Magnetic particle inspection (MT) is a conventional testing method for inspecting surface integrity. It can only be used on ferromagnetic materials such as iron, nickel, etc.
Castings, forged parts, hot-rolled bars and all kinds of finished products are suitable for magnetic testing.
The process of Magnetic Inspection
This process consists of three parts:
- The part to be tested must first be magnetised.
- The inspection media or magnetic particles are then applied.
- Finally, the patterns formed by the particles have to be interpreted. These patterns are formed when attracted by the magnetic leakage field created by discontinuities in the part.
We test magnetic particles with visible or fluorescent media. Particles may be in liquid or powdered form.
Testing with fluorescent magnetic particles is more sensitive than testing with visible media. This method can only be used as a wet method and no contrast dye is used. A UV-A lamp (blacklight) is needed in a darkened room to view and correctly interpret the indications.
We use the yoke method to apply a magnetic field. The electric coil in the yoke creates a longitudinal magnetic field that passes through the legs of the yoke to the part being inspected. This is done with a permanent magnet or with a magnet connected directly to the mains or a battery pack. A yoke is multi-purpose: indoors, outdoors, in drums and tanks and in all positions.
At which stage do we apply Magnetic testing?
Magnetic testing (MT) is applied at every stage of a product’s life cycle:
- production of castings
- welded products
- operational products
The advantages and disadvantages of Magnetic research
- quick method for inspecting large surface areas
- method can be deployed with relatively cheap working equipment
- immediate interpretation of indications
- less critical surface preparation required than with penetrant testing (PT)
- method is not location-bound
- detection of surface faults and faults located just below the surface
- can only be used on ferromagnetic surfaces
power access is needed (from the mains or a battery pack)
- this method cannot be used if the object’s coating is too thick
- some test liquids are highly flammable posing a fire hazard, especially in enclosed or indoor spaces
In which industries do we use Magnetic inspection?
Industries in which magnetic inspection is used:
- construction steel
- oil, gas and wind industry, onshore and offshore
- shipping industry