Magnetic particle inspection (MT)

We provide the single and serial testing of your components for surface defects and near-surface defects with magnetic particle testing.

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How powerful is the magnetic particle inspection ?

Magnetic particle inspection is used to detect defects on the surface and near-surface defects in the material.

Cracks
Lines of slag
Pores
Blowholes near the surface

From 0.01 mm crack width x 0.1 mm crack depth

The material to be tested must be magnetizable (ferromagnetic).

fast and reliable

Non-destructive testing with magnetic particle testing (MT)

Magnetic particle inspection is also called magnetic particle crack detection, flux inspection or fluxing.
It is used to detect flaws in or near the surface.

What is a magnetic particle inspection?

This method is used to detect surface and near-surface defects in ferromagnetic materials and is mainly used for crack detection. The sample, the workpiece or component, is magnetized either locally or as a total. If the material is intact, the magnetic flow is primarily inside the material. However, if there is a surface defect, the magnetic field is distorted, resulting in localized magnetic flux leakage around the defect.

This leakage flux is made visible by covering the surface with very fine iron particles. These are applied either dry or suspended in a liquid. The particles accumulate in the areas of stray flux, creating a cluster that is visually detectable even if the crack opening is very thin. Thus, a crack is indicated as a line of iron powder particles on the surface. The method is applicable to all metals that are highly magnetizable – ferritic steels and iron, but not generally austenitic steels.

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VOGT Ultrasonics is your service provider for magnetic particle testing

Magnetic particle testing can be performed both on site and in the VOGT test center. Magnetic particle testing can be used in particular for the maintenance of components. Since it can be performed relatively easily and the component to be tested, if easily accessible, does not have to be removed from a facility, magnetic particle testing is often used to test components that are subject to high stress. For example, this applies to excavator arms and cranes or to gears and sprockets.

Typical applications of magnetic particle inspection

- Crankshafts for motor vehicles and heavy industrial applications
- Connecting rods
- Cylinder heads
- Industrial iron parts
- Aircraft components
- Basically, magnetic particle inspection is often used to inspect welds or to check highly stressed components for cracks.

Pro & Contra

The advantages and disadvantages of magnetic particle inspection

Magnetic particle inspection is fast and cost-effective, but it also has some limitations.

Advantages

It is mobile and fast
The results of the test are immediately visible on the surface of the material
No strict pre-cleaning is required and post-cleaning also requires little effort
Very sensitive – it can detect flat/ fine cracks in a surface
Magnetic particle inspection can be performed even on very thin layers of paint (indicative value up to about 40µm layer thickness)
Can detect both surface and near surface indications
Flexible – can be used on complex shaped objects, even on surfaces with other materials
Can inspect parts with irregular shapes (external gears, crankshafts, connecting rods, etc.)

Disadvantages

Only ferromagnetic material can be tested with MPI
Only surface and near-surface defects (subsurface defects) can be detected
After the inspection is complete, the material may need to be selectively demagnetized, which can be challenging
Only small sections of a surface can be inspected at a time
Thicker layers of paint must be removed

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Error proneness and the solution

Cracks that are parallel to the magnetic field

Due to the direction of flow of a magnetic field, only cracks that run at a larger angle to the magnetic field can be detected. This is a basic physical problem that cannot be avoided in simple tests. However, this can be corrected by a second test at a different angle, usually 90 degrees. Modern techniques, which already work with different magnetic field orientations during the inspection, are now able to avoid this problem. However, not every component is suitable to be tested with an alternating magnetic field.

So-called magnetic crack test benches take advantage of the fact that the electric field is offset from the magnetic field by 90 degrees. In this way, cracks can be tested in different positions at the same time with a combined electrical and magnetic flow through components.

Dry or wet

Dry and wet magnetic particles

The particles used in magnetic particle inspection are important for indicating defects in the test specimen. The properties of the magnetic particles used in magnetic particle inspection must have high magnetic permeability (magnetizability) so that the particles can be attracted to the stray magnetic fields. Also, you must have low retention (ability of the particles to retain magnetization) so that the particles do not stick to each other or to the surface of the component. Two types of magnetic particles are used:

Dry magnetic particles

This method uses different particle sizes to indicate different sizes of stray flux sites. Smaller particles are more sensitive to flux leakage sites/fields and are therefore able to detect smaller defects/faults, while larger particles are able to identify larger flux leakage sites.

Wet magnetic particles

In this technique, the magnetic particles are suspended in a substance such as water or oil. This method is more sensitive to flux leakage caused by defects and flaws than dry magnetic particles, because the particles are more mobile when suspended. This method can also cover a larger surface area, so more flux leakage locations can be indicated.

Fig.: Magnetic particle inspection with fluorescent magnetic particle

Flaw detection

What magnetization devices are available?

The following magnetization devices are in use for such surface crack testing:

Magnetizing coils

Mostly portable metallic coil-like devices into which the component is immersed or passed through as well as rotated in the process. There are closed and openable coils.

Hand magnets

Electromagnetic yoke magnets with 2, in some cases also 4 magnet points to build up a magnetic field between them.

Mobile high voltage generators

These devices are used in combination with the above-mentioned coils or with magnetization cables that can be flexibly shaped to the component as well as with current electrodes. They can be easily adapted to complicated components or used for different types of magnetization and demagnetization.

Stationary crack detection benches

These stationary devices are versatile, but can also be designed to suit a particular component. They are equipped with coils or even melting electrodes and magnetic electrodes to perform the flexible and, if necessary, combined surface crack tests efficiently and quickly.

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After the inspection

How is the component demagnetized?

After the component has been magnetized for testing, it may need to be demagnetized. This requires special equipment that works in reverse to the magnetizing equipment. Magnetization is usually done with a high current pulse that reaches a peak current very quickly and immediately shuts off, leaving the test part magnetized. To demagnetize a part, the current or magnetic field must be equal to or greater than the current or magnetic field used to magnetize the part.

The current or magnetic field is then slowly reduced to zero, demagnetizing the part. A common method for detecting residual magnetism is to use a Gaussmeter. Theoretically, demagnetization by heating could also be used. However, this is not used due to the risks of damaging the part.

AC demagnetization

AC pull-through demagnetizing coils are operated with alternating current. They generate a strong magnetic field which the component is slowly pulled through by hand or on a conveyor belt. As a result of the movement of the object through the coil and its subsequent slow removal from the coil’s magnetic field, the magnetic field in the component decreases. Many AC demagnetizing coils are so powerful that components must be several meters away from the coils before the demagnetizing cycle is complete, otherwise the part will have residual magnetization.

Decreasing AC demagnetization

This method is widely used. During the process, the part is subjected to an alternating current of the same or higher value compared to the magnetization. Then the current is gradually reduced until the current reaches zero.

As the AC current changes from a positive to a negative polarity, this significantly reduces the magnetization of the part. The ability of AC demagnetization to demagnetize a part can be significantly limited depending on the geometry and alloys used.

Reverse full wave DC demagnetization

Reversing full-wave DC demagnetization is similar to AC demagnetization except that the DC current is reversed at a constant interval, reducing the current by a specified amount. Then the current is passed through the part again. By stopping, reducing and reversing the current, the magnetic areas are randomized.

This process continues until no more current flows through the part. The normal cycle on modern equipment should be 18 seconds or longer. This demagnetization method was developed to overcome the limitations of the AC demagnetization method where part geometry and certain alloys prevent the AC demagnetization method from properly functioning.

DC demagnetization

This method is identical to full-wave DC demagnetization, except that the waveform is half-wave. This demagnetization method is new and not widely used, as its only advantage is that it does not require a powerful DC power source.

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if there are still questions

FAQ

What is magnetic particle testing?

A non-destructive materials testing method that allows surface defects and flaws to be visualized. However, a direct contact with the metallic surface is required.

How is the component demagnetized?

The demagnetization of a component is the most complex part of a magnetic particle inspection, if the component has to be demagnetized at all. There are various methods for this, which must be chosen depending on the component type.

Which materials can be tested with the magnetic particle method?

The method is suitable for all metals that are strongly magnetizable – ferritic steels and iron, but not generally austenitic steels.

How much does a magnetic particle inspection cost?

Magnetic particle testing is a very cost-effective method because it can be performed quickly and often without dismounting a component.