The leakage detection means that after the iron magnetic material is magnetized, the surface and near surface defects are on the surface of the material to form a leakage field. The lossless detection technology of defecting the defect was found by detecting the magnetic field.
The leakage detection of the tube is not high in the surface status of the pipe, and the depth of detection is large. It is used in large quantities in the detection of tube tubes abroad. The detection of domestic oil tubes, especially for oil, has also been commonly used.
In production testing, there have been no phenomenon of welded pipes in the pipe tube. In addition to management and personnel factors, this has something to do with the performance of the instrument, probe performance, and defective size and shape.
The main factors affecting the precision of the whole pipe of the welded pipe are the following points.
(1) Magnetization intensity
When the magnetization strength is low, the magnetic field is small and increases slowly; when the magnetic induction strength reaches about 80%of the saturation value, the peak of the abnormal leakage field will increase rapidly with the increase of magnetization intensity, but when the iron magnetic material enters the entering In the state of magnetic saturation, the increase in external magnetization strength has little effect on the strength of the defect magnetic field.
Therefore, the design of the magnetic circuit should make the measured material as close as possible.
(2) The direction, location and size of the defect.
The direction of defects has a great impact on the accuracy of the detection of magnetic leakage. When the main plane of the defect is perpendicular to the direction of the magnetized magnetic field, the magnetic field produced is the strongest.
The same shortcomings are the largest in the magnetic field when the pipeline surface, and gradually decreases as the burial depth increases. When the burial depth is large enough, the magnetic field will tend to zero.
Therefore, the wall thickness that can be used to detect is generally 6 ~ 15mm; when the sensitivity is reduced, the wall thickness can be detected by 20mm.
The size of the defect also has a great impact on the leakage field. When the width of the defects is the same and the depth is not at the same time, the magnetic field increases with the depth of the defect.
The impact of defect width on the magnetic field is not monotonous. At a very hours of defect width, there is a tendency to increase with the increase in width.
(3) Lifting from value
When the lift value exceeds twice the width of the crack, as the lift height increases, the strength of the leakage field decreases rapidly.
The design of the sensor bracket must keep the probe's lift value of the probe when the surface of the steel pipe is checked is constant. Generally, it should be less than 2mm and often take 1mm.
(4) Detective speed
During the detection process, it should be kept at a uniform speed. Different speeds will cause different shapes of magnetic signal, but generally do not cause misjudgment.
(5) Welding pipe surface quality
The thickness of coating and other coating on the surface of the welded pipe surface has great impact on the sensitivity of the detection. As the thickness of the coating increases, the detection sensitivity decreases sharply.
From the current instrument performance, when the thickness of the coating is ≥6mm, it is no longer possible to obtain effective defect recognition signals.
Different surface roughness of the welded pipe causes dynamic changes in the lift value of the sensor and the surface of the inspection, which will affect the consistency of the sensitivity of the detection. In addition, it will also cause the system to vibrate and bring noise. Therefore Essence
The oxide and rust on the surface of the welded pipe may also generate a pseudo -signal during the detection process. It should be confirmed or re -examination in time.
