Internal Defect Detection in Ferromagnetic Material Equipment Based on Low-Frequency Electromagnetic Technique in 20# Steel Plate

A ferromagnetic material equipment internal defect detection system based on low-frequency (20–50 Hz) electromagnetic technique was proposed. Direct current magnetization was used to make the ferromagnetic material sample (20# steel for instance) be in a saturation magnetization state to decrease its permeability, which is beneficial for increasing the skin depth of the sample and strengthening the signal of the magnetic flux leakage. A low-frequency alternating current source was used to excite the spatial alternating electromagnetic field and a Hall sensor was used to collect the signal of the spatial magnetic field. Experimental results showed that the 6-mm buried depth internal defect with the sensitivity of 0.05 mm can be detected in the 12-mm thick 20# steel plate or pipeline, which is almost 1.5 times higher than that of the current similar sensors.