How to straighten T12 tubes

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Everyone, as a newcomer to ASME, I’ve encountered a rather tricky issue in production; I would be very grateful if you could offer some advice! Our factory is manufacturing an ASME waste heat boiler, and T12 tubes are being used for the superheater. After bending, twisting occurs in the elbow area, resulting in a flatness error of nearly 20 mm. The workshop uses flames to heat the elbow area in order to shape it, and a significant amount of shaping has been done. Upon arriving at the factory, the owner observed this phenomenon and raised two issues. First, bending is not allowed in the elbow area, as there are clear regulations to that effect in ASME. Second, heat bending is not permitted for T12 pipes. My question is: first, is it indeed specified in ASME that heat bending is not allowed in elbow areas? Second, can low-alloy steel pipes such as T12 be thermally shaped? Third, how should low-alloy steel pipes such as T12 be shaped?