Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

What are wormholes and how can they be prevented?


Wormholes are elongated pores which produce a herringbone appearance on a radiograph. Their presence indicates that a large amount of gas has been formed which is then trapped in the solidifying weld metal. Excessive gas will be formed from gross surface contamination or very thick primer or paint coverings. 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 20µm, as a result of overspraying.


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 thickness

Joint geometry

- avoid a joint geometry which creates a cavity

For more information please email: