Gas Welding

GAS WELDING: OXY-FUEL OR OXY-ACETYLENE WELDING

INTRODUCTION

• Instructor: Prof. Dr. Ir. Wim De Waele

• Institution: Laboratory Soete, Ghent University

• Academic Year: 2025 - 2026
  

OUTLINE

1. How it works

2. Equipment

3. The flame

4. Gas welding techniques

5. Applications
   

HOW IT WORKS

• Process Energy:
    - Energy is derived from the combustion of a fuel gas, most commonly acetylene, in the presence of oxygen.
  

EQUIPMENT

• Fuel Gas - Acetylene:
    - Composition: 92.3% Carbon (C) and 7.7% Hydrogen (H) by weight.
    - Properties:
      - Considered dangerous, as it is inflammable and chemically unstable.
      - Highly explosive when under high pressure and when in contact with copper (Cu).
    - Distribution:
      - Acetylene is stored in bottles filled with a porous mass, which contains the gas dissolved in acetone.
      - Typical pressure in acetylene bottles is around 20 bars.
      - A 40L bottle can hold approximately 6000L of acetylene.
      - Bottles, hoses, and fittings are typically red; fittings feature a left-hand thread.
  

• Oxygen Gas:
    - Properties:
      - Bottle pressure can reach up to 200 bars.
      - Available volumes: 10L or 20L (150 bars) and 50L (200 bars).
      - Bottles are typically blue/gray with a white neck; hoses are blue in color; fittings feature a right-hand thread.
      - It is critical to never use grease or oil on gas connections due to the risk of fire or explosion.
  

• Pressure Regulators:
    - Functionality:
      - Designed to reduce high and variable pressure in the gas cylinders to a stable working pressure.
      - Maintain constant pressure at the flame torch.
      - Ensure a consistent gas flow rate throughout the life of the gas charge.
      - Include safety features, such as flame traps or flashback arrestors.
  

• Injector Torch:
    - Consists of a body with valves and a welding chamber equipped with a welding nozzle.
  

• Oxygen Flow Dynamics:
    - The high outlet speed of oxygen from the torch creates a negative pressure in the acetylene gas line, allowing for efficient fuel mixing.
  

• Flashback Arrestors:
    - Installed on both the oxygen and fuel lines to prevent backfire, ensuring safety during operation.
      - Normal Operation: Gas flow moves from the top (oxygen) to the bottom (acetylene).
      - Backfire Situation: Flame traveling from the bottom up, which is dangerous and must be avoided.
  

THE FLAME

• Flame Temperature:
    - The oxy-acetylene flame tip can reach temperatures around 3200°C.

• Flame Types:
    - Reducing Flame:
      - Chemical Reaction: $ext{C}_2 ext{H}_2 + ext{O}_2 ightarrow 2 ext{CO} + ext{H}_2 + ext{heat}$
      - Used for melting plate edges and filler metals.
    - Flame Envelope:
      - Combustion reaction with atmospheric oxygen: $4 ext{CO} + 2 ext{H}_2 + 3 ext{O}_2 ightarrow 4 ext{CO}_2 + 2 ext{H}_2 ext{O} + ext{heat}$
      - Provides preheating and protects the melt pool during welding.

• Safety Precaution:
    - Never work in closed environments to avoid hazardous fumes.

• Carburizing Flame:
    - Only appears during a carburizing flame condition due to excess acetylene.

• Flame Characteristics:
    - Normal or Neutral Flame:
      - Reducing flame, ideal for many welding applications.
    - Oxidizing Flame:
      - Higher temperature with a risk of oxidation, suitable for cutting operations.
    - Carburizing Flame:
      - Lower temperature with excess acetylene, typically used for cast iron and hardenable steels.
  

• Alternative Fuel Gas:
    - MAPP Gas (Methylacetylene-Propadiene Propylene):
      - Composed of various hydrocarbons, primarily methylacetylene and propadiene.
  

GAS WELDING TECHNIQUES

• Forehand Welding:
    - Suitable for plate thicknesses up to 3mm.
    - Torch swings slightly; the process tends to be slower due to toughness requirements.

• Backhand Welding:
    - Ideal for plate thicknesses exceeding 3mm.
    - The filler rod is circled around the weld, making it easy to observe both the weld pool and the keyhole.
    - Ensures molten filler rod stays protected from oxygen for better root fusion.
    - Produces narrow weld seams and consumes less gas.
  

APPLICATIONS

• Metals and Thicknesses:
    - Gas welding is effective for a variety of metals and thicknesses.
    - Applicable materials include steel, cast iron, low and high alloy steels, copper, nickel, aluminum, magnesium, zinc, lead, etc.
    - Not suitable for refractory metals (e.g., Nb, Ta, Mo, W) or reactive metals (e.g., Zr, Ti).
    - Primarily used for thin plates (notably in automobile repairs) and small pipes/tubes (such as in heat exchangers, steam boilers, and heating installations).

• General Versatility:
    - Gas welding is a universal technique, allowing flexible control of heat input.
    - Exemplary for thin plate welding to avoid burn-through (common in electric arc welding) and suitable for very thick cast iron components by using slow heating and cooling approaches.
    - Also used for hardening steels when necessary.

• Workshop Applications:
    - Useful for smaller workshops that require one installation for multiple welding jobs.
    - Mobile installations are beneficial for construction sites, repair, and erection work.

• Derived Techniques:
    - Gas welding techniques can also be adapted for other purposes, including:
      - Heating
      - Straightening
      - Soldering
      - Deposition welding
      - Cutting
  

ADDITIONAL USES

• Cutting:
    - Efficient cutting processes can be executed using oxy-fuel techniques.

• Hardening and Gouging:
    - Can also be used for hardening materials or gouging operations, adding to the versatility of the method.

• Soldering:
    - Useful for soldering applications that require precision and specific heat control.

• Straightening:
    - Employed in metalworkers' practices for straightening warped materials.

• Cleaning:
    - Gas welding techniques can extend to cleaning applications, enhancing its flexibility and utility in workshops and industrial settings.