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GAS
PORE
A gas cavity of essentially spherical shape trapped
within the weld metal.
Gas cavity can be present in various forms:
Isolated
Uniformly distributed porosity
Clustered (localised) porosity
Linear porosity
Elongated cavity
Surface pore
Causes
Damp fluxes/corroded electrode (MMA)
Grease/hydrocarbon/water contamination of prepared surface
Air entrapment in gas shield (MIG/MAG, TIG)
Incorrect/insufficient deoxidant in electrode,
filler or parent metal
Too high an arc voltage or length
Gas evolution from priming paints/surface treatment
Too high a shielding gas flow rate which results in
turbulence (MIG/MAG, TIG)
Prevention
Use dry electrodes in good condition
Clean prepared surface
Check hose connections
Use electrode with sufficient deoxidation activity
Reduce voltage and arc length
Identify risk of reaction before surface treatment
is applied
Optimise gas flow rate
Comments
Note that porosity can either be localised or finely
dispersed voids throughout the weld metal.
WORM
HOLES
Elongated or tubular cavities formed by entrapped
gas during the solidification of the weld metal; they can occur singly or in
groups.
Causes
Gross contamination of preparation surface
Laminated work surface
Crevices in work surface due to joint geometry
Prevention
Introduce pre weld cleaning procedures
Replace parent material with an unlaminated piece
Eliminate joint shapes which produce crevices
Comments
Worm holes are caused by the progressive entrapment
of gas between the solidifying metal crystals (dendrites) producing
characteristic elongated pores of circular cross-section. These elongated pores
can appear as a herring-bone array on a radiograph. Some of them may break the
surface of the weld.
SURFACE
POROSITY
A gas pore that breaks the surface of the weld
Causes
Damp or contaminated surface or electrode
Low fluxing activity (MIG/MAG)
Excess sulphur (particularly free cutting steels)
producing sulphur dioxide
Loss of shielding gas due to long arc or high
breezes (MIG/MAG)
Too high a shielding gas flow rate which results in
turbulence (MIG/MAG, TIG)
Prevention
Clean surface and dry electrodes
Use a high activity flux
Use high manganese electrode to produce MnS, note
free-cutting steels (high sulphur) should not normally be welded
Improve screening against draughts and reduce arc
length
Optimise gas flow rate
Comments
The origins of surface porosity are similar to those
for uniform porosity.
CRATER
PIPE
A shrinkage cavity at the end of a weld run, the
main cause is shrinkage during solidification.
Causes
Lack of welder skill due to using processes with too
high a current
Inoperative crater filler (slope out) (TIG)
Prevention
Retrain welder
Use correct crater filling techniques
Comments
Crater filling is a particular problem in TIG welding
due to its low heat input. To fill the crater for this process it is necessary
to reduce the weld current (slope out) in a series of descending steps until
the arc is extinguished.
Comments
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