Study On The MFL Method For Steel Pipes Inspection Under Circumferential Magnetization Based On FEM

As many large domestic pipe manufacturers have built newly or changed, expanded annual output of 50 million tons continuous casting and rolling pipe production lines, and a number of private enterprises have built annual output of 100,000 tons pipes continuous rolling lines, China has become the world's largest pipe manufacturer and exporter at present. Therefore, if pipe manufacturers can find pipe defects promptly to avoid the follow-up meaningless process, it can effectively control the scraps, save production cost.Because of its high detection sensitivity, high detection speed and good detection for internal and external surface defects to the magnetic materials and so on, MFL nondestructive testing methods showed a significant advantage in pipe and other ferromagnetic components of the rapid automated detection. Currently, imported high-speed magnetic flux leakage testing equipments have almost occupied the domestic markets, while the domestic corresponding technology and equipment have not meet the demand of pipe quality inspection yet, especially for the pipe circumferential and axial magnetic flux leakage detection technology. Therefore, based on three-dimensional finite element method, this dissertation focuses on pipe wall magnetic field distribution for circumferential excitation, analyses the effect of pipe diameter, wall thickness and other structural parameters on pipe wall magnetic field ,discusses the relationship between the different circumferential excitation magnetic field and pipe wall magnetic field, defect leakage field , the background magnetic field, and explores the based on ANSYS magnetic leakage finite element visualization software and secondary development technology.On the basis of magnetic flux leakage finite element theory, simulation and analysis of the magnetic field distribution of pipe circumferential magnetization, and the effect of the external magnetizing field strength on the magnetic field distribution, the effect of pipe thickness and curvature on the magnetic field distribution were studied .The results show that the field of external magnetizing field is much larger for detection axial defects in the circumferential magnetization method comparing to the axial magnetization methods; in the same external magnetizing conditions, the effective detection area of wall will become larger with the wall thickness increasing, but its magnetic flux density decreases rapidly; In the case of external magnetic field keeping unchanged , pipe wall magnetic field intensity gradually decreases, but the wall effective detection area is getting larger, and the effective detection area of the magnetic field more uniform with increasing pipe diameter. In addition, for the magnetic field around the circumferential of pipe, the effect of magnetization field strength and defects orientation on axially aligned defects magnetic flux leakage signals are studied. The results show that magnetic flux leakage field and the background magnetic field strength will increase accordingly with the external magnetization field increasing, only using circumferential satured or supersatured magnetization methods, axial defects in pipes can be achieved better test results. Finally, based on the APDL and VC++, using pipe circumferential magnetic flux leakage testing finite element analysis as the application background, the secondary development of technology of magnetic flux leakage finite element analysis software have been developed. The results show that the software provides a concise graphical input interface, automatically establishes the whole the finite element model , automatically completes the process of the analysis and solution ,and classifies and shows the final results at last.