Wormholes / Crater Pipe

Elongated pores or wormholes

Characteristically, wormholes are elongated pores (Fig. 3) which produce a herring bone appearance on the radiograph.

Fig. 3

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 20µm, 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.