Inspection, Testing, and Defect Correction
Domain 5 - Inspection, testing, and defect identification and correction. You must know what each test finds, what each defect looks like, what causes it, and how it is corrected and accepted.
Inspection sequence
Visual examination before, during, and after welding catches most problems: alignment, fit-up, bead profile, spatter, undercut, surface porosity. Tools include the fillet weld gauge, undercut gauge, hi-lo gauge, magnifier, and a good light.
Test methods and what they detect
- PT (penetrant): surface-breaking defects in non-porous material.
- MT (magnetic particle): surface and near-surface defects in ferromagnetic steel only.
- UT (ultrasonic): subsurface planar and volumetric defects, plus thickness.
- RT (radiography): volumetric defects such as porosity, slag, and incomplete penetration. No method finds everything; a tight lack of fusion can hide from RT and show clearly on UT.
Common GMAW defects and causes
Porosity: damp or oily metal, wet wire, low or turbulent gas flow, long stickout. Lack of fusion or penetration: low heat input, fast travel, wrong angle, poor fit-up. Undercut: high voltage, fast travel, wrong angle. Spatter: high voltage, poor ground, dirty nozzle. Burn-through: excessive current, slow travel, thin metal. Cracking: restraint, hydrogen, small crater. Overlap or cold lap: low current with slow travel.
Correction and acceptance
Defects are removed by grinding or gouging back to sound metal and re-welded to the WPS; cracks must never simply be welded over. Acceptance comes from the applicable code - AWS D1.1 for structural steel, for example - not personal judgement. D1.1 undercut limits are typically 1 mm for material under 25 mm thick and 2 mm for 25 mm and thicker, with length limits.
Worked example: A 6 mm fillet has a 6 mm leg, so throat = 0.707 × 6 = 4.2 mm. An undercut of 0.8 mm on 10 mm plate is inside the 1 mm limit; 1.5 mm would need repair - grind, re-weld, re-inspect, record.
Traps: assuming no slag means no defects; judging acceptance by eye without a gauge; repairing a crack without removing its ends.