What is meant by "root pass" in pipeline welding?

The term "root pass" in pipeline welding refers specifically to the first pass of weld metal that is applied at the joint root area, where the two pieces of material meet. This initial weld is crucial because it establishes the foundation for subsequent layers of weld metal.

The root pass is fundamental for ensuring that there is good penetration into the base material, which is critical to achieving the desired weld strength and integrity. Proper execution of the root pass can help prevent defects such as lack of fusion or insufficient penetration, which could compromise the overall quality of the weld.

This initial pass often forms the underlying structure that later passes build upon, ensuring that the weld joint remains strong and watertight. The root pass may be completed with careful attention to technique and parameters (like heat input and travel speed) to ensure optimal quality. This is why it is important in the context of API 1104, where the standards focus on the quality and reliability of welded joints in pipelines.