Weld Inspection Methods

Weld inspection verifies that the joint meets the requirements specified by the code, drawing, or purchase order. Every weld gets visual inspection at minimum. Critical welds get additional non-destructive testing (NDT) based on the consequences of failure, the welding process, and the code requirements.

Visual inspection (VT) is first and most important. A qualified inspector (CWI, or Certified Welding Inspector, is the AWS credential) examines every weld for cracks, porosity, undercut, incomplete fusion, proper profile, and dimensional accuracy. Tools include a flashlight, mirror, fillet weld gauges, and a hi-lo gauge for pipe root alignment. Good VT catches about 75% of all weld defects without any equipment beyond a trained eye.

Dye penetrant testing (PT) finds surface-breaking cracks and porosity that VT might miss. Apply a red penetrant, wait for dwell time (10-30 minutes), wipe the surface clean, then apply white developer. The developer draws penetrant out of any surface defects, showing them as red marks on white background. PT works on any non-porous material but only finds surface defects.

Magnetic particle testing (MT) detects surface and near-surface defects in ferromagnetic materials (steel, iron). A magnetic field is applied with a yoke or prods, then iron particles (dry powder or wet fluorescent) are applied. Defects disrupt the magnetic field and collect particles, creating a visible indication. MT is faster than PT and finds subsurface defects up to about 1/4 inch deep.

Ultrasonic testing (UT) sends high-frequency sound waves through the weld. Defects reflect the sound back to the transducer, showing up as peaks on a screen. UT excels at finding planar defects (cracks, lack of fusion) and can size defects accurately. Phased-array UT (PAUT) uses multiple elements for faster scanning and better imaging.

Radiographic testing (RT) passes X-rays or gamma rays through the weld onto film or a digital detector. Internal defects show up as dark areas on the image. RT produces a permanent record and detects volumetric defects (porosity, slag) well but struggles with tight planar defects oriented parallel to the beam.