denlow60,
Please note that "non-magnetic" isn't a restriction to perform UT as it is for MT since UT doesn't require the magnetic properties of a steel.
katrinali,
UT is widely used for austenitic stainless steels like SS 316 even though the austenitic structure makes it difficult to detect discontinuities due to its coarse grain as well as its anisotropic crystal structure that result in attenuation, splitting, dispersion and skewing of the beam. In order to overcome that problem low frequency large size probes and shorter ultrasonic wavelengths shall be used. Please have a look at this link: UT on austenitic stainless steel (ASS)
regards
wegm
katrinali,
Regarding your original query, please let me add that the requirement for UT depends on the applicable code and customer requirements
regards,
wegm