Basic Introduction of Magnetic Particle Inspection

Magnetic particle inspection (MPI) is one of non-destructive testing (NDT) methods used to detect cracks at the surface of ferromagnetic materials such as steels and nickel-based alloys. Magnetic particle detection utilizes the interaction of the magnetic leakage field at the defect of the work piece with the magnetic powder. It takes advantage of the difference in magnetic conductivity of surface and near-surface defects of steel investment casting parts, such as cracks, slag, hairlines, etc. After magnetization, the magnetic field at the discontinuity of these materials will be changed. A magnetic leakage field will be generated on the surface of the work piece, where a part of the flux leakage is formed. Thus it attracts the magnetic powder formed and accumulated at the defect. It is called magnetic mark. Under appropriate light conditions, the defect position and shape are shown. Observing and explaining the accumulation of these magnetic powder, inspectors can find out the defects on casting parts.

1. The magnetization may be produced by any of the following methods:
2.  Applying a permanent or electro-magnet to the surface (magnetic flow);
3.  Passing a large current through the specimen, or locally by means of current prods (current flow);
4.  Putting the specimen inside a current-carrying coil, or forming a coil around the specimen;
5.  Making the specimen the secondary loop of a transformer – (induced current) – suitable for ring-shaped specimens;
6.  Placing a current-carrying coil or loop close to the specimen surface;
7.  Threading a current-carrying bar through a hollow specimen.

Magnetic particle inspection is used to ensure that there are no defects on the Metal casting products such as cracks, seams, inclusions, porosity and so on. The method is applicable to all metals which can be strongly magnetized – ferritic steels and irons, but not generally austenitic steels. Compared with other NDT methods, MPI is easy, quick and cheap to conduct because the environment is not limited. Investment casting products with defects tested by MPI will not be delivered to our customers.

The Advantages & Disadvantages of Magnetic Particle Inspection

Advantages:

The sensitivity of the magnetic particle detection is very high. It is able to detect minor defects such as hair lines. The cracks, folds and loose defects can be found. Moreover, the shape, size and position of defects can be visually displayed.

Magnetic particle inspection can test almost all sizes and shapes of steel casting parts.

Compared with other inspection methods, magnetic particle detection is cost efficient and fast.

 

Disadvantages:

Magnetic particle inspection is only applicable to surface or near-surface defects of investment casting products, generally <3mm.

The detection sensitivity has a great relationship with magnetization direction and defect direction.

The covering layer, paint and shot penning layer on the surface of the casting parts, will reduce the sensitivity of the detection.

Demagnetization is generally required after the test.

Fluorescent Flaw Detection: Similar to Magnetic Particle Inspection

Fluorescent flaw detection is similar to magnetic particle detection. Fluorescent flaw detection refers to the method that using the dissolved fluorescent dyes to penetrate into the tiny cracks on the surface of steel casting parts. Fluorescent substances are coated on the surface of steel casting products, which can be observed under ultraviolet light. The yellow and green fluorescent spots or stripes will appear under the ultraviolet lamp, which can be found by inspectors.