WHAT IS UNDERCUT? A DEFECT OR DISCONTINUITY.

Undercut is a surface discontinuity that occurs in the base metal directly adjacent to the weld bead; depending on its size, it may be classified as a discontinuity or a defect. It is a condition in which the base metal has been melted during the welding process but has been insufficiently filled by the filler metal, resulting in a depression. The result is a linear groove in the base metal with a relatively sharp configuration. Since it is a surface condition, it is particularly dangerous for structures that will be subjected to cyclic loading.

The typical appearance of undercut in a fillet weld and a groove weld is evident at the weld toe as a break in the base metal. It is important to emphasize that in groove welds, undercut may occur on either the face or the root of the weld.

The appearance of undercut in a fillet weld can be detected visually more easily; that is, a shadowed area is observed when a light source is placed in an appropriate position. Welding inspectors must be aware of this phenomenon and employ techniques such as illuminating the base metal surface with white light to produce contrast where undercut is present.

Another technique is to visually inspect the part after painting it white or yellow; in this case, when the undercut is observed under normal lighting conditions, it becomes evident. The only drawback of this technique is that once the defect is detected, the paint must be removed before correcting it, in order to avoid the occurrence of another defect such as porosity. To avoid this, a liquid penetrant inspection technique may be used. Remember that it is important to correct this defect, as it is a stress concentration point.

Undercut is normally the result of improper technique on the part of the welder. More specifically, if the travel speed is excessive, insufficient filler metal is deposited to adequately fill the depressions caused by the melting of the base metal adjacent to the weld bead. Undercut may also occur when welding conditions generate excessive heat input, causing excessive melting of the base metal.

When undercut is observed on a radiograph, it appears dark with blurred indications at the edge of the weld reinforcement.

UNDERFILL

Underfill is a surface discontinuity that results in a loss of material in the cross-section of the weld bead. Furthermore, underfill occurs in welds where undercut has been detected in the adjacent base metal; in other words, underfill results when the deposited metal is insufficient to fill the joint.

As with undercut, underfill can occur on either the face or the root of the weld bead.

Underfill at the weld root in pipe welds is sometimes referred to as Internal Shrinkage, since it can be caused by excessive reheating of the root pass when the second pass, or hot pass, is deposited. Also, as with undercut, when the part is illuminated appropriately, a shadow is produced as a result of the surface depression.

The main cause of underfill is lack of welder skill; if welding is performed at an excessive travel speed, the filler metal is not allowed to melt and deposit properly to fill the groove flush with the base metal surface.