When the pipe nipples are correctly spaced and aligned, they are ready to be welded together (see Fig. 8-7). For welding the tack welds and the root bead, an E6010, Е6010ЇР, or E7010-A elec­trode should be used. When the wall thickness of the pipe is % inch, or less, a Vg-inch electrode is used; for thicker-walled pipe a %2'inch electrode is used. Low-hydrogen electrodes cannot be used for downhill pipe welding. A very high current setting should be

Fig. 8-7. Aligning and tack welding two thin-wall pipe nipples using a piece of bent wire as a spacer to obtain the correct root opening.

used when welding the root bead by the downhill method. As an estimate, it should be between 100 and 140 amps DC for the nipples used in practice welding.

The tack welding operation on the pipe nipples is shown in Fig. 8-8. Also shown in this illustration is the correct electrode angle that should be used, 10 to 15 degrees. The arc should be struck in the joint ahead of the weld and immediately lengthened to stabilize it and to form the gaseous shield. As shown in Fig. 8-9, when this has occurred, the arc is buried into the weld joint, meaning that it should be pushed into the joint, with the end of the rod held against the joint with a light pressure. Due to the very high current setting used to deposit the tack weld and the root bead, the arc will continue to burn even when the electrode is in contact with the base metal of the pipe.

TOP VJEW Fig. 8-8. A. Welding the tack weld; B. Top view of pipe nipples showing the correct electrode angle for tack welding.

Holding the electrode at the correct angle while it is buried in the joint, the welder must observe the formation of the puddle and the keyhole. As soon as sufficient metal has been built up to bridge the gap opening, he must begin to move the electrode by sliding its end

in the groove, constantly maintaining a slight pressure on the end of the electrode to keep it buried. The electrode may have a tendency to stick as it is dragged along in the groove, and if this occurs the electrode holder should be wiggled slightly, while keeping the end of the electrode buried in the groove. A small keyhole will form behind the electrode; and this should be watched as the weld progresses, as well as the molten pool of metal. When the bead is about % inch long, the arc is quenched by a quick flip of the electrode away from the keyhole.

PUSH ELECTRODE INTO JOINT Courtesy of the Hobart Brothers Co. Fig. 8-9. Position of the electrode for downhill welding a root bead. The electrode is buried in the pipe joint by holding it in the weld with a fight pressure. The high current setting allows the arc to be maintained when the electrode is in contact with the pipe.

It is rather difficult for a beginner to master this method of welding; therefore, this procedure should be practiced by first weld­ing some flat plates before any attempt is made to weld pipe. It is best to begin practicing with the plates in the vertical, or 3G, position as in Fig. 8-10. Before starting to practice-weld on flat plates, the following section, “Welding the Root Bead,” should be read.

Courtesy of the Hobart Brothers (. o. Fig. 8-10. Set-up for practice-welding root beads by dragging the electrode in a downward direction. A. Side view; B. Welder s view.

On cross-country pipelines, the position of the tack welds and the procedufe used to make them depends upon many factors. Moreover, different crews may vary the procedure. When the pipe is lying in a horizontal position or on a uniformly rising grade, it is lifted upward slightly by a crane to allow the welder to reach the bottom of the pipe. The first tack weld then is made in the 6 o’clock position. After the pipe is lowered, the second tack weld is made in the 12 o’clock position, followed by two additional tacks in the 3 and 9 o’clock positions. If the ground on which the pipes are lying curves upward or downward, the first tack weld is sometimes made in the 12 o’clock position, followed by a tack at the 6 o’clock and then at the 3 and 9 o’clock positions. For practice welding, the pipe nipples should be clamped in the stand in the position shown in Fig. 8-11, with the tacks in the 2, 5, 8, and 11 o’clock positions.

Welding the Root Bead. In preparation for welding the root bead the tack welds should be deslagged and cleaned thoroughly with a wire brush. The ends of the tack weld should be ground to a sharp feather edge with a grinding wheel, as shown in Fig. 8-12.

The procedure for welding the root bead is essentially the same as that for welding the tack welds. A high current setting is used, together with the same types and sizes of electrodes. The electrode

CLAMP Fig. 8*1 1. Method of clamping the pipe nipples in the pipe stand.

should be held at the angle shown (10 to 15 degrees) in the illustration, Fig. 8-13.

To start the root bead, the arc is struck in the 12 o’clock position. It should be struck ahead of the weld in order to preheat the part of the joint in which the bead is to be deposited. A long arc is held until the arc has stabilized and the gaseous shield has formed and then the electrode is buried in the welding joint and kept in place with a light pressure.

The welder must closely observe the formation of the puddle of molten metal and the keyhole. When the liquid base metal and the filler metal from the electrode have formed a bridge of molten metal across the gap, the welder then will start to move the electrode by dragging it in the groove. A small, с rescent-shaped keyhole, shown in Fig. 8-14, will appear at the top side of the electrode.

The keyhole is a necessary element in the formation of the weld bead. It is formed when the faces of the weld joint have melted and it provides the space for the molten metal to flow to the bottom of the weld joint, thereby assuring that the weld bead will penetrate to the root, The solid weld bead should extend below the surface to form a slight crown that must not exceed lAe inch in height (see Fig. 8-15).

metal GROUND AWAY APPROX. 1/16"

The arc force will cause some of the molten metal to flow upward to build the weld bead and to retard its downward movement along the pipe joint. By watching the action of the molten metal and the

GRIND SIDES TO SMOOTH ROUND EDGE GRIND ENDS TO SHARP FEATHERED EDGE

Courtesy of ihe Hobart Brothers Co.

Fig, 8-12. Tack weld ground to facilitate making a tie-in.

VERTICAL AXIS

Courtesy of the Hobart Brothers Co. Fig, 8-13. Correct electrode angle for welding the root bead using the dragging, or buried arc, method.

keyhole, the welder can determine the speed of travel at which the electrode is advanced.

Certain difficulties are encountered when this welding procedure is used. For example, there will always be a slight amount of undercutting at the edges of the weld, as can be seen in Fig. 8-15. However, the undercut is not harmful to the final quality of the weld

Courtesy of the Hoban Brothers Co. Fig. 8-14. A small crescent-shaped keyhole that appears at the top side of the electrode when downhill welding the root bead by dragging the electrode in the weld joint.

ROOT PENETRATION i. ourtesy oj ihe Hobart Brothers Co. Fig. 8-15. Undercutting of root bead caused by dragging the electrode to deposit the bead.

in this case, because it will be removed by the second, or hot, pass, as explained later.

The high current setting together with the very short arc that is used with this welding procedure will cause more current to flow through the electrode than when a longer arc is held, because the resistance of the arc to the flow of the current is less. This will cause the electrode to get very hot. Often the heat will increase to such an extent that the electrode coating starts to break down beyond the end of the electrode. As a result, the arc will tend to blow and the electrode will have a tendency to stick instead of gliding smoothly along in the groove. If this occurs, the electrode should be oscillated slightly from side to side, as shown in Fig. 8-16, left and right.

Slight variations in the root opening often occur on pipe joints that have been fit-up properly. In some places the root opening may

Fig. 8-16. Sidewise wiggle of the electrode, to overcome its sticking when downhill welding the root bead.

be widened very slightly while in others it is slightly narrow. If the root opening is slightly narrow, the speed of travel and the electrode angle also should be reduced slightly. In places where a slightly widened root opening is encountered, the speed of travel should be increased slightly.

Increasing the speed of travel, however, can cause pinholes to form behind the arc, usually at a distance of about y8 inch. When this occurs, the welder must swiftly tilt the electrode holder in the direction of travel and then go back to the original position as in Fig. 8-17. Tilting the electrode causes the arc force to push some of the weld metal into the void. The welder must be alert to recognize pinholes and then immediately take the corrective action. Obviously, if the weld has progressed too far beyond the pinholes, they cannot be eliminated by this procedure.

As the weld progresses the welder must focus his entire attention on his work. He must hold the electrode at the correct electrode angle. He must watch the puddle and the build-up of the weld bead. At all times he must pay careful attention to the keyhole. Should the keyhole decrease in size, the welder must slow down the speed of welding and decrease the electrode angle slightly. If this maneuver does not enlarge the keyhole, welding should be stopped and the current setting increased before restarting the weld.

The keyhole must not be allowed to become greatly enlarged as this will result in excessive penetration or, possibly, burn-through, as

Fig. 8-17. Method of closing pinholes that can develop behind the arc when it is necessary to increase the welding speed. Swiftly tilt the electrode in the direction of welding and back, as shown.

Cannes у of the Hobart Brotherл Co. Fig. 8-18. Burn-through resulting from excessive current flow when drag- welding the root bead.

shown in Fig. 8-18, If burn-through occurs, the defective section should be removed before continuing to weld the root bead. The welding current setting should be reduced slightly when restarting the weld to prevent a recurrence of burn-through.

Stop and Restart. When it is necessary to stop the weld before the bead is completed, the arc is broken by quickly flicking the end of