To reduce the number of welding procedure qualifications that are required, P-Numbers (shortened to P-No) are assigned to base metals according to the characteristics of the material, such as composition, weldability, brazeability and mechanical properties. Materials with similar characteristics are assigned the same P-Number and these are listed in Table QW/QB-422.
Material grades in accordance with standards other than ASME, such as API (American Petroleum Industry), EN (European grades), JIP (Japanese grades) etc, are assigned a P-Number only if their exact designation is listed in QW-QB/422. In recent years, an increasing number of non-ASME base metals have been added to this list (eg S235JR or API 5L X60, both assigned to P-No 1).
Unlisted base metals cannot be assigned to a P-Number, even though they are similar or equivalent to a listed base metal, these are classified as 'unassigned' fore the purpose of welding procedure qualification. Welding procedure qualification using an unassigned material will only qualify that material.
A further useful reference regarding P-numbers is ISO/TR 20173:2008, which provides information on ASME P-numbers as well as ISO grouping for ASME/ASTM materials.
Note on S-numbers: In 2009, S-Numbers were removed from table QW/QB-422. S-Numbers were assigned to materials that were acceptable for use by the ASME B31 Code for Pressure Piping, or by selected Boiler and Pressure Vessel Code Cases, but which were not included within ASME Boiler and Pressure Vessel Code Material Specifications (Section II). Base metals previously assigned S-Numbers were reassigned the corresponding P-Numbers or P-Numbers plus Group Numbers.