Weld Inspection, Testing and Defect Analysis
Methods and their reach
Visual inspection (VT) runs before, during and after welding and catches most defects. Penetrant testing (PT) and magnetic particle testing (MT) find surface and near-surface discontinuities. Ultrasonic (UT) and radiographic (RT) testing find internal ones. Destructive tests include tensile, guided bend, Charpy impact, macro-section and hardness.
Discontinuity to cause
- Undercut: excessive current, long arc, fast travel, wrong electrode angle.
- Porosity: moisture, dirt, long arc, low current, high sulphur in the steel.
- Slag inclusion: incomplete cleaning between passes, low current, wrong angle.
- Incomplete fusion or penetration: low current, fast travel, tight root gap, narrow bevel.
- Cracks: hydrogen (delayed 24-48 hours), high restraint, high carbon equivalent without preheat, crater shrinkage.
Timing matters
Hydrogen-assisted cracks may not appear until the joint has cooled for a day or more. Many codes require inspection after a delay for thick or high-strength joints, so inspecting a warm weld and passing it proves little.
Worked example
Radiography of a pipe girth weld shows a straight, dark line following the root of the joint. That is incomplete penetration, not slag - slag is irregular and typically follows the fusion line. Causes to check: root gap too tight, travel too fast, current too low, wrong electrode angle, root face too large.
Traps
- Calling every dark indication porosity.
- Passing a weld on cap appearance alone without inspecting the root.
- Inspecting for hydrogen cracking before the specified delay has elapsed.