Which factors control an undercut?

• A. Preheating only
• B. Shielding gas variations
• C. Heat input, electrode size, travel speed, and technique
• D. Post-weld heat treatment only

Answer: (C)

Undercut is a groove melted into the base metal right at the weld toe where the weld bead fails to fill, leaving a weak step that can concentrate stress. What controls its formation are four interacting factors: heat input, electrode size, travel speed, and technique.

Heat input determines how much energy is delivered to the joint per unit length. If the heat input is too high or the arc stays in one spot too long, the base metal at the toe can melt away faster than the weld metal can fill it, producing an undercut. Conversely, appropriate heat input helps the weld pool flow and fill the toe properly. Electrode size affects how much filler metal is deposited; a too-small electrode may not provide enough filler to fill the toe, increasing the risk of undercut, while a properly sized electrode helps build a bead that covers the toe. Travel speed matters because moving too fast reduces the time available for filler deposition and bead formation at the toe, leaving a groove behind; slower, controlled travel allows adequate deposition and better toe fill. Technique encompasses arc length, angle, and how the filler metal is directed into the toe; good technique ensures the bead adheres to and fills the toe rather than pulling away from it.

Other factors aren’t the primary controls of undercut. Preheating alters base metal temperature but isn’t the main determinant of whether the toe is filled. Shielding gas variations influence arc stability and contamination risks more than the toe fill itself. Post-weld heat treatment doesn’t prevent or remove undercut; it affects material properties after welding rather than the weld bead geometry.