UT finds internal defects, ET surface/near-surface, PT surface-breaking defects and RT internal structure. Different combinations apply to stainless plate, tube and welds.
Non-destructive testing (NDT) is common in acceptance of stainless plate, tube and welds. The four common methods:
• UT (ultrasonic testing): sound-wave reflection for internal defects (lamination, inclusions, cracks); suited to thick plate, forgings and weld interiors; sensitive to defect orientation.
• ET (eddy current testing): electromagnetic induction for surface and near-surface defects; common for stainless tube and strip inline inspection; fast, but insensitive to deep defects.
• PT (penetrant testing): dye or fluorescent penetrant for surface-breaking defects (cracks, pores); suited to welds and complex shapes; for non-porous materials.
• RT (radiographic testing): X-ray/gamma transmission for internal structure — porosity, lack of penetration, inclusions; radiation safety requirements; thickness-limited.
Selection guidance:
1) Internal plate quality: mainly UT; surface quality: visual + PT/ET.
2) Heat-exchanger or seamless tube: ET or UT per standard.
3) Welds: RT/UT for internal, PT/MT for surface (PT is typical for stainless).
4) State method, coverage, acceptance level and inspector qualification in the acceptance standard.
NDT is sampling, not a substitute for chemical and mechanical verification; combine methods for critical parts.
