6G Pipe Root: Arc Blow and Incomplete Fusion at 4-6 O'Clock
A welder is producing a 168.3 mm OD x 7.1 mm wall pipe joint in the 6G fixed position. The root pass uses 2.5 mm E6010 at 85 A, and the fill uses 3.2 mm E7018. Fit-up is 3.2 mm root gap and 1.6 mm land, with no backing. The pipe is tacked in four places and grounded through the support frame 2 m away. On the first joint, the root at the 4-6 o'clock quadrant shows a 40 mm length of incomplete fusion at the pipe ID; the arc is described as wandering and snuffing when the electrode is near the tack. The welder increases current to 100 A and the arc becomes harsh, with undercut on the upper side. A second welder suggests switching to E7018 for the root and increasing the gap. The joint must pass visual and radiographic inspection; the repair is on a critical line and only one repair is permitted by the client.
1. Diagnose the incomplete fusion and arc instability using the evidence in the scenario.
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The 40 mm incomplete fusion at 4-6 o'clock is in the gravity-assisted quadrant, where the puddle tends to sag away from the root. Contributing evidence: 85 A on 2.5 mm E6010 is at the low end, and the arc snuffs near the tack, which points to magnetic arc blow from the remote ground and the tack. Low current, long arc, and fast travel give poor root penetration and incomplete fusion. The harsh arc at 100 A and upper-side undercut show the welder overcorrected current without fixing arc length or angle. This is not simply a bad-electrode issue; the grounding arrangement and root technique are central.
2. Propose parameter and technique changes. Evaluate the suggestion to use E7018 for the root and widen the gap.
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Fix grounding first: attach the work lead directly to the pipe as close to the weld as practical and move it to balance the field; if arc blow persists, use two grounds or wrap the lead around the joint. For the root, use 90-110 A, a short arc, a consistent keyhole, a 5-15 degree work angle, and steady travel. E7018 is not a drop-in root substitute: it is a low-hydrogen, slag-covered electrode that does not dig an open root as well as cellulosic E6010. It can be used with a backing ring or a carefully controlled land and gap, but switching without procedure qualification and widening the gap increases burn-through and slag-entrapment risk. Do not widen the gap as the first fix.
3. Define the repair sequence and explain how to protect the one-repair limit.
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Grind the 40 mm area to sound metal with a 1:3 taper, confirm by PT, and reweld with a qualified procedure and corrected grounding. Because only one repair is allowed, do not attempt a cosmetic cover pass, and do not start a second excavation elsewhere on the same joint without client approval. Re-radiograph the repaired area. If the root is inaccessible, cut out and refit rather than gouging blindly. Record the arc-blow correction and verify the next joint's root by visual and RT before continuing production, so the same defect is not repeated across the line.