chall is correct.
P-Numbers are the way ASME defines certain "Families" of alloys.
Find a copy of ASME Section IX and look up QW-420.1. Read the whole paragraph, this will help you understand. After you do that, look through the Tables of QW-422 and you will find more specific information about the P-Number you may be interested in.
Good luck, it's a long road understanding the codes, not necessarily difficult but one has to actually read them in order to gain an understanding of how they are structured and how to properly work with them.
It's a REALLY, REALLY good idea to THOROUGHLY read the "Introduction" part of the Code. This may be especially important with ASME Section IX.