5
UNDERCUTTING As
shown in Figure 10-3, undercutting is a defect that appears as a groove in the
parent metal directly along the
edges of the weld. It is most common in lap fillet welds, but can also be encountered
in fillet and butt joints.
This type of defect is most commonly caused by improper welding parameters; particularly
the travel speed
and arc voltage. When
the travel speed is too high, the weld bead will be very peaked because of its
extremely fast solidification.
The forces of surface tension have drawn the molten metal along the edges of the
weld bead and piled
it up along the center. Melted portions of the base plate are affected in the
same way. The undercut
groove is where melted base material has been drawn into the weld and not allowed
to wet back properly
because of the rapid solidification. Decreasing the arc travel speed will gradually
reduce the size of the
undercut and eventually eliminate it. When only small or intermittent undercuts
are present, raising the arc
voltage or using a leading torch angle are also corrective actions. In both cases,
the weld bead will become
flatter and wetting will improve. Continued
on next page... Figure
10-3 – Examples of Undercutting
