The short answer
Acetylene is used in welding because the oxy-acetylene flame is the hottest fuel-gas flame available — about 3,100 °C — and it delivers that heat in a tight, high-intensity inner cone that melts steel quickly. It also burns with a reducing zone that cleans the metal surface, and it needs far less oxygen than propane or LPG. No other common fuel gas combines this heat, flame control and metallurgical friendliness.
The hottest fuel-gas flame
Flame temperature decides how fast and how deep you can weld steel. Acetylene tops the table, which is why it dominates oxy-fuel welding while cheaper gases are used mainly for cutting and heating.
| Fuel gas (with oxygen) | Approx. flame temperature | Best for |
|---|---|---|
| Acetylene | ~3,100–3,300 °C | Welding, brazing, cutting, hard-facing |
| Hydrogen | ~2,800 °C | Thin material, underwater cutting |
| Propane / LPG | ~2,800 °C | Cutting, heating (not fusion welding) |
| Natural gas | ~2,700 °C | Heating, some cutting |
Only acetylene gets hot enough, fast enough, to fusion-weld steel cleanly with an oxy-fuel torch.
Three reasons acetylene beats propane and LPG
- ✓Hotter flame — around 3,100 °C versus about 2,800 °C for propane, so it welds high-strength steels the others cannot.
- ✓A reducing zone — the acetylene flame has a zone that chemically cleans the metal surface as you work; propane does not, which is why propane is used for cutting and heating rather than welding.
- ✓Less oxygen — acetylene burns at roughly a 1.2 : 1 oxygen-to-fuel ratio, against about 4.3 : 1 for propane, so it is more efficient and cheaper to run on oxygen.
The three oxy-acetylene flame types
Adjusting the oxygen-to-acetylene ratio changes the flame — and each setting has a job. Reading the flame is a core welding skill.
| Flame | Ratio | Use |
|---|---|---|
| Neutral | ≈ 1 : 1 | General steel welding — no chemical effect on the weld |
| Carburising (reducing) | Excess acetylene | Hard-facing, high-carbon steel, brazing — adds carbon |
| Oxidising | Excess oxygen | Brass and bronze; hotter but oxidises steel |
What the acetylene flame is used for
- ✓Fusion welding of steel and cast iron with a filler rod.
- ✓Flame cutting — a pre-heat flame plus an oxygen jet cuts thick steel.
- ✓Brazing and braze-welding of steel, copper and brass.
- ✓Flame hardening, straightening and heating of fabricated parts.
- ✓Hard-facing and metal spraying with the carburising flame.
A note on safety
Acetylene’s heat comes with responsibility: it has a very wide flammable range and must be handled with flashback arrestors and non-return valves, correct pressures (never above 15 psig / ~1 bar working) and the right withdrawal rate from the cylinder. Used correctly, it is the safest way to get a welding-hot flame from a portable set.
Frequently asked questions
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