The distortion may be due to lack of sufficient Stiffeners and supports.
Normally the Type of Distortion which occurs is as below.
There are no specific requirements to Distortion while qualifying the WQT. Even WQT's of pipes are being successfully Radiographed. Hence as long as you are able to Radiograph 6inch (150mm) successfully, WQT is considered acceptable.
Thanks & Regards
J.Gerald Jayakumar