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.
USPTO Backgrounds
USPTO Backgrounds