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)

In a B pressure weld root pass, the root must have no undercut. Undercut is a groove melted into the base metal at the weld toes, and when it appears at the root, it reduces the effective throat thickness where the weld bears the load and contains internal pressure. That thinner section becomes a stress concentration and a potential leak path, which is unacceptable for pressure-bearing joints. So the specification requires zero undercut at the root to ensure full cross‑sectional strength and a proper seal. Allowing any undercut or requiring a minimum undercut would weaken the root and risk leakage, which is why zero undercut is the correct standard.

In a B pressure weld root pass, the root must have no undercut. Undercut is a groove melted into the base metal at the weld toes, and when it appears at the root, it reduces the effective throat thickness where the weld bears the load and contains internal pressure. That thinner section becomes a stress concentration and a potential leak path, which is unacceptable for pressure-bearing joints. So the specification requires zero undercut at the root to ensure full cross‑sectional strength and a proper seal. Allowing any undercut or requiring a minimum undercut would weaken the root and risk leakage, which is why zero undercut is the correct standard.