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.
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.
- Lines of slag
- 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.
- 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.)
- 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
Wet magnetic particles
Fig.: Magnetic particle inspection with fluorescent magnetic particle
What magnetization devices are available?
The following magnetization devices are in use for such surface crack testing:
Electromagnetic yoke magnets with 2, in some cases also 4 magnet points to build up a magnetic field between them.
Mobile high voltage generators
Stationary crack detection benches
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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.
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.
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if there are still questions
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.
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.
We stand for quality
Certified quality management for the industry
Certified quality management for the aviation industry
for the inspection of turbine disks (MTU)
for the ultrasonic testing of turbine disks in our testing center in Burgwedel (Hanover, Germany)