21 Nondestructive Examination in Welding

Nondestructive examination (NDE) is the process of determining the suitability of a material or a component for its intended purpose using techniques not affecting its serviceability. Nondestructive examination is performed by a technician who is qualified to conduct specific NDE processes. A technician certified to Level II or III is a person qualified to interpret nondestructive examination results according to the controlling code or standard for the job.

Indications

Indications are categorized as relevant, non-relevant, or false. A relevant indication is an NDE indication caused by a discontinuity that requires evaluation. A non-relevant indication is an NDE indication caused by a discontinuity that does not require further evaluation. A false indication is an NDE indication interpreted to be caused by a discontinuity at a location where no discontinuity actually exists. False indications are non-relevant indications.

Relevant indications are caused by discontinuities. Relevant indications are categorized as continuous line, intermittent or broken line, small dots, or round. Indications may also be categorized as faint or gross, depending on their dimensions. Relevant indications must be evaluated against the requirements of the applicable fabrication code or standard.

All possibilities that the indication is non-relevant or false are first eliminated, after which the cause of the indication may be determined. It is then determined whether the indication is allowable per the applicable fabrication code or standard.

Methods

Common nondestructive examination methods are visual, liquid penetrant, magnetic particle, ultrasonic, radiographic, electromagnetic, and hydrostatic. Visual examination (VT) is the most extensively used method of NDE. It is typically used to determine surface condition, alignment of mating surfaces, conformance to a specific shape, or to locate leakage. VT may be used before, during, and after welding.