What is Welding Porosity?
Weld porosity is defined as a defect in the weld caused by the absorption of nitrogen, oxygen, and hydrogen gases that become trapped in the molten weld puddle and release during the solidification phase, causing cavities or pores either on the surface or within the bead. Porosity can be on the surface of the weld or within the weld bead.
The absorption of nitrogen, very prominent in the case of duplex stainless steels, and oxygen can generally be traced to poor gas shielding. Porosity can be random, oriented randomly, uneven distances apart or isolated, spaced 1″ apart on all sides.
Porosity refers to the presence of cavities in the weld metal caused by the freezing in of gases released from the weld pool during the solidification of the weldment. Porosity can assume several shapes:
- Distributed Porosity: Pores that are distributed in the weld.
- Surface Breaking Pores: Pores that break the surface.
- Wormhole: Elongated pores that appear on the radiograph and resemble a herringbone pattern.
- Crater Pipes: A shrinkage cavity of the Weld Run that is formed by shrinkage during the solidification phase.
Cause and prevention of defects in welding
1. Distributed porosity and surface pores
Discrete porosity generally exists as tiny pores throughout the weld bead. Pores which break the surface usually indicate a high incidence of discrete porosity.
This is also caused by nitrogen, oxygen and hydrogen being absorbed into the weld pool which will inevitably be released when the weld pool solidifies and become trapped in the weld metal.
Nitrogen and oxygen absorption into the weld pool are predominantly due to poor gas shielding. The shielding gas can draw air, 0.5% is enough to induce discrete porosity and 1.0% will produce gross surface breaking pores.
Porosity can also occur due to leaks within the gas line, gas flow rates exceeding the manufacturer/establish limits, draughts or excessive turbulence in the weld pool.
Hydrogen can come from multiple sources including moisture contained in electrodes that have not been properly dried, moisture in fluxes or moisture from the surface of the workpiece, but grease on the surface of the workpiece or filler wire may also emit hydrogen.
Welding fume can also evolve from surface coatings such as primer paints, and surface treatments such as zinc coatings. The chance of entrainment of the evolved gas will be greater in T joints than butt joints, especially when fillet welding on both sides.
With regard to primer paints, special care should be taken with weldable (low zinc) primers. It should be safe to weld without removing the primer; however, if the paint has been put on thicker than the manufacturer recommends, then the risk of porosity will increase, 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:
Air entrainment
- Seal any air leak
- Avoid weld pool turbulence
- Use filler with an adequate level of deoxidants
- Reduce the excessively high gas flow
- Avoid draughts
Hydrogen
- Dry the electrode and flux
- Clean and degrease the workpiece surface
Surface coatings
- Clean the joint edges immediately before welding
- Check that the weldable primer is below the recommended maximum thickness
2. Wormholes
Wormholes are tubular voids in the weld pattern that can be seen radiographically and appear as herringbone pattern. Wormholes are a sign of significant gas generation that is trapped by weld metal solidifying.
Excessive gas generation occurs due to gross surface contamination or excessive thickness of paint or primer. It is more likely that entrapped gas is formed in crevices, like the gap between the vertical member in horizontal to vertical T joint fillet weld on both sides.
When welding T joints in primed plates, it is very important that the thickness of the coating on the edge of the vertical member does not exceed the maximum thickness recommended by the manufacturer, usually 20µm, from overspraying.
How to prevent Wormholes?
Eliminating the gas and cavities prevents wormholes.
Gas generation
- Clean the surfaces of the workpieces at and around the area where the weld is to be made.
- Remove any surface contamination such as oil, grease, rust, and NDT residues.
- Remove any surface coating on the area being joined to expose bright material.
- Confirm the primer thickness is less than the manufacturer’s maximum.
Joint geometry
- Avoid a joint geometry that creates a cavity
3. Crater pipe
A crater pipe occurs during solidification of the weld pool, and is often present with gas porosity.
These discontinuities occur alongside shrinkage during weld pool solidification. Therefore, anything that increases the liquid-to-solid volume change will increase the crater pipe. Stopping, or extinguishing, the welding arc will cause the weld pool to solidify quickly.
In TIG welding, the use of autogenous methods, or stopping the welding wire entering the weld pool prior to extinguishing the welding arc, will influence the crater pipe and could promote the crater pipe imperfection.
How to prevent Crater pipes?
Limiting crater pipe imperfection can be achieved by controlling the speed at which the welding arc is extinguished or by using a welder technique on moving the welding arc and welding wire.
Removal of stop
- Utilize run-off tabs to allow the welding arc to become extinguished outside of the welded joint.
- Grind out the weld run stop crater prior to continuing on with the next electrode or depositing the next weld run.
Welder technique
- Gradually decrease the current during the welding process, to decrease the weld pool size (slope-down or crater fill functions)
- Add filler (tig) to compensate for the shrinkage of the weld pool
How to fix porosity in welds?
Despite all of the precautions taken to avoid porosity, there is always a risk of porosity occurring in your welds. Fortunately, porosity can be repaired in some situations.
To repair porosity, you will first need to establish the presence of porosity through a penetrant or magnetic particle inspection technique, such as radiographic or ultrasonic inspection (for subsurface imperfections).
It will be difficult to establish the existence of small localized pores, especially in a thick section of steel.
Porosity cannot be repaired in the strictest sense. You will be repairing the weld by removing the localized gouging, or grinding it out, assuming the porosity is localized.
If it is widespread, you will need to remove all of the weld, re-preparation the joint, and weld it according to the welding procedure.
How much porosity is acceptable in a weld?
There is no general correct answer for this question, because it depends on your welding construction code.
However, if you follow the American Welding Society, it states that visible porosity should be 3/8 inches (or 9.4mm) or less in any linear inch of the weld, and 3.4 inches (or 19mm) in a 12-inch length of the weld.
When in doubt, check your welding construction code and redo the weld.
FAQs
How is porosity prevented?
To prevent weld porosity, it is important to use the correct welding techniques and to ensure that the welding equipment and materials are in good condition. This includes properly cleaning and preparing the base material, using the correct welding wire, and ensuring that the shielding gas coverage is adequate.
What is the defect of porosity?
Porosity is the formation of holes in the weld pool resulting from gas bubbles that cannot escape. It is usually one of the common welding defects when using shielding gas, which is present in welding techniques such as TIG and stick welding.
What welding process can cause porosity to form?
These voids are usually caused by gases that are trapped in the molten metal during the welding process. Porosity can occur in any type of welding, including MIG, TIG, and stick welding. To weld porosity it can make the weld weak.
Can you burn porosity out of a weld?
Some welders claim you can essentially burn through the porosity by turning your amperage up over 310 and welding back through your bad weld. However, this could potentially lead to all sorts of other problems depending on how it’s performed.
How do you fix porosity in welding?
Remedial action normally needs removal by localised gouging or grinding but if the porosity is widespread, the entire weld should be removed. The joint should be re-prepared and re-welded as specified in the agreed welding procedure.
How do you eliminate porosity?
The techniques used to reduce porosity include mechanical or ultrasound mixing, pressurization of the cement, centrifugation of the mixture and vacuum mixing. All these techniques result in a reduction of porosity from about 8% which is achieved by conventional hand mixing to values below 1% for vacuum mixing.