Research On Eddy Current Thermography Detection Method For Steel Pipe Cracks Based On Biased Magnetization

As an important foundation-bearing component,steel pipe is widely used in oil and gas drilling and transportation.Due to the imperfections of raw materials and processing technology,cracks and corrosions often appear on the inner and outer surfaces of steel pipes in the manufacturing process.Therefore,in order to ensure pipeline quality and service safety,according to national standards,all steel pipes must be 100% nondestructively tested before leaving the factory.Eddy current thermography is widely used in the automatic detection of ferromagnetic components due to its advantages of high efficiency,visualization and large lift.However,due to the skin effect of high-frequency eddy current,thetraditional eddy current thermography is limited in its ability to detect sub-surface defects and quantify deeper surface defects in ferromagnetic pipes.Aiming at the limitations and deficiencies of traditional eddy current thermography method,a new eddy current thermography detection method based on biased magnetization is proposed in this paper.Under biased magnetization,sub-surface and surface defects will cause distortion of the permeability intensity in the skin depth layer,and produce different temperature field responses.Thus,the morphological characteristics of sub-surface and surface defects can be correlated with the surface temperature field distribution.Finally,it breaks through the bottleneck of sub-surface and surface defects detection and quantification of traditional eddy current thermography,which is of great significance for realizing high efficiency,high precision and full coverage detection of steel pipe.Firstly,the theory and simulation of biased magnetization eddy current thermography detection were studied.Combined with the specialized characteristics of ferromagnetic steel pipes,the basic principle of electromagnetic induction heating was explained and the influence law of the relative direction of the biased magnetization field and the high-frequency magnetization field on induction heating was analyzed.The distribution of magnetic field disturbance caused by the sub-surface and surface defects with different depths,the permeability distortion caused by magnetic field disturbance,and the response of the surface temperature field caused by the distorted permeability under the biased magnetization were analyzed by theoretical analytical model and finite element simulation.Thus the detection theory of biased magnetization eddy current thermography method(BM-ECT)is constructed.Secondly,experimental studies on defect characterization of BM-ECT were carried out.The influence mechanism of different magnetization currents and directions on the surface thermal response temperature of the ferromagnetic specimen was analyzed.The response rules of surface temperature field caused by sub-surface and surface defects under biased magnetization were constructed respectively,and the feature parameters of defect characterization and quantification were obtained.The optimal magnetization method and key parameters for optimal thermal contrast of defect were obtained through experiments.Furthermore,a dynamic detection system of BM-ECT was constructed,and a moving image processing algorithm was developed.The Wiener filter deconvolution algorithm and image reconstruction algorithm based on isochronous heating are proposed for the thermal image trailing effect and the spatio-temporal variation of heating.The experimental study on the dynamic detection of the specimen with different direction defects was carried out by BM-ECT.The feature-based method and the morphology-based method were proposed to evaluate the directions of surface defects.Finally,based on the above detection mechanism,magnetization method and image processing algorithm,an automatic detection system for circumferential defect BM-ECT of steel pipe was developed.Experimental studies were carried out on steel pipes with sub-surface and surface defects of different depths.Automatic identification of defect locations was realized by morphological spatial feature and frequency domain feature of the continuous wavelet transform.And the thermal temperature amplitude gradient is used to evaluate the defect depth quantitatively.The eddy current thermography detection method based on biased magnetized proposed in this paper can realize the automatic detection,type differentiation and depth quantification of the sub-surface and surface defects of steel pipes,which is of great significance for the realization of high efficiency,high precision and visual detection of steel pipes and other ferromagnetic components.