What is Welding Porosity?
Weld porosity is a welding defect caused by the absorption of nitrogen, oxygen, and hydrogen gases trapped in the molten weld puddle and release during solidification, which causes pockets or pores on the surface or within the bead. Porosity can occur on the surface of a weld or within the weld bead.
The absorption of nitrogen and oxygen in the weld pool is usually due to poor gas shielding. Porosity can be random: oriented at random, uneven distances. It can also be isolated: spaced one inch apart from all sides.
Porosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms:
- Distributed Porosity – Pores that are distributed throughout the weld.
- Surface Breaking Pores – Pores that break the surface.
- Wormhole – Elongated pores that show up resembling a herringbone pattern on the radiograph
- Crater Pipes – A shrinkage cavity at the end of the Weld Run caused by shrinkage during solidification.
Cause and prevention of defects in welding
1. Distributed porosity and surface pores
Distributed porosity is normally found as fine pores throughout the weld bead. Surface-breaking pores usually indicate a large amount of distributed porosity. It 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 entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface-breaking pores. Leaks in the gas line, too high a gas flow rate, draughts, and excessive turbulence in the weld pool are frequent causes of porosity.
Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes, or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.
Surface coatings like primer paints and surface treatments such as zinc coatings may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides.
Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer’s recommendation, porosity is likely to result especially when using welding processes other than MMA.
How to prevent Distributed porosity and surface pores?
The gas source should be identified and removed as follows:
- Seal any air leak
- Avoid weld pool turbulence
- Use filler with an adequate level of deoxidants
- Reduce the excessively high gas flow
- Avoid draughts
- Dry the electrode and flux
- Clean and degrease the workpiece surface
- Clean the joint edges immediately before welding
- Check that the weldable primer is below the recommended maximum thickness
Characteristically, wormholes are elongated pores that produce a herringbone appearance on the radiograph. Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal.
Excessive gas will be formed from gross surface contamination or very thick paint or primer coatings. Entrapment is more likely in crevices such as the gap beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both sides.
When welding T joints in primed plates it is essential that the coating thickness on the edge of the vertical member is not above the manufacturer’s recommended maximum, typically 20µm, through over-spraying.
How to prevent Wormholes?
Eliminating the gas and cavities prevents wormholes.
- Clean the workpiece surfaces at and adjacent to the location where the weld will be made
- Remove any surface contamination, in particular, oil, grease, rust, and residue from NDT operations
- Remove any surface coatings from the joint area to expose bright material
- Check the primer thickness is below the manufacturer’s maximum
- Avoid a joint geometry that creates a cavity
3. Crater pipe
A crater pipe forms during the final solidification of the weld pool and is often associated with some gas porosity.
This imperfection results from shrinkage on weld pool solidification. Consequently, conditions that exaggerate the liquid-to-solid volume change will promote its formation. Extinguishing the welding arc will result in the rapid solidification of the weld pool.
In TIG welding, autogenous techniques, or stopping the welding wire entering the weld pool before extinguishing the welding arc, will affect crater formation and may promote pipe imperfection.
How to prevent Crater pipes?
Crater pipe imperfection can be prevented by controlling the rate at which the welding arc is extinguished or by the welder technique manipulating the welding arc and welding wire
Removal of stop
- Use run-off tag to enable the welding arc to be extinguished outside the welded joint
- Grind out the weld run stop crater before continuing with the next electrode or depositing the subsequent weld run
- Progressively reduce the welding current to reduce the weld pool size (use slope-down or crater fill functions)
- Add filler (tig) to compensate for the weld pool shrinkage
How to fix porosity in welds?
Even with all the different ways to prevent porosity, there will always be a small chance of it appearing in your welds. Thankfully, porosity can be fixed under certain circumstances.
You first need to detect the presence of porosity by using either a penetrant or a magnetic particle inspection technique, such as radiography or ultrasonic inspection (for subsurface imperfections). Small pores, especially in thicker sections, are difficult to detect.
The porosity itself cannot exactly be fixed. Rather, you can fix the weld by removing the localized gouging, or grinding it out. This is only if the porosity is contained, however. If it is widespread, you need to remove the entire weld, prepare the joint again, and weld according to the welding procedure.
How much porosity is acceptable in a weld?
There is no general sound and true answer to this question, as it depends on your welding construction code. That being said, the American Welding Society mandates the following: diameters of visible porosity should be 3/8 inches (or 9.4mm) or less, in any linear inch of the weld, and be 3.4 inches (or 19mm) in a 12-inch length of the weld.
When in doubt, you need to check your welding construction code and redo the weld.