US3069768A - Method of making coated tubing - Google Patents

Description

Dec. 25, 1962 l. CAPLAN ETAL METHOD OF MAKING COATED TUBING Filed June 16, 1958 3,069,768 METHOB @F MAKEN@ CATED TUBING Isadore Caplan, Gerard T. Rutlin, and Donald K. Van

Zile, Rochester, NX., assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed .lune 1.6, 1958, Ser. No. 742,195 6 Ciaims. (Cl. 29-527) This invention relates to the manufacture of brazed steel tubing. A primary object of the invention is to provide a method of making brazed steel tubing while simultaneously coating the inner surface thereof with a permanent coating of a non-ferrous alloy which is securely bonded to the surface of the tubing.

This invention comprehends making brazed steel tubing from a flat strip of steel which has been previously coated with a suitable nonferrous metal, such as copper. The metal coated steel strip is continuously moved longitudinally through a tubeforming mill which transversely bends it into a tubular configuration. A non-ferrous metal, such as zinc, is progressively introduced into the tubing during its formation. Prior to the complete formation of the tubing and about at the same point on the tube-forming mill where the non-ferrous metal is introduced into the tubing, a small diameter pipe which discharges a non oxidizing gas enters the tubing and extends longitudinally therethrough. After the non-ferrous metal is introduced into the interior of the tubing and the tubing is completely formed, it passes through a sizing device which imparts the desired finished cross sectional configuration to the tubing. After the tubing has been sized it is heated to a suitable temperature to vaporize the metal which was deposited Within the tubing and melt the previously applied metal coating. The vaporized metal alloys with the previously applied coating to form a uniform alloy coating on the inner surface of the tubing. Longitudinally abutting edges on the formed tubing form an open seam which is brazed at this time by migration of the alloying metals. The hot tubing is then passed through a cooling device which causes the alloying metals to solidify. Thus, the tubing is simultaneously brazed and given a permanent coating of an alloy which is securely bonded to the steel.

Further objects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof and from the drawing, in which:

FIGURE l is a diagrammatic View showing a tube-forming mill provided with suitable apparatus for carrying out the present invention;

FIGURE 2 is an enlarged sectional view along the line 2 2 of FIGURE l;

FIGURE 3 is another enlarged sectional view along the line 3-3 of FIGURE l;

FIGURE 4 is a similar view along the line 4-4 of FIGURE 1; and

FIGURE 5 is another enlarged sectional View along the line 5 5' of FIGURE 1.

As shown in FIGURE 1, the tubing T is formed from a iiat steel strip S which has been previously coated with a suitable non-ferrous metal, such as copper. The coated steel strip is coiled on a reel which is positioned at one end of a tube-forming mill 12. The tube-forming mill 12 is composed of a group of forming devices each of which includes a pair of rolls 14. A suicient number of forming devices are included in the forming mill to impart the desired circular cross sectional configuration to the flat steel strip S. These rolls progressively form the flat steel strip S into a tube T having an open seam 16 at the top where edges 18 and 20 of the steel strip S abut. The formed tube T then passes between suitable sizing rolls 22 which further impart the desired cross sectional connite States Patent O ICC figuration to the tubing and then through a swaging device 24 for a final formation operation.

A non-ferrous metal is introduced into the interior of the tubing prior to its complete formation. As shown more clearly in FIGURE 2, a small diameter pipe 26 for conveying a non-oxidizing gas may also be introduced into the interior of the tubing at this point.

From the swaging device 24 the formed tubing T passes into a brazing furnace 28 where the non-ferrous metal which was deposited within the tubing is vaporized. Simultaneously the non-ferrous metal which was previously applied to the steel strip S is melted. The vaporized metal combines or alloys with the molten metal within the hot tubing. The molten vapor metal mixture permeates the inner wall 30 of the tubing and migrates between the open longitudinal seam 16. The tubing then passes through a cooling chamber 32 which solidiiies the alloying metal to deposit an adherent alloy coating on the inner surface of the tubing and effect a brazing of the open seam 16. A protective atmosphere is preferably maintained Within the furnace 28 and the cooling chamber 32 to prevent oxidation of the outer surface 34 of the tubing T and insure against oxidation within the tubing. Upon cooling, the coating metals solidify on the inner surface 30 of the tubing and they can then be cut into Whatever lengths are desired. lf cut in double lengths the tubing is rolled on large rollers but it can be cut in short lengths and stored in straight pieces.

The particular manner in which the steel strip is coated prior to the forming operation forms no part of this invention. This coating 31 can be applied in any convenient manner, as by means of electrodeposition, immersion in molten metal baths, or any of the other methods of coating which are Well konwn in the art. The preferred method of application, of course, is primarily dependent upon the nature of the metal being applied. It may even be desirable to coat the steel strip immediately prior to the tubeforming operatoin. In such instance, it may be advantageous to pass the steel strip initially through a coating apparatus and then directly into the tube-forming mill 12. The thickness of the coating applied will vary with the particular application of the invention but, in general, coating thicknesses of 0.001 inch to 0.003 inch can be used.

In other circumstances it may be necessary to clean the previously coated strip prior to formation of the tubing. The coated strip can lbe degreased in the usual way, as by means of suitable solvents such as trichloroethylene. After degreasing the coated steel strip, if it is severely rusted, it can be pickled in a suitable acid solution in the known and accepted manner for such cleaning.

Our invention is successfully practiced by introducing a suitable non-ferrous metal, such as zinc, into the interior of the hollow annular cylindrical tubing in any convenient form. Our invention is especially more conveniently practiced if this metal is introduced into the tubing as a wire 36, as shown in FIGURES 2, 3 and 4. Generally, it is preferred to introduce the coating metal into the tubing immediately before it is fully formed. A strand 38 of Wire 36 from a coiled roll 40 mounted on the tube-forming mill 12 is fed through a directing tube 42 into the interior of the tubing by means of a pair of driving rolls 44. The specific construction of this mechanism is not a part of this invention and any suitable device can be used to introduce the coating metal at a substantially uniform rate. The :particular rate at which the metal is introduced is variable to some eXent and is governed by the size and speed of movement of the tubing being formed.

As shown in United States Patent No. 2,771,669, filed in the names of I. W. Armstrong, R. W. Spears and R. D. Williams and which is assigned to the assignee of the present invention, particles of metal, such as zinc, can also be introduced uniformly into the interior of the tubing employing a suitable hopper or funnel having a narrow outlet which extends into the tubing prior to the complete formation thereof.

Obviously, in order to provide a uniform coating the amount of Zinc or other metal which is introduced into the interior of the tubing must vary in direct proportion with `both the size and speed of movement of the tubing T which is to be alloy coated. Moreover, coating cornpositions of varying proportions can be produced by varying the amount of coating material introduced into the interior of the tubing. In general, highly satisfactory results are obtained if the coating metal is introduced as a wire which moves at the same speed as the tubing. Satisfactory brass coatings are obtained when using tubing which was formed from a steel strip having a copper coating of about 0.002 inch in thickness. To produce a uniform brass coating at this `sped on 1A inch tubing, it is desirable to use a Zinc wire having a diameter of 0.03 inch. Similarly, 5/s inch tubing can be effectively brass coated using 0.04 inch diameter zinc wire moving at the `same rate of speed as that of the tubing.

A non-oxidizing gas from a lsuitable reservoir (not shown) passes through a small diameter pipe 26 which passes into the tubing T at approximately that point on a tube-forming mill where coated metal is deposited within the tubing. The small diameter pipe 26 has a part 46 which extends axially within the interior of the tubing T towards the swaging device 2d to a point where the `tubing is completely formed. The discharge end of the pipe is located at a point where the tubing is completely formed so that the reducing atmosphere emitted therefrom will be substantially contained within the interior of the tubing. Thus, a protective atmosphere is established within the tubing as it moves along to the various following operations. It is also desirable to employ a protective atmosphere outside the tubing in the operation following tube formation. Heating the tubing in an oxidizing atmosphere may cause a deleterious corrosion and spalling of the outer surface. Thus, as shown in FIGURE l, the non-oxidizing gas can also be introduced into the brazing furnace 28 and cooling chamber 32 by means of additional piping 4S from the source of supply (not shown).

After the tubing passes through the swaging device 24 it progresses through a brazing furnace 2h where it is heated above ythe vaporization temperature of the metal deposited within the tubing during its formation. The vaporization of this metal permits rapid alloying with the previously applied metal coating to produce an exceedingly uniform tenacious alloy coating. Accordingly, when the metal deposited within the tubing is zinc, it is preferred to heat the tubing to a temperature above about 1665 F., the vaporization temperature of Zinc. It has `been found that satisfactory brass coatings are attained when the tubing is heated to a temperature between 1700 F. and 2200 F. to vaporize the Zinc. In this temperature range the steel tubing is not only satisfactorily brass coated but simultaneously annealed. At these temperatures the Zinc is converted into zinc vapor and the copper coating on the inner surface of the tubing is reduced to the molten state. Molten copper alloys with the vaporized zinc and both migrate between abutting edges of the formed tubing to braze the open seam 16. Thus the heating operation not only effects a combination of the two coating metals to produce a corrosion-resistant alloy but it also effects a brazing of the open seam of the tubing.

From the brazing furnace the tubing passes through a cooling chamber 32 which serves to solidify the alloying metals. Continuously circulated cooling water in a jacket (not shown) which surrounds the tubing is one effective means by which the alloying metals can be rapidly converted to the sol-id state. It has been observed that the alloy coating formed by this invention 4 has an insignificant difference in thickness at the top and Ibottom of the tubing with the thickness at the bottom only negligibly greater. When the tubing is cooled, the alloy coating solidies so rapidly that there is substantially no tendency for the coating metal to run toward the bottom of the tubing.

It is especially desirable to heat the tubing to the brazing temperature in a protective atmosphere to avoid eX- cessive scaling, spalling or corrosion of the outer surface of the steel tubing. Accordingly, it is desirable to maintain such a protective atmosphere not only in the interior of the tubing but also in the brazing furnace and the cooling chamber. A gas which protects the interior and exterior of the tubing is preferably 20% to 25% reducing 4in nature. For example, highly satisfactory results are obtained with a gas which has the following analysis: 10% carbon monoxide, 18% hydrogen, 41/2 carbon dioxide, 1% methane and the balance nitrogen, all proportions by volume. However, substantially pure mixtures of hydrogen, carbon monoxide, nitrogen, helium, argon, etc. can also be used.

Steel tubing having alloy coatings of various compositions can be formed by this invention when the metal deposited within the tubing, such as zinc, has a vaporization temperature below the melting point of steel. The previously applied alloying metal, of course, must have a lower melting point than that of steel and be metallurgically compatible with the other lalloying metal. By metallurgically compatible we refer to metals which are mutually soluble in the liquid state and which readily alloy with one another at elevated temperatures, such as copper and Zinc, and coper base and zinc base alloys. Accordingly, the terms copper and zinc, for example, are used in the appended claims as respectively including not only the pure metals but also copper base alloys and zinc base alloys, preferably those alloys in which the al-loying constituents are present in only very minor proportions.

The specific characteristics of the alloy coating formed on the inner surface of a hollow steel article by our invention are that a superior corrosion-resistant and exceptionally adherent coating is formed. The heating of steel tubing as it passes through the furnace `or other heating evice at the ordinary speed of operation of a tubing mill is sucient fto effect Vapoization of the coat- `ing metal. Thus, the normal speed of the forming mill operation does not have to be slowed down to employ our method of coating in conjunction therewith. When the described method is carried out the interior of the tubing will be coated with a thin layer of corrosionresistant metal effectively bonded to the inner surface of the steel tubing. In producing a brass coating, for .example, :the vaporized zinc actively combines with the molten copper coating on the steel to form a brazed alloy which is securely anchored to the inner surface of the tubing. In cooling, the alloy coating metals form a continuous, superior non-corrosive brass coating on the inner surface of the tubing. Moreover, the coating is not only secured by means of a superior physical adherence to the steel surface but the high temperatures of the coating metals additionally effect an alloy bonding with the steel. Accordingly, our invention provides a rapid commercially applicable method of alloy coating the inner surface of steel tubing during the manufacture of the tubing which permits simultaneous brazing, annealing and coating of the tubing in one continuous rapid operation.

The term abutment is used herein in its primary meaning; i.e., indicating touching or contacting. Thus, overlapping edges of a lap seam is also comprehended within the phrase abutting edges as well as non-overlapping oontacting edges, such as shown in the drawing. Our invention therefore encompasses making tubing having abutting edges yfrom a metal strip having scarfed longitudinal edges, the surfaces thereof being non-perpendicular to the major surface of the steel strip, as well as from a steel strip having its longitudinal edges perpendicular to the major surface of the strip.

It is to be understood that although our invention has been described in connection with single walled brazed tubing and other certain specific examples thereof, no limitation lis intended thereby except as defined in the appended claims.

We claim:

1. The method of coating the interior surfaces of hollow steel articles with a uniform adherent non-ferrous alloy coating by vaporizing a metal onto a previously coated steel surface, said method comprising introducing a non-ferrous metal into the interior of a hollow steel article which has a coating on its inner surface of a second non-ferrous metal that is metallurgically compatible with the first and has a lower melting point than steel, said first non-ferrous metal having a vaporization point lower than the melting point of steel, heating said steel article to a temperature which is sufficient to simultaneously Vaporize said non-ferrous metal and melt said coating so that said non-ferrous metals will alloy on the inner surface of said steel article, and finally cooling the article sufficiently to cause said alloying metals to solidify as a continuous uniform adherent non-ferrous alloy coating.

2. The method of coating the interior of annular cylindrical steel articles with a uniform adherent br-ass coating by vaporzing zinc onto a copper coated steel surface, introducing zinc into the interior of an annular cylindrical steel article which has a copper coated inner surface, introducing7 a non-oxidizing gas into the interior of said hollow steel article, heating said steel article to a temperature which is sutiicient to simultaneously melt said copper and vaporize said zinc so that said metals will combine to form a brass alloy, and finally cooling the steel article sufficiently to cause said alloy metals to solidify as a continuous uniform adherent brass coating.

3. The method of coating the interior of steel tubing with a uniform adherent non-ferrous alloy coating during the formation of the tubing from a substantially flat steel strip having a previously applied metal coating thereon, said method comprising the steps of longitudinally moving a substantially flat steel strip which has a coating thereon of a non-ferrous metal which has a melting point lower than that of steel, during said longitudinal movement continuously transversely bending said coated fiat steel strip into an internally coated tubular configuration having abutting edges, introducing a second non-ferrous metal into the tubing while it is being formed, said second non-ferrous metal being metallurgically compatible with said first non-ferrous metal and having a vaporization point lower than the melting point of steel, heating the tubing after it is fully formed to a temperature which is sufficient to simultaneously melt said first non-ferrous metal and vaporize said second non-ferrous metal, and finally cooling the tubing sutiiciently to cause said alloying metals to solidify as a continuous adherent alloy coating.

4. The method of coating the interior of steel tubing with a uniform adherent non-ferrous alloy coating during the formation of the tubing from a substantially fiat steel strip having a previously applied metal coating thereon, said method comprising the steps of longitudinally moving a substantially flat steel strip which has a coating thereon of a non-ferrous metal having a melting point lower than that of steel, during said longitudinal movement continuously transversely bending said coated flat steel strip into an internally coated tubular configuration having abutting edges, introducing a second non-ferrous metal into said tubing while it is being formed, said second non-ferrous metal being metallurgically compatible with said rst non-ferrous metal and having a vaporization point lower than the melting point of steel, introducing a non-oxiding gas into the tubing after it is fully formed so as to establish a non-oxidizing atmosphere therein, subsequently heating the tubing in a non-oxidizing atmosphere to a temperature which is sufiicient to simultaneously melt said first non-ferrous metal and vaporize said second non-ferrous metal within the tubing causing it to alloy and permeate pores of the steel while migrating between said abutting edges, and finally cooling the tubing sufficiently to cause said alloying metals to solidify as a continuous adherent alloy coating.

5. A method of coating the interior of steel tubing with a uniform adherent brass coating during the formation of the tubing from a substantially flat steel strip having a previously applied copper coating thereon, said method comprising the steps o-f longitudinally moving a substantially flat copper coated steel strip, during said longitudinal movement continuously transversely bending said copper coated flat steel strip into an internally coated tubular conguration having abutting edges, progressively introducing a zinc metal into the tubing while it is being formed, subsequently heating the tubing after it is fully formed to a temperature which is sufficient to simultaneously melt said copper and vaporize said zinc within the tubing so that said copper and said zinc will combine to form a brass alloy, and finally cooling the tubing sufficiently to cause said alloying metals to solidify as a continuous adherent brass coating.

6. A method of coating the interior of steel tubing with a uniform adherent brass coating during the formation of the tubing from a substantially fiat steel strip having7 a previously applied copper coating thereon, said method comprising the steps of longitudinally moving a substantially flat copper' coated steel strip, during said longitudinal movement continuously transversely bending said copper coated tiat steel strip into an internally coated tubular configuration having abutting edges, progressively introducing a zinc wire into the tubing as it is being formed, introducing a non oxidizing gas into the tubing after it is fully formed so as to establish a non-oxidizing atmosphere therein, subsequently heating the tubing to a temperature of about 1700 F. to 2200 F. in an inert atmosphere to simultaneously melt said copper and vaporize said Zinc within the tubing causing it to form a brass alloy and permeate the pores of the steel while migrating between said abutting edges, and finally cooling the tubing sufficiently to cause said alloying metals to solidify as a continuous adherent brass coating.

References Cited in the tile of this patent UNITED STATES PATENTS 845,606 Anderson Feb. 26, 1907 1,156,169 Monnot Oct. 12, 1915 1,712,090 Murphy May 7, 1929 1,892,607 Bundy Dec. 27, 1932 2,085,313 Guthrie June 29, 1937 2,771,669 Armstrong Nov. 27, 1956 2,822,291 Hahn Feb. 4, 1958

Claims (1)

1. 4. THE METHOD OF COATING THE INTERIOR OF STEEL TUBING WITH A UNIFORM ADHERENT NON-FERROUS ALLOY COATING DURING THE FORMATION OF THE TUBING FROM A SUBSTANTIALLY FLAT STEEL STRIP HAVING A PREVIOUSLY APPLIED METAL COATING THEREON, SAID METHOD COMPRISING THE STEPS OF LONGITUDINALLY MOV-

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