What is Magnetic Particle Testing in Welding?

15 January 2019

Magnetic particle testing, also known as MPT, uses a magnetic field to identify weld discontinuities. The field is invisible, which is why a second test element is needed if the results are to be visually confirmed. To facilitate this phase, iron-rich particles are spread across the weld site. As the field flux leaks from a weld discontinuity, the iron filings are drawn to the damage.

Generating the Magnetic Field

A portable kit is packed prior to a welding inspector’s trip. Heading out to take care of his daily duties, the kit is pulled out so that the tests can commence. In this case, the tester takes the form of two prods. The current generating terminals touch the weld surface, and an electrical current generates a magnetic field. Ordinarily invisible, the results of the test don’t become observable until the inspector pulls out the next piece of test kit. A plastic bottle swoops into sight, and the hand of the inspector gently presses its sides. Floating out of the bottle nozzle, a fine mist, rich in ferrous particles, is attracted to the test site, where it then gathers in strange patterns. The discontinuities, hidden just below the weld’s surface, leak flux energy, and it’s those leak zones that the powdery metal grains gather around. Just like that, a magnetically charged weld defect is revealed.

Expanding the Magnetically Charged Kit

Nondestructive by design, the magnetic flux and iron particles react to the charged electrical probes as they’re manipulated around welds. Alternatively, there’s a yoke-shaped variant. It’s considered the most used method out of the handful of MPT approaches. The portable instrument operates on AC current or DC, with direct current preferred when looking for deeper-seated weld irregularities. For pipe welds and other longitudinally mounted parts, a stationary solution is favoured. This method uses a circular field element, which delivers its magnetic field as a pipe is fed into its core. Finally, before closing out this post, there are two forms of magnetic particle testing in use. The dry method has been covered already, and it’s accurate. But, for increased accuracy, the ability to detect fatigue cracks and other hard-to-detect weld irregularities, the wet method uses a finer mass of water-suspended iron particles.

We’d be remiss in our duties if we were to miss out this final test requirement. Since this is an electromagnetically powered procedure, all tested welds must be ferromagnetic. Plentifully imbued with iron, test instrument flux can spread without interruption until it’s attenuated by a visually concealed weld defect. In summary, this inexpensive test technique is non-destructive, it’s fast, and it’s scalable.

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