Visual Testing Applications and Interpretations
From Detection to Decision
Visual testing is applied to welds, castings, forgings, machined parts, and in-service components. The examiner must distinguish between actual discontinuities and artifacts.
Key concepts:
- Weld inspection: Look for porosity, undercut, overlap, cracks, incomplete fusion, spatter, and arc strikes. Each has a characteristic appearance. Undercut is a groove melted into the base metal; overlap is a protrusion of weld metal.
- Castings: Surface porosity, cold shuts, hot tears, and sand inclusions. Cast surfaces may be shot-blasted, which can mask or create indications.
- In-service inspection: Cracks, corrosion, erosion, and deformation. Lighting and access are often challenging.
- Interpretation vs. evaluation: Interpretation is determining what the indication is; evaluation is deciding if it is acceptable per code. The Level III must ensure both are done correctly.
- False indications: Lighting reflections, machining marks, or surface roughness can mimic discontinuities. Changing the viewing angle or lighting can help differentiate.
Traps in questions:
- Confusing a linear indication (e.g., crack) with a linear surface condition (e.g., scratch).
- Assuming all indications are defects; many are acceptable per code.
- Overlooking that evaluation requires knowledge of the acceptance criteria, which may be in a different document.
- Misinterpreting slag lines as cracks; slag is often smoother and may be removable.
Rule of thumb: If an indication disappears when the viewing angle changes, it may be a reflection or surface artifact. True discontinuities typically remain visible from multiple angles.
Worked example: A weld shows a dark line along the toe. Under magnification, it appears as a sharp, irregular line. The inspector suspects a crack. To confirm, the inspector uses a penetrant test or magnetic particle test. Visual alone may not be definitive.