Codes like ASME B31.3 read like a wall of clauses, but the intent is simple: a pipe should hold what it carries, at the pressure and temperature it will see, for as long as the plant runs.
What the code actually asks of a fabricator
Qualified welders working to an approved welding procedure, the right filler for the base metal, and testing that proves the joint. Documented, so anyone can check it later.
When post-weld heat treatment is non-negotiable
Certain materials and thicknesses lock in stress as they cool. PWHT relieves it. Skip it where the spec calls for it and you are building in a failure that shows up years later.