A Magnetic Flux Leakage Testing Method Of Steel Pipes Based On AC Magnetization Modulation

The magnetic flux leakage(MFL)testing method has been widely applied in nondestructive testing of steel pipes.However,there are still challenges to internal defects detection of large thickness-to-diameter ratio and rough-surfaced steel pipes.To solve this problem,a novel MFL testing method based on AC modulated magnetization is proposed,which further expands the theory of MFL testing and solves engineering application problems.First of all,the magnetic permeability disturbance is used as the characteristic physical quantity and information transmission parameter.The calculation model of the local magnetic permeability disturbance caused by internal defects is established.Then,simulations and experiments are carried out to verify the influence of magnetic permeability disturbance on the leakage magnetic field of internal defects.The results show that under certain conditions,when the leakage magnetic field generated by internal defects is transmitted to the outside of the tube,the local magnetic permeability disturbances do not hinder the transmission of the leakage magnetic field.On the contrary,It has an enhancement effect.For this reason,the feasibility of the MFL tesing method to detect deep buried internal defects is theoretically clarified.The theory of internal defects detection is deepened in DC MFL testing.To realize the detection of weak leakage magnetic field of inner surface defects in thick-walled steel pipes,a new magnetic flux leakage detection method based on AC magnetization modulation is proposed.The AC magnetization field added under DC magnetization can only act on the external defect,but it will change the characteristics of the leakage magnetic field generated by the internal defect outside the tube.By this method,the magnetic permeability of the surface layer changes with time.After the internal defect leakage magnetic field transmits through this layer,the time-invariant disturbance magnetic field is changed to a time-varying magnetic field,so that the induction coil can measure the internal defect leakage magnetic field statically and with high sensitivity.Compared with the AC magnetic flux leakage method,the new method can detect internal defects with a greater buried depth.Compared with the existing DC magnetic flux leakage method,it has higher internal injury detection accuracy.The surface of the thick-walled steel pipe is rough after heat treatment,and the weak leakage magnetic field signal of the inner surface defect will be completely submerged in the background noise.It is found that the direction of the leakage magnetic field of external defects will periodically change while that of internal defects will remain unchanged under appropriate DC and AC magnetizing currents by simulation analysis of the space-time characteristics of the leakage magnetic field of internal and external defects under the new detection method.Therefore,it is possible to demodulate and separate the outer layer leakage magnetic field signal,whick will improve the sensitivity and signal-to-noise ratio of the internal defects detection signal and achieve higher precision and deeper buried depth detection.Finally,an induction coil probe technology based on the near-zero lift-off of the iron core is proposed and applied in the new testing method proposed in this paper.On the one hand,the measurement sensitivity of the coil is improved by the iron core.On the other hand,the iron core passing through the wear-resistant layer achieves near zero lift-off.This technology can increase the thickness of the wear-resistant layer of the probe and increase the service life of the probe without causing sensitivity fluctuations due to the gradual consumption of the wear-resistant layer,which has high application value.