What is undercut, and why is it undesirable?

What is undercut, and why is it undesirable?

• A. A groove in the weld root caused by excessive heat or slow travel; weakens the joint and concentrates stress.
• B. A void in the weld caused by gas entrapment.
• C. A raised bead that strengthens the joint.
• D. A layer of oxide at the weld surface.

Answer: (A)

Undercut is when a groove forms along the edge of the weld where the base metal has melted away but isn’t filled by weld metal. This happens when heat input is too high or the travel speed is too slow, causing the molten metal at the toe to erode the edge faster than the filler metal can fill it. The result is a notch-like recess that reduces the effective cross-section of the weld at the root and creates a stress concentration. Under load, especially cyclic loading, this makes the joint weaker and more prone to cracking. Other conditions—gas porosity (voids from gas entrapment), an overbuilt raised bead, or an oxide layer—are different imperfections and don’t describe the groove formed at the toe due to excess heat and deposition issues. To prevent it, adjust heat input and travel speed, and ensure proper technique to promote full fusion and fill at the weld toe.

Undercut is when a groove forms along the edge of the weld where the base metal has melted away but isn’t filled by weld metal. This happens when heat input is too high or the travel speed is too slow, causing the molten metal at the toe to erode the edge faster than the filler metal can fill it. The result is a notch-like recess that reduces the effective cross-section of the weld at the root and creates a stress concentration. Under load, especially cyclic loading, this makes the joint weaker and more prone to cracking. Other conditions—gas porosity (voids from gas entrapment), an overbuilt raised bead, or an oxide layer—are different imperfections and don’t describe the groove formed at the toe due to excess heat and deposition issues. To prevent it, adjust heat input and travel speed, and ensure proper technique to promote full fusion and fill at the weld toe.