Application of NDT Technology in Iron and Steel Industry
Molten Steel through smelting, casting, rolling, heat treatment and other processes, can be obtained with different specifications of the finished product. But in the production process, steel bars are easy to appear a variety of internal and external defects, such as shrinkage, white spots, internal cracks, scarring, cracks, depression and folding.
At present, the technologies used to detect the internal defects of steel bars are X-ray testing and ultrasonic testing respectively, and some of them, such as penetration testing, ultrasonic testing, magnetic flux leakage testing and eddy current testing, etc. , is the most common in the detection of surface defects.
1 Ultrasonic testing technique
The ultrasonic testing technique is effective by the physical phenomenon that the sound beam entering the material is reflected at the heterogeneous interface. Because ultrasound technology can play a role not only in the detection of surface and near-surface defects, but also in the detection of internal defects; in addition, for some millimeter-scale macro-defects, for example, white spots, internal cracks, shrinkage holes, and even microscopic defects of ten microns, such as silicates and silicon oxides, can be accurately detected.
Therefore, in the steel industry, ultrasonic detection technology has been highly sought after. The frequency of the transducer used to detect macroscopic defects is usually between 2 and 10 mhz. For microscopic defects, it needs to be expanded to 10 to 300 mhz. According to the steel bar detection of different acoustic wave, detection and longitudinal wave, shear wave detection of these two categories. It plays an important role in the detection of internal defects and surface or near-surface defects of steel bars.
2 Eddy current testing technology
Eddy current testing technology is a kind of testing method based on electromagnetic induction, which is common in the testing of conductive materials. Because of the high detection rate of eddy current testing technology on working surface and near surface defects, and the linear expression in a certain range is ideal, it can directly evaluate the size of defects, the coil is separated from the workpiece in the testing process, so it is difficult to achieve the automatic testing effect without the participation of the coupling medium.
Steel Rod eddy current testing generally uses two forms of coil, respectively, point-type, through-type coil. Through-type coil detection, steel rod is a straight line displacement, the position of the coil does not change, before the detection of steel bar magnetic saturation, treatment, etc. , the magnetic asymmetry will be completely eliminated, so that the depth of eddy current penetration increases, after the inspection also need to demagnetization is the complete process. Because of the high difficulty in demagnetizing and magnetic explosion of large size steel bars, the demagnetization of steel bars can not exceed the limit value of φ65mm by using through coil. Point coil detection, steel rod straight line translation, probe rotation or steel rod spiral rotation, probe position unchanged.
3 The magnetic flux leakage (MFL) testing technique
The magnetic flux leakage (MFL) testing technique is used to detect the defects on the surface of ferromagnetic material after it is magnetized, it is common in magnetic conductive materials. Considering the correlation between the MFL signal and the size and shape of the defect, it is feasible to quantify the defect, through the computer automatic identification, alarm defects, so that the probability of human intervention to a minimum, so that the accuracy of detection can be greatly increased.
4 Infrared thermography inspection technology
The technology of infrared thermography is to make the temperature difference between the defect and the normal part by special heating method. The infrared thermography is used to monitor the surface temperature in real time, and then the defects found during the period are recorded in real time, and stored in the form of video. The advantages of this technique lie in the strong intuitiveness of the test results, the lack of contact with the tested parts, the short test time and so on. According to the principle of existing UAV, there are active and passive infrared detection technology.
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