How does oxy-acetylene gas welding join metals?

Edges are melted by a flame of acetylene burning in oxygen, with filler rod added to the pool

An electric arc melts the edges

Pressure and current melt a nugget

The filler melts but the edges remain solid

A flame with excess acetylene and a feather beyond the inner cone is:

Carburising (reducing), for hard-facing and aluminium

Which is a limitation of gas welding?

Wide heat-affected zone, distortion and gas hazards

Practice · 8 marks

Oxy-acetylene gas welding

Short answer

Oxy‑acetylene welding melts metal edges with a flame from acetylene and oxigen, adding filler rod. Flames: neutral (O₂:C₂H₂ ≈ 1:1) for steel, carburising (excess acetylene) for hard‑facing, oxidising (excess oxigen) for brass. Advantages: portable, cheap, no electricity. Limitations: slow, wide heat‑affected zone, not for thick plates.

Explain the oxy-acetylene gas welding process with a neat sketch of the set-up. Describe the three types of flame and their uses, and state the advantages and limitations of gas welding.

Gas welding joins metals by melting the edges with a flame produced by burning a fuel gas (acetylene) in oxygen; a filler rod is added to the molten pool.

Set-up

 O2 cylinder      C2H2 cylinder
   |regulator       |regulator
   +------hose----------hose-----+
                    |             |
                 [ blow pipe / torch ]
                          \__ nozzle -> flame
                  work + filler rod

Cylinders of oxygen (about 125 to 150 bar) and dissolved acetylene (about 15 bar) with regulators and pressure gauges.
Hoses, flashback arrestors, blow pipe (torch) with mixing chamber and tip.
Goggles, spark lighter, filler rods and flux (for non-ferrous metals).

Types of flame

Flame	Oxygen : acetylene	Temperature	Use
Neutral	1 : 1	About 3200 °C	Steel, cast iron, copper, most welding
Carburising (reducing)	Excess acetylene	About 3000 °C	Hard-facing, aluminium, monel, high carbon steel
Oxidising	Excess oxygen	About 3300 °C	Brass and bronze (to prevent zinc fuming), cutting

The neutral flame has a bright inner cone and a pale outer envelope; the carburising flame has a feather beyond the inner cone.

Welding technique

Leftward (forward) method for thin plates; rightward method for thick plates (more economical, better strength).

Advantages

Cheap, portable equipment; flexible; no electricity needed; good control of heat; can weld, braze, solder and cut.

Limitations

Slow, wide heat affected zone and distortion; not suited for thick plates; fire and explosion hazard of gases; lower strength than arc welds.

✓ Answer checked by Claude Opus 5.5 (high effort)

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