US2191598A - Method of bonding dissimilar metals - Google Patents

Description

Feb. 27, 1940.

c. aswAm z ET AL METHOD OF BONDING DISSIMILAR METALS Filed Nov. 5, 1938 Leadi Silk/er I @J &

y mm m '10 or coating Patented" Feb. 27, 1940 UNITED STATES P TENT OFFICE 2,191,598 METHOD or nonmnc mssminlm METALS Carl E. Swartz, Cleveland Heights, and Elmore J.

Dockstetter, East Cleveland, Ohio, asslgnors to The Cleveland Graphite Bronze Company, Cleveland, Ohio, a corporation of Ohio Application November 5, 1938, Serial No. 239,115

4 Claims.

The present invention relates to a method of bonding or permanently uniting two dissimilar metals which normally are of such characteristics and, properties as to have relatively low aflinity I one for the other, and are therefore essentially immiscible in either liquid or solid states and do not form chemical compounds or alloys, commonly known as intermetallic phases. More particularly, the invention relates to the bonding of non-ferrous metals such as lead or silver to a ferrous metal such as iron or steel.

We have found that such dissimilar-metals can be bonded or permanently joined one to the other by the provision of certain operative conditions, and without requiring the presence of an additional bonding or fiuxing agent such as solder, tin or copper plate or the like. Such operative conditions, as embodied in our present method, also eliminate certain prior steps heretofore generally found necessary, in the bonding or coating of one metal with another, such as scrubbing, cleaning, pickling and rinsing. In our co-pending' applications Serial No. 91,730, filed July 21, 1936, and Serial No. 195,986, filed March 15, 1938, and of which the present one is a continuation in part, it is indicated that lead or silver can be directly bonded to steel by the employment of temperatures substantially above that of the melting point of lead.

The present invention is based upon the discoveries (1) that actual bonding begins only at an elevated temperature with respect to the melting'point of the lead and at a temperature which approximates that of the gamma iron formation temperature of the iron or steel to which the lead is to be bonded, (2) that the bonding becomes easier to effect as the temperature is further increased up to at least 2400 F., and (3) that actual bonding only occurs between such a metal as lead and chemically clean steel of low and medium carbon content, say 50%, such as is used for boiler plate, deep drawing steel and the like and structural steels and the like in the form of sheets, strips, rods and wire, that is such steels in finished or semi-finished form ready for fabrication or for further finishing operations. We have also found that the utilization of such a bonding temperature range and other conditions is applicable not only to the bonding of lead and steel, but also to the bonding of a ferrous metal with silver.

To the accomplishment of the foregoing and related ends, said invention, then, consists'of the steps hereinafter fully described and particulli' larly pointed out in the claims; the annexed (Cl. 91-703) I drawing and the following description setting forth in detail one approved method of carrying out the invention, such disclosed method, however, constituting but one of the various ways in which the principle of the invention may be used. 5

In. said annexed drawing: i

Fig. 1 is a more or less diagrammatic view illustrating one form of apparatus for practicing the process of our invention; Fig. 2 is a photo-micrograph vof steel coated with lead according to our l0 present process; and Fig. 3 is a photo-micrograph of steel coated with or bonded to silver according to our process.

In the practice of our process, the ferrous metal, such as steel or iron, is introduced into a 16 controlled atmosphere heating furnace. The fer rous metal may be in any form, unfinished, finished or semi-finished or even fabricated condition, such as a'sheet, wire, rod, bar, .woven wire screen or the like. In Fig. 1 of the drawing this 20 step is illustrated by way of the continuous steel strip I led into the heating furnace 2. The atmosphere in this furnace is determined by the introduction of gas, such as commercial annealing hydrogen, or partially burnt natural or arti ficial gases produced by commercial controlled atmosphere units, into the inlet 3. The temperature and concentration of the gases in the inte-' rior ofthe heating furnace 2 are such as to reduce the oxides in the surface of the steel strip l and to place it in achemically clean condition so that there will be no foreign or obstructing substance tending to interfere with the proper contact of the subsequently applied non-ferrous metal.

The following chemical equation illustrates the nature of the deoxidizing reactions taking place in the heating furnace 2:

In order to prevent the access of air to the surface of the steel strip I, the lower end 5 of the heating furnace, which may be in the form of a quartz tube, is led under the surface of the nonferrous metal bath 6. The temperature of the non-ferrous metal bath is maintained at any temperature between its melting point and the temperature of the steel strip. We have discovered that by maintaining the steel strip at its gamma iron transformation range or above, that 50 it is possible to secure a direct bond between the non-ferrous metal and the steel without necessity of applying a flux to the latter. Reference to the iron carbon diagram will show that this gamma iron transformation temperature 6 grain boundaries of the steel; in other words,

range begins at 133"! for complete transformation, depending upon the carbon content of the steel or iron. Therefore, the temperature of the steel strip I emerging from the heating furnace 2 should at or'above its gamma iron transformation t mperature range at the time it is introduced into the non-ferrous metal bath 6. -We have found that thehigher the temperature the faster and more firm will be the non-ferrous metal bond with the ferrous metal.

After the steel strip I, with its coating of nonferrous metal is led out of the non-ferrous metal bath 6, it may be quenched by means of the water spray 1. Any suitable cooling means may be employed since'the bond is determined and affected, not by the cooling but by the temperature of the two metals and bythe condition and carbon content of the ferrous metal.

A different type of apparatus may be employed for the performance of our process, as long as the required conditions of operation are maintained. Thus a centrifugal casting apparatus such as shown and described in our co-pending application Serial No. 91,730, filed July 21, 1936 or U. S. Patent No. 1,923,075, may be employed, with appropriate change in conditions of temperature and atmosphere as hereinabove described.

Where lead is the non-ferrous metal employed in the bath 6, a bond will result as illustrated in the photo-micrograph of Fig. 2. Similarly, where a silver bath is employed, the bond as shown in the photo-micrograph of Fig. Both of these photo-micrographs illustrate the unusual irregularity of the bond line, thus showing that there is an extremely effective physical adhesion between the two dissimilar metals. The photomicrographs also illustrate the preferential effect which the non-ferrous metals have for the the non-ferrous metal has penetrated into the surface of the steel, and in some areas, it will be noted that in the case. of lead (Fig. 2) that the bonding line is so irregular that there are in effect peninsulas of steel surrounded by lead, and that in the case of silver (Fig. 3) the ferrite grains are completely isolated in the silver layer. This difference as to the amount of penetration of lead as compared to silver into the steel may 1 be possibly explained by the fact that the solubility of iron in lead is not quite as great as the solubiliy of iron in silver, although for other than theoretical purposes, the solubilities are nil. Thus, it has been determined that the solubility of of iron in lead (at elevated temperatures) is in the order of 2 to i -times 10- per cent, and that the solubility of iron.in silver is 4 to, 6 times 10- per cent.

This preferential j-efiect of the non-ferrous metal along the grain' boundaries of the steel apparently accounts for the irregular bond line. The possible use of a decarburizing atmosphere in the treatment of the steel before it is immersed in the non-ferrous metal bath will enhance the production roughened and irregular surface.

A possible explanatory, factor pointing to the of the direct bond between the steel as lead and sileflect on the steel and the non-ferrous metal such F. and proceeds to 1663 F. I

process herein 3 results.

ver, and at the critical temperature range specifled, is to be found in the fact that the precipitation of ferrite from the solid solution of austenite occurs in this temperature range. Thus, the line ordinarily referred to in the iron carbon diagram as A3 is sometimes called the ferrite solubility line. Therefore, as steel is heated to a range at orabove this ferrite solubility line (vim, 1337? to 1663 F.) the ferrite begins to enter solution. Since, as shown in Figs. 2 and 3, the non-ferrous metals such as lead and silver have a preference for the grain boundaries rather than the grains of ferrite themselves, it can be concluded that the absence of ferrite, or its entering into solution, enhances the bonding or adhesion effect of the non-ferrous metal to the steel.

Other modes of applying the principle of our invention may explained, change being made as regards the steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

We therefore particularly point out and-distinctly claim as our invention:

1. The method of coating ferrous metal sheets, strips and the like containing not more than .50% carbon with lead comprising the steps of heating the ferrous metal at or above its gamma iron transformation temperature range and in an atto produce a chemically clean mosphere suflicient surface thereon, and then, while the metal is at or above said temperaturerangaand before the metal has been exposed to the air, conducting it into and through a coating bath of molten lead.

2.. The method of coating ferrous metal sheets, strips and the like containing not more ,than 50% carbon with lead, comprising the steps of heating the ferrous metal to a temperature not less than that of the gamma iron transformation tempera? ture range and not exceeding 2400" F. and in an atmosphere sufiicient to produce a chemically clean surface thereon, and then, is at such temperature and before it has been exposed to the air, conducting it into and through a coating bath of molten lead.

3. The method of lead coating ferrous metal sheets, strips and the like, containing not more than 50% carbon, comprising the stepsof heating the ferrous metal to a temperature not less than about 1337" F. and not more than about 2400 F. and in an atmosphere sufiicient to deoxidize the surface thereof, and then, metal is at such temperature and before exposing the metal to oxidizing influences, conducting it into and through a coating bath of molten lead.

4. The method of coating ferrous metal sheets, strips and the like containing not more than .50% carbon with lead comprising the steps of heating the ferrous metal at or above its gamma iron transformation temperature range and in an atmosphere sufiicient to produce a chemically clean surface thereon, and then, while the metal is at or above said temperature range,-and before the metal has been exposed to the air, applying a coating of molten lead thereto.

CARL E. SWAR'IZ. EL'MORE J.

be employed instead of the one disclosed, provided the step orwhile the while the metal

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