解析深沟球轴承装配自动线
来源：南京博克纳自动化系统有限公司 日期：2017-09-26 关键词：涡流探伤，涡流探伤仪
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Eddy current testing is suitable for conductive materials testing. The common metal materials can be divided into two categories: non ferromagnetic materials and ferromagnetic materials. The former is copper, aluminum, titanium and its alloy and austenitic stainless steel; the latter is steel, iron and alloy. The essential difference between them is material permeability is about 1 or greater than 1. In the power plant, a small number of pipelines, such as copper, titanium and austenitic stainless steel, are used in a small number of pipes, such as copper, titanium, austenitic stainless steel and other non ferromagnetic materials.
Conventional eddy current testing applied to non ferromagnetic tubes is a very mature technology. It can not only detect defects, but also use impedance plane technology to analyze the location and depth of defects. However, the simple application of this method to ferromagnetic steel tubes has failed to achieve the desired results. What are the reasons for this? This is due to the ferromagnetic material >>1, according to the standard formula of eddy current penetration:
D = 503.3 / V f r.
In this case, the vortex can only be concentrated on the surface, can not penetrate into the internal material. In addition, the domain structure of ferromagnetic materials will cause great interference to the eddy current detection signal, which is enough to completely submerge the defect signals and can not obtain useful information.
There are two ways to overcome the influence of ferromagnetic metal permeability on flaw detection: one is to use far field eddy current testing method, and the other is to detect the saturation magnetization of steel tubes. The former method needs to update the instrument, the latter method only needs to add the magnetic saturation device on the basis of the conventional instrument to detect the steel pipe, and has the advantages of less investment. After the magnetic material magnetic saturation after treatment can be treated with non ferromagnetic materials.
Usually used by steel pipe eddy current testing type magnetic saturation reactor. It is composed of the coil passes through the DC to produce high magnetic field, and with the help of the guide sleeve etc. high magnetic components will be diverted to magnetic flaw detection parts of steel tube, so as to achieve magnetic saturation. In order to make full use of the magnetic field produced by the coil, the device generally has a shell made of ferromagnetic material (such as pure iron). Because of the great value of pure iron, reluctance is very small, in the space of the magnetic flux leakage will be iron collection, was also detected in the parts of the pipe dredge.
Due to the strong magnetizing current passing through the coil of the magnetic saturation coil, the coil will be heated, so good heat conduction measures should be taken to prevent the coil from burning down.
In addition to producing a strong dc magnetic field, the magnetic saturation device is often used to clamp the magnetic saturation device, so the structure of the magnetic saturation device is closely related to the shape of the detection coil. In the through eddy current testing, the guide sleeve of the magnetic saturation device must keep concentric with the detection coil, otherwise it will cause large circumferential sensitivity, resulting in missed detection and false detection.
Magnetic saturation eddy current testing method should make the flux density near the detection coil reach more than 80% of the required magnetic flux density for the saturation magnetization of the steel tube. Therefore, before testing the pipe materials and specifications should be based on the selection of magnetization current. Magnetizing current selection usually is carried on by comparing the sample under the state. Theoretically, the magnetic flux density required for the saturation magnetization of the tested steel tube should be calculated first, and then the magnetization current should be adjusted according to the requirements mentioned above. In the actual operation, a simple adjustment method can be adopted, that is, during the trip through the contrast sample, with the gradual increase of the magnetizing current, the noise signal and the artificial defect signal displayed by the instrument can be observed. When the noise signal is minimum and the artificial defect signal is maximum, the magnetizing current is basically appropriate. According to the general law, the larger the diameter, the thicker the wall thickness, the softer the magnetic properties of the material, the greater the magnetizing current is required, and vice versa
EM series of magnetic saturation device is specially designed for steel pipe eddy current flow situation. It is composed of magnetic saturation magnetization and constant current power supply. The conventional magnetic saturation device by the magnetizing coil and iron components, large volume and heavy weight, suitable for use in the manufacture of steel tube plant sites, in this case, the magnetic saturation device without moving, volume and weight are not considered, so using ordinary materials, to reduce the cost. For power plants and petrochemical plants, eddy current testing of steel tubes is usually carried out on the mobile site rather than in the workshop. The device must be portable and efficient for ease of use and movement. This specially designed EM series magnetic saturation device, using a compact design, high permeability materials and careful processing, greatly improving the efficiency of magnetic device, the weight is only 40% of the general device, the volume of less half. In addition, the magnetizing power supply uses the constant voltage constant current power supply, it can avoid the voltage change or the magnetization coil heating causes the resistance to change greatly, and changes the magnetization current the malady.
