What is the purpose of a root pass in a groove weld?

• A. To finish the final weld bead.
• B. To preheat the joint.
• C. To remove slag from the weld.
• D. To establish root fusion and penetration between the base metals.

Answer: (D)

The root pass is the initial weld that fuses the base metals at the bottom of the groove, establishing root fusion and penetration. This first bead creates a continuous metal path through the joint, tying the two pieces together from root to toe. Proper root fusion and penetration are essential for a sound weld; without them, the joint can have lack of fusion or insufficient penetration, weakening the overall weld and undermining strength and service life. The root pass sets up the weld for the remaining passes (filler and cap) that build up and finish the weld. It isn’t about finishing the final bead, preheating, or slag removal, though those factors can influence how effectively the root is achieved.