---
title: "Oxy-acetylene gas welding"
subject: "Workshop Technology"
chapter: 10
key: "semester_2/workshop_technology:gas-welding-flames-equipment"
source: https://www.bctengineer.com/q/semester_2/workshop_technology/gas-welding-flames-equipment
site: BCT Engineering
---

# Oxy-acetylene gas welding

Workshop Technology · Chapter 10: Metal Joining Process

- Asked in: year not given, 8 marks
- Video: [GAS WELDING | Oxy-acetylene welding](https://www.youtube.com/watch?v=-SA4D098u-Q) — ADTW Study
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/workshop_technology/gas-welding-flames-equipment
- Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-10

## Question

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.

## 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.

## Answer

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
```text
 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does oxy-acetylene gas welding join metals?
   A. An electric arc melts the edges
   B. Edges are melted by a flame of acetylene burning in oxygen, with filler rod added to the pool
   C. Pressure and current melt a nugget
   D. The filler melts but the edges remain solid
   Answer: B. The flame melts the edges and a filler rod is added to the molten pool. (Key point: Principle of gas welding)

2. Which flame has an O2 : C2H2 ratio of 1 : 1 and is used for steel, cast iron and copper?
   A. Carburising
   B. Oxidising
   C. Neutral
   D. Reducing with feather
   Answer: C. The neutral flame has equal gases and is used for most welding. (Key point: Neutral flame)

3. Why is an oxidising flame used for brass and bronze?
   A. It prevents zinc fuming
   B. It adds carbon to the weld
   C. It is the coolest flame
   D. It cleans oxide off aluminium
   Answer: A. The excess oxygen forms a protective oxide film that prevents zinc from fuming. (Key point: Oxidising flame use)

4. A flame with excess acetylene and a feather beyond the inner cone is:
   A. Neutral, for steel
   B. Oxidising, for brass
   C. Carburising (reducing), for hard-facing and aluminium
   D. Hottest flame, for cutting
   Answer: C. Excess acetylene gives a carburising flame with a feather, used for hard-facing, aluminium and high-carbon steel. (Key point: Carburising flame)

5. Which is a limitation of gas welding?
   A. Needs electricity
   B. Equipment is costly and bulky
   C. Wide heat-affected zone, distortion and gas hazards
   D. Cannot weld steel
   Answer: C. The slow, wide heating causes distortion, and the gases pose fire and explosion risk. (Key point: Limitations of gas welding)

## Related in Workshop Technology

- [Resistance spot welding heat and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Aresistance-spot-welding-heat-numerical) — Workshop Technology, chapter 10

## Related in other subjects

- [Gas flames, arc welding and heat input](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aoxyacetylene-flames-smaw-heat-input-numerical) — Manufacturing and Production Processes, chapter 12
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