Magnetic particle inspection is a detection method that uses the properties of ferromagnetic materials (such as steel, iron, cobalt, nickel, etc.) to be magnetized. When the metal casting is strongly magnetized by the magnetic field, if there is a defect perpendicular to the magnetization direction on the surface or near the surface of the casting, part of the magnetic field lines will overflow here, forming a leakage magnetic field to generate new magnetic poles. At this time, the surface of the casting is poured with suspended magnetic liquid or sprinkled with dry magnetic powder, and the magnetic powder particles show traces of defects due to the attraction of magnetic poles.
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.
Characteristics and scope of application of magnetic particle inspection:
1. Magnetic particle inspection has the highest sensitivity for detecting surface or near-surface defects of castings, but its sensitivity decreases rapidly with the increase of defect depth.
2. This detection method is only applicable to the detection of ferromagnetic materials, but cannot be used for non-magnetic castings such as austenitic stainless steels.
3. The equipment of magnetic particle inspection is simple, and the portable instrument is easy to operate on site.
4. Magnetic particle inspection has higher requirements on the roughness of the casting surface.
5. After the casting inspection, it is necessary to clean the surface and remove the residual magnetic powder in time. If necessary, demagnetization treatment is required.
Post time: Sep-24-2022