Magnetic Particle Testing
Magnetic Particle Testing (MPT/MPI/MT) is a non-destructive examination method for ferromagnetic materials that reveals surface and near-surface discontinuities such as cracks, laps, seams, and inclusions. The part is magnetized, and ferromagnetic particles are applied; particles gather at flux leakage fields around discontinuities, forming visible indications. It is widely used on steel and iron parts after machining, grinding, heat treatment, or welding.
On a CNC or fabrication shop floor, MPT is a go-to final and intermediate inspection step for ferrous parts after machining, grinding, heat treating, or welding. Because those processes can create tight cracks, grinding burns, quench cracks, or weld defects that are invisible to the naked eye, an inspector magnetizes the part and applies dry or wet particles to make discontinuities stand out. The method works best when the magnetic field is oriented perpendicular to the suspected flaw, so parts with complex geometry often require multiple magnetization directions. In practice, the part must be clean and free of oil, scale, and grinding swarf to avoid false indications. After the test, demagnetization is typically required before welding, further machining, or assembly, since residual magnetism attracts chips, distorts weld arcs, and interferes with gauging or rotating equipment.
- Masked indications on linear defects: When the field runs parallel to a crack, flux leakage is weak, particles don't gather, and the flaw stays invisible. Re-magnetizing perpendicular is essential; otherwise defects are missed.
- False backgrounds and missed cracks: Oil, scale, paint, or grinding swarf traps particles and creates confusing background patterns, lowering sensitivity. A dirty surface can hide real indications and lead to false rejections or missed defects.
- Residual magnetism gone wrong: Skipping demagnetization leaves parts magnetized, causing arc blow during welding, chip attraction during machining, and interference in assembly or inspection. This creates costly downstream rework and functional problems.
What material can be tested with MPT?
Only ferromagnetic materials, such as steel and iron, can be effectively tested because they can be magnetized or strongly attracted by a magnetic field. Nonferrous metals like aluminum are not suitable for MPT.
What defects does MPT actually detect?
MPT is strongest for surface and slight subsurface discontinuities like cracks, laps, seams, and inclusions. It does not characterize deep internal defects the way volumetric methods such as ultrasonic or radiographic testing do.
Why choose wet fluorescent MPT in production?
Wet fluorescent particle systems are commonly used when higher sensitivity and better contrast are needed, especially for production inspection of critical ferrous parts under controlled lighting.