Welding Fuel Gas — Acetylene and Other Gases Used in Welding

Acetylene (ethyne): • Chemical formula: C₂H₂ • IUPAC name: Ethyne • A colourless gas with a distinctive garlic-like odour (pure acetylene is odourless; commercial-grade has impurities) • Highly flammable and explosive

Combustion reaction (complete combustion with oxygen): 2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O

Oxyacetylene flame temperature: ~3500°C This is hot enough to melt all common metals including iron (melting point 1538°C) and steel.

Acetylene is stored in cylinders dissolved in acetone (because pure acetylene is unstable above 1.5 atm pressure). The cylinder valve releases acetylene gas as the pressure drops.

1. Hydrogen: • Burns with oxygen to give an oxyhydrogen flame (~2800°C) • Clean combustion — produces only water • Used for underwater welding and cutting aluminium 2H₂ + O₂ → 2H₂O

2. LPG (Propane/Butane): • Used in blowtorches and brazing • Oxy-propane flame: ~2820°C • Cheaper and easier to handle than acetylene

3. Natural Gas (Methane, CH₄): • Lower flame temperature (~2770°C with oxygen) • Used for cutting rather than welding thick metals

4. MAPP Gas (Methylacetylene-Propadiene Propane): • Safer alternative to acetylene • Flame temperature: ~2925°C • Used in metal cutting and some welding

Acetylene is preferred over other welding gases because:

1. Highest flame temperature (~3500°C with oxygen) — melts even the most difficult metals.

2. Concentrated flame — allows precise control of heat input.

3. Carburising, neutral, and oxidising flames can be produced by adjusting the oxygen-to-acetylene ratio: • Carburising flame (excess C₂H₂): used for case-hardening metals • Neutral flame (1:1 O₂:C₂H₂): used for welding most metals • Oxidising flame (excess O₂): used for brass and bronze

4. Versatile — same equipment used for cutting (by oxidising), welding, and brazing.