US2950991A

US2950991A - Method and apparatus for coating ferrous metal with aluminum

Info

Publication number: US2950991A
Application number: US810174A
Authority: US
Inventors: Harvey W Seymour
Original assignee: American Chain and Cable Co Inc
Current assignee: FKI Industries Inc
Priority date: 1959-04-30
Filing date: 1959-04-30
Publication date: 1960-08-30
Anticipated expiration: 1977-08-30

Description

Aug. 30, 1960 H. w. SEYMOUR 2,950,991 7 METHOD AND APPARATUS FOR COATING FERROUS METAL WITH ALUMINUM Filed April 30, 1959 High Prasuro Atomizinq Air Water High Pressure ir Supply Water Supply.

INVENTOR. Harvey W. Seymour BY ATTORNEYS Q L L ,4 t

Patented Aug. 36,196il ice WTHOD AND APPARATUS FOR COATING FERROUS METAL WETH ALUMKNUM Harvey W. Seymour, Belle Vernon, Pa., assignorto Amen iean Chm & Cable Company, Inc., a corporation of New York Filed Apr. 30, 1959, Ser. No. 810,174

1% Claims. (Q1. 117-119A) This invention relates to the manufacture of alumihum-coated ferrous metal articles, and is particularly directed to an improved method and improved apparatus for aluminum coating by a hot dip procedure. This application is a continuation in part of my copending application Serial No. 544,215, filed November 1, 1955, now abandoned.

The invention provides an improved method and improved apparatus for rapid cooling and congealing of the aluminum coating promptly upon emersion of the ferrous metal article from the molten aluminum bath, Without distorting or roughening the coating, and it is a particular object of the invention to provide for such cooling in a continuous aluminum coating operation for coating ferrous metal wire, rod, Sheet, strip, and other articles. The scope of the invention, however, is not confined to continuous operation, for it may be applied to batch operations as well.

Numerous proposals have been made heretofore to form a protective coating of aluminum on ferrous metal articles by dipping the article in molten aluminum and then withdrawing the aluminum coated article from the molten bath. A method which is particularly efiective for thus forming an adherent aluminum coating, free from pinholes and similar defects, is described in U.S. Patent No. 2,686,355 to Harald Lundin. The Lundin method is characterized by the use of a flux comprising a fluoride compound of zirconium or titanium to promote formation of a tightly adherent continuous aluminum coating on the steel or other ferrous metal article. Other hot dip methods, some using other flux compositions, and some using no fluxes at all, have also been proposed and used with varying degrees of success to form aluminum coatings on steel articles.

Much difficulty has been encountered heretofore in forming adequately smooth and adequately thick aluminum coatings, possessing adequate ductility, by hot dip methods, particularly on wire and strip of the heavier gauges. The temperature at which the molten aluminum bath must be held in the hot dip coating method is high enough to have a stress relief annealing effect on ferrous metal articles immersed therein. Even though the time of immersion is short, the ferrous metal shape generally is heated substantially throughout its entire cross-sectional area to the temperature of the molten aluminum. Consequently it is frequently desirable to subject the freshly coated ferrous metal to cold working to develop the hardness and tensile strength that may be required for specific applications. Cold working results in reducing the thickness of the aluminum coating, and unless the freshly formed aluminum coating is quite thick, the coating on the cold worked product will be too thin to meet certain commercial requirements.

Heavy gauge wires, sheets, etc., which have been heated to the temperature of the molten aluminum during formation of the aluminum coating do not cool rapidly in air, because of the considerable mass of the ferrous metal. It has been observed that an appreciable period of time is required for the ferrous metal to cool in air to a temperature below that at Which the aluminum coating ceases to flow. This is particularly true in high speed continuous coating operations, for such operations unavoidably result in subjecting the moving wire or sheet to vibration which tends to induce flow of the coating metal even when the latter is in a mushy or semi-solid state. Flow of the aluminum coating on the ferrous metal results in reduction in the thickness of the aluminum coating by drainage of coating metal back into the aluminum bath; and it also commonly results in the formation of coatings of uneven thickness and of rough surface.

Another objectionable effect which is traceable to slow cooling of the ferrous metal is that the aluminum coating thereon displays a marked lack of ductility. Such lack of ductility results from an excessive thickness of a brittle intermetallic iron-aluminum compound which forms by interdifiusion of the aluminum coating and the ferrous metal base. The amount of such compound which is formed and hence the thickness of the brittle layer depends on the length of time during which the aluminum and the ferrous metal are held at an elevated temperature. Since the relatively heavy gauges of steel wire and sheet, and other correspondingly heavy articles, cool only slowly to a temperature below that at which the brittle intermetallic compound forms, the coating on such wires, sheets and otherarticles is subject to cracking and peeling when the article undergoes substantial deformation.

Other difficulties heretofore encountered in hot dip aluminizing methods are due to the rather coarse grain structure of the coating layer and the accumulation of iron in the coating bath. These factors have been found to be related to each other and to the composition of the aluminum bath. The bath may contain substantial silicon (say about 2.5%) to inhibit formation of the brittle aluminum-iron layer between the coating and the ferrous metal base. However, as iron builds. up in the bath (from the ferrous metal passed through it), an undesirable compound of iron, aluminum and silicon, in a form which adversely afiects coating ductility and adherence, occurs in the coating. Thus, steel wire which has been aluminized by passage through an aluminum bath containing about 2.5% silicon and in which considerable iron has accumulated, and which has then been air-cooled, is found to have a coating comprising rather coarse particles of an aluminum-silicon-iron compound in a matrix of aluminum-silicon. The coarse segregated aluminum-iron-silicon compound substantially impairs the adherence and ductility of the coating. To keep the adverse effect of such segregations Within reasonable bounds, it has been necessary heretofore to maintain the iron content of the aluminum bath at a low value (generally below 1%) either by making periodic additions of pure aluminum-silicon alloy, or by discarding it when the iron content has become too high.

Owing to the foregoing circumstances, much difficulty has been encountered heretofore in producing aluminumcoated ferrous metal wires and sheets of good quality in any but thin gauges. Such success as has been attained with the heavier gauges has been under special manufac turing conditions which have in general resulted in low production output and correspondingly high manufacturing costs.

The present invention provides an improved method and improved apparatus for producing aluminum-coated ferrous metal articles by a hot dip method which avoids the disadvantages heretofore encountered. By means of the invention it is possible to produce adequately thick and smooth aluminum coatings by hot dip methods on even heavy gauges of Wires and sheets, and yet the ironaluminum inter-facial layer is thin enough so that the coatings possess adequate ductility to withstand severe mechanical deformation without cracking or peeling. Moreover, the invention leads to an improved coating grain structure in which the iron-aluminum-silicon compound is uniformly distributed as small spheroids through the aluminum-silicon matrix, in which form the compound has relatively little effect on coating ductility and adherence. Consequently the invention makes it possible to tolerate up to 2% or even more of iron in the aluminum bath. These results are attainable in accordance with the invention at high coating speeds and with correspondingly low manufacturing costs.

The foregoing advantages are attained in accordance with this invention by subjecting the aluminum coated article, immediately upon its withdrawal from the bath of molten aluminum, to a quenching operating in a dense mist of liquid coolant droplets. It has been proposed heretofore to quench articles that have been coated with aluminum by a hot dip method, but the conventional quenchingprocedures which such proposals have entailed are unsatisfactory because they result in serious roughening of the aluminum coating. For example, if an ordinary stream or spray of coarse drops of water are directed at the aluminum coated article before the aluminum coating thereon has fully solidified, the impact of the liquid coolant on the aluminum coating deforms and distorts the aluminum coating and congeals it in the distorted form, so that the article is objectionably roughened. Quenching with a blast of air or other gaseous coolant behaves similarly. We have found, however, that very rapid and effective quenching of the aluminum coating can be achieved by means of a dense mist of exceedingly fine liquid coolant droplets which impinge on the aluminum coated article at low velocity. Quenching in this fashion results in cooling the aluminum coating almost as rapidly as conventional cooling with a heavy stream or spray of liquid coolant, but does so without impairing the surface smoothness of the aluminum coating.

Briefly stated, the invention provides, in a method for coating a ferrous metal shape with aluminum by immersing the shape in a bath of molten aluminum and then withdrawing the shape from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises introducing the coated shape immediately upon withdrawal from the aluminum bath and while the aluminum coating thereon is still fluid into direct contact with a dense mist of liquid coolant. The mist comprises a gaseous suspension of very finely divided coolant droplets, and is characterized by (i) very fine particle size (ii) low particle velocity, and (iii) high particle density. The particle size of the droplets forming the mist are sufficiently small so that settlement thereof from. suspension is negligibly slow even in a quiet atmosphere and does not occur to any substantial extent in a moderately turbulent atmosphere. The velocity with which the droplets impinge on the aluminum coated shape is sufficiently low so that substantially no deformation of the aluminum coating thereon, even while it is still fluid, is caused thereby. The density of the mist is at least great enough to cool an aluminum coated steel wire 0.1 inch in diameter from 1250 F. to below 900 F. in less than fifteen seconds; and preferably it is great enough to cool whatever article is being coated (usually an article substantially more massive than a wire 01 inch in diameter) to below 1000 F., or better, to below 900 F., in less than one half minute. As a result, the aluminum coating is quenched and congealed before flow of the molten coating metal can result in a coating having substantial irregularities in thickness and surface smoothness, and in a manner which itself does not contribute to deforming or roughening the coating. Preferably the coated shape is maintained in contact with the mist coolant until its temperature has been reduced to below 1000 F., and

advantageously to below 900 F., so that growth of brittle iron-aluminum alloy by interdiifusion of the coating and the ferrous metal is minimized.

The liquid coolant employed in accordance with the invention is most advantageously water, and is delivered into direct contact with the freshly coated ferrous metal in the form of a mist of fine liquid water droplets. The term mist is used herein in its customary sense to mean a suspension of very fine water or other coolant droplets in air or other gaseous suspending medium, in which the liquid droplets are larger than at present in a fog so that there is some tendency for settlement of the droplet particles to occur in a quiet atmosphere, but in which the coolant droplets are not large enough for the fairly rapid settlement that characterizes a gentle rain.

Apparatus according to the invention for coating ferrous metal wire, rod, sheet, strip, and other articles comprises a vessel for containing a bath of molten aluminum, and means for continuously passing such article through said bath and for withdrawing the article upwardly from the bath in a substantially vertical direction. Spray nozzles are directed at the vertical path of travel of the article immediately above said vessel,

so that promptly upon emergence of the article from the bath and while the aluminum coating thereon is still fluid, the article enters the zone covered by spray from said nozzles. The lowermost of the nozzles is directed at the path of travel of the article in the region immediately above said vessel and the uppermost of the nozzles is directed at said path of travel at a substantial distance above said vessel, and the intermediate nozzles are spaced so that substantially the entire length of said path of travel between the lowermost and uppermost nozzles is covered by spray therefrom. However, it is desirable for the nozzles to be so arranged that there is substantially no overlap of the spray delivered from one nozzle with the spray delivered from an adjacent nozzle where said sprays meet the path of travel of the aluminum coated article. Means are of course provided for delivering a liquid coolant to said spray nozzles, and means connect the nozzles with a source of atomizing gas under pressure, whereby the coolant may be discharged from the nozzles in the form of a mist of particles so small that settlement thereof from gaseous suspension is negligibly slow. With such apparatus the coating is quenched and congealed before fiow thereof on the continuously moving ferrous article can impart substantial irregularities to the thickness and surface smoothness of the coating.

An advantageous embodiment of the invention is described below with reference to the single figure of the accompanying drawing. The drawing shows in schematic elevation, and partially in section, a form of apparatus according to the invention which is particularly well suited for coating one or more wires by the method of the invention.

The apparatus shown in the accompanying drawing is particularly designed for continuously coating a numberof wires W with aluminum in accordance with the invention. Although only one wire W is shown in the elevational view of the drawing, it is of course evident that a group of wires can be arranged in fairly closely spaced relation in planes perpendicular to the plane of the drawing and can be passed through the apparatus simultaneously.

The apparatus shown comprises a pot furnace-10 of more or less conventional design, which contains a bath of molten aluminum 11. Conventional heating means (for example an induction heating assembly) provide for maintaining the aluminum molten and at the temperature required for the coating operation.

' A sinker roll 12 is mounted inthe furnace pot well below the surface of the molten aluminum bath. The sinker roll is supported at each end in a bearing carried by a'bracket'arm 3.3, the upper end 14 of which is secured to the upper side edge of the pot and the lower end of which extends down into the pot to beneath the surface of the molten aluminum. The wire W travels, as indicated by the arrows, downwardly into the bath of molten aluminum, around the sinker roll 12, and thence vertically upward from the pot.

Adjacent the path of the vertically ascending wire is a framework comprising one or more vertical columns 15. This framework carries a supporting structure of angle irons 16 on which a plurality of

spray nozzle assemblies

17a, 17b, 18a, 18b, capable of producing a mist of very fine coolant droplets in gaseous suspension, are supported in vertically spaced array, directed substantially toward the path of wire travel. Spray nozzle assemblies of the character used in paint spraying operations, and capable of producing a mist as fine as a paint spray mist, are well suited for use as the nozzle assemblies. Although the spray nozzle assemblies are all identical, the nozzles 17 in the lower half of the array are connected in one operating group, and the nozzles 18 in the upper half of the array are connected in a separately operated group. This arrangement of the nozzles in two opertaing groups is primarily for operating flexibility and convenience.

Liquid water is delivered to each of the nozzles 17a, 17b in the lower group through a water header 19, and compressed air for atomizing the water and delivering it from the nozzle assemblies in the form of a spray is supplied to each of these nozzle assemblies through an atomizing air header 26. Additionally, compressed air for actuating the control mechanism of the nozzles, by which the spray from the nozzles is turned On and off, is delivered to each nozzle through a control air header 21. Water is supplied to the water header 19 from a water supply main 22 through manual shut-

0E Valves

23 and 24, and through a pressure regulator valve 25. A water pressure gauge 26 is provided to show the pressure of water entering the header 19. Compressed air is supplied to the two

compressed air headers

20 and 21 from a high pressure air supply main 27 through a manual shut-off valve 28 and a commercial air transformer 29 which includes pressure regulator valves and pressure gauges to show the pressure at which compressed air is supplied to the headers. The admission of compressed air into the control air header 21 is governed by a quick acting control valve 30, by operation of which the spray nozzles 17 can be rapidly turned on or oif. The air and water controls for the lower group of spray nozzles 17 are advantageously grouped at a single control panel 31.

Similarly, water is supplied to the spray nozzles 18 in the upper group through a water header 19a, and air for projecting an atomized spray of water from the nozzles is delivered through an atomizing air header 29a. Compressed air for actuating the nozzle shut-ofif mechanism is supplied to each nozzle through a control air header 21a. Water is delivered to the water header 19a from the main 22, and compressed air is supplied to the air headers 24M and 21a from the high pressure air main 27, through control mechanism of the same character as is employed in conjunction with the nozzles 17 of the lower group. The controls for the upper group, like the controls for the lower group of nozzles, are for convenience trouped at a single control panel 32. It is advantageous of course for the two

control panels

31 and 32 to be located directly adjacent to one another.

As indicated above, the nozzles 17, 18 are arranged along the path of vertical travel of the Wire W as it emerges from the molten aluminum in the pot 10. The angular diameter of the spray cones delivered from the nozzles is preferably great enough so that the sprays from each adjacent pair of nozzles merge together just before reaching the path of wire travel. Thereby the entire path of wire travel is blanketed with the spray of water droplets emerging from the nozzles, but there is no substantial overlap of the spray from one nozzle with that from an adjacent nozzle at the path of travel of the Wire W. The lowermost nozzle 17a is positioned closely adjacent the upper'edge of the pot furnace 10, so that the wire W enters the zone covered by the water sprays promptly after its emergence from the bath 11 of molten aluminum. The uppermost nozzle 18 is located a substantial distance above the molten aluminum so as to provide a spray zone of sulficient lengthto insure adequate cooling of the wire even when it is passed through the apparatus at its maximum rate of travel.

An arrangement of splashboards 33 is positioned adjacent the path of travel of the vertically moving wire on the side thereof opposite the sprays 17, 18. The Water droplets in the mist projected from the sprays which pass by the Wire W impinge on the splashboards, coalesce into larger drops, and drain downwardly into a pan 34 which is supported just above the pct 10. Drainage water collecting in the pan 34 is carried away through a drain pipe 35.

In order to damp out oscillations of the wire W as it moves vertically upwardly from the pct 10, a damping roll 36 is provided. The damping roll is supported at each end in a bearing carried by a bracket arm 37 which projects from the column 15, and it is held thereby in light pressure contact with the upwardly moving wire. The damping roll 36 should of course be positioned far enough above the pot 10 so that the aluminum coating on the wire is frozen hard before coming in contact with it.

A head sheave 38 is mounted at the upper end of the supporting structure carried by the column 15, in position to receive the Wire W at the upper end of its path of vertical travel. The head sheave 38 overlies one end of a quench tank 39. A sinker sheave 40 is mounted adjacent the center portion of the quench tank, and a takeout sheave 41 is mounted adjacent the end thereof remote from the head sheave. The Wire W is carried around the head sheave 38, thence under the sinker sheave 40, and finally over the take-out sheave 41, whence it passes to a take-up reel.

Water is delivered into the quench tank 39 through an inlet pipe 42 in volume sufficient to maintain the tank substantially full of water. A drain stand pipe 43 is provided to prevent the quench tank from overflowing. The wire W, after passing over the head sheave 38, is carried by the sinker sheave to beneath the surface of the body of water in the quench tank, thereby to effect final cooling of the wire to a desirably low temperature.

Operation of the above-described apparatus is as follows: The wire W to be coated with aluminum is passed continuously into the bath of molten aluminum 11, under the sinker roll 12, and thence vertically upwardly and out of the bath. Promptly after the wire emerges from the bath it enters a dense mist of water droplets directed toward its path of travel by the nozzles 17, 18. The direct contact thus established between the hot wire with its fresh still-fluid aluminum coating and the liquid water droplets of the mist results in quick quenching and congealing of the aluminum coating. M

The use of a coolant mist of very fine particles which impinge at low velocity on the aluminum coated article, but which are present in high density, is a particular feature of the invention. The mist particles are, as stated above, of such small size that they settle but slowly in still air and hardly at all if the air is moderately turbulent. Such a mist is created by atomizing the water with air at quite high presssure (say 75 to pounds per square inch) as it emerges from the spray nozzles, and its velocity at the nozzle is high. However, the wide angle of the spray cone and the great increase in its cross-sectional area at the path of wire travel as compared with the nozzle aperture results in a correspondingly low impingement velocity of the mist against the wire. The velocity of the spray, by the time it reaches the path of travel of the wire, therefore is sufficiently low so that neither the air nor the small water droplets of the mist impinge on the wire with sufiicient force to cause any objectionable physical distortion of the aluminum coating before it freezes.

The density of the water particles in the mist (ie the number of particles per unit volume of air) can best be specified in terms of the cooling power of the mist. For effectively rapid cooling of the aluminum coated article, the density of water particles in the mist when it comes in contact with the article should be at least sufficient to cool a continuously advancing aluminum coated wire 0.1 inch in diameter from a temperature of 1250 F. to below 900 F. in less than fifteen seconds. A mist of such density is quite effective for cooling small wires (up to say inch or slightly larger). For relatively heavier articles a more dense mist is advantageous. In order to assure rapid enough cooling of wire, rod, sheet, strip, and other shapes in general, the mist density advantageously is high enough to cool whatever shape is aluminum coated from the temperature it attains during coating (generally above 1250 F.) to below 1000 F. and preferably to below 900 F., in less than one-half minute. Relatively massive articles will require times up to one-half minute in the mist to be cooled to the extent indicated, whereas small diameter wires may be cooled to below 900 F. in a few seconds.

Thus the quenching efiiciency of the water mist is high and the aluminum coating is rapidly congealed to a nonfiowing solid state. As a result the coating on the wire is held at substantially the thickness with which it is drawn from the aluminum bath 11, for it is solidified before it can become appreciably reduced in thickness by drainage down the wire. Moreover, the quenching effect of the mist congeals the aluminum coating sufficiently rapidly so that uneven coating thickness and rough surface characteristics are substantially avoided, even when the wire is travelling at its maximum velocity.

Although the aluminum coating layer is quite completely solidified upon cooling to a temperature below about 1200 F., the array of sprays 1'7, 18 should be high enough tocool the wire as rapidly as possible to a temperature below 1000 F. and preferably below 900 F., so as to minimize the thickness of brittle iron-aluminum alloy that forms by interdiffusion of the ferrous metal wire and the aluminum coating. It has been found that the formation of such a brittle intermetallic compound continues to occur even at temperatures substantially below the freezing temperature of the aluminum layer. The rate at which such layer forms decreases with temperature, but the rate does not become very small until the temperature has been decreased to below about 1000 F., and some formation of the intermetallic compound continues to occur by diffusion until the wire has been cooled to below about 900 F. Accordingly, for optimum results a mist of the above specified density should be provided throughout the entire height of the array of sprays, and such height should be correlated with the maximum linear speed with which the wire W is passed through the apparatus, and with the thickness of the wire, so that the temperature of the wire is reduced to below 900 F. before the wire passes out of the mist formed by the sprays. Advantageously the height of the array of sprays is suflicient to effect cooling of the wire to a temperature even considerably below 900 F. before the wire passes beyond the spray zone, in order to be sure of attaining the maximum benefits of the invention.

It is of course important that the sprays effect sufiicien-t cooling of the wire in the lower portion of the spray zone so that the coating on the wire is fully solidified before it comes in contact with the damping roll 36. 'Otherwise the damping roll will mechanically deform the coating and thus lead to an inferior product.

As the wire passes out of the spray zone and over the head sheave 38, it is carried by the sinker sheave 40 to beneath the surface of the body of liquid water in the quench tank 39. While quenching of the wire at this point is not necessary in accordance with the invention, it has the advantage of insuring that the wire is brought to a temperature near room temperature preparatory to coiling and handling.

As an exmple according to the invention, a steel wire having a diameter of 0.132 inch has been provided with a heavy aluminum coating, approximately 0.0015 inch in thickness, by passing it at a velocity of 115 feet per minute through a bath of molten aluminum containing 1.94% iron and 2.93% silicon, and then upwardly through a mist of water droplets of the size, velocity and density stated above, formed by an array of water sprays of sufiicient height to quench and congeal the aluminum coating promptly and to cool the wire to the temperature below 900 F. in a distance of approximately 24 feet from the point at which the wire emerged from the molten aluminum (i.e. within about 0.2 minute, or about 12 seconds, after emergence of the wire from the aluminum bath). The coating on the wire was sufiiciently adherent so that the wire could be wrapped on a mandrel of its own diameter without cracking or peeling of the coating. The coating moreover was quite uniform in thickness and exhibited smooth commercially acceptable surface qualities. The wire could be drawn successfully to smaller sizes without damaging the coating. The coating on the wire, as drawn, while thinner than when applied at larger diameter, was reasonably uniform, adherent and ductile and, therefore, commercially useful. An identical wire coated in the same manner but cooled in air, without the rapid cooling attendant with the use of the water mist, had undesirable coating characteristics in that the coating was more irregular and rough as well as less adherent and more brittle. Moreover, the coating on this wire, after drawing to smaller size, was found to be non-adherent, brittle and, therefore, commercially unsatisfactory.

Although the invention has been particularly described above with reference to the coating of wires, it is of course apparent that the method of the invention is equally applicable to coating other forms of ferrous metal articles such as rod, sheet, strip, and other articles.

, It is also apparent that the apparatus described above can be modified, without departing from the invention. For example, the spray nozzles 17, 18 instead of being all arranged on one side of the wire and directed against a single splashboard, can be mounted so as to direct the sprays toward different faces of the article being coated. In particular, in coating sheet and strip, it is desirable to have sprays directed against each side of the article as it emerges from the bath of molten aluminum. Likewise, although the method of the invention has been particularly described with reference to a continuous coating operation, it is equally applicable to batch coating operations in which articles are dipped below the surface of the molten aluminum bath, and then, as they are withdrawn from the coating bath and while the coating thereon is still fluid, are brought into direct contact with the liquid coolant.

I claim:

1. In a method for coating a ferrous metal shape with 9 aluminum "by immersing the shape in a bath of molten aluminum and then withdrawing the shape from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises introducing the coated shape immediately upon withdrawal from the aluminum bath and while the aluminum coating thereon is still fluid into direct contact with a dense mist of liquid coolant, said mist comprising a gaseous suspension of very finely divided coolant droplets, the particle size of said droplets being sufficiently small so that settlement thereof from suspension is negligibly slow and the velocity with which said drop-lets impinge on said shape being sufficiently low so that substantially no deformation of the aluminum coating thereon while it is still fluid is caused thereby and the density of said mist being sulficient to cool an aluminum coated wire 0.1 inch in diameter from 1250 F. to below 900 F. in less than fifteen seconds, whereby said coating is quenched and congealed before flow of the molten coating metal can result in a coating having substantial irregularities in thickness and surface smoothness.

2. In a method for coating a ferrous metal shape with aluminum by immersing the shape in a bath of molten aluminum and then withdrawing the shape from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises bringing the coated shape promptly after it emerges from the aluminum bath and while the coating thereon is still in the fluid condition into direct contact with a dense mist of liquid coolant, said mist comprising a gaseous suspension of very finely divided coolant droplets, the particle size of said droplets being sufiiciently small so that settlement thereof rom suspension is negligibly slow and the velocity with which said droplets impinge on said shape being sumciently low so that substantially no deformation of the aluminum coating thereon while it is still fluid is caused thereby, the density of said mist being sutficient to cool said coated article to below l000 F. in less than one-half minute, and maintaining the coated shape in said mist until the temperature of said shape has been reduced to below 1000 F, whereby said coating is quenched and congealed before flow of the molten coating metal can lead to irregularities in the thickness and surface smoothness of the coating and whereby growth of iron-aluminum alloy by interdiifusion of the coating and the ferrous metal is minimized.

3. In a method for coating a ferrous metal shape with aluminum by continuously passing the shape into a bath of molten aluminum and continuously withdrawing the shape from the bath with a coating of molten aluminum adheringthereto, the improvement which comprises bringing the coated shape promptly after it emerges from the aluminum bath and while the coating thereon is still in the fluid condition into direct contact with a dense mist of liquid water, said mist comprising a suspension in air of water droplets of sufficiently small particle size so that settlement thereof from suspension does not occur to any substantial extent, and the velocity with which said droplets impinge on said shape being sufficiently low so that substantially no deformation of the aluminum coating prior to solidifying is caused thereby, the density of said mist being sufiicient to cool said coated article to below 900 F. in less than one-half minute, and maintaining the coated shape in said mist until its temperature has been reduced to below 900 F., whereby said coating is quenched and congealed before flow of the molten coating metal can lead to irregularities in the thickness and surface smoothness of the coating and whereby growth of ironaluminum alloy by interdiifusion of the coating and the ferrous metal is minimized.

4. In a method for continuously coating ferrous metal wire, rod, sheet, strip, and other articles with aluminum by continuously passing such article through a bath of molten aluminum and continuously withdrawing such article from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises delivering a dense mist of a liquid coolant into direct contact with the continuously moving article immediately upon its emergence from the bath of molten aluminum and while the aluminum coating thereon is still fluid, said mist comprising a gaseous suspension of very finely divided coolant droplets, the particle size of said droplets being sufliciently small so that settlement thereof from suspension is negligibly slow and the velocity with which said droplets impinge on said shape being suificiently low so that substantially no deformation of the aluminum coating thereon while it is still fluid is caused thereby, and the'density of said mist being suflicient to cool an aluminum coated iron wire 0.1 inch in diameter from 1250 F. to below 900 F. in less than fifteen seconds, whereby said coating is quenched and congealed before flow of the molten coating metal on the continuously moving ferrous article can impart substantial irregularities to the thickness and surface smoothness of the coating.

5. In a method for continuously coating ferrous metal wire, rod, sheet, strip, and other articles with aluminum by continuously passing such article through a bath of molten aluminum and continuously withdrawing such article from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises directing a fine mist of liquid water into contact with the continuously moving article as it'emerges from the bath of molten aluminum and while the aluminum coating thereon is still at a temperature above 1200 F., said mist comprising a suspension in air of water droplets of sufliciently small particle size so that settlement thereof from suspension does not occur to any substantial extent, the velocity with which said droplets impinge on said shape being sufliciently low so that substantially no deformation of the aluminum coating prior to solidifying is caused thereby and the density of said mist being sufficient to cool said coated article to below 900 F. in less than one-half minute, and retaining the article in said mist until the temperature of the article and of the coating has been reduced to below 900 F., whereby said coating is quenched and congealed before flow of the coating metal on the continuously moving ferrous article can impart substantial irregularities to the thickness and surface smoothness of the coating, and whereby growth of iron-aluminum alloy by interdilfusion of the coating and the ferrous metal is minimized.

6. In a method for continuously coating ferrous metal wire, rod, sheet, strip, and other articles with aluminum by continuously passing such article through a bath of molten aluminum and continuously withdrawing such article from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises continuously withdrawing the article upwardly from the molten aluminum bath in a substantially vertical direction, and spraying a fine mist of a liquid coolant on to the upwardly moving article promptly after its emergence from said bath and while the coating thereon is still fluid, said mist comprising a gaseous suspension of liquid coolant droplets in which the particle size of the droplets is sufficiently small so that they tend to settle from suspension only very slowly, and said mist being directed into contact with the article in the form of a wide-angle spray the velocity of which as it impinges on the upwardly moving article is sufliciently low so that substantially no deformation of the aluminum coating is caused thereby, and the density of said mist being sufiicient to cool an aluminum coated iron wire 0.1 inch in diameter from 1250 F. to below 900 F. in less than fifteen seconds, whereby said coating is quenched and congealed before flow of the molten coating metal on the continuously moving ferrous article can impart substantial irregularities to the thickness and surface smoothness of the coating.

7. In a method for continuously coating ferrous metal wire, rod, sheet, strip, and other articles with aluminum by continuously passing such article through a bath of molten aluminum and continuously withdrawing such article from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises continuously withdrawing the ferrous article upwardly from the aluminum bath in a substantially vertical direction, spraying a fine mist of water droplets on to the upwardly moving article promptly after its emergence from said bath and while the coating thereon is still fluid, said mist comprising an air suspension of liquid water droplets of sufiiciently small particle size so that settlement thereof does not occur to any substantial extent, said mist being delivered into contact with the article in the form of a wide-angle spray of such low velocity where it impinges on the upwardly moving article that substantially no deformation of the aluminum coating is caused thereby, the density of said mist being sufiicient to cool said coated article to below 900 F. in less than one-half minute, and maintaining the advancing article in the water mist spray until the temperature of the article and of the coating has been reduced to below 900 E, whereby said coating is quenched and congealed before flow of the coating metal on the continuously moving ferrous article can impart substantial irregularities to the thickness and surface smoothness of the coating, and whereby growth of iron-aluminum alloy by interdiffusion of the coating and the ferrous metal is minimized.

8. In a method for continuously coating ferrous metal, wire, rod, sheet, strip and other articles with aluminum by continuously passing such article through a bath of molten aluminum and continuously withdrawing such article from the bath with a coating of molten aluminum adhering thereto, the improvement which comprises delivering a dense mist of a liquid coolant into direct contact with the continuously moving article immediately upon its emergence from the bath of molten aluminum and while the aluminum coating thereon is still fluid, said mist comprising a gaseous suspension of very finely divided coolant droplets, the particle size of said droplets being sufliciently small so that settlement thereof from suspension is negligibly slow and the velocity with which said droplets impinge on said shape being sufiiciently low so that substantially no deformation of the aluminum coating thereon While it is still fluid'is caused thereby, said mist comprising a plurality of sprays from a plurality of point sources so spaced that there is substantially no overlap of spray delivered from one point source with the spray delivered from an adjacent source at the point where said sprays meet the path of travel of said continuously moving article, the density of said mist being sufficient to cool said coated article to below 1000 F. in less than one-half minute, and maintaining the coated article in said mist until the temperature thereof has been reduced to below 1000 F., whereby said coating is quenched and congealed before the flow of the molten coating metal on the continuously moving ferrous article can impart substantial irregularities to the thickness and surface smoothness of the coating and whereby growth of iron-aluminum alloy by interdilfusion of the coating and the ferrous metal is minimized.

9. Apparatus for coating ferrous metal wire, rod, sheet, strip, and other articles with aluminum, comprising a vessel for containing a bath of molten aluminum, means for continuously passing such article through said bath and for withdrawing the article upwardly from the molten aluminum bath through a substantially vertical path of travel, an array of atomizing spray nozzles arranged in vertically spaced relation and all directed at said path of travel, the lowermost of said nozzles being directed at said path of travel in the region thereof immediately above said vessel and the uppermost of said nozzles being directed at said path of travel at a substantial distance above said vessel and the intermediate nozzles being spaced so that substantially the entire length of said path of travel between the lowermost and uppermost nozzles is covered by spray therefrom, means for delivering a liquid coolant to said spray nozzles, and means connecting said nozzles with a source of atomizing gas under pressure, whereby said coolant may be discharged from said nozzles in the form of a mist of particles so small that settlement thereof from gaseous suspension is negligibly slow.

10. Apparatus for coating ferrous metal wire, rod, sheet, strip, and other articles with aluminum, comprising a vessel for containing a bath of molten aluminum, means for continuously passing such article through said bath and for withdrawing the article upwardly from the molten aluminum bath through a substantially vertical path of travel, an array of atomizing spray nozzles arranged in vertically spaced relation and all directed at said path of travel, the lowermost of said nozzles being directed at said path of travel in the region thereof immediately above said vessel and the uppermost of said nozzles being directed at said path of travel at a substantial distance above said vessel and the intermediate noz zles being spaced so that substantially the entire length of said path of travel between the lowermost and uppermost nozzles is covered by spray therefrom, means for delivering a liquid coolant to said spray nozzles, and means connecting said nozzles With a source of atomizing gas under pressure, whereby said coolant may be discharged from said nozzles in the form of a mist of particles so small that settlement thereof from gaseous suspension is negligibly slow, said nozzles being so arranged that there is substantially no overlap of the spray delivered from one nozzle with the spray delivered from an adjacent nozzle where said sprays meet the path of travel of said article.

References Cited in the file of this patent UNITED STATES PATENTS 1,863,809 Hapkins June 21, 1932 2,034,348 Lytle Mar. 17, 1936 2,069,658 Renkin- Feb. 2, 1937 2,126,244 Cook Aug. 9, 1938 2,166,510 Whitfield et a1. July 18, 1939 2,243,979 Reynolds June 3, 1941 2,569,097 Grange Sept. 25, 1951

Claims

1. IN A METHOD FOR COATING A FERROUS METAL SHAPE WITH ALUMINUM BY IMMERSING THE SHAPE IN A BATH OF MOLTEN ALUMINUM AND THEN WITHDRAWING THE SHAPE FROM THE BATH WITH A COATING MOLTEN ALUMINUM ADHERING THERETO, THE IMPROVEMENT WHICH COMPRISES INTRODUCING THE COATED SHAPE IMMEDIATELY UPON WITHDRAWAL FROM THE ALUMINUM BATH AND WHILE THE ALUMINUM COATING THEREON IS STILL FLUID INTO DIRECT CONTACT WITH A DENSE MIST OF LIQUID COOLANT, SAID MIST COMPRISING A GASEOUS SUSPENSION OF VERY FINELY DIVIDED COOLANT DROPLETS, THE PARTICLE SIZE OF SAID DROPLETS BEING SUFFICIENTLY SMALL SO THAT SETTLEMENT