The characteristic features and principal
causes of porosity imperfections are described. Best practice guidelines are
given so welders can minimise porosity risk during fabrication.
Identification
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
- surface breaking pores
- wormhole
- crater pipes
Cause and prevention
Distributed porosity and surface pores
Distributed porosity (Fig.
1) is normally found as fine pores throughout the weld bead. Surface
breaking pores (Fig. 2) usually indicate a large amount of distributed
porosity
 |
Fig. 1. Uniformly distributed porosity |
 |
Fig. 2. Surface breaking pores (T fillet weld in primed
plate) |
Cause
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 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 (see
Fig 2). 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.
Prevention
The gas source should be identified and removed as follows:
Air entrainment
- - seal any air leak
- - avoid weld pool turbulence
- - use filler with adequate level of deoxidants
- - reduce 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
Wormholes
Elongated pores or wormholes |
 |
Characteristically, wormholes are elongated
pores (Fig. 3) which produce a herring bone appearance on the
radiograph.
Cause
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 20µ, through over-spraying.
Prevention
Eliminating the gas and cavities prevents wormholes.
Gas generation
- - clean the workpiece surfaces
- - remove any coatings from the joint area
- - check the primer thickness is below the manufacturer's maximum
Joint geometry
- - avoid a joint geometry which creates a cavity
Crater pipe
A crater pipe forms during the final solidified weld pool
and is often associated with some gas porosity.
Cause
This imperfection results from shrinkage on weld
pool solidification. Consequently, conditions which exaggerate the liquid to
solid volume change will promote its formation. Switching off the welding
current will result in the rapid solidification of a large weld pool.
In TIG welding, autogenous techniques, or stopping the wire before switching
off the welding current, will cause crater formation and the pipe imperfection.
Prevention
Crater pipe imperfection can be prevented by removing the
stop or by welder technique.
Removal of stop
- - use run-off tag in butt joints
- - grind out the stop before continuing with the next electrode or
depositing the subsequent weld run
Welder technique
- - progressively reduce the welding current to reduce the weld pool size
- - add filler (TIG) to compensate for the weld pool shrinkage
Porosity susceptibility of materials
Gases likely to cause porosity in
the commonly used range of materials are listed in the Table.
Principal gases causing porosity and recommended cleaning methods
| Material |
Gas |
Cleaning |
| C Mn steel |
Hydrogen, Nitrogen and Oxygen |
Grind to remove scale coatings |
| Stainless steel |
Hydrogen |
Degrease + wire brush + degrease |
| Aluminium and alloys |
Hydrogen |
Chemical clean + wire brush + degrease + scrape |
| Copper and alloys |
Hydrogen, Nitrogen |
Degrease + wire brush + degrease |
| Nickel and alloys |
Nitrogen |
Degrease + wire brush + degrease |
Detection and remedial action
If the imperfections are surface breaking,
they can be detected using a penetrant or magnetic particle inspection
technique. For sub surface imperfections, detection is by radiography or
ultrasonic inspection. Radiography is normally more effective in detecting and
characterising porosity imperfections. However, detection of small pores is
difficult especially in thick sections.
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 procedure.
Copyright by TWI, 1999
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