When working with ASME Section IX, P numbers are families of base metals with similar weldability. In the case of carbon steels and high strength steels, the carbon equivalency falls with a range that ensure the materials have good weldability.
The group number is a subdivision of the base metals within the P number family. The group number is used or applies when the WPS is qualified and notch toughness requirements are imposed by the construction code.
With a standard WPS, one where notch toughness is not a requirement, qualifying a P1 joined to a P1 base metal qualifies the WPS all the base metal specifications included in that P number.
When the WPS is qualified and it includes notch toughness, not only must one consider the P number, but one must also consider the group number. So, if the WPS is qualified by welding a P1, Grp. 1 to a P1, Grp 1 base metal, the WPS is valid for any P1, Grp 1 base metals welded to any other P1, Grp 1 base metal. In essence, when notch toughness is required, the range of base metals qualified is more restrictive.
Keep in mind that the provisions of Section IX can be and often are modified by the construction code, i.e., Section I, Section VIII, B31.3, etc. An example to consider is when the construction code is B31.3 Process Piping and the fluid service is specified as High Pressure. In that case, the range of base metals qualified is very restricted. In that case, the WPS is only qualified for the material specification used and recorded on the PQR. While SA-106 is carbon manganese steel pipe and is classified as a P1 Grp 1, the procedure is only qualified for SA-106 welded to SA-106. As I said, the construction code is very restrictive.
AWS B2.1 uses M numbers in place of P numbers. Generally, the P1 materials are included in the family M1 in B2.1. However, the base metal specifications included the family M1 is more extensive than the base metal specifications included in the family P1.
In NAVSEA S9074-AQ-GIB-010/248, the base metals are grouped by S numbers. In many respects, the S1 family is very similar to P1 and M1 base metals, i.e., carbon and high strength low alloy steels, except the S1 are base metals approved for use in Naval vessels and include materials that meet military standards.
Jon, it is so good to see you back in the fray.
Best regards - Al