This testing method is called MT (Magnetic Testing) or MPT (Magnetic Particle Testing), and suitable for the surface / subsurface flaw inspection of ferromagnetic materials.
Principle of MPT is as follows:
- When the workpiece to be inspected is magnetized, magnetic flux is induced.
- If there is a flaw on the surface, magnetic flux leaks into the air at the position of the flaw.
- Then magnetic particles (dyed or fluorescent encapsulated) are applied to the surface.
- These magnetic particles will migrate to the flaw where the magnetic flux leaks and forms a flaw indication that is several tens of times of actual flaw width.
- The inspector visually identifies the flaw.
Procedure of MPT
Procedure of MPT is "Pre-cleaning" - "Magnetization" - "Applying magnetic particle" - "Observation" - "Post-cleaning".
Oil, paint, rust and other foreign materials on the surface to be inspected not only prevent attraction of magnetic particles to the flux leakage, but also lead to form a false indication.Therefore such materials will be cleaned chemically or mechanically before magnetization process.
The workpiece is magnetized as direction of magnetic flux is orthogonal to direction of a flaw.
Following method is applied for proper magnetization.
- Axial current method : To pass electric current longitudinal direction of the workpiece
- Cross current method : To pass electric current cross direction of axis of the workpiece
- Prod method : To pass electric current between two prods contacted inspection area of the workpiece
- Through conductor method : To pass electrical current through the hole of the workpiece
- Coil method : Put the workpiece in a magnetizing coil
- Yoke method : Put the workpiece between magnetic poles
- Through flux method : To pass magnetic flux through the hole of the workpiece
(3) Applying magnetic particle
- Types of magnetic particle
Easy magnetization and migration to flux leakage and discriminative flaw indication are required for performance of magnetic particle.
There are two types of magnetic particle, one is non-fluorescent particle (white, red, black) for observation under visible light, and the other is fluorescent magnetic particle for observation under UV light.
Dry method, magnetic particle is applied to the surface as it stands, and wet method, magnetic particle is dispersed in oil or water, are applied.
- Applying timing of magnetic particle
There are two methods. Magnetic particle is applied during magnetization of the workpiece, or applied after ceasing magnetization. In later case residual magnetism is utilized for forming flaw indication.
UV light is used for observation of the workpiece under dark environment in case of using fluorescent magnetic particle for inspection.
Demagnetization, removing residual magnetic particle and rust proofing of the workpiece are done if required.