Quality Control/PWHT-P-91

In the process of welding P-91 pipe, for the thickness of the material it is required that we maintain a preheat of between 400^f & 600^F. If we lose the preheat - interupted welding - we are required (per ASME B31.1) to apply a heat treatment. The Code does not specify what the range of temperatures and holding time is for that intermediate heat treatment. At present, the intermediate heat treatment is 600^F for the case in which we lose the preheat. However, when performing an informational X-Ray, we bring the preheat down to 200^F and hold it there until the radiography is complete and then bring thep[reheat back up to 400^F. The question is: Is not the rduction of preheat for the radiography also a loss of preheat condition that should be subject to the intermediate heat treatment simalrly to the loss of preheat due to loss fo power to the heating elements (Heating elements attached to the pipe is used). Also, what is the justification for using 600^F as the intermediate heat treatment protocol?