I would suggest you review ASME SA 530 or ASTM A 530 Specification, titled Specification for General Requirements for Specialized Carbon Steel and Alloy Steel Pipe. This specification provides the requirements for under tolerance of pipe wall (12.5%) and OD variations based on the NPS designator.
The answer to your second question depends on the ASTM/SA specification for the cast pipe. The ASTM/SA 530 Specification would normally apply to both wrought and cast pipe specifications unless the cast pipe specification has specific requirements which differ from 530. In this case, the specific requirements in the cast pipe specification would need to be satisfied.
The B31 Codes include the 12.5 percent wall thickness variation in their calculation for minimum required wall thickness for SEAMLESS pipe. This requirement adjusts the calculation to account for the method of seamless pipe manufacture. When seamless pipe is made, a mandrel is pushed through a hot billet (or "bloom") of metal to create the "hole" that is the inside diameter. In this process the mandrel may "wander" (slightly off-course) as it is pushed through the billet. The result of this slight side-to-side movement of the mandrel is that the pipe wall may be 12.5 percent thicker in the wall on one side and 12.5 percent thinner in the wall directly across the diameter. To be sure that the thinnest wall permitted by this manufacturing methodology the B31 Codes include in the "mill tolerance" in the minimum required wall thickness calculation.
Pipe that is made of seam welded plate has much less thickness variation. However, unless the pipe's longitudinal seam weld is 100 percent radiographed (NDE), the required pipe wall thickness calculation must include the weld efficiency coefficient for the type of weld used in the seam. Of course, there is also much more "out-of-roundness" in seam welded pipe.
