Ignite Your Welding Genius with the 2026 Metallurgy Mastery Test!

Fast travel speed cuts the heat input per unit length. With less energy entering each section of the joint, the weld pool becomes narrower and cooler, so fusion at the weld toe with the base metal is reduced. That limited fusion and filling along the toe can create a groove in the base metal where the weld metal doesn’t fully support or fill the edge—a defect known as an undercut. So, the faster you move, the more likely you are to see undercut, because the reduced heat input prevents adequate filling at the toe. While reduced energy input and lower penetration/deposition can accompany fast travel, the most direct and characteristic consequence is the increased tendency for undercut.