Horizontal welding: how to avoid undercut along the top edge
A horizontal weld runs across a vertical surface. The pool is pulled downwards, so the upper edge ends up undercut and the lower one gets an overlap. The whole job is to prop the pool up with the arc.
Where this position turns up
On tanks and pipes erected upright, on shell courses, on columns and anywhere the part cannot be laid flat. On pipework it matches the 2G setup: the pipe stands vertical and the weld runs round the circumference.
How to run it
Point the electrode slightly upwards, 5–15° above horizontal: the arc then holds the pool from below and stops it running. Keep the arc short and the beads narrow, with no wide weaving — every wide weave turns into undercut along the top. Take the current as for the flat position or 5–10 % below it.
Guide settings for a rutile electrode
| Thickness, mm | Electrode diameter, mm | Current, A |
|---|---|---|
| 2,0 | 2,5 | 70–90 |
| 3,0 | 3,2 | 90–120 |
| 4,0 | 3,2 | 110–140 |
| 6,0 | 3,2–4,0 | 130–170 |
| 8,0 | 4,0 | 150–190 |
Multi-run welds
Lay the runs as ledges from the bottom up: the first against the lower edge, each following one resting on the previous and overlapping it by about a third. That stacking supports the pool and keeps it from sagging. At every break, clean the slag off the boundary between runs — that is exactly where lack of fusion sits.
Typical defects
Undercut along the top edge is defect number one here: it comes from a long arc, too much current and wide weaving. An overlap at the bottom means you travelled too slowly. Lack of fusion between runs shows up where the next bead was placed too low or laid over unclean slag.
Designation and qualification
The horizontal position is PC to EN ISO 6947, 2G on pipe. A qualification passed in PC covers neither PF nor PE: they are separate positions with separate test pieces. The range follows from the test details.
Frequently asked questions
Why does a horizontal weld undercut the top edge?
Because the pool sags and leaves the edge bare, and the arc burns it away. The cure is always the same: a shorter arc, less current, a narrower bead and the electrode angled slightly upwards.
How are the runs stacked in a multi-run weld?
As ledges from the bottom up: the first against the lower edge, each following one resting on the previous and overlapping it by about a third.
Does a PC qualification cover vertical and overhead?
No. PC, PF and PE are three different positions to EN ISO 6947 and three different test pieces to EN ISO 9606-1. None of them replaces the others.
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