Cavities

a.      Gas Pore

A gas cavity of essentially spherical shape trapped within the weld metal.

This cavity can be present in various forms :

-         isolated

-         uniformly distributed porosity

-         clustered (localized) porosity

-         linear porosity

-         elongated cavity

-         surface pore

Causes	Prevention
Damp fluxes / Corroded electrode (MMA)	Use dry electrodes in good condition
Grease/hydrocarbon/water contamination or prepared surface	Clean prepared surface
Air entrapment in gas shield (MIG/MAG TIG)	Check hose connection
Incorrect/insufficient deoxidant in electrode, filler or parrent metal	Use electrode with sufficient deoxidation activity
Too high an arc voltage or arc lenght	Reduce voltage and arc length
Gas evolution from priming paints/surface treatment	Identify risk of reaction before surface treatment is applied
Too high of a shielding gas flow rate which results in turbulance (MIG/MAG TIG)	Optimise gas flow rate

a.      Worm Holes

Elongated or tubular cavities formed by entrapped gas during the solidification of the weld metal.

Causes	Prevention
Gross contaminated of preparation surface	Introduce preweld cleaning procedures
Laminated work surface	Replace parent material with an unlaminated piece
Crevices in work surface due to joint geometry	Eliminate joint shapes which produce crevices

Wormholes 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 radiograph

a.      Surface Porosity

Gas Pore that breaks the surface of the weld

Causes	Prevention
Damp or contaminated surface or electrode	Clean surface and dry electrode
Low fluxing activity (MIG/MAG)	Use  high activity flux
Excess sulphur (particulary free cutting steels) producing sulfur dioxide	Use high manganese electrode to produce MnS, note free-cutting steels (high sulfur) should not normaly be welded
Loss of shielding gas due to long arc or high breeze (MIG/MAG)	Improve screening againts draughts and reduce arc length
Too high of shielding  gas flow rate which results in turbulance (MIG/MAG/TIG)	Optimise gas flow rate

a.      Crater Pipe

 A shrinkage cavity at the end of a weld run. The main cause is shrinkage during solidification

Causes	Prevention
Lack of welder skill due to using process with too high a current	Retrain welder
Inoperative crater filler (Slope out) TIG	Use correct crater filling techniques

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