The Eddy Current Testing System Design For Steel Tube And Study On The Identification Of Different Holes

In engineering practice, Eddy current testing(ECT) often be used for steel tube’s quality, and our country formulates a standard GB/T 7735-2004 about ECT for steel tube. In this paper, according to the standard GB/T 7735-2004, processing steel specimens with through-hole or internal and external blind hole. ECT experiments of the through-hole or internal and external blind hole of steel tube under different magnetization current are carried on. The experiments is to study on the variation of ECT signal of through-hole or internal and external blind hole under different magnetization current, and improve the defect evaluation system of the standard GB/T 7735-2004 for steel tube.Consistent machining diameter steel tube in the same material, the different depth of internal and external blind hole, for steel tube under the unsaturated and saturated magnetization of ECT found the internal and external blind hole can effectively be checked out. However, this test uses exteriorly-thru-type probe, can be seen from the effective penetration depth of the eddy current, the internal blind hole should not be checked out. The trial against the phenomenon, through theoretical analysis curve extrapolated steel permeability and magnetizing current, because of the proliferation of non-uniform at around permeability to the steel surface of the coil impedance of an impact, this makes the internal blind hole to check out.According to the experiment phenomenon, the curve of relationship between steel tube permeability and magnetizing current can be gotten through theoretical analysis. It is concluded that the reason for the internal blind hole can be checked out is the non-uniform permeability around the flaws diffusion to the surface of steel tube, the coil impedance is influenced, then it makes the internal blind hole to check out.Firstly, considering the needs of ECT experiments of steel tube, designing the ECT instrument. The instrument includes the ECT coil, excitation module and amplifier module, using hardware to accomplish the design of them. The follow-up signal processing mainly based on the Python language for the design of the digital phase-sensitive detection, signal synthesis and interface display.Set up ECT experiment platform with the above ECT instrument and associated mechanical devices.An experiment of the through-hole of steel tube under the ECT experiment platform. The results showed that, under the region of unsaturated magnetization, the signal amplitude of different through-hole diameters increased first and then decreased with the increase of the magnetization current, signal phase increased gradually with the increase of the magnetization current. Under the region of saturated magnetization, for the smaller through-holes’ diameter(e.g. Φ1.5,2,2.5mm), the signal amplitude decreases with the increase of the magnetization current, the signal phase fluctuates with the increase of magnetization current, but the volatility difference is small; for the larger through-holes’ diameter(e.g. Φ4.5,6,10,15mm), the signal amplitude increases slightly with the increase of magnetization current, the signal phase reduced greatly with the increase of magnetization current, and the variation of signal phase is disorder.Finally, studying on signal characteristics at different depths of the internal and external blind hole in a certain time of the magnetizing current,Proposing under the region of unsaturated magnetization, choosing the magnetization current range in which the signal phase of internal and external blind hole comparatively large, and within the internal and external blind hole’s different phase to set two alarm zone in the ECT instrument, and then distinguish the internal and external blind hole. This method is expected to provide some valuable references for ECT on the steel tube in engineering practice.