Magnetic flux leakage testing of high sensitivity, can well distinguish the pipe outer wall defects, but a long tube, large wall thickness of the pipe after degaussing magnetic flux leakage inspection to be done to deal with; eddy current flaw detection speed, but by the skin effect, the difficult to find defects in the depths of the workpiece; ultrasonic flaw penetrating ability, defect positioning accuracy, low cost, high speed, but testing operations to be coupled, the coupling in the northern winter environment welded easy to freeze, causing inconvenience to the inspection work . In the manufacture and use of welded pipe, the non-destructive testing is performed to ensure the weld quality is particularly important. Pipe commonly used non-destructive testing methods are: for more than 5 mm from the pipe surface offline whole tube of magnetic flux leakage testing, eddy current and ultrasonic testing; verify more than 5 mm from the surface of the pipe welding quality online magnetic flux leakage inspection and eddy current inspection; applicable in thick-walled pipe weld the whole tube offline ultrasonic flaw detection; verification thick-walled pipe welding quality ultrasonic flaw detection.
Refers to the magnetic flux leakage inspection of ferromagnetic material is magnetized, the surface and near-surface defects in material surface leakage magnetic field, discovered by detecting leakage field NDT techniques defects. Magnetic flux leakage inspection of the surface state of the pipe does not ask, detected a greater depth, are widely used in pipe detection abroad, especially oil with domestic pipe detection has also been widely used. When the magnetization is low, the leakage magnetic field is small, and increases slowly; When the magnetic flux density reaches about 80% of the saturation value, the peak of the defect with the increase of the leakage magnetic field the magnetization increases rapidly, but when a ferromagnetic material into the the magnetic saturation magnetization increases outside little influence on the magnetic field strength of the defect. Therefore, the magnetic circuit should be developed so that the material being tested as nearly saturation magnetization state. A huge influence on the direction of the leakage defects in the magnetic detection accuracy when the defect perpendicular to the main plane of the magnetization direction of the magnetic field, generated by the leakage magnetic field is the strongest. The same weakness leakage magnetic field maximum surface of the duct, and with increasing burial depth gradually decreases when the burial depth is sufficiently large, the leakage magnetic field will tend to zero. Defect size is also a great influence on the leakage magnetic field, the same width and the depth when the defect is not the same, with the increase of the leakage magnetic field to increase the depth of the defect in a certain range of the approximate linear relationship between the two.
For pipe weld often produce "inner burr adsorption", eddy current testing equipment often erroneously reported as defects. Frequently appeared in eddy current testing process glitch due to electromagnetic forces adsorbed on the seam, leaving the eddy current magnetic field exposure steel automatic degaussing device, there is no magnetic attraction after the glitch automatically fall, and then manually re-defect detection and no awkward questions displayed. Due to the skin effect, as the distance between the detection coil and the pipe is subject, the detection sensitivity is gradually reduced, so that, for the same size of the defect, as reflected in the pipe wall will be smaller than the amplitude of a signal tube wall defects on. Pipe full pipe eddy current testing equipment capacity in the probe and the inner surface of the outer surface of the pipe defects regard is determined by many factors, but the most important thing is dependent on the wall thickness of the pipe was seized and eddy current excitation frequency and magnetic saturation intensity ; the same time, the choice of instrumental parameters should consider the impact of the physical parameters of the subject to be welded conductivity and magnetic permeability, etc. of the eddy-current flaw detection precision.
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