Magnetic NDT method is used when searching the surface and subsurface for micro-defects in welds, details and constructions made of ferromagnetic materials. For this purpose, the product is magnetized and covered with magnetic powder, which settles on the irregularities of the magnetic field in the area of defects, forming a visible "footprints" of defects.
This method allows detecting thin, invisible to the eye surface material defects such as cracks (hardening, welding, sanding, fatigue, stamping, casting, etc.), hairline, hairline cracks, tears, straightening fractures, some types of bundles.
Under magnetic particle inspection suspension with suspended particles of the ferromagnetic powder is applied to the item. In this case nearby particles are attracted to defective places and form defective sites visible to the naked eye rollers of powder, since the width of these rolls is several times greater than the opening width of defects. By the nature of powder deposition (shape and size rolls) can be judged the extent of the defects, and in some cases the type of these defects.
Magnetic particle inspection includes the following stages:
- surface preparation
- magnetization of details
- applying suspension or dry powder on the surface of detail inspected
- inspection of parts
Sensitivity is determined by the characteristics of the magnetic material of controlled devices (magnetic induction (B), the remanence (Br), maximum magnetic permeability (μmax), and coercive force (H0), the surface roughness, the strength of the magnetizing field and its orientation relative to the plane of the defect, quality of defectoscopic equipment and brightness of controlled surface.
Magnetic method is widely used in many industries: aviation, engineering, shipbuilding, automobile manufacturing and other.