Courte. vy of the Hobart Brothers Co. Fig. 4-4. Method of spacing two pipe nipples in preparation for tack welding (a); Correct alignment of pipe nipples in preparation for welding (b); and maximum allowable misalignment (c).

To fit-up the pipe nipples, first determine the required width of the root opening. Next, find a piece of wire having a diameter equal to the width of the root opening and bend it into a vee-shape, as shown in Fig. 4-4 (a). Place one of the nipples on end on the welding table (b) and place the other nipple on top of it with the wire between them to act as a spacer. With the spacer wire in place, align the two pipes with a straight edge (c). The maximum misalignment allowed (d) is Vie inch, which is specified by the ASME Code. When properly aligned, the two pipe nipples are ready to be tack welded together.

Tack Welding. After the pipes are aligned, four tack welds, evenly spaced around the pipe, are made in the root of the weld using a Vg - inch Е60Ш electrode in this case. Each tack weld should be about % inch long. Since the tack welds will remain as part of the root

bead, they must be sound throughout. They must be strong, and they should penetrate to the root of the weld from start to finish.

With the pipe nippies properly aligned, one tack weld is made in the root of the joint, as shown in Fig. 4-5 A. The spacer wire is then moved so that only the bent end is between the nipples, as shown in Fig. 4-5B. Then the second tack weld is made on the side opposite from the first tack weld.

Remove the spacer wire entirely from the joint and inspect the root opening. If the opening on one side is only slightly wider than the other, weld the wide side next. The shrinkage of the tack weld will equalize the spacing. If the space is too wide to correct by welding, bump the pipes on the table as shown in Fig. 4-5C until the openings are equalized. Then weld the third and fourth tack weld 90 degrees from the first two tack welds.

In most cases the ends of the tack welds should be ground to a feather edge to facilitate the tie-in with the root bead. This is not, however, always done. Sometimes a grinder is not available on the job and the welder must be able to make a tie-in on tack welds and other welds that have not been ground to a feather edge. If he can do this, he will have no difficulty in making a good tie-in on weld beads that have been ground. For this reason, when learning to weld, it is recommended that ends of the tack welds not be ground.

The correct procedure for welding the tack welds will be described in the following paragraphs. It will be assumed that the correct welding rod has been selected and that the welding machine setting is also correct.

It is important to start any weld correctly, whether tack welding or welding a longer bead. When the arc is struck, it should not be shortened immediately. Time should be allowed to stabilize the arc and to allow the gaseous shield to form.

If an unstable arc is brought close to the root face, the electrode may stick or small globules of filler metal will be deposited on the beveled edge which can restrict the manipulation of the arc - The gaseous shield protects the molten metal from oxidation. In the absence of the gaseous shield, the molten metal in the puddle will combine with oxygen in the air to form oxides and thus will not readily flow into the root opening. When this occurs, the first deposit is usually a large lump of metal that is seldom properly fused, and in

almost all cases the penetration into the root opening is insufficient. The result is a defective deposit that must be removed from the weld.

For the tack weld, the correct procedure is to strike the arc ahead of the weld, or that part of the groove in which the weld bead is to be deposited, as seen in Fig. 4-6A. Maintain a long arc and move the electrode along the groove to the position where the weld bead is to start. This serves to preheat this surface as well as to provide time for the arc to stabilize and for the gaseous shield to form. During this time the globules of metal transferred from the electrode will be deposited as spatter outside of the weld. On larger pipes, where it is often necessary to deposit a tack weld in the overhead position, no filler metal will be transferred when a long arc is maintained in this position.