Magnetic Particle Testing (MT) in Belgium
Magnetic particle testing is a non-destructive testing method that enables the detection of surface and near-surface defects in ferromagnetic materials. It relies on the disturbance of a magnetic field by discontinuities, made visible through the accumulation of magnetic particles.
What is Magnetic Particle Testing?
Magnetic particle testing (MT), also known as magnetic particle inspection, is a non-destructive testing method specifically designed for the detection of surface and near-surface defects (up to approximately 2 to 3 mm below the surface) in ferromagnetic materials such as carbon steel, low-alloy steels and cast iron. The principle relies on magnetising the component under test: when a magnetic field passes through a ferromagnetic material and encounters a discontinuity, the magnetic flux lines are diverted, creating a leakage field at the surface. This leakage field attracts and retains fine magnetic particles (iron powder) applied to the surface, forming a visible accumulation that outlines the shape of the defect.
The magnetic particles used may be coloured (black, grey or red) for observation under white light, or fluorescent for observation under ultraviolet (UV-A) light, with the latter option offering significantly higher detection sensitivity. Magnetisation of the component can be achieved through various techniques: direct current flow through the part, use of an electromagnet (magnetic yoke), magnetic coil or threading bar. The choice of magnetisation technique depends on the geometry of the part, the presumed orientation of the defects being sought and the accessibility conditions.
LCNDTEST maintains a comprehensive range of magnetic particle testing equipment and operates on all types of sites in Belgium and across Europe. Our certified technicians are proficient in the various magnetisation techniques and systematically select the most appropriate method to ensure optimal defect detection in your steel structures and components.
Inspection Methodology
The magnetic particle testing procedure begins with surface preparation: the surface must be clean, dry and free from any substance that could mask indications (thick paint, grease, heavy scale). Light grinding or wire brushing may be necessary. The surface is then visually inspected prior to magnetisation.
Magnetisation is performed by selecting the appropriate technique based on the direction of the defects being sought. Magnetic particle testing is most sensitive to defects oriented perpendicular to the magnetic flux lines. It is therefore common practice to perform two successive magnetisations in perpendicular directions to cover all possible defect orientations. The most commonly used techniques on site are the magnetic yoke (portable AC/DC electromagnet) and current flow magnetisation. During magnetisation, magnetic particles are applied in the form of a liquid suspension (wet method) or dry powder (dry method) to the surface under inspection.
The inspector then carefully examines the surface to identify accumulations of particles that indicate the presence of defects. Each indication is evaluated, characterised (linear, rounded or aligned indication) and compared against the acceptance criteria defined by the applicable standard or specification. After the inspection, the part is demagnetised if necessary (particularly for parts that will undergo subsequent machining or be placed in service in an environment sensitive to residual magnetic fields) and the results are recorded in a comprehensive inspection report.
Industrial Applications
- Weld inspection on steel structures: structural steelwork, bridges, storage tanks, pipelines
- Inspection of steel forgings and castings in the mechanical engineering and shipbuilding industries
- Detection of fatigue cracks on in-service mechanical components: shafts, gears, crankshafts, springs
- Inspection of railway rails and track components
- Inspection of pressure equipment in carbon steel and low-alloy steel (boilers, vessels, columns)
- Post-weld repair verification and preventive maintenance inspection on industrial installations
Advantages of Magnetic Particle Testing
- Reliable detection of surface and near-surface defects (up to 2-3 mm depth)
- Rapid and efficient method, applicable on site with portable equipment (magnetic yoke)
- High sensitivity to fine cracks, even on relatively rough surfaces
- Fewer surface preparation requirements than penetrant testing (tolerance to a thin paint layer)
- Immediate results with direct interpretation of indications by the inspector
Reference Standards
EN ISO 9934-1 • EN ISO 23278 • EN ISO 17638 • EN ISO 9934-2 • EN ISO 9934-3 • ASME Section V Article 7
Frequently asked questions
Magnetic particle testing is a non-destructive testing method used to detect surface and near-surface defects (up to approximately 2-3 mm below the surface) in ferromagnetic materials. The component is magnetised, and fine magnetic particles are applied to the surface. Where a defect disrupts the magnetic flux, a leakage field forms that attracts and holds the particles, creating a visible indication that outlines the shape and location of the defect.
Magnetic particle testing is highly effective at detecting surface-breaking and near-surface defects such as cracks (fatigue cracks, grinding cracks, welding cracks), laps, seams, inclusions near the surface and lack of fusion in welds. It is particularly sensitive to linear defects oriented perpendicular to the magnetic field, making it an excellent method for detecting cracks in welds, forgings and castings.
Yes, magnetic particle testing can only be applied to ferromagnetic materials such as carbon steel, low-alloy steels and certain types of cast iron. It cannot be used on non-ferromagnetic materials such as austenitic stainless steels, aluminium, copper, titanium or nickel alloys. For these materials, penetrant testing (PT) is the recommended surface inspection method as it works on any non-porous material regardless of its magnetic properties.