Very high-quality pipe joints can be made using the uphill welding method to weld both large - and small-diameter thin-wall pipe. It
is recommended for welding high-alloyed pipe and in other cases where only the highest-quality welded joint will be accepted.
For large-diameter pipe, the procedure is exactly the same as that described in Chapters 5, 6, and 7. The root bead may be welded by the shielded me tal-arc-welding process or by the gas tungsten arc - welding (GTAW) process. The procedure for welding the intermediate and cover passes is the same as previously described.
When welding small-diameter thin-wall pipe, the root bead and intermediate passes are welded as previously described; however, a slightly different procedure must be used to weld the cover pass. Because more heat is retained in the weld zone when welding small-diameter thin-wall pipe, the temperature of the weld is often very high when the cover pass is deposited.
In this case, a semicircular weave, shown in Fig. 8-1, is used. The current setting should be slightly decreased when using this weave pattern. It is made with a continuous motion, there being little or no pause at the end of each weave. This allows the area that is seen encircled in Fig. 8-1 to cool and lose a considerable amount of fluidity when the arc is on the opposite side. When the arc is returned
Fig. 8-1. Semicircular weave used to weld the cover pass when welding small - diameter pipes by the uphill-welding method.
to the encircled area and the filler metal is added, the metal will not drip as a result of overheating. The electrode angle remains constant when welding around the pipe, as shown in Fig. 8-2.
When this weave pattern is to be used, it is very important to clean the weld joint thoroughly beforehand. Since there is little or no hesitation at the end of the weave, some danger exists that slag may become entrapped, causing incomplete fusion to occur in this area. The welder, therefore, must take a position which enables him to manipulate the electrode comfortably and precisely. This is especially important when welding small-diameter pipe because of the continuous change in the electrode position. A perfect cover pass is shown in Fig. 8-3.
Fig, 8-2. Correct electrode angle when welding the cover pass on small-diameter pipe by the uphill-welding method.