66 Underfill

Underfill is a discontinuity in which the weld face or root surface extends below the adjacent surface of the base metal. Underfill occurs when a groove weld is not filled completely.

Underfill reduces the cross-sectional area of the weld below the amount required in the design.

Underfill tends to occur primarily in the flat position in groove welding and in the 5G and 6G pipe welding positions.

Underfill creates a region susceptible to structural failure from insufficient cross section to support the load. In fillet welds, underfill is exhibited by a less than normal throat as measured by the length of the leg. This is referred to as concavity. Underfill and concavity are detected by visual examination (VT).

Underfill Prevention

Underfill is prevented by reducing welding current and voltage, reducing arc length and arc travel speed, and adding sufficient filler metal.