Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Unless done properly, removal of surface roughness by mechanical means
can result in further scattering problems. Small curved gouges left by a
grinding wheel used to remove spatter or machining grooves can form small
lenses. The affect of grinding can be unpredictable. Some of the lensing may
concentrate the beam thereby increasing signal amplitude, or, the lens effect
may be a de-focusing of the beam, again resulting in lower than expected
signal amplitudes.
Uniform surface preparation by sand or shot blasting usually provides a good
surface for ultrasonic testing. Removal of excess metal by a hand held
grinding wheel is commonly used on weld caps and roots. When a pipe weld
has had its root ground flush and inspection can only be performed from the
outside diameter, quality of grinding can result in unnecessary repair calls if
grinding has been along the weld axis. The small grooves made by the
grinding wheel run parallel to the root edge and are easily confused with lack
of fusion, missed edge or undercut defects.