An excerpt from ASME IX
QW-420 Material Groupings
QW-420.1 P-Numbers and S-Numbers. Base metals have been assigned P-Numbers or S-Numbers for the purpose of reducing the number of welding and brazing procedure qualifications required. In addition, ferrous base metals have been assigned Group Numbers creating subsets of P-Numbers and S-Numbers that are used when WPSs are required to be qualified by impact testing by other Sections or Codes. These assignments are based essentially on comparable base metal characteristics, such as composition, weldability, brazeability, and mechanical properties, where this can logically be done. These assignments do not imply that base metals may be indiscriminately substituted for a base metal that was used in the qualification test without consideration of compatibility from the standpoint of metallurgical properties, postweld heat treatment, design, mechanical properties, and service
requirements. The following table shows the assignment groups for various alloy systems:
When a base metal with a UNS number designation is assigned a P- or S-Number or P- or S-Number plus Group Number, then a base metal listed in a different ASME material specification with the same UNS number shall be considered that P- or S-Number or P- or S Number plus Group Number. For example, SB-163, UNS N08800 is P-No. 45; therefore, all ASME specifications listing a base metal with the UNS N08800 designation 74 shall be considered P-No. 45 (i.e., SB-407, SB-408, SB-514, etc.) whether or not these specifications are listed in table QW/QB-422. When utilizing this provision, only base metals listed in table QW/QB-422 may be used for
test coupons since a minimum tensile value is required for procedure qualification.
QW-420.2 S-Numbers. S-Numbers are assigned to materials that are acceptable for use by the ASME B31 Code for Pressure Piping, or by selected Boiler and Pressure Vessel Code Cases, but which are not included within ASME Boiler and Pressure Vessel Code Material Specifications (Section II). Material produced under an ASTM specification shall be considered to have the same S-Number or S-Number plus Group Number as that of the P-Number or P-Number plus Group Number assigned to the same grade or type material in the corresponding ASME specification (i.e., SA-240 Type 304 is assigned P-No. 8, Group No. 1; therefore, A 240 Type 304 is considered S-No. 8, Group No. 1). Some variables and figures may not specifically address S-Numbers. When this occurs, the requirements regarding P-Numbers and P-Number Group Numbers shall apply equally to materials that are assigned to corresponding S-Numbers and S-Number Group Numbers. However, if procedure qualification testing was done using material assigned an S-Number or S-Number Group Number, the range qualified is limited to materials that are assigned S-Numbers or S-Numbers Group Numbers (i.e., qualification using a P-Number material qualifies corresponding S-Number materials; qualification using an S-Number material qualifies corresponding S-Number materials but not corresponding P-Number materials; qualification of welders using a P-Number material qualifies them to weld on corresponding S-Number materials and vice versa).