Hi guys
Materials in pressure bearing parts must be listed in those standards otherwise they are not approved as pressure bearing material, thereby you can`t use that material. I think you can verify your own material according to the standards but then you have the obligation to prove that it can be used as pressure bearing material and in which pressure/temp combinations. If you can convince standard institute about the material you can get a approval to use it, that approval is only valide for your company.
Monel K500 is not a pressure bearing material, One name of Monel as casting is M-35, I think it is ASTM A494 M-35.
In the innerparts K500 is often used in stems and other parts that take loads, then Monel 400 can be used in parts that are not subjected to higher loads.
What I mean is that it is necissary to stick to valid materials in pressure bearing parts and then in inner parts you can those any material you want.