What is magnetic particle testing (MT)?
Magnetic particle testing (MT) is a non-destructive material testing (NDT) method that uses iron oxide particles to detect material defects that are near the surface or open to the surface. Magentic particle testing is a simple and reliable method for ferromagnetic materials. Components made of iron (low or unalloyed steels, cast steel and cast iron) as well as cobalt and nickel, for example, can be tested by magnetic particle testing.
Areas of application
Magnetic particle testing is used particularly frequently in the automotive industry for testing of safety-relevant components such as drive shafts, springs, steering and engine components. In mechanical engineering crankshafts, gear wheels and axles are tested. In the oil and gas industry tests ensure safety in the upstream (production/extraction), midstream (transportation and storage) and downstream (refining) sectors.
In some cases, magnetic particle testing is also known colloquially as fluxing. It is a classic method of non-destructive material testing. It therefore enables testing without affecting the specimen itself.
In addition to MT (magnetic particle testing), the abbreviation MPI (magnetic particle inspection) is also commonly used internationally.
Fluorescent Magnetic Particles
Black/White Magnetic Particles
Basics and functionality of magnetic particle testing
Magnetic particle testing, also known as magnetic particle crack testing, MT testing (magnetic testing) or simply fluxing, utilizes the properties of ferromagnetic materials. During magnetization, the component is simultaneously flushed with magnetic particles.
The magnetization of a component usually takes place by applying a magnetic field using an external magnetic source. Such a magnetic field can be applied, for example, by current flow, by means of a coil or by magnets. Depending on the type of magnetic field used, the applied magnetic field is located on the surface (skin effect) or floods the entire cross-section of the component. In the case of large workpieces where complete magnetization is not possible, only the partial area to be tested is magnetized.
The resulting field lines normally run parallel to the surface of the workpiece. However, if there is a defect such as a crack in the material, the magnetic field is disturbed locally. So-called stray fields then emerge from the material at such imperfections.
When finest iron oxide particles are applied to the surface (e.g. by using a magnetic particle suspension), these particles accumulate at the stray field. Due to the color contrast between the magnetic particle particles and the component surface, the surface defects become visible and can be documented. If the particles are coated with fluorescent dyes, for example, the component can now be viewed under UV radiation. The fluorescent particles thus make the stray fields and therefore the defects visible.
In addition to fluorescent magnetic particle testing with the aid of UV lamps, magnetic particle testing can also be carried out in daylight. Colored, usually black magnetic particles are used here. Sufficient contrast is achieved by first applying a white contrast paint.
This is why magnetic particle testing under daylight conditions is also colloquially known as "black and white testing".
Advantages of magnetic particle testing
- Immediate test results: Defects are instantly visible
- Direct recognizability of the defect location
- Flexible usability, independent of the shape, size, surface roughness and condition of the workpiece to be inspected
- Stationary and mobile usability
The process of magnetic particle testing
The procedure described here for carrying out the magnetic particle test complies with the descriptions of DIN EN ISO 9934-1. The following are required for a reliable test process and correct results
- precise adherence to the process steps
- use of suitable magnetization methods
- use of suitable accessories
- use of approved consumables for magnetic particle testing
In addition, the test must be carried out by test personnel qualified e.g. acc. to DIN EN ISO 9712. This is the only way to ensure that the requirements above are met and that the results are interpreted correctly.
Process steps of magnetic particle testing
1. Precleaning
Contaminants such as scale, rust, oil, grease, paint and water shall be removed if necessary using mechanical or chemical methods (e.g. with PFINDER 890), or a combination of these. It is important to ensure that the test surface is dry after precleaning.
2. Application of the white contrast paint (only for MT visible at daylight / white light)
The white contrast color(e.g. PFINDER 280) increases the contrast. It is applied evenly to the test surface. The coating layer should be as thin as possible (max. 50 μm/dry). If the coating layer is too thick, the test result may be negatively affected. Allow the white contrast paint (only for MT visible at daylight / white light) to dry.
3. Magnetization and application of the magnetic particle suspension
Magnetization can be performed by hand magnets (e.g. PFINDER 15-0). Usually a tangential field strength of about 2 kA/m² is required. The magnetic particle suspension must be applied shortly before and during magnetization. The spraying / rinsing must be completed before the magnetization is switched off. The test surface must be sprayed / rinsed with so little pressure that indication is formed undisturbed. After the application, the magnetic particle suspension should be run off in a way that the visibility of indications is improved, e.g. by tilting the test surface.
4. Inspection
After the formation of the indications, they can be evaluated and documented in daylight / white light (> 500 lx) when using black magnetic powder and under UV light (≥ 1000 μW/cm² und < 20 lx) when using fluorescent magnetic powder. Documentation may be done by any adequate method.
5. Post-cleaning and further treatment
In order to use the specimen for its intended purpose, it may be required to remove the suspension and the white contrast paint from the test surface (e.g. with PFINDER 890). It may be necessary to demagnetize the specimen and/or apply a suitable corrosion protection.
Standards and specifications
The procedure and practical application of magnetic particle testing is described and defined in various standards and specifications. This ensures that a reliable and consistent result is achieved with the tests. Standardization also ensures that tests produce comparable results worldwide.
The following standards describe the procedure, practical implementation, requirements for the consumables, viewing conditions or personnel qualification for magnetic particle testing.
- DIN EN ISO 9934-1, 9934-2, 9934-3
- ASTM E709, E1444,
- AMS 2641, 3041, 3042, 3043, 3044, 3045, 3046
- ASME V Art. 7
- AS 4792
- EN 3059
- DIN EN ISO 9712
In Germany, the German Society for Non-Destructive Testing (DGZfP) also plays a central role in the dissemination and training of this and other test methods. In addition, there are numerous other, often national or even company-internal specifications and regulations, e.g. on workpiece quality classes.
PFINDER is a member of the following standards committees and is also involved in various process-related technical committees:
National
NA 062-08-20 AA „Zersörungsfreie Prüfung, Obleuteausschuss“, (Non destructive testing, chaipersons committee)
NA 062-08-24 AA „Elektrische und magentische Prüfverfahren“, (Electrical and magetic test methods)
NA 062-08-25 AA „Oberflächenverfahren“, (Surface methods)
International
CEN/TC 138/WG 4 „Penetrant testing”
CEN/TC 138/WG 5 “Magnetic particle testing”
ISO/TC 135/SC 2 “Surface Methods”
ISO/TC 135/SC4 “Eddy current testing” (cares as well about MT)
Limitations of magnetic particle testing
Only ferromagnetic materials can be tested using magnetic particle testing. Magnetic particle testing does not work on materials such as high-alloy (austenitic) steels, light metal alloys, copper, plastics and composite materials, as these cannot be magnetized.
Special attention in magnetic particle testing is paid to the alignment of the field lines relative to the expected defects. As the field lines run parallel to the surface, defects that lie transverse to these field lines are particularly easy to detect, as they disturb the magnetic field the most and thus generate a significant stray field. Cracks in the longitudinal direction, on the other hand, generate less pronounced stray fields and are therefore more difficult to detect. This requires precise alignment of the magnetization to ensure that all possible defects are reliably detected.
Particularly large or geometrically very complex workpieces require special handling and careful planning, as the magnetic field must be maintained uniformly over the entire length of the workpiece. This can make magnetization more difficult or make an efficient test procedure impossible. In this case, penetrant testing may be a more suitable method.
In the case of very rough surfaces or test pieces that have been heavily contaminated by previous processes, the sensitivity of the test may be impaired. It is therefore important to have the magnetic particle testing process planned and carried out by qualified test personnel. The correct test methods, equipment and consumables must be selected and the results interpreted correctly.