What is the purpose of the root pass in a multi-pass weld?

• A. It establishes penetration and fusion at the joint root, providing a base for subsequent fill and cap passes.
• B. It is only for reducing workpiece warpage.
• C. It seals the joint with no fusion.
• D. It is solely for cosmetic appearance.

Answer: (A)

The root pass is the initial bead that penetrates to the joint root to fuse the base metals. Its main job is to establish full penetration and fusion at the root so the joint has a solid foundation for the remaining passes to build on. With a proper root, the subsequent fill and cap passes can reliably fuse to that base and create the required weld thickness and strength. If the root doesn't achieve proper fusion or penetration, gaps or lack of fusion can occur, weakening the weld and potentially leading to cracks. Warpage control and cosmetic appearance are not the primary purposes of the root pass, and sealing the joint with no fusion would defeat its purpose; the emphasis is on achieving penetration and fusion at the root.