Root Concavity - Suck Back

A shallow groove that may occur in the root of a butt weld.

Common causes

Root concavity is caused by shrinkage of the weld pool in the through-thickness direction of the weld. Melting of the root pass by the second pass can also produce root concavity.

This imperfection is frequently associated with TIG welding with the most common cause being poor preparation leaving the root gap either too small or, in some cases, too large. Excessively high welding speeds make the formation of root concavity more likely.

Acceptance

The root concavity may be acceptable. This will depend on the relevant standard being worked to. For example:

BS 2971 requires that:

a) there is complete root fusion

b) the thickness of the weld is not less than the pipe thickness.

ASME B31.3 requires that the 'total joint thickness, including weld reinforcement, must be greater than the weld thickness.

BS EN 25817 sets upper limits related to the quality level, eg Moderate, D, h  1.5mm and for Stringent, B, h  0.5mm. Furthermore, a smooth transition is required at the weld toes.

In effect the standards require that the minimum design throat thickness of the finished weldment is achieved. If the first two conditions of acceptance are met but the weld face does not have a sufficiently high cap, additional weld metal may be deposited to increase the throat.

Avoidance

It is important to ensure that joint fit-up is as specified in the welding procedure and that the defined parameters are being followed. If welder technique is the problem then retraining is required.