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

• A. To finish the weld and hide defects.
• B. To establish initial fusion at root, provide a stable base for subsequent passes, and reduce porosity risk.
• C. To apply final coating.
• D. To cool the weld quickly to reduce energy.

Answer: (B)

The root pass is about establishing a solid foundation at the bottom of the joint. It penetrates to the root to ensure fusion across the full thickness and sets the stage for the remaining passes. A proper root pass provides a stable base that the subsequent passes can build on, helping to maintain correct joint geometry and minimizing the risk of defects like lack of fusion and porosity as you add more weld metal. When done with the right joint preparation, heat input, and shielding, it helps keep the root clean and fully fused, which is essential for a sound overall weld. Finishing, coating, or rapid cooling are not the goals of the root pass, which is focused on forming a reliable root foundation for the rest of the weld.