Practical Inspection and Nondestructive Examination (Part B)
Visual examination comes first
Visual inspection (VT) applies to essentially every weld and happens in three stages: before welding (joint preparation, fit-up, cleanliness, consumable condition), during welding (root condition, interpass cleaning, preheat/interpass checks), and after welding (final size, profile, and surface discontinuities). Typical tools: fillet weld gauge, undercut gauge, V-WAC-type gauge, taper gauge, magnifier, flashlight, mirror, borescope, tape, and straightedge.
Verify lighting rather than assuming it. A commonly cited minimum for weld inspection is about 100 foot-candles (roughly 1,000 lux) at the inspection surface, but the governing procedure controls; use a meter.
Choosing an NDE method
PT: surface-breaking discontinuities only, on nonporous materials; requires proper cleaning, dwell time, and developer. MT: surface and near-surface discontinuities, ferromagnetic materials only; AC magnetization is biased to the surface, DC reaches deeper. UT: volumetric and internal; angle-beam search units (45°, 60°, 70°) are used on welds; requires couplant and reference blocks; strong on planar defects such as lack of fusion. RT: volumetric; relies on density differences, so it reliably reveals porosity, slag, and inclusions but can miss tight planar defects oriented parallel to the beam; radiation safety controls the area.
Measurement rule of thumb
For a 90° fillet, theoretical throat = 0.707 × leg. A 1/4 in (6 mm) leg therefore gives about 0.177 in (4.5 mm) of throat.
Worked example: a gauge shows a 5/16 in leg with a visibly concave face. Record the leg and the profile; concavity reduces effective throat, so compare both against the WPS and acceptance criteria instead of accepting on leg alone.
Common traps: swapping PT and MT capabilities, treating RT as a universal detector, calling an indication a defect, and ignoring weld profile while measuring size.