Magnetic materials are used for Magnetic Particle Inspections/Testing (MPI/MT) of ferrous parts. All these materials must be used along with a magnetizing device such as a yoke, coil and Magnetic Particle Testing equipment.
MPI/MT methods with ease detect surface cracks and can even discover some subsurface discontinuities. Visible indications appear when particles are attracted to magnetic leakage fields that occur whenever a flaw is present. There are two methods of Magnetic Particle Testing :
- Wet method fluorescent magnetic materials, which must be used with a UV LED lamp / black light, can locate very fine surface flaws or slightly sub-surface discontinuities, such as inclusions, seams, shrink cracks, tears, laps, flakes, and grinding, quenching or fatigue cracks.
- Dry method visible magnetic materials, which do not require a black light, are more sensitive for finding sub-surface defects on components that have rough surfaces, such as large forgings and castings.
Experts well-acknowledged Magnetic particle testing of magnetizable metals (mostly steel, less often nickel or cobalt) is a very reliable method. For inspection, the part needs to be magnetized. Larger work-pieces are not possible to do complete magnetization; it only to magnetize in the partial areas.
The field lines resulting from the magnetization run parallel to the surface within the component. Cracks that lie transverse to the field lines disturb them and thus generate a stray magnetic field or also called leakage flux.
It means that the field lines emerge from the ferromagnetic material on one side of the defect and re-enter on the other side. The results formation of magnetic poles. If iron powder spreads over in a stray field, it accumulates due to the iron powder attracted by the magnetic poles. Cracks that run in parallel field lines do not generate in a stray field, therefore, cannot be detected. Hidden cracks below the surface can locate at a certain depth.
The processes for magnetization of workpieces roughly divide into a field and current flow. The application of both flow methods in one test cycle is also called combined magnetization.
If a component is exposed to a magnetic field, usually in a longitudinal direction, this is referred to as field flow. With the help of one or more current-carrying coils, a magnetic field is generated in a U-shaped iron yoke. The workpiece is clamped in this iron yoke The component creates a magnetic field in the longitudinal direction. Cracks at right angles to this, so-called transverse cracks, form a flux leakage and are displayed.
Depending on the test task, however, cracks in the longitudinal direction of the component need to be detected as well. For this a second kind of magnetization is used, the so-called current flow. During the current flow, an electrical current passes through the workpiece to be tested generating a ring-shaped magnetic field. This allows longitudinal cracks to be displayed on the test specimen.
For many components, there is no bound crack orientation. Therefore, a preferred direction can not be expected. In the case of large quantities, for example for vehicle construction, testing takes place on testing machines. They allow field and current flow, which can be activated individually or combined (both simultaneously), so cracks of any orientation can be detected.
During magnetization, the fluorescent magnetic particles are applied. In most cases, this is done by wetting with an aqueous suspension. The fine particles, often these are ferromagnetic iron oxides, stick to the emerging magnetic fields. In order to increase the displayability, the particles are mixed with a fluorescent dye that glows yellowish-green under ultraviolet light (UV light). In a darkened room, this compound creates a luminous, high-contrast (crack) display, as a result of which even the finest imperfections can be identified.