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Manufacturing Processes > Joining processes > Gas Welding(GW)

 

Gas Welding(GW)

 

Process description

High pressure gaseous fuel and oxygen are supplied by a torch through a nozzle where combustion takes place, providing a controllable flame. The high temperature generated (greater than 3000°C) is sufficient to melt the base metal at the joint area. Shielding from the atmosphere is performed by the outer flame. Filler metal can be supplied to the weld pool if needed.

Materials

  • Commonly ferrous alloys: low carbon, low alloy and stainless steels and cast iron.
  • Also, nickel, copper and aluminum alloys, and some low melting point metals (zinc, lead and precious metals).
  • Refractory metals cannot be welded.
GW

Process variations

  • Commonly manually operated, portable and self-contained welding sets.
  • Can use forehand or backhand welding procedures.
  • Gas fuel commonly used is acetylene for most welding applications and materials, known as oxyacetylene welding.
  • Hydrogen, propane, butane and natural gas used for low temperature brazing and welding small and thin parts.
  • Air can be used instead of oxygen for brazing, soldering and welding lead sheet.
  • Flux may be necessary for welding metals other than ferrous alloys.
  • By regulating the oxygen flow, three types of flame can be produced:

    • Carburizing: for flame hardening, brazing, welding nickel alloys and high carbon steels
    • Neutral: for most welding operations
    • Oxidizing: used for welding copper, brass and bronze.
  • Braze welding: base metal is pre-heated with an oxyacetylene or oxypropane gas torch at the joint area. Brazing filler metal, usually supplied in rod form, and a flux is applied to joint area, where the filler becomes molten and fills the joint gap through capillary action. Although no fusion takes place, very high temperatures are required, typically 700°C. Some finishing may be necessary to clean flux residue and excess braze.
  • Pressure gas welding: heat from oxyacetylene burner is used to melt ends of the parts to be joined and then applied pressure creates the weld.
  • Gas cutting: an oxyacetylene or oxypropane flame from a specially designed nozzle is used to preheat the parent metal and an additional high pressure oxygen supply effectively cuts the metal by oxidizing it. Can perform straight cuts or profiles (when automated) in plate over 500mm thickness.

Economic considerations

  • Weld rates very low, typically 0.1 m/min.
  • Lead times very short.
  • Very flexible process. Same equipment can be used for welding, cutting and several heat treatment processes.
  • Economical for very low production runs. Can be used for one-offs.
  • Automation not practical for most situations.
  • Tooling costs low to moderate. Little tooling required and jigs and fixtures are simple for manual operation.
  • Equipment costs low to moderate.
  • Direct labor costs moderate. Skilled operators may be required.
  • Finishing costs low to moderate. No slag produced, but cleaning may be required.

Typical applications

  • Sheet-metal fabrication
  • Ventilation ducts
  • Small diameter pipe welding
  • Repair work

Design aspects

  • Moderate levels of complexity possible. Capability to weld parts with large size and shape variations.
  • Typical joint designs possible using gas welding: butt, fillet, lap and edge, in thin sheet.
  • All welding positions possible.
  • Design joints using minimum amount of weld, i.e. intermittent runs and simple or straight contours wherever possible.
  • Balance the welds around the fabrication’s neutral axis.
  • Distortion can be reduced by designing symmetry in parts to be welded along weld lines.
  • The fabrication sequence should be examined with respect to the above.
  • Sufficient edge distances should be designed for and avoid welds meeting at the end of runs.
  • Minimum sheet thickness, commonly:

    • Carbon steel =0.5mm
    • Cast iron =3 mm.
  • Maximum sheet thickness, commonly:

    • Carbon steel and cast iron =30mm
    • Low alloy steel, stainless steel, nickel and aluminum alloys =3 mm.
  • Multiple weld runs required on sheet thicknesses ≥4 mm.
  • Unequal thicknesses possible.

Quality issues

  • Good quality welds with moderate but acceptable levels of distortion can be produced. Repeatability can be a problem.
  • Access for weld inspection important.
  • Attention to adequate jigs and fixtures when welding thin sheet recommended to avoid excessive distortion of parts by providing good fit-up and to take heat away from the surrounding metal.
  • Heat affected zone always created. Some stress relieving may be required for restoration of materials original physical properties.
  • Surface preparation important to remove any contaminates from the weld area such as oxide layers, paint and thick films of grease and oil.
  • Gas flow rates should be pre-set and regulated during production. Even gas mix gives the neutral flame most commonly used for welding. Even heating of joint area required for consistent results.
  • Shielding integrity at the weld area not as high as arc welding methods and some oxidation and atmospheric attack may occur.
  • ‘Weldability’ of the material important and combines many of the basic properties that govern the ease with which a material can be welded and the quality of the finished weld, i.e. porosity and cracking. Material composition (alloying elements, grain structure and impurities) and physical properties (thermal conductivity, specific heat and thermal expansion) are some important attributes which determine weldability.
  • Surface finish of weld fair to good.
  • Fabrication tolerances typically ±1 mm.

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