Fillet welding in position PB: the T-joint lying down
PB is the commonest position in a fabrication shop: welding a rib, a gusset or a post to a plate is done exactly this way. It is also the most deceptive — it looks easy and produces more overlap at the bottom and undercut at the top than any other.
When PB, and when to roll it into the boat
If the part can be turned so the corner faces up, turn it: that is PA, the boat position, where the pool settles into the corner by itself and the same current gives a smoother weld. PB is for when turning is not possible — the part is heavy, already assembled or already welded on the other side. In a shop that is most of the time, which is why PB ends up being the workhorse.
How to run it
Hold the electrode at 45° to both legs and tilt it 10–15° in the direction of travel. The key correction: take the angle slightly above 45°, towards the vertical leg — that balances gravity, which otherwise drags the pool down. Keep the arc short and never dwell at the bottom leg: any pause down there is overlap. A throat up to 8 mm goes in one run; beyond that, split it — the first into the corner, the rest as a triangle from the bottom up, each overlapping the last by a third.
Guide settings, current 5 % below the flat position
| Thickness, mm | Electrode diameter, mm | Current, A |
|---|---|---|
| 1–2 | 1,6 | 40–55 |
| 1,5–3 | 2,0 | 50–75 |
| 2–4 | 2,5 | 65–95 |
| 3–5 | 3,0 | 75–125 |
| 3–6 | 3,2 | 85–135 |
| 6–12 | 4,0 | 125–180 |
| 10–20 | 5,0 | 170–240 |
| 16+ | 6,0 | 220–305 |
Leg size: what to specify and how to measure it
The leg z and the design throat a are different sizes of the same weld: a = z / √2, roughly 0.7 of the leg. The designer sets a and writes it into the ISO 2553 designation; the shop measures z with a gauge. Specifying generously is expensive: deposited metal grows with the square, and z = 8 instead of z = 6 means 78 % more metal, time and distortion. If the strength is short, lengthening the weld is cheaper than thickening it.
What to weld it with
A rutile electrode holds the pool well in PB and gives a smooth toe — this is the position where rutile suits better than basic. Basic is for where toughness is required or thick material under preheat. In MAG the fillet runs in spray transfer on an argon-rich mixture: pure CO₂ throws coarse droplets and leaves a rough toe, and the toe is exactly what decides whether you get undercut. Keep the stick-out short — a long one pulls the arc away and cuts penetration into the corner.
The usual defects and where they come from
Overlap on the bottom leg means the pool ran: current too low, travel too slow, or the electrode held too flat. Undercut on the vertical leg is the opposite: current too high, arc too long, or a pause at the top. Lack of penetration in the corner means the electrode was not aimed into the corner, or was too thick — it has to reach the root, not bridge it. An uneven leg shows up on the gauge at once and almost always means the 45° was not held.
Designation and qualification
PB to EN ISO 6947, 2F in the American system. A test passed in PB covers PA and PB — the flat position and this horizontal-vertical fillet. It does not work the other way: a test in PA does not open the horizontal fillet. So for a shop welding ribs and gussets, PB is the least test position that makes sense.
Frequently asked questions
How does PB differ from welding in the boat?
By how the part sits, not by technique. The boat position is PA: the corner faces up and the pool settles by itself. In PB the T lies down and the pool is pulled onto the bottom leg the whole time. If the part can be rolled over, roll it: the same current will give a smoother weld and less distortion.
What throat can go in a single run?
Up to 8 mm comfortably. Beyond that, split it: the first run into the corner, the rest as a triangle from the bottom up with a third overlap. Trying to lay 10 mm in one run gives overlap at the bottom and a lack of penetration in the root.
Why does it undercut the top leg?
Current too high, arc too long, or the electrode dwelling at the top. Start with the current: take 10–15 A off and shorten the arc. If the undercut stays, check the angle — it should sit slightly above 45°, but not so far that the arc hangs on the vertical leg.
What does a test in PB cover?
PA and PB to EN ISO 9606-1. A test in the flat position does not cover the horizontal fillet, so for welding ribs and gussets PB is the least position that makes sense.
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