What is the difference between a root pass and a fill pass?

• A. Root pass creates the initial weld to fuse base metals; fill passes add material to build the weld to final thickness.
• B. Fill passes add material to build the weld to final thickness.
• C. Root pass uses filler material only; fill pass uses base metal.
• D. Root pass is done after cleaning; fill pass is done before cleaning.

Answer: (A)

In multi-pass welding, the root pass and fill passes serve distinct roles. The root pass is the first weld that fuses the base metals along the joint, establishing the initial bond and root penetration. Fill passes are added after that to deposit more weld metal, building the weld up to the final thickness and shape. This sequence ensures a solid foundational bond at the root, followed by controlled material buildup to achieve the required size and strength. The other ideas mix up the purposes or sequencing—for example, filling all thickness with fills alone, or misrepresenting which material is used in each pass—so they don’t accurately describe how a weld is built.