My intuition tells me the fabrication of flange is not covered by the construction section of pressure vessel (Section VIII) but the material specification of Section II or equivalent.

For example, if you need to repaire an SA-105 flange, you will refer to the following from Section II, Part A, SA-105:

10. Repair by Welding
10.1 Repair of defects by the manufacturer is permissible
for forgings made to dimensional standards such as
those of ASME or for other parts made for stock by the
manufacturer. Prior approval of the purchaser is required
to repair-weld special forgings made to the purchaser’s
requirements.
10.2 Weld repairs shall be made by a process that does
not produce undesirably high levels of hydrogen in the
welded areas.
10.3 All forgings repaired by welding shall be postweld
heat treated between 1100°F [593°C] and the lower
transformation temperature for a minimum of 1⁄2 h/in.
[1⁄2 h/25.4 mm] of maximum section thickness, or alternatively
annealed, normalized and tempered, or quenched and
tempered. If the forging was not previously heat treated,
the original tempering temperature was exceeded, or the
forging was fully heat treated in the post weld cycle, then
the forging shall be tested in accordance with Section 7
on completion of the cycle.
10.4 The mechanical properties of the procedure-qualification
weldment shall, when tested in accordance with
Section IX of the ASME Boiler and Pressure Vessel Code,
conform with the requirements listed in Table 2 for the
thermal condition of repair-welded forgings.

Jovi