NDE Coordination: Methods, Timing, and Acceptance
Selecting and sequencing NDE
The procedure and applicable code define which nondestructive examination methods apply, how much of the weld is examined, and when. Common methods are visual (VT), liquid penetrant (PT), magnetic particle (MT), ultrasonic (UT), and radiographic (RT). Know the limits: PT detects surface-breaking indications in nonporous materials; MT detects surface and near-surface indications in ferromagnetic materials; UT and RT detect internal indications, with RT better for volumetric defects and UT better for planar defects and thick sections.
Timing and prerequisites
The trap candidates miss most often is timing. Examination must occur after the weld has cooled sufficiently, after any required post-weld heat treatment where the code specifies, and after surface preparation appropriate to the method. RT and UT generally require completed welds and a qualified technique; PT and MT are often used for intermediate checks and for repair verification. Examination of the root pass, hot pass, fill, and cap may be specified separately.
Personnel, technique, and records
NDE must be performed by qualified personnel using a written technique or procedure, with equipment in calibration. Acceptance criteria must match the code and the WPS; an indication acceptable under one standard may be rejectable under another. Records must identify the weld, the method, the extent of examination, the results, and the examiner.
Worked example
A specification requires 100 percent RT of a butt weld, but the joint is a partial-penetration T-joint with a backing bar. Rule of thumb: RT is generally unsuitable for T-joints; UT or another approved method is usually required. Flag the conflict before welding rather than after, since method suitability is a planning decision.
Common traps
Applying acceptance criteria from the wrong code; examining before the required cooling or heat treatment; using an uncalibrated instrument; assuming 100 percent RT means the entire weld volume is inspected without dead zones; failing to re-examine repaired areas.