Porosity

Porosity in welding results when bubbles of gas are trapped in the weld pool as it solidifies

These pores or holes can be inside the weld or surface breaking and take several forms that can be identified by their shape.

The most common, rounded version is ‘spherical’ porosity, but if the holes are elongated, the problem could be described as ‘wormholes’ or piping. The good news is that porosity is preventable. Read on for the causes and how you can deal with weld porosity.

What causes porosity in welding?

Porosity generally results from the absorption of nitrogen, oxygen and hydrogen in the molten weld pool, which becomes trapped in the solidifying weld metal.

Nitrogen and oxygen in the weld pool usually result from inadequate shielding due to leaks in the gas line, incorrect shielding gas flow rates and draughts and turbulence in the weld pool. In fact, as little as just one per cent air in your shielding gas will cause porosity.

Hydrogen, meanwhile, originates from moisture from your workplace shielding gas delivery hoses, or oil-based (hydrocarbon) surface contamination.

Surface paints, coatings and treatments such as zinc, will also ‘gas off’ during welding.

The size, amount and location of the porosity can help to identify the possible cause.

How to deal with porosity in welding

Other causes of porosity include:

  • Poor/wrong choice of shielding gases
    Those containing hydrogen and/ or nitrogen can be beneficial for some materials, but cause porosity in others.
  • Rust, paint, water, oil and grease on the material being welded
    These can decompose in the heat of the welding arc to form gases which become trapped as the weld cools.

  • Poor equipment maintenance
    Loose hose fittings, gas leaks in the power source or torch, contamination of the nozzle from spatter, dirty wire or wire left on a welding machine.
  • Poor welder technique
    Too long a stick-out or bad torch angle.
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How can porosity be prevented?

Porosity is best controlled by eliminating the source of the gas that is generating the holes. Careful cleaning of your joint surfaces can also help.

  • Clean components to be welded
    Remove rust or paint by grinding, and oil, grease or hydrocarbons with degreasing agents.
  • Ensure your equipment is well maintained
    Ensure all of your gas hoses are leak free before commencing work.
  • Avoid weld pool turbulence
    Make sure your welding current is high enough to stabilise droplet transfer.

  • Use deoxidised filler wires where applicable
    Make sure wire isn’t contaminated by wiping a small sample with a clean cloth containing an acetone-based cleaning solution.
  • Use the correct shielding gas for your process and the right flow rate
    Too high a flow rate and air can get entrained into the weld; too low and the shielding gas cannot protect the cooling weld metal from the atmosphere. Using a flowmeter at the nozzle can ensure the flow remains at the recommended rate.
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How to remedy porosity in welding

Small areas of porosity can normally be removed by localised gouging or grinding.

If it’s widespread the entire weld should be removed and the joint re-prepared and re-welded.

Are you using the best welding gas for your job?


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