US2926103A - Aluminum cladding process and apparatus - Google Patents

Description

Feb. 23, 1960 R. M. BRICK ALUMINUM CLADDING PROCESS AND APPARATUS 2 Sheets-Sheet 1 Filed Jan. 21, 1958 wuszs iu/ :Ezzi:

0m |n .[l mm INVENTOR BY 9% 1W ATTORNEYS Feb. 23, 1960 R. M. BRICK- ALUMINUM CLADDING PROCESS AND APPARATUS Filed Jan. 21, 1958 2 SheetsSheet 2 (D In INVENTOR IE8;- 7n, B

ATTORNEYS United States Patent Can Company, Inc., New York, N.Y., a corporation of New York Application January 21, 1958, Serial No. 710,292

16 Claims. (Cl. 117-51) This invention relates to the coating of base metal with aluminum by treatment with the molten metal, and is particularly concerned with the finishing of the coated material.

Proposals are known, by which a steel strip is coated with aluminum by passing it continuously through molten aluminum. One form of this is to run the steel strip through a molten high-gravity salt bath at about 1400 degrees F., with a layer of molten aluminum floating on the bath surface, and to bring the strip through the molten aluminum. The salt bath operates to remove surface oxide from the strip so that clean surfaces are presented to the molten aluminum, and both sides of the strip are coated as it emerges from the molten material. There is a wide variation in the thickness of the aluminum coatings.

Another way is to pass the heated strip through a chamber having a hydrogen atmosphere, by which the surface oxides are reduced; then into a bath of molten aluminum; thereafter between a pair of steel wiping ro'lls intended to remove excess of the aluminum coating; and finally through a cooling region. The steel wiping rolls have to be maintained hot in order to keep them wettable by the liquid aluminum, e.g. by positioning them in the bath itself; and hence thin aluminum coatings are not obtained, but instead the coatings are of the order of 2 to mils. In addition, the steel rolls deteriorate when kept at 1200 to 1300 degrees F. (650 to 700 degrees C.) by the attack of aluminum upon the steel and the continual growth of an iron-aluminum alloy layer thereon; and a like difliculty exists when the roll surfaces are of most other metals, due to dissolution of the roll metal into the molten aluminum and the growth of the alloy layer.

Another difliculty encountered in such operations is that aluminum acts upon the base surface, e.g. of a steel sheet, to form an iron-aluminum intermetallic compound which is hard and brittle; and the extended time of contact with molten aluminum causes the formation of a thick intermediate layer of this type, which breaks during fabrication of the coated sheet, with resultant loss of adhesion of the coating at sharp bends and production of an abrasive dust which causes excessive wear of fabricating tools.

According to the present invention, the wiping or applying is accomplished by devices having surfaces wettable by the molten aluminum, with positioning of these devices outside of a bath or in a bath, for transferring from the bath to a metal base strip and eifective upon the coated base metal before the aluminum has congealed, with a rapid cooling of the material immediately following the application of the coating, sothat coating thicknesses of 0.2 to 0.5 mil may be applied as continuous adherent films with a minimum thickness of the intervening aluminum alloy layer.

Illustrative forms of practice of this invention are shown on the accompanying drawings, in which:

Fig. 1 shows an upright section of a conventionalized apparatus in which a thin coating of aluminum is proice 2 vided upon the surfaces of a continuously moving strip of steel.

Fig. 2 is a conventionalized apparatus for preparing the wiping devices.

Fig. 3 is a transverse section, on, a larger scale, of a wiping roller shown in Fig. 1.

V Fig. 4 is an upright section of a second illustrative em bodiment. v

For producing an adherent coating, it is requisite that the base, e.g. steel plate, be absolutely clean and that its surface be at a temperature closely approximating the melting point of aluminum, ie in thevicinity of 1220 degrees F. (660 degrees C.).

In Fig. 1, a roll 10 of a steel strip delivers the strip S into a chamber 11 having a hydrogenatmosphere. This atmosphere is provided by gas admitted by a valved conduit 11a. This chamber has heating means such as the induction coils 12 for raising the strip to a temperature, e.g. 1000 to 1200 degrees F. (540 to 650 degrees C.), at which the hydrogen is effective to reduce surface o'xides. The reaction temperature is selected to assure a proper reaction rate for the specific stock and removal of the oxide present. Near its exit from thechamber 11,'the strip is heated further, e.g. by induction coils 13, to about 1200-1350 degrees F., and then enters the bath chamber 14- which has a bath 15 of molten aluminum in its lower po'rtion; this bath being maintained at a temperature of 1200 to 1350 degrees F. by suitable means such as the gas flames provided from a burner 16. The gas can pass from the chamber 11 into chamber 14- along with the strip.

The strip passes over a guide roll 17 and thence downward into the bath, and upward again after passing the immersed guide roll 18. After emerging from the bath, the aluminum coating is still in molten form, and in part drains off and in part is wiped away by the driven rolls 19 which are mounted and held apart a distance determined by the thickness of the aluminum coatings. Spring loaded wiper bars 19a are in contact with the rolls 19 and are effective to prevent accumulation of stripped metalon these rolls, and return the material to the bath. The

rolls 19 are presented closely above the surfaceof the bath so that they engage the coated strip quickly after it leaves the bath; and this distance is maintained by conof driven chilling rollers 21 which can be hollow and of copper for good heat conduction.

metal quickly to a temperature below 1000 degrees F.

' (540 degrees C.) for restricting undesirablegrowth of In the illustrative form, the rollers 21 are enclosed in' an exit chamber 22, one of whose Walls is provided by the. bafile 20; and one roller 21 serves as a guide roll low the melting point of aluminum.

The wiping rollers 19 have surfaces wettable by molten aluminum, and are kept at the temperature of the bath chamber 14, that is, atabove the melting point of the aluminum. .Since steel surfaces would not remain wettable, these rollers are made with surfaces of wet table aluminum carbide. Thus a hollow body of graphitic carbon is prepared and formed with a smooth cylindrical surface; the body is then heated to about 2500 degrees F. (1370 degrees C.) or above in an atmosphere of non-oxidizing nature, such as hydrogen or inert, gas,

The surfaces of the rollers 21 are for example chromium-plated and polished,

while supplying aluminum thereto. This may be done as shown in Fig. 2, by immersing the body in molten aluminum for a pre-determined time dependent upon the selected temperature and the desired thickness of aluminum compound deposit, and then removing the coated body, draining, and permitting it to cool. The coated roll is then ground to a smooth cylindrical contour. The molten aluminum 25 is in a bath vessel 26 which is heated by a burner 27, and has an apertured bathe 28 through which the body can be lowered by a cable 29; after coating, the body is withdrawn through the bafiie 28 for primary cooling before the vessel cover 30 is removed. An inert atmosphere can be supplied through the valved conduit 31.

An assembled roller 19 is shown in Fig. 3, with its central driving shaft 35, which has been inserted in the central hole, the body 36 of carbon, and the surface layer 37 of aluminum carbide, noting that this surface layer 37 is not accurately definable, being aluminum carbide at the exposed surface, with introfication of aluminum into the structure along the carbon grains and conversion thereat to carbide form. The carbon body may be made of the material employed for carbon electrodes for electric furnaces; and small amounts of the normal binders cause no operational difiiculties.

The rolls 19 alternately may be coated with refractory compounds which are stable and wettable at the operating temperature, as for example refractory silicides and borides of metals such as chromium, tungsten and titanium.

The procedure is also applicable to the roll coating of sheets. In Fig. 4, a steel strip S is passed between rolls 40 which are individually in contact with supply rolls 41. These supply rolls 41 are partly immersed in baths 42 of molten aluminum which may be held at a temperature of 1250 degrees F. (675 degrees C.) by the electrical resistance heaters 43. The rolls 40, 41 have wettable surfaces. The rolls 40, 41 are driven in the direction shown by the arrows, so that rolls 41 take up molten aluminum for the baths 42. Wiper bars 44 are provided, to reduce and regularize the films of aluminum on the supply rolls; and this regularization is promoted by maintaining the axes of the pairs of coopcrating rolls 40, 41 so that only a thin film, of the desired thickness, remains on the rolls 40 as they turn and deliver aluminum from the film to the surfaces of the strip S. A housing 45 encloses the rolls 40, 41, the baths 42, and the wiper bars 44. As the coated strip CS leaves the rolls 40, it is engaged by the cooled chilling and smoothing rolls 46, likewise within the housing '45, and then passes through chamber sealing means such as the rollers 47. Wiper bars 46a may be employed with the rolls 46.

In practice, the strip S can be drawn from a supply 48, advanced into the chamber housing 45 between inlet sealing rollers 49, passed between the coating rolls 40 and the chilling rolls 46 and then taken up upon the driven rewinding device 50. The interior of the chamber housing has a non-oxidizing atmosphere of an inert gas or a reducing gas such as hydrogen; preferably at a pressure slightly above atmospheric: which is supplied through a conduit 51 under control of a valve 52. The strip S can be heated to a temperature of about 1200 degrees F. (650 degrees C.) by the heaters 53 if a reducing atmosphere is prescntin the chamber, it then serves to assist in rendering the surfaces of the strip S clean and ready for the coating.

The rolls 40, 41 have surfaces of the wettable materials set out above; and therewith maintain their dimensions and surface conditions. The chilling or quenching rolls 46 may here also be madewith a plating of chromium or like metal which has a melting point above the operating temperature; and'which is not wetted by the coating metal at such temperature.

' It is also feasible to effect the rapid chilling either by passing the coated strip CS into a cooling liquid or by use of such cold liquid bath after the action of chilling rolls. Thus in Fig. 4, the strip CS upon leaving the chilling rolls 46 passes around a roller 54 and downward into a water bath 55, moves around the roller 57 and moves upward to the take-up device 50. This water bath 55 is contained in a closed auxiliary housing 56 having a depending bafiie wall 58 so that the bath 55 acts as an outlet seal.

7 It is obvious that the illustrated form is not restrictive, and that the invention can be practiced in many ways within the scope of the following claims.

I claim:

1. The method of coating base metal with aluminum which comprises preheating the base metal to a temperature of about 1000 to 1200 degrees F., applying molten aluminum in excess thickness to a surface of the base metal, wiping the coated metal by a device having a contacting surface of refractory material essentially insoluble in and wettable by the aluminum at the operating temperature, said material being selected from the group consisting of aluminum carbide and the silicides and borides of chromium, tungsten, and titanium, and immediately chilling the coated metal and thereby solidifying the aluminum.

2..The method of coating steel strip with aluminum which comprises passing the strip through a mass of molten aluminum, withdrawing the strip from the molten mass and wiping its sides While the aluminum is molten by rollers having surfaces of aluminum carbide, removing the wiped-off aluminum from the wiping rollers and immediately chilling the coated strip and thereby soliditying the aluminum.

3. The method of coating a steel strip with aluminum which comprises passing the strip through a bath of molten aluminum, withdrawing the strip from the bath into an inert atmosphere, wiping the sides of the coated strip by rollers having surfaces of aluminum carbide while the aluminum is molten, and immediately after the wiping contacting the sides with polished chilling rollers effective for smoothing and solidifying the aluminum coating, said chilling rollers having contactive surfaces of refractory material not wetted by the molten aluminum.

4. The method of coating steel with aluminum, which comprises passing the steel in contact with a roll having a contacting surface of refractory material essentially insoluble in and wettable by the aluminum at the operating temperature and thereby applying molten aluminum to the steel, said material being selected from the group consisting of aluminum carbide and the silicides and borides of chromium, tungsten and titanium, Wiping excess molten aluminum from the steel by a roller of material selected from the said group, and immediately chilling the coated steel and thereby solidifying the aluminum.

5. The method as in claim 4, in which the contacting surface is of aluminum carbide.

6. The method as in claim 4, in which the applying and chilling are accomplished in successive chambers closed from the atmosphere, the chilling being accomplished by contacting the molten metal with a roller having a surface of a refractory material which is not wetted by molten aluminum at the operating temperature.

7. An apparatus for coating a base metal strip with aluminum comprising a chamber containing a bath of molten aluminum, means for supplying and guiding the strip so that it passes through the molten aluminum and is withdrawn therefrom, rollers having surfaces of aluminum carbide and positioned for engaging and wiping the coated strip after it has emerged from the bath, and means for cooling and effecting solidification of the molten aluminum after the coated strip has left the wiping rollers.

8. An apparatus as in claim 7, in which the cooling means comprises cooling rollers between which the coated strip is passed immediately after leaving the wiping rollers.

9. An apparatus for coating a base metal with aluminum comprising means for supplying molten aluminum and for applying the same to the base metal, said means including a roll having a surface provided by a refractory material selected from the group consisting of aluminum carbide and the silicides and borides of chromium, tungsten and titanium, and cooling means eifecti-ve to reduce the temperature of the aluminum to below its melting point and being located close to the said applying roll.

10. An apparatus for coating a base metal with aluminum comprising means for supplying molten aluminum, and for applying the same to the base metal, said means including a roll having a surface provided by a refractory material selected from the group consisting of aluminum carbide and the silicides and borides of chromium, tungsten and titanium, and cooling means including a chilling roll having a contacting surface of refractoiy material which is not wettable by the aluminum at the operating temperature, said cooling means being elfective to reduce the temperature of the aluminum to below its melting point and being located close to the said applying roll.

11. An apparatus as in claim 9, in which the cooling means includes a chilling liquid bath into which the coated metal is immersed.

12. An apparatus as in claim 11, in which a housing encloses the applying means and the cooling means, a baflle is provided at the chilling liquid bath, and means to guide the base metal into and out of the bath and beneath the baffle whereby the bath provides a liquid seal at the outlet of the housing.

13. The method of coating base metal with aluminum which compnses applying molten aluminum in excess thickness to a surface of the base metal, wiping the coated metal while the aluminum is still molten by a device having a contacting surface of refractory material essentially insoluble in and wettable by the aluminum at the operating temperature, said material being selected from the group consisting of aluminum carbide and the silicides and borides of chromium, tungsten and titanium, and immediately chilling the coated metal and thereby solidifying the aluminum prior to the establishment of an iron-aluminum alloy layer of significant thickness between the base metal and its coating.

14. The method as in claim 13, in which the chilling is effected by contacting the coating metal with an element of refractory material which is insoluble in and not wetted by the molten coating metal and thereby reducing the temperature of the base metal and coating below 1000 degrees F.

15. The method as in claim 13, in which the applying and wiping is performed in a first and hot chamber having a bath of molten aluminum therein, and in which the chilling is effected in a separate second chamber having atmosphere-excluding communication with the said first chamber and itself containing a bath of cooling liquid, the coated metal being immersed in the bath during its passage, and in which the bath is employed for sealing the second chamber from the atmosphere.

16. The method of coating base metal with aluminum which comprises applying molten aluminum in excess thickness to a surface of the base metal, wiping the coated metal with a roller having a coating of aluminum carbide while the aluminum is molten, removing the wiped-01f aluminum from the wiping roller, and immediately chilling the coated metal and thereby solidifying the aluminum.

References Cited in the file of this patent UNITED STATES PATENTS 2,465,161 Little Mar. 22, 1949 2,592,282 Hodil Apr. 8, 1952 2,817,312 Westby Dec. 24, 1957

Claims (1)

1. THE METHOD OF COATING BASE METAL WITH ALUMINUM WHICH COMPRISES PREHEATING THE BASE METAL TO A TEMPERATURE OF ABOUT 1000 TO 1200 DEGREES F., APPLYING MOLTEN ALUMINUM IN EXCESS THICKNESS TO A SURFACE OF THE BASE METAL, WIPING THE COATED METAL BY A DEVICE HAVING A CONTACTING SURFACE OF REFRACTORY MATERIAL ESSENTIALLY INSOLUBLE IN AND WETTABLE BY THE ALUMINUM AT THE OPERATING TEMPERATURE, SAID MATERIAL BEING SELECTED FROM THE GROUP CONSISTING OF ALUMINUM CARBIDE AND THE SILICIDES AND BORIDES OF CHROMIUM, TUNGSTEN, AND TITANIUM, AND IMMEDIATELY CHILLING THE COATED METAL AND THEREBY SOLIDIFYING THE ALUMINUM.

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