Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
1999-06-22
2001-09-18
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S520000, C148S521000, C228S150000, C228S199000, C228S202000, C228S223000
Reexamination Certificate
active
06290786
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to improved methods and apparatus for coating the inner exposed surface of the seam of a welded metal tube with a protective metal coating. The tube may be formed from a coated metal strip such as a ferrous strip having a coating of zinc or aluminum or various alloys. However, the coating on the surface of the seam will burn off or vaporize during welding because the melting temperature of the coating, such as zinc or aluminum and their alloys, has a much lower melting temperature than the tube which is generally formed of steel. More specifically, the improved method and apparatus of this invention utilizes a unique coating and heating process which results in an improved bond between the protective metal coating and the exposed seam surface which extends the life of the tube.
Methods of continuous or in-line forming of seamed metal tubes such as steel from a continuous strip are well known. In a conventional tube forming mill, the strip is rolled to form an open seam tube having nearly abutting edges located at the top of the tube. The edges are then welded together by one of several conventional methods which generally include heating the edges and then either forging the edges together with squeeze rolls and/or flux welding a seam. The edges of the tube may be heated, for example, by resistance welding, electric arc or by high frequency induction welding. High frequency induction welding is a form of electronic resistance welding, wherein the open seam tube is received through an electric work coil which creates a strong magnetic field, which in turn induces a current to flow around the tube and in the “Vee” formed as the edges of the strip are welded. An impeder is generally located within the tube, which forces the current down the nearly abutting edges of the open seam tube, heating the tube edges to the hot forging temperature. The tube edges are then forged by squeeze rolls which drive the molten edges of the open seam tube together to form an integral seam.
In-line galvanizing and coating or painting processes which apply a protective coating to the tube are also well known. The strip may be galvanized or painted on one or both sides prior to forming and welding, or the outer surface of the welded seamed tube may be galvanized by immersing the tube in a molten zinc bath. Where the strip is coated with a protective coating prior to seam welding, the coating on the seam area will burn-off or vaporize because the welding operation involves the melting of the tube material, which is generally steel. The temperature of the adjacent edges of the open seam steel tube are heated to melting temperature of steel, which is 2300° F. or greater. Where the strip is coated with protective metal coating, such as zinc or aluminum, the protective metal coating will melt and vaporize during welding and the protective metal coating near the weld will flow away from the seam, which is located at the top of the tube in a conventional tube mill. Zinc coating solutions have also been used to “paint” the exterior surface of the seam. However, such coatings have poor adherence and are mainly cosmetic. “Metallizing” of the seam surface has also been attempted; however, such coatings are mainly mechanical and do not provide a metallurgical bond between the protective metal coating and the steel tube. The failure of the prior commercial processes to fully coat and thus protect the tube seam is evident by the fact that the welded area is generally the first to fail in accelerated corrosion tests. Metalized surfaces covering the exterior seam surfaces have been known to fail because of mechanical stresses associated with tube fabrication.
The improved method and apparatus for coating the interior seam surface of a welded tube of this invention assures complete coating of the inner surface of the seam of a welded metal tube and a metallurgical bond between the protective metal coating on the surface of the weld in a continuation or in-line process.
SUMMARY OF THE INVENTION
Where the metal strip or skelp which forms the tube (generally formed of iron or steel) is coated with a protective metal coating (usually zinc) prior to forming and welding the tube, it is important in the method of this invention to remove the metal coating from adjacent the weld area prior to welding to avoid galvanealing of the remaining zinc adjacent the weld which would prevent or inhibit the flow and the bonding of a zinc coating applied to the galvanealed surface. The presence of zinc and other protective metal coating at the surfaces of the metal tube to be welded may also inhibit or prevent the formation of a good weld between the adjacent edges of the open seam tube. Thus, the protective metal coating is thus conventionally removed or scarfed from the lateral edges of the strip prior to welding. In the process of this invention, it is also desirable to remove the coating from adjacent the weld to prevent vaporization of the coating or galvanealing. The preferred width of the exposed areas of the strip adjacent the lateral edges will depend upon the thickness and diameter of the tube, the speed of the line and the means used to heat the lateral edges during welding. Where the welding heat can be localized, the exposed areas can be relatively small. However, it is important to avoid formations of a substrate/coating alloy or galvanealing of the zinc adjacent the weld area to form a good metallurgical bond between the coating such as zinc applied to the interior surface of the weld area in the method of this invention.
The preferred method of coating the interior surface of the weld area with a protective metal coating of this invention may be utilized in combination with the method of coating the exterior surface in an in-line process or the methods may be used independently to coat either the interior or exterior surface of the weld. In the most preferred process of coating the interior surface of the seam, the tube is welded with the seam located in the lower portion of the tube, most preferably at or very near the bottom. However, it is also possible to weld the seam in the upper portion of the tube as is presently done in conventional tube forming mills and then “locating” the weld in the lower portion of tube during the seam coating process of this invention utilizing a conventional tube twisting apparatus, for example. In the most preferred method of coating the entire interior surface of the weld area of this invention, where the strip is precoated with a protective metal coating such as zinc, aluminum or their alloys, the lateral edges of the strip are pre cut to expose the edges of the tube to be welded and to form a well for receipt of the protective metal coating. The coating adjacent the weld should also be removed as described above. The protective metal coating on the strip may also be thicker adjacent to but spaced from the edges of the strip, such that the protective metal coating will melt and flow downwardly over the seam during the two stage heating of the method of this invention. During heating of the weld area in the lower portion of the tube to a temperature above the melting temperature of the metal coating, the coating metal adjacent the seam will melt and flow toward over the seam.
The method of this invention is particularly adapted to apply an adherent protective metal coating over the internal surface of a welded seam of a metal tube. The method and apparatus of this invention is particularly, but not exclusively, adapted to a continuous or in line tube forming process, wherein the strip or skelp is coated on at least one surface with a protective metal coating prior to forming and welding. Where the tube is formed of steel, for example, the protective metal coating is preferably formed of zinc, aluminum, copper, their alloys or other metals or alloys having a melting temperature substantially below the melting temperature of the tube material. Zinc is the most common protective coating for ferrous tubes which is generally alloyed wit
Brown Curt
Krengel Theodore
Howard & Howard
The IDOD Trust
Wyszomierski George
LandOfFree
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