Weld Quality, Inspection, and Testing
Weld Quality, Inspection, and Testing
This domain tests how weld quality is specified, inspected, and tested. You need to distinguish discontinuities from defects, select inspection methods, and apply acceptance criteria from the applicable code or WPS.
Core rules
- Inspection happens before welding (joint fit-up, cleanliness, consumables), during welding (preheat, interpass, parameters), and after welding (visual, dimensional, NDT).
- Visual inspection is the most common method: check weld profile, leg size, throat, undercut, overlap, spatter, cracks, and arc strikes. Use weld gauges and a good light.
- PT detects surface-breaking discontinuities in nonporous materials; it works on stainless and aluminum. MT detects surface and near-surface discontinuities in ferromagnetic materials only. UT detects internal planar defects and requires a trained operator. RT detects internal volumetric defects and requires radiation safety controls.
- Destructive tests include bend, tensile, nick-break, and macroetch; they verify procedure or welder qualification, not production welds.
- A discontinuity is not automatically a defect; it becomes a defect when it exceeds the acceptance criteria.
- WPS and PQR: the WPS must be supported by a qualified procedure; welders must be qualified for the process and position.
Worked example
A stainless steel tank weld shows a surface-breaking crack. PT is suitable because the crack is open to the surface and the material is nonmagnetic. MT would not work because austenitic stainless is not ferromagnetic. If the concern were internal lack of fusion, PT would be inadequate; UT or RT would be needed, depending on the code and geometry. The reasoning is method capability first, then code acceptance.
Traps
- Using MT on austenitic stainless or aluminum.
- Calling every porosity a rejectable defect without the code.
- Inspecting before the weld has cooled or before slag removal.
- Using a weld gauge on a curved surface without accounting for curvature.
- Confusing a procedure qualification with a welder qualification.
- Skipping pre-weld inspection of fit-up and consumables.