Electric heating – Metal heating – By arc
Reexamination Certificate
1999-08-18
2001-01-09
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
By arc
C219S130100
Reexamination Certificate
active
06172333
ABSTRACT:
INCORPORATION BY REFERENCE
The present invention involves the use of two side-by-side consumable electrodes for welding the root pass defined by the gap between two spaced ends of pipe sections being welded together in the field. In pipe welding, the consumable electrode is moved toward the space between the pipe sections from a wire feeder while the torch carrying the electrode and introducing electrical current to the electrode moves around the pipe sections. This movement is accomplished by a mechanical arrangement often referred to as a “bug.” Such torch moving mechanism used in the present invention is illustrated in Parker U.S. Pat. No. 5,676,857 wherein a single electrode is moved by a welding bug on a track supported adjacent the gap being welded. This patent is incorporated by reference herein showing the environment to which the present invention is directed, so that the bug and guide track used with the present invention need not be repeated. The invention involves the use of two consumable electrodes that are moved in unison for the purpose of laying the initial bead in the gap between the pipe sections. The use of two or more consumable electrodes is known in electric arc welding. Such tandem electrodes are operated in a front to back orientation and are shown in Shutt U.S. Pat No. 4,246,463 and Fratiello U.S. Pat No. 5,155,330. These patents are also incorporated by reference herein to show tandem electrodes driven by a single power supply, even though the electrodes are disposed from back to front so that a first bead is laid and then a second bead is laid over the first bead. Pipe welding using tandem consumable electrodes for laying a first bead in the open root joint followed by a second bead over the joint is the general subject matter of prior co-pending application Ser. No. 336,804 filed Jun. 21, 1999 by applicant. This prior co-pending application is not prior art; however, it does disclose the use of two separate power supplies for driving each of two separate consumable electrodes. These electrodes are in separate, spaced torches that are isolated from each other. Each electrode is driven by a separate power supply, such as an STT welder manufactured by The Lincoln Electric Company of Cleveland, Ohio. An STT welder is disclosed in Stava U.S. Pat. No. 5,001,326, incorporated by reference herein. Since the present invention relates to the use of two separate power supplies, preferably two separate STT welders, the disclosure of prior application Ser. No. 336,804 filed Jun. 21, 1999 is also incorporated herein by reference so that the separate power supplies contemplated for use by the present invention need not be repeated herein.
BACKGROUND
When forming a pipe line in the field, two sections of pipe are brought together defining a circular joint which is welded by an appropriate arc welder. When performing this pipe welding procedure, the ends of the pipe sections are normally chamfered and brought together until the pipe sections abut each other. To create a high strength, quality weld between the pipe sections, one section is then moved away from the other section a slight amount to define an open root or gap, which is filled by the initial pass around the cylindrical joint. After the initial open root pass has filled the space between the ends of the sections, the welding operation continues by laying a succession of molten metal beads on top of the previously created weld bead. This procedure is repeated until the outwardly tapering joint between the pipe sections is filled. A critical operation in the pipe welding procedure is welding the initial open root pass. Welding the open root presents substantial variables. The welding procedure must form a bead between the pipe sections and the bead must penetrate through the pipe to fill the open root. However, molten metal can not protrude into the interior of the pipe through the gap. Consequently, there is substantial development work in the type of power supplies and the consumable electrode configurations to control heat of the open root pass to, thereby, create a quality open root joint. This bead must be laid rapidly and consistently in the field. Rapid open root welding of pipe sections is now done using a short circuit welder, such as the STT welder manufactured by The Lincoln Electric Company of Cleveland, Ohio. A single STT power supply and a single wire feeder directs the consumable electrode toward the gap between the pipe sections as the torch carrying the electrode is moved or guided around the pipe joint by a mechanism, as shown in Parker U.S. Pat. No. 5,676,857. Using an STT arc welder and the mechanical bug and track shown in the Parker patent, formulates a root pass bead of exceptional quality. The consumable electrode is centered above the weld puddle bridging the open gap between the pipe sections as the open root bead is being formed. Centering of the electrode in this pipe welding procedure is important so that the molten metal from the consumable electrode is properly fused in the gap between the two spaced ends of the adjacent pipe sections. Since the welding arc is concentrated in the weld puddle, the bug traveling around the pipe and carrying the torch supporting the electrode must travel at a relatively slow speed. If the bug moves the electrode too rapidly as it is attempting to lay a bead in the open root, the electrode moves faster than formation of the puddle causing a blow through of the electric arc. Thus, the speed of the welding operation using the STT welder and a standard mechanical device for moving the electrode around the open root is limited by the physics of the surface tension forces. The speed at which the weld puddle is formed by the molten metal of the electrode limits the travel speed of the torch. This same limitation on the speed of the welding process exists when the welding process is modified to provide tandem electrodes, as shown in Fratiello U.S. Pat. No. 5,155,330. Since the electrodes are tandem back to front, the molten metal bead is formed at a speed determined by the surface tension forces interacting at the front consumable electrode. However, the tandem consumable electrode concept which has not yet been fully adopted for pipe welding is an advance in the art. By using tandem electrodes, the second, or following, electrode deposits additional material on top of the initial root pass of the first electrode to provide an increased deposition rate. Such tandem electrode welding does increase the amount of metal being deposited during the initial pass around the pipe sections; however, it presents substantial difficulties with respect to interference between the two adjacent welding processes. Prior application Ser. No. 336,804 filed Mar. 21, 1999 is directed to use of two tandem consumable electrodes with separate power supplies that are individually adjusted to prevent interference between the welding processes. This procedure has advantages and is not prior art to the present invention; however, it presents a limitation on the speed at which the open root pass bead can be laid. The initial bead is created by the front tandem electrode that is centered between the edges of the pipe section and can not move along the gap at a speed greater than the advancing molten metal puddle. These limitations exist in the rapidly advancing development work regarding use of STT welders for welding the ends of pipe sections in the field.
THE INVENTION
The present invention further improves a pipe welding operation implemented by an electric arc welding process utilizing an STT welder. When the electrode is centered in the gap, the torch can move along the gap at a speed controlled by the surface tension transfer of metal to the molten metal puddle being formed during the welding process. This is a speed limitation, as explained above, experienced even when a higher deposition rate is obtained by the use of two or more tandem electrodes. In accordance with the present invention, two consumable electrodes are directed to the puddle from a single torch. However, th
Lincoln Global Inc.
Shaw Clifford C.
Vickers Daniels & Young
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