Oxy-acetylene welding 311
The process that was supposed to disappear and did not. On thin sheet, on repairs and wherever there is no power, an oxy-acetylene flame still does what the arc cannot: it heats slowly and lets you watch the pool without hurrying.
Where it still wins
- Sheet from 0.5 to 3 mm. The flame heats more gently than an arc, so burning through is harder, not easier.
- No electricity. Two cylinders and a torch work where a generator is not and will not be.
- Repairs and small-bore pipework, where a welding torch simply will not reach.
- Braze welding of cast iron and galvanised sheet — the same torch, a different process and a different number.
Number 311 is oxy-acetylene welding; 312 is oxy-propane, 313 oxy-hydrogen. The full list is in the ISO 4063 table.
The flame: three settings, one correct
| Flame | How it looks | What for |
|---|---|---|
| Carburising excess acetylene |
A second, soft feather reaches out of the inner cone | Hardfacing alloys, stellite |
| Neutral roughly 1:1 |
A sharply drawn inner cone, no feather | Non-alloy steel — the default |
| Oxidising excess oxygen |
Shorter cone, louder and harsher flame | Brass; harmful on steel |
It is always set in the same order: open the acetylene, light it, add oxygen until the feather disappears. The point where it disappears is the neutral flame, and that is where you stop. For steel, anything else is a mistake rather than a style.
Filler and thickness
Filler diameter follows the rule d = s/2 + 1 — 2 mm sheet
takes 2 mm filler, 4 mm takes 3 mm. Filler that is too thick
draws heat out of the pool and gives adhesion rather than fusion — which looks
like a weld until someone bends it.
Up to 3 mm you weld leftward: the torch follows the filler, the flame runs over cold metal, penetration stays shallow. Above that, rightward: the flame runs over the finished bead, soaks it and gives deeper penetration. Changing direction is not a matter of comfort but a way of controlling how much heat goes in.
What this process never forgives
Acetylene is flammable and unstable under compression; oxygen does not burn itself but violently supports burning. Hence two rules with no exceptions: never handle an oxygen cylinder with greasy hands — oil under oxygen pressure ignites on its own — and flashback arrestors go on both regulators, not only on the acetylene side.
An acetylene cylinder works upright. Laid down, it gives off the acetone the acetylene is dissolved in, and the torch then behaves in ways no setting will explain. The rest of the rules — hot work and fire safety.
What next
Thin sheet by other methods — welding thin sheet. Process numbers and abbreviations — the ISO 4063 table. The arc alternative for the same repairs — MMA.
Frequently asked questions
What filler diameter should I take?
The rule is simple: d = s/2 + 1, where s is the sheet thickness in millimetres. For 2 mm sheet that gives 2 mm filler, for 4 mm it gives 3 mm. Filler that is too thick chills the pool and gives you a stuck-on bead instead of fusion.
How do you recognise a neutral flame?
By the inner cone. With excess acetylene a second, soft cone reaches out of it — that is a carburising flame. Backing the acetylene off until that feather disappears leaves a sharply drawn inner cone, and that is neutral. Backing off further gives an oxidising flame: shorter and noisier.
How does gas welding differ from braze welding?
By whether the parent metal melts. In welding (311) the edges and the filler melt together. In braze welding (971) the edges stay intact and the joint is held by a filler of lower melting point that wets them. That is why braze welding is used on cast iron and galvanised sheet, where melting the parent metal would do harm.
Updated: