Frequently Asked Questions
Porosity is a fairly common defect which can be influenced by many factors. Sometimes it is clearly visible as pinholes in the weld surface, at other times it is below the surface and is revealed only by X-ray examination or ultrasonic testing. Unless it is gross or preferentially aligned, porosity is unlikely to be harmful.
Common causes of porosity are:
- Contamination of joint surfaces with oil, paint, grease, hydrated oxides, etc. These decompose in the arc to give gaseous products which can cause elongated 'wormhole' porosity often located along the centreline of the weld.
- Damp flux: flux should be kept dry. It is good practice to dry all fluxes before use and store them in a heated hopper. The manufacturer's recommendations regarding drying temperatures should be observed. Note that if a flux recovery unit, driven by compressed air, is used the compressed air should be dried thoroughly.
- Insufficient flux burden can expose the arc and molten weld pool to atmospheric contamination.
The surface of a weld may sometimes contain small depressions known as surface pocking or gas flats. These are harmless and while the exact cause is not fully understood it is linked to conditions which cause generation of gas or make it difficult for gas to escape; for example, moisture or lack of deoxidants and too many fines in the flux to allow gas to pass readily.