NDT would not prevent superheater tubes from coming loose from sliplugs etc. If you are having problems with welds failing at the lugs on relatively new superheaters I would suggest looking at the design or the service. I have seen a few missing or broken lugs in various types of superheater designs and often times we repair or replace thosewith welds intended to fail at the weld.
If you are experiencing underbead cracks that are subsequently being pulled out, I am pretty confident this could be detected with UT SHear Wave but that is only a theory. PT/MT would show those at or near the surface however mt is limited on some materials used forlugs(SS, INconel etc.)
Have a nice day
Gerald Austin
Have a nice