Magnetic inspection is widely used for diagnostics of ferromagnetic metals and for detection of faults. This technology allows for detecting faults like cracks, hairlines, alien inclusions, flakes, losses of fusion, etc. The method is perfectly suited for detecting superficial faults and hidden faults lying at the maximum depth of 2–3 mm.
This control method is based on the study of magnetic fields, which dissipate around the inspected area after being magnetized. In fault spots, magnetic flows are redistributed in such a way that they form magnetic flux leakage fields.
Magnetic control means application of magnetic powder or suspension onto the studied area. The suspension has the form of fine magnetic particles suspended in liquid. When the ferromagnetic powder enters a magnetic flux leakage field, it is attracted to a discontinuity and settles around it.
The width of the line surrounded by the powder may be larger than the real width of the fault. Thanks to this, magnetic control is suitable for finding even narrow cracks not visible with a naked eye. The indication pattern is registered either visually or using special devices.
NDT inspectors use stationary or portable magnetic inspection instruments for magnetic particle inspection. They have integrated meters of magnetizing current. Besides that, a specialist's toolkit includes a magnifier, an endoscope, a microscope and other gear.
Depending on the required sensitivity level and the properties of the studied material, magnetization is performed in one of the two ways:
· Residual magnetism method. Powder is applied only after removal of magnetism.
· Applied field method. Powder is applied simultaneously with magnetization.
As a result of magnetization and impact of resultant forces, material particles are arranged in lines. After the indication pattern has been formed, the item is visually inspected. For enhanced efficiency, optical means can be used to magnify the image. Local lighting should be provisioned alongside with general one. If fluorescent powders have been used, UV lamps and flashlights are a must.
Main advantages of magnetic particle control:
· relatively low labor;
· high productivity;
· efficient for detecting sub-surface and surface faults;
· detects not only hollow discontinuities, but also discontinuities filled with alien matter;
· diagnostics during manufacturing and use of parts.