Geometric Shape Imperfections - Undercut

This is an irregular groove at the toe of a run in the parent metal. The figure shows undercut at surface of a completed joint but it may also be found at the toes of each pass of a multi-run weld. The latter can result in slag becoming trapped in the undercut region.

Common causes

When arc and gas welding, undercut is probably the most common shape imperfection. With single-sided pipe welds it may also be found at the bore surface. It may also be seen on the vertical face of fillet welds made in the horizontal vertical position.

A wide spreading arc (high arc voltage) with insufficient fill (low current or high travel speed) is the usual cause. However, welder technique, especially when weaving, and the way the welding torch is angled can both cause and be used to overcome undercutting (ie angled to push the weld metal to fill the melted groove). High welding current will also cause undercut - this is generally associated with the need for a high travel speed to avoid overfilling of the joint.

Acceptance

Largely because this imperfection is widespread, most standards permit some level of undercut although they do require that a 'smooth transition is required. The limits in BS EN 25817 range from 0.5mm (stringent) to 1.5mm (moderate) while AWS D1.1 has a limit of 1mm. Standards may also place limits on the length of the undercut. For example, BS 2640 says, 'not exceed a total of 25mm in any 100mm length of weld'.

Measuring undercut can be a problem because of the small size of the imperfection compared with the general environment where there can be mill scale, irregularities in the surface and spatter. In critical applications the imperfection can be 'corrected' by blend grinding or by depositing an additional weld bead.

Avoidance

This imperfection may be avoided by reducing travel speed and/or the welding current and by maintaining the correct arc length.