Hey fellas, I'm new to this forum and pretty new to the ASME section IX code. I'm trying my best everyday to understand this book. My question today is in QW-422 some P numbers have the same number but a different letter ie; P5b - P5a. What do these letters signify? The group numbers are in regards to impact tests?

Read QW-420.1 P-Numbers and S-Numbers. (NOTE: S Numbers are no longer in current edition of code, they have magically become P 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.

P Numbers are, essentially, families of alloy "groups"
Steel and Steel Alloys are assigned to P1 through P11 (and all in between), note that there are no P Number 2 materials. :)

P21 through P25 are aluminum and aluminum allys.

P31 through P35 are copper and copper alloys

P41 through P49 are nickel and nickel alloys

Read QW-420 MATERIAL GROUPINGS to get the complete picture.

The sub-numbering system has specific meaning.

Good luck. By the way, as far as I know, assignment of P Numbers has nothing to do with impacts.

Try a websearch and read many excellent articles on ASME Section IX Chairman, Walt Sperko's website: www.sperkoengineering.com