API and AISI 4130

Anyone of you can you please explain what is difference between Flange API 6A 45K and Flange AISI 4130 .
where can i find P numbers for those materials??

Unassigned metal.
you can check API 6A

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Table QW-422 are not listed for these material, so consider “Unassigned metal”.
P-Number qualification, see QW-424.