TWI Frequently asked questions
by Chris Miller
To reduce the number of welding procedure qualifications that are required, P-Numbers are assigned to base metals according to the characteristics of the material. Materials with similar characteristics are assigned the sameP-Number and these are listed in Table QW/QB-422.
Materials that have been assigned P-Numbers are confined to those given in ASME Section II (Material Specifications). Thus, material grades in accordance with other standards such as BS EN, ASTM, etc cannot be assigned a P-Numberand are classified as unassigned materials.
Table QW/QB-422 lists some materials that are allowed by ASME/ANSI B31 Codes, or by selected Code Cases, but are not listed in ASME Section II and these materials have been assigned S-Numbers. A material with a particular S-Numberwill have similar characteristics to a material with the same P-Number but P-Numbers and S-Number materials cannot be indiscriminately interchanged for welding procedure qualifications.
QW-420-2 gives rules regarding the use of P-Number and S-Number materials for welding procedure qualification, namely a P-Number material will qualify a material with the same S-Number but not vice versa.
Welding procedure qualification using an unassigned material will only qualify that material.
