ASME Weld Categories and Types

Many requirements for welds in the ASME code depend on a weld’s category and its type, but those two classifications often get mixed up. So what are they, and what is the difference between them? A weld’s category is based on its location on the vessel, and a weld’s type is determined by how it is welded. For example, longitudinal shell seams are category A, circumferential seams are category B, and other categories are shown in Figure UW-3:

A Type 1 vessel weld is a butt joint, welded from both sides to achieve full penetration. A Type 2 weld is a butt joint welded from one side, with a permanent backing strip left in place. Other types are defined in Table UW-12. 

The joint efficiency of a weld is determined by its type and the amount of non-destructive examination of the joint. The weld’s category will determine which design formulas should be used to calculate stress in the weld.

Contact us if you have any questions on weld categories, types, or any other code questions.