US2325659A - Composite metal stock - Google Patents

Description

Aug- 3, 1943- l T. B. cl-mcla:` y 2,325,659

cMPosITE METAL sTocK Filed Oct. 30, 1959 Patented 3, 1943 UNITED STATES PATENT OFFICE COMPOSITE METAL STOCK Thomas B. Chace, Winnetka, Ill., assignor to Clad Metals Industries, Inc., Chicago, lll., a corporation of Illinois Application October 30, 1939, Serial No. 301,978

2 Claims.

i My invention relates, generally, to composite metal stock for cooking utensils and the like, and it has particular relation to such composite metal ,stock comprising a relatively thick layer of copper bonded to a relatively thin layer of chromium ferrite alloy. This application is a continuation inl the cost thereof is a consideration of great importance.

To date no single metal or alloy appears to have satisfactorily met all of the above requirements, and the logical solution has been to resort to composite metal stock having a plurality of united layers of different metals each having been selected with reference to some particularly desirable property or properties thereof. I have found that composite metal stock comprising a thin layer of chromium ferrite alloy bonded to a relatively thick layer of copper more fully and adequately meets the above requirements than any composite metal stock provided heretofore. By chromium ferrite alloys I refer to alloys containing suicient chromium without the presence of nickel, except in residual amounts, so as to be resistant to straining and corrosion and useful for such articles as cooking utensils, containers, and the like. 'Generally this includes stainless iron ranging from 5% to 22% chromium. I have found that 16% to 18% chromium with a maximum of 0.12% carbon to be particularly suitable. -The elements, silicon, manganese, sulphur, and phosphorus are usually present in these alloys, the remainder being substantially iron. To improve the workability or corrosion resistance for some applications, such elements as vanadium, molybdenum, aluminum, copper, titanium and columbian may be added up to 1.0% without departing from the invention.

I have found that nickel chromium alloys, although having slightly better properties in respect to deep drawing and severe corrosion resistance, are very diicult to process with copper to provide a composite sheet. The hot rolling temperature range of 18-8 chromium nickel steel, for

instance, is approximately 2250 F. to 1600 F. Since the maximum rolling temperature is limited by the melting point or hot rolling temperature of copper, the permissible hot rolling range of a composite slab of these two metals is very narrow, being from about 1700" F. to 1600 F Obviously, this necessitates many reheatlngs during the reduction of such a slab. Furthermore, in the case of such a composite metal slab, the annealing temperatures for cold rolling of 18-8 `chromium nickel steel are approximately 1350a F. which develops large grain growth in the copper layer. Since the permissible cold rolling reductions between anneals for 18-8 chmomium nickel steel are about l0 to 15%, alarge number of anneals are required to bring the material to finished gauge thereby favoring large grain lgrowth and making for an excessive production cost. And if these rolling and annealing temperatures are exceeded to a substantial degree in processing, the 18-8 chromium nickel steel cracks over the entire surface thereby exposing the copper therethrough.

On the other hand, I have found that a chromium ferrite alloy or stainless iron, having 16 to 18% chromium with low carbon content, rolls very nicely with copper with a much wider hot rolling range, it being approximately 1600 F. down to 1100 F., or until the material loses most of its color. For cold rolling, such a composite slab comprises a layer of copper and a layer of chromium ferrite alloy, reductions of as much as 50% or more per anneal are permissible without injuring the composite metal, and without rolling the copper excessively over the ends and edges of the chromium ferrite alloy layer. Anneals for cold rolling of such composite metal are in the vicinity of 1400" F. to l450 F., which is a good annealing temperature for copper and does not favor excessive grain growth in the copper as in the case for instance' wherel88 chromium nickel steel and copper are reduced together, requiring a minimum annealing temperature of around 1800o F. I have found that large composite slabs of copper and chromium ferrite alloy can be hot rolled to break down `gauge with one or two heatings, and cold rolled to finish gauge with one or two intermediate anneals, as against 10 to 12 reheats for hot rolling and 15 to 20 anneals for cold rolling for the corresponding 18-8 chromium nickel composite slabs. Furthermore,l I find that straight chromium irons or chromium ferrite alloys do not dissolve in non-ferrous metals readily, so thatiron and chromium .pick-up in copper is maintained at a minimum in the bonding operation when copper is cast on to the chromium ferrite alloy layer. Whereas, the 18-8 chromium nickel steel type dissolves in copper to the extent of 1% or more of iron, chromium, and nickel. As one of the essential properties of com- Dosite metal stock for cooking utensils and the like is a high rate of heat transfer, and as the pick-up in the copper layer of iron and chromium greatly reduces its heat conductivity, this also becomes an important factor.

The significance of this freedom of the chromium ferrite from going into solution is not at once apparent, but it is of the utmost importance, as will appear from the following:

1. The conductivity of the copper in the plane of the sheet is of primary importance in spreading the heat which is applied at a. particular point over an extended area. Hence the presence of impurities in the copper, which greatly decrease the conductivity is highly undesirable.

2. The perfection of the bond between the copper layer and the facing-metal controls the transmission of heat transversely of the plane of ythe sheet. Unless the ybond is invariably intimate and complete throughout, the conduction of heat from the copper layer to and through the facing layer or vice versa is impeded, andthe resulting vessel or utensil is unsatisfactory.

3. I have discovered how to make abond -between chromium ferrite and copper by fusion. The molten copper is poured onto the cleaned and lluxed face of the solid layer of coating metal. Heretofore, it has been difficult, if not impossible, to secure any bond at all between copper and vchromium ferrite, vbut after I discovered how to perfect the fusion bond, it turned out that the thing which made the bond difficult was the thing which kept the copper from being contaminated. In other words, if the facing metal contains constituents like nickel which are readily soluble in copper, a fusion bond which is necessary to make a good heat conducting union tends to destroy the desired conductive property of the copper.

Hence, the particular combination of ya substantially uncontaminated copper layer fusion bonded to a chromium ferrite facing on one or both sides provides a new and superior article. And that article has the further advantages of superiority for hot and cold rolling into a finished sheet and for drawing into the finished form of a cooking vessel or utensil.

The results of actual production tests on making composite sheet stock using both chromium ferrite alloy and 18-8 chromium nickel steel will be described hereinafter.

In brief, I have found that chromium ferrite alloys in combination with copper for producing composite metal stock have the desirable properties of rolling and annealing well with copper, of dissolving only to a slight extent in the copper during bonding, having excellent surface properties as regard to taking a high permanent polish, and not contaminating food products. Such composite metal stock has also been found to be well adapted to be formed into cooking utensils and the like.

The object of my invention, generally stated, is to provide composite metal stock for cooking utensils and the like, which has high tensile strength, suitable corrosion resistance, high heat conductivity, good deep drawing characteristics, a high degree of toughness and ductility, and

being relatively inexpensive from both the production and material standpoint.

application of which will be indicated in the appended claims.

For a more complete understanding of the nature and scope of my invention reference may be had to the following detailed description, taken in connection with the accompanying drawing, in which:

Figure 1 is a perspective view of a mold assembly for producing a composite metal slab;

Figure 2 is a perspective view of a modified mold assembly;

Figure 3 is a sectional view of a. composite metal slab adapted to be rolled into composite metal stock;

Figure 4 illustrates diagrammatically the rolling of the composite metal slab shown in Figure 3; and

Figure 5 is a sectional view of a cooking utensil made from my. composite metal stock.

Chromium ferrite alloys are diicult to bond to other metals because of the weld-interferring chromium oxide film which forms almost instantaneously on exposure to atmosphere at room temperature. This oxide is not readily dissolved by the usual fluxes which are suitable for subsequent bonding operations such as heating to high temperatures. To overcome this problem of the chromium oxide illm and to secure an inseparable bond, which will withstand rolling strains, I rst dissolve the chromium oxide with a 35% to 50% cold hydrochloric acid solution. This can be accomplished by pouring sufficient amount of the acid pickle into the mold to cover the bonding surface to a depth of one-eighth of an inch or more. A period of about twenty minutes is required to fully dissolve the chromium oxide, after which the excess acid is poured off. While the surface is still wet with a thin acid lm, it is quickly covered with a sealing fiux such as borax or boric acid'. Such a flux covering prevents exposure of the bonding surface to the atmosphere with the resultant formation of the weld interferring oxide film. I prefer to apply the borax or boric acid flux in powdered form which may be quickly spread over the surface before the acid lm dries. The acid solution is purposely used cold so that it does not evaporate and dry as quickly as would a less concentrated hot solution. Usually powdered flux to the depth of one-half to one inch is sufficient. 'I'he mold with the flux covering may then be subjected to furnace temperatures of about 2150 F., at which the flux covering melts and provides a viscous liquid-tight seal during preheating and the subsequent casting operations. When the copper facing is cast to fill the mold the flux rises to the surface and is skimmed olf before the copper solidiiles. In brief, the ,essential steps of bonding copper alloys to chromium ferrite alloys are: dissolving the chromium oxide with acid; keeping the surface sealed with acid lm untilv covered with the powdered ux; and, keeping the surface sealed with flux during transfer to the furnace, during preheating and transfer to the casting platform, and until the ux is replaced with molten copper.

vReferring now to Figure 1 of the drawing, a

mold assembly is shown generally at I comprising a slab I I of chromium ferrite alloy having end and side thin metal welding strips I2 and I3, respectively. welded around the sides thereof. The end and side strips lI2 and I3 form a liquid-tight mold space around the top surface I4 of the slab II. To produce a compositemetal slab, the surface I( may be first cleaned by acid, as outlined above, and the surface Il then covered with a powdered flux such as borax or boric acid. The mold assembly I0 with the surface I4 thus cleaned and covered, may then be preheated to melt the ux and heat the slab Il above,the melting point of copper. Having been thus preheated, molten copper is poured into the mold space of the mold assembly I0 and allowed to integrally bondto the surface Il. 1

If the slab of chromium ferrite alloy is suffir ciently thin, a mold assembly `may be provided by an alternative method. In Figure 2 of the F. and1`500 F., sixteen reheatings were required.

After such rolling it was found that the resulting sheet was very unsatisfactory and that large areas of copper showed through the 18-8 chromium nickel stainless steel. `Thus, the superil ority of chromium ferrite alloy or chromium iron drawing such a mold assembly is indicated generally at I5. The m`old assembly I5 comprises a slab of chromium ferrite alloy IB having its ends I1 and I3 upturned as shown. Thin metal side strips I9 are welded on opposite sides of the slab I6 and the upturned ends I1 and- I3, as shown, to provide a liquid-tight mold space around the top surface of the slab I6, The mold assembly I5 may be prepared and the mold space filled with molten copper as outlined above in connection withthe mold assembly I0 of Figure 1 of the drawing.

Refen'ing now to Figure 3 of the drawing, a composite metal slab is designated generally at .25 which may have been formed as outlined above in connection with either'- Figure 1 or Figure 2 of the drawing. The composite metal slab 25 comprises a relatively thin layer '26 of chromium ferrite alloy and a relatively thick layer 21 of copper integrally bonded: to the layer 26. The layer 21 is preferably pure copperl with` perhaps a deoxidizing agent such as manganese or silicon added to an extent oi' not more than 0.5%. vFor example, silicon has been found to be a suitable deoxidizer, since amounts up to 0.15% :do not materially the conductivity. Other deoxidizing agents knownintheartmayalsobeused. However, the amount of added deoxidizing agent should belimitedto assmall amount asisl as it tends to decrease the heat conductivity'of the copper. i

The composite slab 25 may be rolled. according. to conventional rolling mill practice into thin composite metal stock of desired thickness as shown ditically in-Flgure 4 of the drawing. In this figure the composite` slab 25 is shown being passed between the'rolls 23 and 29 to give a resultant decrease in thickness. A composite metal slab one and one-half inches in thickness of copper bondedto a layer of chromium ferrite alloy, or chromium iron. containing 18% chromium was made up. The chromium iron layer was constituted a 10% `of the thickness of this slab. This slab wasrolled down to 0.185 inch within a temperature range of 1500 P. to 1100 F. in one heating. It was then annealed at 1450 F. and cold rolled to finished gauge of 0.062 inch. The resultant finished composite sht was of very good quality. The layers were uniform in thickness, integrally bonded and of very good surface character. For purposes of comparison, a

` mium ferrite consisting of from 5% to 22% chrosecond com-y over chromium nickel stainless' steel was clearly demonstrated.

Referring now to Figure 5 of the drawing, a cooking utensil is designated generally at 30 which was formed from composite metal stock 3l, such as may have been rolled from the composite metal slab 25 of Figure 3 of the drawing as` described above. The composite metal stock 3| comprises a thin layer 32 of chromium ferrite alloywith a relatively thick outer layer 33 of c opper integrally bonded thereto. `The inner layer 32 is readily polished and is easily maintained in a clean condition. The relatively thick outer layer 33 of copper gives the cookingutensil 30 a high lateral heat conductivity thereby minimizing the possibility of local overheating. The layer of chromium ferrite alloy or chromium iron usually comprises from 5% to 20% of the thickness of the composite metal stock. The particular thickness used in any instance will depend upon the desired specification required.

It will be understood that although composite metal stock has been shown and described having a single layer of chromium ferrite alloy, a composite metal stock could easily be likewise provided having thin layers of chromium ferrite alloy on opposite sides of the relatively thick intermediate copper layer. This would be within the scope of the invention.

Since certain further changes maybe made in the foregoing constructions and techniques, and different embodiments of the invention may be made without departing from the scope thereof, it is intended that all matter shown in the accompanying drawings or described hereinbefore shall be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. Composite metal stock comprising a layer of high conductivity copper fusion bonded to a layer of chromium ferrite and characterized by the thickness of the layer of alloy comprising 5% to 20% of the thickness ofthe stock, said chromium, not to exceed 0.12% carbon and the remainder iron with any otheringredients not. to exceed 1.0%, said stock `having the ability to withstand severe reductions by hot rollingwithin the range of approximately 1600 F. to 11'00 F. and

to anneal for cold rolling within the range of approximately 1400 F. to 1450 F.

2. Composite metal stock comprising a relatively thin layer of chromium ferrite consisting of 16% to 18% chromium with 0.12% maximum carbon and the remainder iron with any incidental elements not to exceed a total of 1.0%, and a relatively thick layer of 4high conductivity copper fusion bonded to said first layer, said copper being substantially free of alloying ingredi.. ents picked up from the chromium ferrite. THOMAS B. (7HACE.

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