You need to do a PWHT procedure. The following criteria must be known in order to correctly perform heat treatment to satisfy Section VIII, Div 1
1. P Number& Group Number of the material
2. Heat treatment method
3. Maximum charge temperature for furnace heat treatment
4. Nominal thickness...
You'll have to use Code B16.5. Go to Table 1A to find the
Material Group the flange is made from. Use that material Group to find the rating in Tables 2. The pressure at the particular temp will give you the MAWP.