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

• A. To establish root fusion and a stable base for subsequent filler passes.
• B. To cap the weld with final bead.
• C. To remove slag from the surface.
• D. To reduce heat input.

Answer: (D)

The root pass is about establishing a solid foundation for the weld by creating fusion at the root of the joint and ensuring proper penetration. This first layer ties the base metals together along the groove, providing a stable base for the subsequent filler passes to fill and cap. If the root isn’t fused adequately, later passes may not bond correctly and defects such as lack of fusion or porosity can occur, compromising strength. The final cap is what finishes and strengthens the top surface, while slag removal and other cleanup steps happen after each pass. Heat input is a factor in all passes, but the primary purpose of the root pass is to secure a good root fusion and a stable base for the remaining weld layers.