Welding Tests: Magnetic Particle Inspection

wissco-5764In welding, a few small problems can be dangerous. This is a skill that demands high scrutiny. In other words everything will be inspected, so don’t take it personally.

One of the most popular ways to test welds is called, Magnetic Particle Inspection (MPI), also known as Magna-fluxing. This is a non-destructive inspection, meaning it does not destroy the weld. It is used on ferromagnetic materials like iron or steel.

Basically, you magnetize the weld surface, pour a specialized powder on it, and watch to see if and where powder gathers, because that indicates a problem. Now let’s dive a bit deeper into the Magnetic Particle Inspection process.

How it works

This is a popular way to inspect welds, because it’s inexpensive, doesn’t require complicated machinery, and checks the surface and sub-surface of a weld. During the welding process, if a crack develops, flux leaks from the spot, but it is not visible with the naked eye. The Magnetic Particle Inspection searches for these flux leaks.

A device is used to create a magnetic field on the weld surface. A dry iron oxide powder or wet iron mix is poured over the surface. Two types of devices are generally used for testing – a stationary horizontal system or a portable magnetic yoke. In each case, either an Alternating Current (AC) or Direct Current (DC) flows through the testing devices.

Why it works

The magnetic field causes the iron powder or wet mix to cluster around any spot where flux has leaked. The size and shape of the defect – such as a crack – are then clearly displayed, whether on the surface or beneath. The AC is generally used to detect surface flaws because it does not penetrate very deep into the material. DC is used for deeper penetration.

The magnetic yoke can be used with either AC or DC, but the drawback here is that testing must be done twice. The device must be rotated once in order to detect flux particles located on the surface and a second time to find cracks that did not leak flux because they run parallel to the surface.

When using a stationary horizontal system with DC, it can be hard to magnetize big sections for testing. In order to be effective, inspectors must be able to calculate the level of the current by judging the size of the material, what type of defect might be present, and how far the magnetic field can penetrate.

Both of these inspection systems can be used with fluorescent or non-fluorescent coloring in the materials. There are differences between using wet particles and dry powder:

  • Wet mix is infused with ultra-violet (UV) pigment and must be viewed in a dark room using UV goggles or glasses in order to filter the UV light. It can also be a bit messy.
  • Dry powder can be seen in regular light with the naked eye. When finished, the powder can simply be wiped or blown away.

Leading fabricators like LeJeune Steel Company make quality control a priority. As a company that fabricates some 40,000 tons of steel each year, getting the work right has led to a solid reputation. Magnetic Particle Inspection is one of the tests methods they use. Sometimes the defect comes by the hands of the welder but sometimes it’s just a matter of stress or tension created by the welding process. Either way, defects can be dangerous, and Weld Magnetic Particle Inspection is an effective way to spot them.

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For more than 70 years, LeJeune Steel has been an industry leader in steel fabrication. Since 1944, we have grown to become one of the largest structural steel fabricators in the Midwest, with more than 40,000 tons of steel fabricated annually by our shops in Minnesota and Wisconsin.

Contact LeJeune and learn how you can partner with a leading steel company.