What is the purpose of a root pass?

• A. To complete the final weld and seal the joint.
• B. To establish root fusion and penetration in the joint.
• C. To remove slag from the interior of the weld.
• D. To verify alignment before filling passes.

Answer: (B)

The key idea is that the root pass creates the initial weld at the joint root and ensures fusion and penetration there. It is the first layer placed from the root side that fuses the base metals together at the base of the joint, forming a solid foundation for the rest of the weld. If the root pass achieves proper fusion and penetration, the subsequent filler passes can build up the weld without risking lack of fusion at the root. The other options describe outcomes that happen later (sealing the joint with the final weld) or steps performed before or after welding (slag removal or fit-up/alignment verification), rather than the fundamental purpose of the root pass.