and depositing electrolytically on such surface, a closely adhering metal coating, building up such coating with a facing of electrolytically deposited iron, and thereafter heating the foundation metal and coating to effect metallic diffusion between them.

2. A process as specified in claim 1, in which the electrolytically deposited layer comprises iron of a thickness at least as great as the thickness of the foundation metal.

3. A process as specified in claim 1, in which the said electrolytically deposited coating comprises an outer layer of iron and aninner film of metal selected from the group comprising silver, copper, nickel and cobalt.

4. The process of producing composite metal bodies which comprises preparing a mass of high melting point foundation by pickling a surface thereof in a bath comprising free hydrochloric acid, maintaining such surface wet with hydrochloric acid and thereby protecting such surface against exposure to the air or other oxidizing agent immersing such surface, while still wet, in

chloric acid and depositing electrolytically on such surface while substantially oxide free a closely adhering coating of iron adapted to diffuse with the foundation metal on heating.

5. A process as specified in claim 4, in which the high melting point foundation metal is an alloy steel and in which such foundation metal is substantially surrounded by the coating of electrolytically deposited iron as a procedure preparatory to forging, drawing or the like.

6. The process of producing composite metal bodies which comprises treating a surface of a foundation mass of metal, comprising metal selected from the group consisting of iron and nickel, with hydrochloric acid and continuing such treatment until such surface is substantially clean, transferring such metal to an electroplating bath comprising free hydrochloric acid while maintaining such surface covered with such acid, s'ubmerging such surface in the plating bath while so' protected from oxidizing conditions, promptly depositing electroyltically on such surface a mass of iron while maintaining conditions adapted to cause such deposited iron to bond strongly to the surface of the foundation metal and thereafter heating.

7. A process as specified in claim 6, in which the foundation mass of metal is an alloy comprising chromium and iron.

8. The process of producing composite metal acid to remove oxide from a surface thereof, -maintaining such surface wet with a liquid comprising such free acid and electroplating on such surface from a chloride plating bath comprising free hydrochloric acid, a protective layer of metal, and thereafter heating to effect metallic diffusion. 9. A process as specified in claim 8 in which the pickling bath is maintained at approximately room temperature. 10. The process of electroplating melting point metals which comprises cleaning a surface of such a metal, subjecting such surface to the action of a bath comprising free hydrochloric acid of a strength at least equal to that of a 3 normal solution and electroplating a metal onto such surface from a chloride bath without washing from such surface the reaction products resulting from the action of hydrochloric acid on the high melting point metal.

PERCY A. E. ARMSTRONG. RAYMOND R. ROGERS.

onto high

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