Depositing Root Bead—Heavy-Wall Pipe, Large Diameter Pipe
These pipes are known for their higher strength. These low alloy, higher strength pipe require the use of certain higher strength electrodes that include E-8010, E-7010, and E-7048. The last of these is a low-hydrogen electrode used for downhill welding. In all cases preheating to specified temperature of interpass temperatures is required.
The technique to weld the root pass with the E-8010 electrode is similar to using the E-6010 electrode, except the movement of the electrode from side to side is slightly limited. However, the speed of travel is directly in the hands of the welder. Therefore, the arc should not be allowed to get far ahead of where the weld deposit is or more than one sixteenth of an inch from where the fusion of the root openings is taking place.
On-the-job, welders are equipped with grinders that use thin blades (discs). Sometimes welders do not like the look of the edges of the root bead. When changing electrodes and restarting the root pass it is necessary to use the grinder to feather the leading edge of the crater. They can also use the grinder for quick touch-ups, then resume welding. The method for cleaning, wire brushing, and grinding is the same
as previously described. After brushing, the welder should inspect the finished root bead, looking along the fusion line on both sides, and especially the extent of undercut. If undercuts along the fusion line exceed 10 percent of the weld, the welder may have to adjust the personal welding technique, as well as reduce the current setting slightly, and slow the speed of travel when making future welds. Welder helpers should be able to help ensure that the welder’s need for higher or lower current is instantly met.
Controlling the amount of undercut is important because, like the electrode, the base metal has a high tensile strength. The undercut at the fusion line raises the stress. If the (preheating) temperature is allowed to drop, the stress in the areas of the undercut increases; the weld is likely to crack because it fails to withstand the applied load. Such stress also points to why preheating is so essential. Normally, if such a condition presented itself during the welding of a carbon steel pipe (undercut) with the same dimensions, it would not pose a problem, because the steel is of a lower carbon content with high ductility. Running or depositing root beads on today’s pipe materials is quite different On pipelines where the undercut must be very limited, preheating must be applied and maintained during welding. Considering the composition of the low alloy steel and its carbon content, there are reasons to believe that. without preheating and maintaining the required temperature, the unfinished weld is likely to crack. Grinding the surface of the root bead in preparation for depositing the hot pass compounds the problem.
Depositing: The Hot pass
The hot pass follows the root bead after the root bead is prepared. This weld layer is deposited with utmost care not to create an undercut, but a weld with proper fusion along the fusion line on both sides. The third pass is deposited in a similar fashion, using a controlled arc length and a very slight weave.