Which statement about the B pressure weld root pass undercut is correct?

Which statement about the B pressure weld root pass undercut is correct?

• A. 0 inches undercut is required
• B. 0 inches undercut is prohibited
• C. Undercut must be at least 1/16 inch
• D. Undercut is not specified

Answer: (A)

Undercut at the weld root weakens the joint by removing material right at the fusion line, creating a notch that concentrates stress. For a root pass in a pressure weld, you want a solid, uninterrupted root fusion with no thinning of the base metal at the joint’s origin. That’s why zero undercut is required—the root must be free of any groove along the weld toe or root to maintain full thickness and strength under pressure.

Other statements would either imply allowing some undercut, requiring a minimum undercut, or providing no specification at all, all of which could compromise the joint’s integrity under pressure. Even a small amount of undercut at the root can become a weakness, especially in a pressure weld.