Defects/Imperfections in Welds – Type of Porosity
Porosity is the presence of rounded cavities in the weld metal caused by the entrapment of gas as the weld solidifies. Porosity can occur in several forms:
- Uniform porosity
- Isolated porosity
- Group porosity
- Linear porosity
- Surface porosity
- Crater pipe
CAUSES AND PREVENTION:
Uniform, isolated, linear, group and surface porosity
Uniform porosity is normally found as fine pores throughout the weld. Isolated porosity occurs infrequently throughout the weld. Linear porosity occurs in a straight line and is also indicative of lack of fusion. Surface breaking pores usually indicate a large amount of uniform porosity.
Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal. Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air in the shielding gas will cause uniform porosity and greater than 1.5% may result in surface breaking pores. Draughts and excessive turbulence in the weld pool are also frequent causes of porosity.
Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the work piece surface. Grease and oil on the surface of the work piece or filler wire are also common sources of hydrogen.
The gas source should be identified and removed as follows: Avoid weld pool turbulence - reduce excessively high gas flow - avoid draughts - dry the electrode and flux - clean and degrease the work piece surface - clean the joint edges immediately before welding - check any weldable primer, if used, is below the recommended maximum thickness
Wormholes are elongated pores.
Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal.
Eliminating the gas and any cavities prevents wormholes - avoid a joint geometry which creates a cavity
A crater pipe forms during the final solidified weld pool in a weld crater.
This imperfection results from shrinkage on weld pool during solidification. Rapid termination of the arc will result in the rapid solidification of a large weld pool. In GTAW welding, autogenous techniques, or stopping the wire before switching off the welding current, will cause crater formation and may result in a crater pipe imperfection.
Crater pipe imperfections can be prevented by preventing any rapid arc termination and filling the crater by improving welder technique - use run-off tag in butt joints - grind out the stop before continuing with the next electrode or depositing the subsequent weld run. For welder technique - gradually reduce the welding current to reduce the weld pool size - add filler (GTAW) to compensate for the weld pool shrinkage.
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