US2818360A - Method for the aluminum cladding of ferrous base metal and product thereof - Google Patents

Description

w. J. PORTER 2,818,360 METHOD FOR THE ALUMINUM CLADDING OF FERROUS BASE Dec. 31, 1957 METAL AND PRODUCT THEREOF- 3 Sheets-Sheet 1 Filed March 19, 1952 55%; @2585 l o 950: mic. .m dc 0Q 00 0o 2. 00 On O# OM ON 9 r O r o m T m m r 034 $.02 I II I EDEELEP 2:225 11 cos d vi fill o0: +32 2.5 III u\.i I13 4 1325125 2:22.: mwl nw 00M. J. 0

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TEMPERATURE F ATTORNEY Dec. 31, 1957 w. J. PORTER 2,818,360

METHOD FOR THE ALUMINUM CLADDING 0F FERROUS BASE Filed March 19, 1952 METAL AND PRODUCT THEREOF 5 Sheets-Sheet 2 ||60 INVENTOR. W\LL\AN\ J. PORTER ATTORNEY.

Dec. 31, 1957 w. J. PORTER 2,818,360

METHOD FOR THE ALUMINUM CLADDING OF FERROUS BASE METAL AND PRODUCT THEREOF Filed March 19, 1952 3 Sheets-Sheet 3 H6 q "60 INVENTOR.

' WILLIAM J. PORTER ATTORNEY.

United States PatentO METHOD FOR THE ALUlVIlNUM CLADDING OF FERROUS BASE METAL AND PRODUCT THEREOF William J. Porter, Penn Township, Allegheny County, Pa.,

assignor to Jones & Laughlin Steel Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Application March 19, 1952, Serial No. 277,394

7 Claims. (Cl. 14812.1)

This invention relates to a bimetallic article having a ferrous base and an aluminum coating, and to a process num on ferrous base metals have been advanced, and

a certain amount of aluminum coated sheet-s so produced have been sold commercially, but these sheets all exhibit to a greater or less degree the defect that the aluminum coating tends to crack or peel oif when the sheet is deformed. It is believed that this undesirable characteristic is a result of. a brittle alloy of aluminum and iron which forms at the interface between these metals during the hot dipping operation. Some degree of control over the formation of this aluminum-iron alloy may be achieved by control of the chemical com position of the aluminum coating. For example, the addition of silicon to the aluminum coating metal appears to be beneficial in this respect. appear to be possible to provide by the hot dipping process a coatingof commercially pure aluminum which will adhere to steel when the latter is deformed. Furthermore, it is very difficult, if not impossible, to produce by hot dipping a thin aluminum coating Without pin holes.

Attempts'have been made to produce aluminum coated steel sheets by cladding; that is, uniting a sheet of aluminum to one of steel by subjecting them to rolling pressure followed in some cases by annealing. None of these has produced a fully successful product. The mechanical bond created by rolling alone will not withstand deformation, and when the mechanically bonded product is annealed to bring about chemical bonding by alloying, the previously mentioned brittle alloy of iron and aluminum is formed;

The physical properties of alloys of aluminum and iron have been investigated by C. Sykes and J. W. Bampfylde and reported by them in the Journal of the Iron and Steel Institute;.volume CXXX, page 389, 1934. It appears that alloysof ironcontaining from to 34% of aluminum consist of a homogeneous solid solution.

The above-mentioned investigators found, however, that alloys containing more than 17% of aluminum could not be'worked in any way; that alloys containing from S to 16% of aluminum: could be worked hot but were quite brittle when cold; and that only alloys containing less: than 52% of aluminum were. sufiiciently ductile'to be worked cold-.- Although iron. and aluminum. can alloy in any. proportion, all such. alloys containing. more than about aluminum are seen to be brittle when sub- However, it does not tioned above.

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jected to cold deformation and therefore undesirable in aluminum coated steel sheets or similar articles'intended to be cold formed.

It is an object of my invention, therefore, to provide a bimetallic article having a ferrous base covered with a coating of aluminum of uniform controlled thickness which will adhere to the base when the latter is deformed. It is another object to provide a process for producing such a bimetallic article. Other objects will appear in the course of the following description of my invention.

My invention may best be described in terms of a present preferred process for producing aluminum coated' steel sheets comparable to tin plate, and the properties of such product. I prefer to use as base metal either hot rolled low carbon steel strip or cold rolled and annealed steel strip. The aluminum for cladding is provided in the form of thin sheet or strip of uniform thickness, thereby insuring uniformity of thickness of the coating of the finished product and absence of pin holes in such coating. This aluminum sheet or strip is applied to the surface of the properly prepared steel base and the composite sheet or strip is cold reduced by'rolling to a substantial degree of reduction. This cold reduction effects a mechanical bonding of aluminum to steel, but this bond will not withstand any substantial deformation. I find that heat treatment of the cold reduced material greatly improves the adhesion of aluminum to steel, presumably by effecting chemical bonding between the two. This heat treating. operation, however, in the absence of precautions to the contrary, causes the formation of the brittle aluminum-iron alloy men- The process of my invention, therefore, includes a novel step of providing between iron and aluminum a non-metallic film which inhibits the formation of a brittle aluminum-iron alloy layer during heat treatment but does not prevent chemical bonding of aluminum to the underlying iron.

The first step in the preparation of base metal for cladding with aluminum according to my invention that of roughening its surface. I prefer to etfect this step mechanically by grit blasting with steel grit although the strip may be roughened by other means if they are adjusted to produce sharp undercut projections and reentrant cavities similar to those formed by grit blasting. If acid pickling is employed, precautions should be taken to inhibit hydrogen absorption by the steel, otherwise blisters will form during heat treatment.

The steel base metal with roughened surface is then treated to provide thereon a substantially continuous alloy inhibiting non-metallic film. I have found that phosphate films have the desired properties. Such films may be formed by treating the steel base with an aqueous solution containing the acid radical of phosphoric acid. This treatment may take a form of a dip in an aqueous solution of phosphoric acid or readily obtainable phosphate such as trisodium phosphate. Such a solution forms on the surface of the steel a visible film of a phosphate of iron which will be discussed in more detail in a following paragraph. Since a very thin film on steel is sufiicient for my purpose, it is not necessary for the steel to remain immersed longer than is required to insure its complete wetting so that the film will. form same dimensions as the base metal sheet or strip or preferably slightly narrower, and laidthereon on one orboth sides as desired. The composite strip or sand; wich so formed is then cold reduced in a rolling mill, or

by other appropriate means, a substantial amount which I prefer to be at least about 35%. In this reducing operation both the aluminum coating and the steel base are reduced in thickness, and the roughened surface of the steel prevents the aluminum from crawling or creeping over the steel base so that it is reduced in thickness as uniformly as is the steel. This cold reduction eifects mechanical bonding between the aluminum and the steel, as has been mentioned.

The cold reduced composite material, which may be in the form of sheet or coiled strip, is then heat treated to bring about chemical bonding between base metal and coating metal. I have indicated that the non-metallic film I provide between aluminum and steel prevents the formation of a brittle aluminum-iron alloy during heat treatment, but it will be understood that there are temperature limits to the effectiveness of this film. I find that a heat treating temperature of 1100 F. to 1125 F. is safe under most conditions. Under circumstances to be discussed in a following paragraph, somewhat higher or lower temperatures may be employed. The composite material should be held at heat treating temperature for a time sufficient to allow chemical bonding of aluminum and steel to take place, which at 1100 P. requires an interval of only a few minutes if the material is heat treated in the form of a strand. It will be understood by those familiar with the art of heat treating sheet and strip that if the clad material is heat treated in the form of coils or piles of sheets the overall time required to bring the mass of metal to temperature, to soak it, and then cool it to a temperature below the sealing point will be measured in tens of hours or in days and will in fact be comparable with the time required to anneal sheet and strip in the conventional manner. The heat treatment contemplated by my process will also modify the physical properties of the clad material in a manner similar to conventional annealing and may be adjusted to obviate any further heat treatment. The coated strip after heat treatment may be given the conventional skin rolling or light reduction in a cold rolling mill if desired.

Reference is now made to the attached figures which indicates the range of conditions for heat treating made possible by the process of my invention and illustrate the effect of my process in inhibiting the formation during heat treatment of an aluminum-iron alloy layer between aluminum and steel.

Figures 1, 2, and 3 may be described generally as representing graphs of heat treating temperatures plotted against time of heat treatment for indicated conditions of aluminum-iron alloy formation, both for aluminum clad steel produced by the process of my invention and for such a product made in the conventional manner.

Figures 4 through 9 are photomicrographs of the above-mentioned aluminum clad steel subjected to the indicated heat treating temperature for a uniform time of 20 hours. In all cases the steel was covered with aluminum foil .002" thick, and steel and aluminum cold reduced 40% before heat treatment. All these photomicrographs are taken at a magnification of 1,000 diameters and etched in the same way with 3% nital for steel and 1% sodium hydroxide for aluminum.

Figure 1 is a plot against time of the minimum heat treatment temperature which will bring about formation of aluminum-iron alloy detectable with a microscope having a magnification of 1,000 diameters. The continuous line represents these conditions for a clad product provided during its processing with a phosphate film between steel and aluminum in accordance with the process of my invention. The broken line represents conditions for a clad product identical with the first mentioned except that it was not provided with a phosphate film in processing. It will be observed that the broken line curve, representing conditions for conventional practice, is everywhere below the solid line curve representing conditions for the process of my invention, and falls rapidly in the time interval between 4 and 20 hours. The significance of this is that for heat treating times of 20 hours or more, which as has been mentioned are commonplace for coiled strip or stacked sheets, the heat treating temperature in conventional practice must not exceed 950 F. or less if formation of a brittle aluminum-iron alloy is to be avoided. As will be shown, this heat treating temperature is insufficient to remove in a reasonable time the effects of the cold working the steel has received. The continuous curve, however, shows that the product of my invention may be heat treated at temperatures as high as 1150 F. for up to 20 hours without danger of brittle alloy formation, and at 1100 F. for up to 40 hours, for example, which in either case is sufficient to render the steel base fully ductile. The area between the two curves is a net gain in heat treating practice made possible by my invention.

Figure 2 is a plot otherwise similar to that of Figure 1 but representing the minimum temperatures for formation of a substantially continuous layer of aluminumiron alloy between the steel base and aluminum coating. In production, heat treating conditions should be held within the limits indicated in Figure 1 as the presence of any substantial amount of the brittle aluminum-iron alloy is generally undesirable, but the curves of Figure 2 indicate that the phosphate film of my invention is effective to a considerable extent at the higher temperatures there shown.

Figure 3 is of interest as showing the minimum heat treating temperature which will convert all the aluminum coating into aluminum-iron alloy. It will be noted that the minimum temperatures of Figure 3 are only very slightly higher than those of Figure 2.

Figure 4 is a photomicrograph of aluminum clad steel of my invention heat treated at a temperature of 940 F. The aluminum coating 1 is in direct contact with the steel base 2, no aluminum-iron alloy being found at the interface. The heat treatment given this material has been insuflicient to recrystallize the grain structure of the cold worked steel as is obvious from the shape and size of the grains in the steel 2. This material is not sufiiciently ductile for most cold forming operations. The relation of the microstructure of Figure 4 to the heat treating conditions plotted in Figure l is indicated by point A on the latter figure.

Figure 5 is a photomicrograph of material similar to that of Figure 4 but heat treated at 1115 F. Here again the aluminum 3 is in direct and continuous contact with the steel 4, no alloy being present. The steel 4 here exhibits a fully recrystallized grain structure indicating its suitability for cold forming purposes. This microstructure corresponds to point B on Figures 1 and 2.

Figure 6 is a photomicrograph of material similar to that of Figures 4 and 5 but heat treated at 1160 F. as indicated by point C on Figures 1 and 2. The aluminum 5 is not in continuous contact with the steel 6, being separated from it in some regions by a layer of aluminum-iron alloy 7. Heat treating conditions determined by point C are below the continuous curve in Figure 2 for minimum temperature of continuous alloy formation, and it will be seen from Figure 6 that the alloy zone 7 is not continuous.

Figure 7 corresponds to Figure 4 except that the material here was clad by conventional processes and was not provided with a phosphate film between aluminum and steel. The heat treating temperature of 940 F. corresponding to point A on Figure 1 was below the minimum temperature for aluminum-iron alloy formation at the point, as shown by the broken line curve, and so the aluminum 8 is in full contact with the steel 9, no alloy being present.

Figure 8 corresponds to Figure 5 with the same exception noted for Figure 7 above. Where Figure 5, however, shows only aluminum 3 and steel 4, Figure 8 PM 229 shows aluminum 10, steel 11, and a considerable amount of aluminum-iron alloy 12 between coating and base metal. The structure shown in this figure corresponds to point B on Figures 1 and 2 just as one does the structure of Figure 5. Point B is between the continuous and broken line curves of Figure 1 and just below the broken line curve of Figure 2; the alloy 12 in Figure 8, although substantial in amount, is not continuous.

Figure 9 corresponds to Figure 6 but is from a clad product not provided with a phosphate film in processing. The aluminum 13 is separated from the steel 14 by a thick continuous layer of aluminum-iron alloy 15. Point C which corresponds both to Figure 9 and Figure 6 is seen to be above the broken line curve of minimum temperature for continuous alloy formation in Figure 2 but below the continuous curve in that figure.

Figures 1 through 9, above referred to, illustrate the effect of the phosphate film of my invention in inhibiting the formation of brittle aluminum-iron alloy between base metal and coating under various heat treating conditions. Although the presence of aluminum-iron alloy is an indication of poor coating adhesion, it does not necessarily follow that absence of this alloy indicates good coating adhesion. Such adhesion can only be determined by mechanical testing.

The adhesion of a coating to a base is difficult to measure quantitively, and there are no tests of this property as generally recognized and employed as the Brinell and Rockwell test for hardness, for example, or the Izod and Charpy tests for impact strength. A simple bend test may be used with deformable material for qualitative estimation of coating adhesion, poor adhesion being manifested by cracking or flaking of the coating at the bend and good adhesion by the absence of such. The so-called handkerchief test is a form of bend test readily applied. A square or rectangular sample of the sheet is folded once so that two opposite edges coincide, then folded a second time so that the other pair of opposite edges of the once-folded specimen come together. The corner formed by the double folding is so severely deformed that the steel frequently cracks or breaks; absence of cracking or flaking of the coating at this point is an indication of very good adhesion. This test was used to evaluate the effect on coating adhesion of varying amounts of cold work and of variation in heat treating temperature set out below.

A set of samples of low carbon cold rolled and annealed steel sheet, .020" in thickness, was grit blasted, treated with an aqueous solution containing 1% phosphoric acid and 4% potassium dichromate by weight, covered with aluminum foil .00065" thick, cold rolled to the degree of reduction listed below, and then heat treated at a temperature of about 1100 F. for one hour with the results listed in Table I.

Table I Approximate Percent Reduction Coating Adherence Poor. Poor. Fair to Good. Fair to Good. Good. Good. Good. Good.

Table II Heat Treating Temperature, "F. Coating Adherence Good.

Good.

Good.

Good.

Good.

Poor-Some alloy formation. Poor-Blisters and alloy. Poor-All alloy.

This series of tests shows that coating adherence is impaired if the annealing of the material is carried out at temperatures above 1125 F. Comparable adhesion tests on samples clad without a phosphate film between steel and aluminum showed much poorer adhesion in all cases, even for samples heat treated at temperatures below the minimum for alloy formation as shown graphically in Figure 1.

My phosphate-containing film may be readily provided by treating the steel base metal with an aqueous solution containing about 1% to about 4% phosphoric acid by weight. Above and below these limits the effectiveness of the treatment falls ofi gradually, a fair degree of coating adhesion being obtained with as little as and as much as 10% phosphoric acid. The steel may be treated by dipping or spraying with the solution or the solution may be applied by other means. If the temperature of the solution is maintained between about F. and F., the steel need be immersed a short time only, say 60 seconds or so, for the deposition of the phosphate-containing film. Under the proper conditions an easily recognizable solid grayish-blue phosphate film forms on the surface of the steel.

In place of phosphoric acid, a phosphate may be used. I have obtained good results with phosphates of lithium, sodium, potassium, barium, calcium, cobalt, iron,nickel, zinc, aluminum, manganese, copper, silver, and ammonium. Since the film required by my process is extremely thin, I have not been able to determine its composition, but the experiments with the treatments above mentioned lead me to believe that this film must contain a substantial amount of iron phosphate or some more complex phosphate of iron and other elements. From this I conclude that any such film containing a substantial amount of such phosphate or phosphates will be effective in my process, and that such film may be provided by treating the steel with an aqueous solution contain ing an acid radical of phosphoric acid.

I have also found that chromic acid or a chromate such as sodium chromate or potassium dichromate may be added to the phosphate solution with good results, although these chromates themselves do not form films as do phosphoric acid and phosphate solutions. The optimum amount of such a chromate appears to be about 4% by weight. A very satisfactory solution contains 1% or 2% phosphoric acid and 4% chromic acid by weight.

I find it generally desirable to proceed with the cladding operation without undue delay after the ferrous base has been coated with a phosphate containing film as described above, since contamination of the phosphated surface may impair the adhesion of the coating to the base.

As I have mentioned, I prefer to roughen the surface of my steel base by blasting with steel grit. The size of grit employed is governed by the thickness of the aluminum to be applied so that the ridges and projections formed on the steel surface by the grit blasting do not puncture the aluminum coating upon cold rolling. For very thin aluminum foil, such as that of .0005" thickness previously mentioned, I prefer to grit blast with grit not larger than 120 mesh. When thicker foils are used, correspondingly larger particles of grit may be employed. For foil .002" in thickness, for example, I find 60 mesh grit satisfactory.

It will be understood thatthe thickness of aluminum sheet or foil used to produce a clad product having an aluminum coating of a desired thickness will depend upon the amount of cold reduction to which the clad strip is subjected. In my process the aluminum and the steel are reduced in the same degree so that if 50% cold reduction, for example, is contemplated, the aluminum sheet or foil should be about twice the thickness of the aluminum coating desired.

Since the aluminum coating is applied to the product of my invention as a rolled sheet or strip of metallic aluminum, its thickness is extremely uniform and it is free from pores or pin holes. It can therefore be characterized as preformed to distinguish it from hot dip and electrolytic coatings of aluminum which are formed in place on the base metal and may be of non-uniform thickness. It can also be characterized as continuous as it is not interrupted by the pores or pin holes found in both hot dip and electrolytic aluminum coatings. Furthermore, it can be characterized as directly bonded to the steel since my product exhibits no layer of aluminumiron alloy between aluminum and steel.

While I have shown and described what I presently consider to be a preferred embodiment of my invention, it will be understood that various modifications may be made thereof without departing from the spirit and scope of my invention.

I claim:

1. The process of cladding a ferrous base metal with aluminum comprising roughening the base metal, providing between base metal and aluminum a phosphatecontaining film, cold reducing together the base metal and aluminum, and heat treating the cold reduced product for a time and at a temperature sufiicient to recrystallize the ferrous base metal but not exceeding the temperature determined to correspond to the said time from the continuous curve of Fig. 1.

2. The process of cladding a ferrous base metal with aluminum comprising toughening the base metal, forming thereon a phosphate-containing film, applying thereto a layer of aluminum, cold reducing together the base metal and aluminum, and heat treating the cold reduced product for a time and at a temperature suificient to recrystallize the ferrous base metal but not exceeding the temperature determined to correspond to the said time from the continuous curve of Fig. 1.

3. The process of cladding a ferrous base metal with aluminum comprising mechanically roughening the base metal, treating it with an aqueous solution containing an acid radical of phosphoric acid so as to form a phosphate-containing film thereon, drying the base metal, applying a layer of metallic aluminum thereto, cold reducing together the base metal and aluminum, and heat treating the cold reduced product for a time and at a temperature sufiicient to recrystallize the ferrous base metal but not exceeding the temperature determined to correspond to the said time from the continuous curve of Fig. 1.

4. The process of cladding a ferrous base metal with aluminum comprising mechanically roughening the base metal, treating it with an aqueous solution containing an acid radical of phosphoric acid and an acid radical of chromic acid so as to form a phosphate-containing film thereon, drying the base metal, applying a layer of metallic aluminum thereto, cold reducing together the base metal and aluminum, and heat treating the cold reduced product for a time and at a temperature sufiicient to recrystallize the ferrous base metal but not exceeding the temperature determined to correspond to the said time from the continuous curve of Fig. 1.

5. The process of claim 3 in which the cold reducing of base metal and applied layer is effected by cold rolling so as to reduce the thickness thereof by not less than about 35%..

6. The process of claim 3 in which mechanical roughening of the base is accomplished by grit blasting with steel grit in a size range between about and about mesh.

7. A ductile bimetallic article comprising a ferrous base and a preformed aluminum coating in direct contact therewith, and which is produced by roughening the ferrous base, providing between ferrous base and aluminum a phosphate-containing film, cold reducing to gether the base and coating, and heat treating the cold reduced article for a time and at a temperature sufficient to recrystallize the ferrous base but not exceeding the temperature determined to correspond to the said time from the continuous curve of Fig. 1.

References Cited in the file of this patent UNITED STATES PATENTS 213,319 Brown et al. Mar. 18, 1879 1,211,218 Parker Ian. 2, 1917 1,317,351 Chadwick Sept. 30, 1919 1,434,081 Beck et al. Oct. 31, 1922 2,171,040 Merritt et al. Aug. 29, 1939 2,219,738 Copson Oct. 29, 1940 2,271,374 Mackay Jan. 27, 1942 2,369,146 Kingston Feb. 13, 1945 2,410,060 Goodale Oct. 29, 1946 2,490,543 Robertson Dec. 6, 1949 OTHER REFERENCES Metal Progress, January 1950, pages 59-63.

Claims (1)

1. THE PROCESS OF CLADDING A FERROUS BASE METAL WITH ALUMINUM COMPRISING ROUGHENING THE BASE METAL, PROVIDING BETWEEN BASE METAL AND ALUMINUM A PHOSPHATECONTAINING FILM, COLD REDUCING TOGETHER THE BASE METAL AND ALUMINUM, AND HEAT TREATING THE COLD REDUCED PRODUCT FOR A TIME AND AT A TEMPERATURE SUFFICIENT TO RECRYSTALLIZE THE FERROUS BASE METAL BUT NOT EXCEEDING THE TEMPERATURE DETERMINED TO CORRESPOND TO THE SAID TIME FROM THE CONTINUOUS CURVE OF FIG 1.

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