US2102394A - Method of making composite ferrous metal articles - Google Patents

Description

Dec. 14, 1937. M. R. TREMBOUR METHOD OF MAKING COMPOSITE FERROUS METAL ARTICLES Filed Dec 3, 1931 2 Sheets-Sheet l mvsurro'R Dec. 14, 1937.

M. R. 'TREMBOUR METHOD OF MAKING COMPOSITE FERROUS METAL ARTICLES Filed Dec.

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WITNESSES 2 Sheets-Sheet 2 '(Illlllllllll!IIIIIIIIIIIIIIIIIIIIIo III'IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII4 INVENTOR Patented Dec. 14, 1937 PATENT OFFICE METHOD OF MAKING COMPOSITE FERROUS METAL ARTICLES Max R. Trembour, Beaver, Pa., assignor to J essop Steel Company, Washington, Pa., a corporation of Pennsylvania Application December 3, 1931, Serial No. 578,661

Claims.

This invention relates to the production of ply metal and especially to stable surface articles formed of a body of base ferrous metal, such as ordinary steel, having one or more surfaces protected by a more noble metal, or alloy, and especially by chrome-bearing ferrous base alloys such as the stainless steels.

The stainless irons and steels, for example containing chromium, have been exploited widely because of their excellent surface stability, as manifested by resistance to staining, rusting and attack by certain acids, resistance to oxidation at elevated temperatures, and other forms of corrosion. Articles manufactured from these stainless ferrous alloys have been substantially more expensive than those made from the steels commonly used, not only because the chromium content increases the cost of the alloys, but particularly because these alloys present fabrieating difficulties and the surfaces of the articles must be carefully ground and polished after rolling or forming in order to develop the property of surface stability.

In order to reduce the expense of such articles and at the same time to obtain the advantages due to surface stability, it has been proposed to use ply metal comprising a body of relatively cheap ferrous metal, such as iron or ordinary steels, provided with a surface covering of stable surface metal, the cheap ferrous metal forming the major portion of the composite structure. In this manner the desirable surface properties would be had with the use of a relatively small amount of stainless material.

Ply metals have been known for many years, and various procedures for their production have been proposed. However, experience has shown that none of the means proposed heretofore for the production of ferrous metal surfaced with stainless metal are capable of commercially satisfactory results. For instance, it has been proposed to apply pressure welding to sheets of, for example, mild steel and stainless steel, but ordinary pressure welding methods have not been successful for this purpose.

The failure of ordinary pressure Welding methods in this field led to the belief that the stainless materials are not pressure weldable, and that it is necessary to produce a preliminary weld by a casting procedure. Particularly it has been recommended to cast one of the materials against a plate of the other so as to incipiently fuse the surface of the plate. However, so far as I am aware none of these procedures has been I successful, chiefly because it is difficult, if not impossible, to obtain a weld in this manner, and the poor adherence between the ferrous base and stable surface metals interferes with or prevents subsequent satisfactory working.

My experiments have shown that a weld is not obtained in the casting of such ingots, and that this is so even when the ratio of the cast metal to the insert is as great as 10:1. That is, in an ingot so prepared there is no sign of a perma-.

nent and uniform bond between the two metals, and in the ingots of the type proposed heretofore it has not been possible to produce satis factory welding upon their being rolled.

In general the procedures suggested for this purpose contemplate casting stainless steel against a mild steel insert supported in a mold. To be economical such processes must use a relatively large insert of the cheaper material, and this enhances the difficulty of obtaining a weld by such casting. Even if such ingots could be made commercially such procedures would be unsatisfactory, for the stainless material tends tocrack, particularly at the corners of the ingot. Furthermore, all of those procedures entail the expense which burdens the production of articles consisting wholly of stainless steel, for the stainless material is exposed during heating and rolling so that defects in surface due to rolling in of scale and the like result. Thus such products are subject to the expensive grinding and polishing operations which have constituted the major part of the cost of articles consisting of stable surface alloy.

It has also been proposed to cement sheets of stainless steel to ordinary steel, by processes resembling soldering or brazing. Procedures of this type do not provide as permanent or as strong a union as that produced by welding, and are complicated and require very careful practice.

Therefore, in general, they are not adapted for general practice, and particularly for making large plates.

Likewise, gas and arc welding processes are inapplicable to this field, chiefly because they can not be applied to the production of articles of the type here contemplated, especially large sheets, or plates, bands, and the like.

It is among the objects of this invention to provide stable surface articles formed of ply metal comprising a body of relatively cheap ferrous base metal having satisfactorily welded to its surface an adherent layer of stable surface metal, which may be produced readily and economically, in which exceedingly thin layers of stable surface metal may be used, and in the produetion of which many of the disadvantages of prior procedures are overcome.

The invention may be described in connection with the accompanying drawings, in which Fig. l is a plan view of an ingot mold used in the practice of the invention; Fig. 2 a vertical section through the mold shown in Fig. 1, taken on line II-II thereof; Fig. 3 a cross section of an ingot formed in the mold shown in Figs. 1 and 2; Fig. 4 a plan view of a plate rolled from the ingot shown in Fig. 3; Fig. 5 an unfolded cross sectional view of ply metal sheets formed from the plate shown in Fig. 4; Fig. 6 a plan view of a mold showing a further embodiment of the invention; Fig. 7 an unfolded cross sectional view of plates formed from an ingot'made in the mold shown in Fig. 6; Fig. 8 a plan view of an ingot mold showing another embodiment; Fig. 9 an unfolded view of the product obtained from an ingot made in the mold shown in Fig. 8; Figs. 10 and 11 views similar respectively to Figs. 8 and 9 showing another embodiment; and Figs. 12 and 13 two views like Figs. 8 and '9 representing still another embodiment.

The invention is predicated upon my discovery that, contrary to the prevailing belief, ferrous base metals and stable surface metals may be pressure welded readily and economically, and that ply metal formed of these materials may be made by providing and maintaining clean metallic weldable contact surfaces and pressure welding them while in a plastic condition at ordinary rolling or forging temperatures.

The invention will be described with particular reference to low carbon,-or mild, steel, but it will be' understood that it is equally applicable to other relatively cheap irons and steels suitable for use as the body of the ply metals to which the invention pertains. All such materials are comprehended in the term "ferrous metal as used herein. For most purposes it is preferred to use stainless steels as the surface member of these ply metals. However, other ferrous alloys of chromium may be used also, as well as metallic chromium, or metallic nickel and its alloys, all of which are more noble than the ferrous metal to which they are applied. It will be understood that, for brevity, reference to one of these comprehends the others.

This invention is predicated on my discovery that the disadvantages of prior processes are overcome, and a permanent union between plain carbon and stainless steels or other stable surface metal obtained, by completely enveloping within cast ferrous metal an insert of stable surface metal having the surface to be welded in clean, metallic, weldable condition, and having another surface covered with a weld-preventing material, and then effecting welding by applying pressure to force the surfaces to be welded into welding contact at ordinary rolling or fabricating temperatures, '1. e. while the two materials are in a plastic condition. After welding has been effected the product is parted to provide one or more sheets surfaced with stainless steel. The number of such surfaced sheets produced is governed by the type of insert used, as will appear hereinafter. The production of the weld is dependent upon the provision and maintenance of clean weldable surfaces between the cast metal and the surfacing insert which is to be welded, and this precaution is particularly to be observed in the case of the insert. After pressure welding of the composite ingot containing an insert enveloped within ferrous metal, the weldable surface of the insert is satisfactorily welded to the ferrous metal, while the surface covered with weld preventing material is not welded. This permits parting to form the finished product in a I manner presently to be described.

Thus it will be observed that the invention differs from prior procedures for the production of stable surfaces ferrous articles in that the cheaper material, e. g. steel, is cast about inserts of stable surface metal and the ingot rolled or forged with the plain steel exposed, in that welding is produced, not by casting, but by pressure, and also in that welding conditions are maintained by completely enveloping the insert in the ferrous metal.

Various types of insert may be used in the practice of the invention. Their thickness is governed by the size of the ingot and the thickness of coating desired, as will be explained more fully hereinafter. In one embodiment of the invention the insert comprises a pair of sheets or plates of stable surface metal fastened in fixed back-toback relation, to prevent them from separating or buckling during casting of the ingot. In the use of such inserts the adjacent faces of the plates are provided with a weld-preventing material. Although the scale formed on hot rolled stainless steel plates is sufficient for this purpose it is usually preferred to remove the scale and to apply to the cleaned surfaces a coating of more suitable weld-preventing material. This is done preferably by painting the cleaned contacting surfaces with a thin coating of a dilute aqueous solution of sodium silicate, and then dusting a finely powdered inert oxide, for instance very finely powdered silica, alumina, magnesia, or chromic oxide on to the wet surfaces. The sheets are then dried and the excess oxide is dusted off, whereupon they are ready to be fastened together to form the completed insert.

It is important that the surface of the insert which is to be welded be cleaned to present weldable metallic surface to the cast metal. To this end they are cleaned thoroughly to remove oxide scale, grease, etc. Welding is favored also by having such surfaces in an unpolished condition, and for the best results they should be slightly roughened. In the preferred embodiment the entire surfaces of the insert are prepared for welding by sand blasting. This procedure not only removes the oxide and other dirt, but also roughens the surface sufficiently for the purposes of the invention.

In another embodiment of the invention the insert consists of a single plate having one side in a cleaned weldable condition, and the other coated with weld-preventing material, as described in connection with the two-part insert. It will be observed that it is characteristic of all of the inserts used that they present a clean weldable surface to be united with the cast metal by pressure-welding, and a surface conditioned to prevent such welding. The latter surface is for a purpose described in detail hereinafter.

I have found, also, that welding is favored if the low carbon steel or other cheap ferrous metal used for casting about the inserts is carefully deoxidized and freed from oxidation products prior to casting the ingot. That is, the metal not only is deoxidized, but also is freed from such products as alumina, aluminum silicates, etc., produced when aluminum is used for deoxidation. In general, it is preferred to avoid the use of aluminum and similar materials which produce light deoxidation products which tend tov remain in the metal. These, I have found, tend to segregate at the surface between the two metals and to prevent suitable welding. Although this precaution is not essential, it contributes to production of the most satisfactory results, and is preferred, therefor.

In the practice of the invention inserts of either type are supported in a mold with their edges spaced from the bottom and sides of the mold, and ferrous metal, e. g., mild steel, is then cast around the insert to entirely surround it within a continuous gas-tight envelope of the ferrous metal. Normal casting temperatures may be used in this procedure.

The surfaces of the two metals in ingots prepared in this manner are not permanently welded, but, on the contrary are, in general, slightly separated. A permanent bond is effected between the surface to be welded, however, by applying pressure to force those surfaces together, the ingot being heated to normal fabricating temperature. Preferably the pressure is applied by rolling. This forces the clean weldable surfaces of the ingot and insert into intimate contact, and under the influence of heat permanent welding is produced. The ingot should not be edge rolled until after the permanent bond has been formed in the manner just explained, as this tends to break the envelope and to force the surfaces of the two metals further apart.

After welding has been completed the ply metal sheets may be formed by cropping off the excess ferrous base metal at the sides and ends, to expose the insert, and then separating along the parting plane formed by the insert surface coated with weld preventative. Where two-part inserts are used parting takes place between the stainless plates, but in the case of single plate inserts it occurs between one side thereof and the contiguous ferrous metal. Thus there are formed plates comprising a body of ferrous metal provided on one face with an adherent welded coating of stainless steel. These may be used directly, or reduced in section, as desired, although as will be explained, it is preferred to complete section reduction before parting.

The invention may be understood further by reference to the accompanying drawings, in which Figs. 1 to 7 represent the use of two-part inserts. As shown in Figs. 1 and 2, an insert I is supported in a mold 2 of appropriate size by means of spacer blocks 3. If the insert is held in the center of the mold the resultant sheets will be of the same thickness, but the results can be varied according to need by shifting the position of the insert in the mold. This insert comprises two stainless steel plates 4 and 5 held in fixed back-to-back relation, as by spot welding, or in any other suitable manner. The adjacent surfaces of the plates are separated by a layer of weld-preventing material 6, for instance, by coating them in the manner explained above. For purpose of illustration layer 6 is exaggerated in the drawings. Preferably it is as thin as possible.

The entire surfaces of the plates forming the insert are preferably cleaned before applying coating 6, and after the plates are connected to make the insert the exposed outer surfaces are sand blasted. Preferably the inner surfaces are finished smooth before coating to remove all surface imperfections such as scale pits, roll marks, scratches, and the like. because the surface finish of the dual product depends largely upon their initial condition when they are made up into the insert.

Ferrous metal, such as mild steel, is now poured into the mold to fill it and entirely surround the insert. As shown by the drawings the edges of the insert are spaced from the edges, top and bottom of the mold, so that the ferrous metal completely surrounds the insert in the cast ingot. As stated previously, there is no welding in the ingot as cast, and the ferrous metal does not make complete contact with the insert. Sections taken from the ingots have shown that usually complete contact is obtained only at the edges of the insert, the metal being spaced from the faces, for instance as shown in Fig. 3, where the ferrous metal envelope 1 is withdrawn from the faces of the insert.

In order to produce complete welding between the steel and the stainless metal insert the ingot is then heated to ordinary fabricating temperature, for instance about 2200 F. Pressure is then applied, to bring the surfaces into welding contact, by rolling in the customary manner. When welding has been produced,the sheets may be separated, by parting along the plane between plates 4 and 5, or, and most suitably, the ingot may be rolled further to produce sheets of any desired thickness.

In order to part the sheets the ingot is cropped to remove the excess metal and expose the edges of the insert, as by shearing off at the edges and ends of the rolled composite plate, as shown by broken lines aa and bb, Fig. 4. Thereupon the slab or plate is parted, as shown in Fig. 5, to provide two sheets each composed of a main body portion formed'of steel 1 surfaced on one side with a layer 8 of stainless steel.

If desired, the invention may be applied to the production of a plurality of layers of ply metal from a single ingot. One such embodiment is shown in Figs. 6 and 7, in which three two-part inserts id identical with that just described are supported in the same manner in an ingot mold H, and ferrous metal is cast around them to envelop them in the same way. Upon rolling this ingot it may be separated to provide two sheets [2 each having an inner body of ferrous metal IS with both surfaces covered with stainless steel [4, while the two outer sheets l5 are similar to those described in the preceding embodiment.

The use of single plate inserts is shown in Figs. 8 to 10. In Figs. 8 and 9 one insert 20 having one of its surfaces, 2|, in a clean weldable condition, and the other provided with a weld-preventing coating 22 is supported as before in a mold 23. It is completely enveloped in ferrous metal, and rolled as just described. Upon cropping off excess ferrous metal the slab is parted along surface 22. This forms a ply metal plate having a body of ferrous metal 24 surfaced on one side with stainless material 25, and a plain sheet 26 of ferrous metal. Generally, of course, it will be desirable to keep sheet 26 of minimum thickness, by appropriately positioning the insert.

Like the two-part inserts, these single plate inserts may be used in multiple. One such use is shown in Figs. and 11. Here a mold 30 contains three inserts 3| each having a surface coating of inert oxide 32 on one side, the opposite surface being clean and weldable. The sheets formed on parting this ingot are shown in Fig. 11, the particular combination providing three ply metal sheet: each surfaced on one side with stable surface metal.

portion, leaving a thin strip of stable surstable surface material 33, and a sheet 34 of plain ferrous metal.

These inserts may be arranged in various combinations, as will be understood, to vary the results, i. e. to produce single or double coated sheets, or various combinations thereof, and the single and two-part inserts may be used also in a single insert for varying the result.

It has been found that there is a tendency for the carbon of medium and high carbon steels to migrate to stainless steel when these materials are used together. This may, of course, cause the stainless steel to lose its property of surface stability. I have found that this result may be minimized or eliminated by interposing a thin sheet of low carbon steel between the cast metal and the surface of the insert which is to be welded to it. This prevents rapid migration of carbon into the stainless steel. Where the inserts used are of heat-resisting metal and the product is to be subjected to elevated temperatures it is desirable to use separatingsheets which resist carburizing better than low carbon steel. These separating sheets are, of course, cleaned to provide weldable surfaces, and upon rolling the ingot they become welded both to the stainless and ferrous metals.

This embodiment is shown in Figs. 12 and 13 where a two-part insert 35, like that described hereinabove, is supported in a mold 36. The welding surfaces of the insert are covered by plates 31 of low carbon steel, carefully cleaned, and high carbon steel is poured to make the ingot. Rolling and parting proceed as before, to provide two sheets each having a body 38 of high carbon steel, a stainless steel surface 39, and an intermediate sheet 40 of low carbon steel welded to the outside sheets.

The ratio of the size of insert to the ingot dimensions is chiefly dependent upon the thickness of stable surface coating desired, although in general it is desirable to avoid the use of very heavy inserts, as they tend to warp in casting, or to crack in working. In general, inserts representing from about 5 to 15 percent of the weight of the cast metal suflice for all practical purposes. It is feasible, and is generally preferred to produce very thin layers of stainless steel, and by this process stainless steel coatings as thin as 0.001 inch may be made by hot rolling the ingots containing insert plates of appropriate thickness. The economy in the use of such,small amounts of stainless material is apparent.

- The articles produced by my process may take the form of bands, strips, plates, sheets, bars, and other appropriate rolled, pressed or forged forms, depending upon the manner in which they are wrought, and the degree of working, as will be understood by those skilled in the art. For brevity of reference these and other forms are designated herein as sheet.

The two metals in the sheets prepared in accordance with the invention are so welded as to provide satisfactory adherence, so that they are fitted to be used or shaped as desired. This adherence may be demonstrated by immersing a part of thesheet in an acid which dissolves the :ferrous base metal without attacking the ,After the ferrous base metal has been wholly dissolved from the ex- ,Jnetahi. no sign of separating of the two layers appears at the unexposed boundary, and

they resist pulling apart at that point. Thus the invention is applicable to the, production of substantially all articles where it is desired to take advantage of the properties of these stable surface materials.

Anespecial advantage of the invention results from fabrication with the cheap ferrous base metal in contact with the rolls. This is in distinction to prior processes where the surface of the stable surface material was damaged by roll marks, scale, tools, stains due to oxidation, and the like. In accordance with this invention the stainless steel is not exposed during rolling, but is protected against oxidation, scale, grit, roll and tong marks, staining and the like. Consequently, the plates comprising the insert are reduced in thickness while completely protected against surface-deteriorating influences. It is for this reason that it is preferred to postpone separating the ingot until after reduction has been completed. By using insert plates which initially have adjacent surfaces of suitable character, i. e. smooth or free from imperfections, the duplex sheets produced by my process-have non-corrodible surfaces which are ready for polishing as soon as the ingot has been parted. To do this it is necessary only to remove imperfections in the relatively small inserts, a much less troublesome and expensive operation than has previously been necessary. Thus the heavy expense of finishing that has attended prior pro cedures is greatly reduced. 1

Other advantages will be understood by tho, e skilled in the art. For instance, fabricating difiiculties are minimized by having the body of the ingot composed of easily workable ferrous metal. For some purposes these ply metals possess better physical properties than articles consisting f stainless steel and the like, and they are easier to form, machine and heat treat. particular importance, it is possible to make a plurality of sheets, either of the same or varying thickness, and coating thickness, from a single ingot. The production costs are reduced substantially, both by making the body of the sheet of cheap material, and by reduction of fabricating and finishing difiiculties and costs.

It is well known that the annealing of stainless sheets, etc. is one of the most difficult problems of the entire processing of this material, on account of the difficulty of removing the scale formed by this operation, so that special very expensive furnaces have beendesigned to overcome this difficulty. This difficulty, and the need for such special furnaces, is minimizedor eliminated by the present invention, according to which the rolled sheets may be annealed before separating, that is, with the stainless surfaces still enclosed in a gas tight envelope. This is an important advantage, for sealing of the stainless surfaces is prevented.

According to the provisions of the patent statutes, I have explained the principle and mode of performing my invention and. have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. A method of making ply metal comprising supporting in a mold an insert formed of chromium alloy steel, the insert having a clean roughened weldable surface and a surface provided with a weld-preventing coating, casting steel other than said chromium steel around the in- Also, and of sort to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot while heated to working temperature to press the surfaces together and cause welding between said steel and the weldable surface of said insert and to efiect a desired change in section, then cropping off excess steel to expose the edges of the insert, and separating along the parting plane formed by said weld-preventing coating to form said ply metal.

2. A method of making ply metal comprising supporting in a mold an insert formed of a pair of plates of chromium alloy steel held in fixed back-to-back relation, the insert having a cleaned unpolished weldable exterior surface and the adjacent faces of said plates being separated by a weld-preventing coating, casting ferrous metal other than said chrornium steel around the insert to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, working said ingot by application of pressure to its faces while heated to working temperature to cause welding between said ferrous metal and the weldable surface of said insert, cropping off excess ferrous metal to expose the edges of the insert, and separating along the parting plane formed by the coating between said plates to form separate ply metal sheets each composed of a body of said ferrous metal provided with an adherent welded surface of said chromium alloy steel. 7

3. A method of making ply metal comprising supporting at spaced intervals in a mold a plurality of inserts each formed of a pair of plates of stable surface alloy steel held in fixed backto-back relation, the inserts having a clean roughened weldable exterior surface and having the adjacent faces of said plates separated by a coating of weld-preventing material, casting another steel around the inserts to envelop them and form a composite ingot in which said insert is completely enclosed within said cast metal, applying pressure to the faces of the ingot heated to working temperature to cause welding between said steel and the weldable surfaces of said inserts, cropping off the excess of said other steel to expose the edges of the inserts, and separating along the parting planes formed by said coatings to form separate ply metal sheets composed of a body of said steel provided with an adherent welded surface of said stable surface alloy steel, the outer sheets being thus surfaced on their inner faces and the inner sheets being surfaced on both faces.

4. A method of making ply metal comprising supporting in a mold an insert formed of a pair of plates of stable surface steel held in fixed backto-back relation, the insert having a clean roughened weldable exterior surface and the adjacent faces of said plates being separated by a weldpreventing coating, casting a ferrous metal other than said stable surface steel around the insert to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot while heated to working temperature to cause welding between said ferrous metal and the exterior of said insert and to effect a desired change in section, then cropping off excess ferrous metal to expose the edges of the and separating along the parting plane body of said ferrous metal provided with an adherent welded surface of said stable surface steel.

5. A method of making ply metal comprising supporting in a mold an insert formed of chromium alloy steel and having a clean roughened weldable surface and a surface free from surface imperfections and provided with a weld-preventing coating, casting ferrous metal other than said chromium steel around the insert to envelope it and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot While heated to working temperature to cause welding between said ferrous metal and the weldable surface of said insert, continuing said rolling to effect a desired change in section, then cropping off excess ferrous metal to expose the edges of the insert, and separating along the parting plane formed by said coating to form ply metal sheet composed of a body of said ferrous metal provided with an adherent welded smooth surface of said chromium alloy steel which is substantially free from rolling defects.

6. A method 'of making ply metal comprising supporting in a mold an insert formed of a pair of plates of chromium alloy steel held in fixed back-to-back relation, the insert having a clean roughened weldable exterior surface and, the adjacent faces of said plates being free from surface imperfections and being separated by a coating of weld-preventing material, casting ferrous metal other than said chromium steel around the insert to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot while heated to working temperature to cause welding between said ferrous metal and the weldable surface of said insert, continuing said rolling to effect a desired change in section, then cropping off excess ferrous metal to expose the edges of the insert, and separating along the parting plane formed by said coating to form ply metal sheets composed of a body of said ferrous metal provided with an adherent welded smooth surface of said chromium alloy steel which is substantially free from rolling defects.

7. A method of making ply metal comprising supporting in a mold an insert formed of stable surface steel, the insert having a clean roughened weldable surface and a surface covered with a weld-preventing coating, casting another steel deoxidized and freed from deoxidation products, such as alumina, around the insert to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot heated to working temperature to cause welding between said other steel and the weldable surface of said insert, continuing said rolling to effect a desired change of section, then cropping off the excess of said other steel to expose the edges of the insert, and separating along the parting plane formed by said coating to form said ply metal.

8. A method of making ply metal according to claim 4 in which a plurality .of said inserts are spaced in parallel relation inthe mold, whereby to produce a plurality of said ply metal sheets.

9. A method of making ply metal comprising supporting at spaced intervals in a mold a plurality of inserts each formed of a pair of plates of chromium alloy steel held in fixed back-to-back relation, each of said inserts having a clean roughened weldable exterior surface and having the adjacent faces of said plates separated by a weldpreventing coating casting another steel deo idized and freed from deoxidation products, such as alumina, around the inserts to envelop them and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot heated to working temperature to cause welding between said steel and the weldable surface of said inserts, cropping off the excess of said other steel to expose the edges of the inserts, and separating along the parting plane formed by said coatings to form separate ply metal sheets each composed of a body of said steel provided with an adherent welded surface of said chromium alloy steel. the outer sheets being thus surfaced on their inner faces and the inner sheets being surfaced on both faces.

10. A method according to claim 9, the adjacent coated surfaces of said plates being initially free from surface imperfections, whereby the chromium steel surfaces of the ply metal sheets are substantially free from surface defects.

11. A method of making ply metal comprising supporting in a mold a plurality of inserts formed of a pair of plates of chromium alloy steel held in fixed back-to-back relation, each of the inserts having a clean roughened weldable surface and having the adjacent faces of said plates free from surface imperfections and separated bya weld-preventing coating, casting another steel deoxidized and freed from deoxidation products, such as alumina, around the insert to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot heated to working temperature to cause welding between said other steel and the weldable surfaces of the inserts, cropping off the excess of said other steel to expose the edges of the insert, and separating along the parting plane formed by said coatings to form separate ply metal sheets each composed of a body of said steel provided with an adherent welded surface of said chromium alloy steel, said chromium steel surfaces being free from surface defects.

12. A method of making ply metal comprising supporting in a mold an insert formed of stainless steel, the insert provided with a clean roughened weldable surface and a surface coated with a weld-preventing material, and having in contact with said weldable surface a. sheet. of low carbon steel having its entire surface in clean weldable condition, casting steel containing a relatively high percentage of carbon around the insert to envelop it and form a composite ingot in which said insert is completelyenclosed. within said cast metal, rolling said ingot heated to working temperature to cause welding between said relatively high carbon steel, low carbon steel and the weldable surface of said insert, continuing said rolling to effect a desired change of section, then cropping off excess high carbon steel to expose the edges of the insert, and separating along the parting plane formed by said coating to form ply metal sheet composed of said low carbon steel interposed between and welded to a body of said high carbon steel and a surface layer of said stainless steel.

13. A method of making ply metal comprising supporting in a mold an insert formed of a pair of stainless steel plates held in fixed back-to-back relation with their outer faces held in contact with a pair of low carbon steel sheets, the entire surface of said low carbon steel sheets and the outer surfaces of said stainless steel plates being in a clean roughened weldable condition, and the adjacent surfaces of said stainless steel plates being separated by a weld-preventing coating, casting steel containing a relatively high percentage of carbon around the insert to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, rolling said ingot heated to working temperature to cause welding between said relatively high carbon steel, low carbon steel and the weldable surface of said insert, continuing said rolling to effect a desired change of section, then cropping off excess high carbon steel to expose the edges of the insert, and separating along the parting plane formed by said coating to form ply metal sheet composed of said low carbon steel interposed betweenand welded to a body of said high carbon steel and a surface layer of said stainless steel.

14. A method of making a metallic body faced with a layer of another metal which comprises positioning a sheet of metal having one face covered with a weld-preventing coating and the other face clean, and another metal, in a molten condition, relatively to one another in such a manner that the molten metal is in contact with both faces of the sheet and a considerably larger portion of the molten metal on the side of the sheet which is clean than on the coated side; and removing that part of said other metal which is in contact with the'coated side of said sheet after the molten metal has solidified.

15. A method of making ply metal composed of a body of ferrous metaland a surface covering of chromium alloy steel, comprising sand-blasting one surface. of each of two plates of said chromium alloy steel and supporting said plates in an ingot mold in fixed back-to-back relation with said sand-blasted surfaces exposed and with a weld-preventing coating separating the contiguous faces of the plates, casting said ferrous metal around the insert to envelop it and form a composite ingot in which said insert is completely enclosed within said cast metal, hot rolling the ingot to cause welding between said ferrous metal and said insert, croppin off excess ferrous metal to expose the edges of the insert, and separating along the parting plane formed by the coating between said plates to form said 00 ply metal sheets.

' MAX R. TREMBOUR.

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