Ten misunderstandings of magnetic particle detection

Here, we have directly listed 10 common misconceptions we have seen in this area in recent years.

1. Magnetic particle detection can be applied to all metals

What material is the part you want to test? It is important to know what metal your part is made of, as only iron, nickel and cobalt can be magnetized. Most of their alloys and many kinds of steel also have magnetic properties that can be magnetized.

Aluminum is expected to account for 80 percent of the material currently used to make aircraft because it is light, strong, predictable and cheap. It is also one of the most common metals that cannot be magnetized but is misused for magnetic particle detection.

If you're not sure if something can be magnetized, just put a permanent magnet on it and see if it holds.

2. The magnetic suspension you use does not need to be replaced

No matter how clean your air and artifacts are, contaminants are still present in every environment. Oil, grease, sand, dust, and other solids will be carried into your magnetic suspension by the test item.

Oil mixed in a hydromagnetic suspension, or water mixed in an oil magnetic suspension, are major issues of concern. These excess contaminants can affect detection performance and mask magnetic marks showing:

The most common way to determine whether a magnetic suspension needs to be replaced is to examine a precipitated magnetic suspension sample during routine magnetic suspension concentration tests. You can spot contamination by looking for different colors, layers, or bands in the carrier fluid or magnetic sediment. If there is a strip that fluoresces more brightly than most materials, there is too much fluorescent pigment that has been stripped from the fluorescent magnetic powder, and the magnetic suspension should be replaced.

If the volume of impurities (including bands and streaks) exceeds 30% of the volume of the precipitated magnetic powder, the magnetic suspension should be replaced.

3. The bigger the magnetizing current, the better

It is not always better to run more current through the workpiece.

If you apply a larger amperage current to the workpiece to magnetize, you will most likely not see the magnetic mark of the defect. This is because the fluorescent magnetic powder will be attracted not only by the leakage magnetic field at the defect (forming a display), but also by the entire workpiece, which will form a bright background that will mask the magnetic marks that the injured need to look for.

Due to the need to have a sharp contrast with the background in order to find the defect display, it is not appropriate to use too much current to magnetize the workpiece and cover up the defect.

It is important to work with Level 3 magnetic particle inspection personnel to develop the correct magnetic exploration specifications for different workpieces.

4. All materials are magnetized with the same specifications

If the workpiece looks the same, but the material (including the material grade and heat treatment state) is different, the same magnetization specification should not be used. This is because different materials have different magnetic permeability and retentivity (coercivity).

The greater the permeability of the material, the more easily the workpiece is magnetized, the greater the magnetic flux generated, and the larger the small leakage flux formed due to defects that can attract magnetic powder. The coercivity of a material affects the ability of a material to retain remanence after being magnetized.

To ensure that the workpiece is properly inspected, carefully examine its material and do not assume that the same magnetization specification can be used for similarly shaped workpieces.

5. The pre-cleaning of the workpiece is not necessary

The pre-cleaning of the workpiece is a key link to ensure the highest detection rate in the magnetic particle detection process. If you do the correct pre-cleaning of the workpiece, it will reduce the contamination of the magnetic suspension used and improve the recognition of the defect display on the workpiece.

Prepare the workpiece with a cleaner or remover before sending it to the magnetic exploration area. This not only improves the quality of your tests, but also reduces the frequency with which you need to replace the magnetic suspension.

6. Can take the coating magnetic probe, regardless of the specific conditions of the coating

According to ASTM E709-15, "Thin, non-conductive coatings, such as 1 or 2 mil(0.02 to 0.05mm) paint, generally do not impede display formation." However, for direct electrified magnetization, the coating at all electrical contacts must be removed first.

Conductive coatings also mask discontinuities on the workpiece, and it must be demonstrated that discontinuities on the workpiece surface under the coating can be detected.

7. It must always be magnetized twice

When both longitudinal and transverse cracks need to be detected, most magnetic detectors need to magnetize and inspect the workpiece twice. This is because the magnetic field generated by each magnetization is only suitable for detecting defects that have a certain direction relationship with it, and it will reduce the display formed by magnetization in the other direction. There are two ways to magnetize and inspect the workpiece in both directions.

The first way is to detect cracks in one direction at a time. Magnetic exploration specifications are determined by certified Level 3 personnel based on the geometry and intended use of the workpiece.

The second way to avoid re-energized magnetization is to use a device with a multidirectional magnetization device. The polyphase circuits of these devices can magnetize in both directions at the same time, and the magnetic fields in both directions will not suppress each other as long as they are properly adjusted.

If you are inspecting a large number of workpieces and need to check for cracks in both directions, then a multi-directional wet magnetic probe is the best choice. It only needs to be magnetized once for each workpiece, and only needs to be checked with one ultraviolet lamp instead of two.

8. Routine system performance checks are not important

There's a reason for this: it's a quality check that ASTM and NADCAP standards require to be performed and documented.

Performing routine inspections is essential to ensure that your magnetic suspension and machine are in proper working order. The operator must check the characteristic conditions, such as magnetic suspension concentration and UV lamp intensity, at least once per day or per shift. There are many quality inspection items, such as ambient white light, which has a great impact on the inspector's detection defect display.

Routine checkups can save time and money in the long run. There have been such examples, because the machine has not been correctly calibrated, no workpiece has been magnetized, so it must be re-tested.

9. Adding magnetic powder can make the display easier to spot

More magnetic particles is not always better.

Adding too much magnetic powder to the magnetic suspension will cause its concentration to be too high. Due to excessive background, the ability to detect small defects is lost.

It is best to replace the entire magnetic suspension and slowly add the magnetic powder until the correct concentration is reached.

10. Multidirectional magnetized devices are always better

If you only test a few parts, then it may not be worth the time to adjust the multidirectional magnetization current of the device, because if you are using the same magnetization method to test a large number of parts, a multidirectional magnetization device will only reduce your inspection time by half.

For a small number of complex parts, it is best to use a standard universal magnetic probe and magnetize and check twice in a vertical direction.