This invention relates generally to welding, and more particularly to a gas metal arc welding method for repairing misaligned tubesheet holes.
Heat exchangers are often comprised of a multiplicity of heat exchange tubes which extend between and are generally supported by at least one tubesheet. The tubesheet has a series of holes cut therein and the tubes are inserted into the holes in the tubesheet. A tubesheet may contain up to, or more than, a thousand tube holes each of which must be precisely machined in order to allow proper tube alignment and intertube clearance. If a tube hole is drilled off location, preferably the tubesheet is repaired and then the hole is redrilled. Otherwise, precise tube alignment is impossible for that hole, and the surrounding tubes may not have the proper clearance for satisfactory fluid flow around the tube.
Gas metal arc welding (GMAW) is a relatively rapid and inexpensive method of filling an opening in a metal work. In GMAW welding, a metallic, consumable wire is continuously fed through a nozzle. An arc is developed between the tip of the wire and the work, melting the wire and producing a pool of filler weld, which subsequently solidifies. As the weld is produced, an inert gas is directed to the weld area, shielding the weld to prevent contamination of the weld by oxygen, dirt, or other particles which would otherwise be present in the atmosphere adjacent to the weld. This shielding inert gas is heated by the arc which is maintained between the tip of the consumable wire and the work. Inside the constrained confinement of a small diameter tube hole, the heated, inert gas tends to concentrate near the bottom of the nozzle. Prior art GMAW welding devices have not been designed to withstand the temperature which can be reached within the tube hole; and, accordingly, the prior art devices have not been suitable to repair misaligned tube holes.
Heretofore misaligned tubesheet holes have been commonly repaired either by cutting out a relatively large segment of the tubesheet and welding a replacement piece into its position, or by gouging out a large volume of material in the vicinity of the misaligned tube hole and welding a filler piece to the tubesheet. These techniques, in addition to being expensive, time consuming, and requiring extensive manual labor, may cause distortion of surrounding holes. This results in difficulty in installing and sealing heat exchange tubes to the tubesheet. Thus, in the past, repairing misaligned tubesheet holes has been a slow and expensive process and has customarily led to unsatisfactory results.
In accordance with the present invention, it has been learned that by properly electrically and thermally insulating portions of a gas metal arc welding (GMAW) apparatus, the apparatus can be successfully used within the constrained confinement of a tube hole having a diameter in the vicinity of 5/8 of an inch. By constructing and using a gas metal arc welding apparatus according to the present invention, both the cost and time required to fill misaligned tube holes are substantially reduced. Moreover, it has been learned that by imparting circular type motion to the consumable wire as it exits the welding apparatus to bring the tip of the wire closer to the sidewall of the tube hole, an excellent metallurgical bond can be developed between the wire as it melts in the tube hole and the tube hole sidewall, leading to a very satisfactory filling of the misaligned hole.
The most relevant prior art appears to be U.S. Pat. Nos. 4,047,656 and 3,567,900 and U.S.S.R. Certificates of Inventorship Nos. 288,949 and 258,429. The latter three references are relevant in that they disclose welding apparatus wherein a consumable wire exits the nozzle of the apparatus at an angle to the centerline of the nozzle. Further, the last two of these cited references show the consumable wire exiting the nozzle of the welding apparatus at an offset from, as well as at an angle to, the centerline of the nozzle assembly. The first of the above listed references is relevant because it shows a welding apparatus including an eccentric mounting for providing circular motion. In addition, all of these references are relevant to the present disclosure in that they show welding apparatus wherein the consumable electrode is moved through an oscillatory or circular path during the welding process. These features, as discussed in greater detail below, are also incorporated into the welding method of the present invention.
None of the above cited references, however, suggests the unique type of nozzle construction disclosed herein which permits the welding method of the present invention to be used to fill a misaligned, small diameter tube hole. Furthermore, these references do not suggest the combination, which is disclosed herein, of a consumable electrode exciting the nozzle at an offset from and at an angle to the centerline of the nozzle, an orbital type of motion wherein the nozzle orbits the centerline of a tube hole, and a rotational type of motion wherein the nozzle rotates about its own axis as it orbits the centerline of the tube hole. This combination of factors results in the electrode being constantly positioned almost directly against the tube hole sidewall, resulting in the production of an excellent metallurgical bond between the tube hole sidewall and the weld which is deposited in the tube hole. With these features, the welding method of the present invention can be used to effectively fill misaligned tube holes in a very simple, inexpensive, and efficient manner. Accordingly, it is believed that the present disclosure patentably distinguishes from the above cited prior art.
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