Magnetic Particle Inspection In the Detection of Internal Defects in Castings

Magnetic particle inspection is a non-destructive testing method that detects defects on the surface or near the surface of ferromagnetic materials by the accumulation of magnetic particles in the leakage magnetic field near the defect. The workpiece made of magnetic materials such as steel is magnetized, and the magnetic leakage energy of the defect part is used to absorb the characteristics of magnetic powder, and the surface defects and near-surface defects of the detected object are displayed according to the distribution of magnetic powder. The flaw detection method is characterized by simplicity and intuitive display.

Magnetic particle inspection uses the interaction between the leakage magnetic field at the workpiece defect and the magnetic powder, because the magnetic permeability of the surface and near-surface defects (such as cracks, slag inclusions, hairlines, etc.) The continuous magnetic field will be distorted, and a leakage magnetic field will be generated on the surface of the workpiece where part of the magnetic flux leaks, thereby attracting the magnetic powder to form the magnetic powder accumulation at the defect - magnetic trace, which shows the position and shape of the defect under appropriate lighting conditions.

When performing magnetic particle inspection on castings, a strong magnetic field is generally generated by energizing. According to different energization methods and current waveforms, the magnetization methods can be divided into direct magnetization and indirect magnetization, DC magnetization and AC magnetization. According to the direction in which the magnetic field is formed and the way to generate the magnetic field, there are magnetization methods that can be divided into circumferential magnetization and longitudinal magnetization, continuous magnetization and residual magnetization. In the actual inspection, the foundry can choose a variety of AC and DC composite magnetization methods according to the size of the casting, the distribution of defects and other factors.

Magnetic powder is a material that forms magnetic traces and shows defects, and its material is usually high-valent iron oxide with high magnetic permeability, low remanence and low coercivity, such as ferric oxide and ferric oxide. The particle size of the magnetic powder is preferably 80 - 300 μm for the method of testing with dry magnetic powder. For wet and fluorescent inspection, the particle size of the magnetic powder can be finer. Minor casting defects should choose fine magnetic powder. The shape of the magnetic powder should be mainly spherical magnetic powder, and then matched with a certain proportion of strip magnetic powder.

Magnetic powder suspension is a mixture of magnetic powder and dispersion in a certain proportion. The volume fraction of ordinary magnetic powder is 1.3% - 3.0%, and the volume fraction of fluorescent magnetic powder is 0.1% - 0.3%. The dispersion liquid can be selected from water agent, kerosene and mixture of kerosene and transformer oil, which have the ability to prevent rust, wetting and defoaming.

The Operation Method of Magnetic Particle Inspection:

Place the object to be tested in a strong magnetic field or apply a large current to make it magnetized. If there are defects (cracks, folds, inclusions, etc.) on or near the surface of the object, because they are non-ferromagnetic, the resistance to the passage of magnetic lines of force is very high. Large, the magnetic field lines will generate magnetic flux leakage near these defects. When magnetic powder with good magnetic permeability (usually magnetic iron oxide powder) is applied to the object, the leakage magnetic field near the defect will attract the magnetic powder and accumulate to form visible magnetic powder traces, thereby showing the defect.

1. Pre-cleaning

The surface of all materials and test pieces should be free of grease and other impurities that may affect the normal distribution of magnetic powder, and affect the density, characteristics and clarity of magnetic powder deposits.

2. Defect Detection

Magnetic particle inspection shall be carried out to ensure satisfactory detection of any detrimental defect. The magnetic field lines are made to traverse as far as practicable any defects that may be present in the test piece.

3. Selection of Flaw Detection Methods

1) Wet method: The magnetic suspension should be applied to the test piece by the hose pouring or dipping method, so that the entire surface to be inspected is completely covered, the magnetizing current should be maintained for 1/5~1/2 second, and then the magnetizing current should be cut off. The magnetic suspension is applied by hose pouring or dipping.
2) Dry method. The magnetic powder should be directly sprayed or sprinkled on the inspected area, and the excess magnetic powder should be removed, and the test piece should be shaken gently to obtain a relatively uniform magnetic powder distribution. Care should be taken to avoid using excessive amounts of magnetic powder, which will affect the effective display of defects.
3) Detect near-surface defects. When detecting near-surface defects, the wet powder continuous method should be used. Because the leakage flux caused by non-metallic inclusions is the smallest, the dry powder continuous method can be used when detecting near-surface defects in large castings or welded parts.
4) Circumferential magnetization. The center conductor method should be used when inspecting the inner surface of any cylindrical specimen; there should be a gap between the specimen and the center conductor to avoid direct contact with each other. When current is passed directly through the test piece, care should be taken to prevent burns at the electrical contact surfaces, and all contact surfaces should be clean.
5) Longitudinal magnetization. When magnetizing a test piece with a solenoid, in order to obtain sufficient magnetization, the test piece should be placed in an appropriate position within the solenoid. The size of the solenoid shall be sufficient to accommodate the specimen.

4. Demagnetize.

The part is placed in a DC electromagnetic field, continuously changing the direction of the current and gradually reducing the current to zero. Large parts can be demagnetized in zones with moving electromagnets or solenoid coils.

5. Post Cleaning.

After inspection and demagnetization, all magnetic particles on the test piece should be cleaned; care should be taken to completely remove all blockages in holes and cavities.

Characteristics and Scope of Application of Magnetic Particle Inspection:

Magnetic particle inspection has the highest detection sensitivity for casting surface or near-surface defects, but its sensitivity decreases rapidly with the increase of defect depth. This detection method is only suitable for the detection of ferromagnetic materials, but cannot be used for non-magnetic castings such as austenitic steels. The equipment of magnetic particle inspection is simple, and the portable instrument is easy to operate on site. Magnetic particle inspection has higher requirements on the roughness of the casting surface. After the casting inspection, the surface needs to be cleaned in time and the residual magnetic powder should be removed. If necessary, demagnetization treatment should be carried out.

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