In China, there is no good performance for cold-drawn seamless pipe flux leakage testing equipment produced, once in use that need to be imported. Imported magnetic flux leakage testing equipment is expensive and is unacceptable to most domestic enterprises. However, hydrostatic testing is inefficient and labor-intensive, especially when the operator's sense of responsibility is not high. It can be seen to achieve cold-drawn seamless steel pipe testing has become a metallurgical steel pipe industry to be solved.
Cold-drawn seamless pipe are characterized by large diameter and relatively thick wall thickness. Therefore, the "blind spot" flaw detection can be realized by fully utilizing the characteristics of the inner and the eddy current detection surfaces and the secondary surfaces according to the characteristics. Through the use of "pipe rotating in situ, the detection of the probe forward combination", not only to solve the detection problems, but also to solve the problem of reducing the space occupied by the site.
In the automatic inspection of sanitary stainless steel pipe, lift-off effect and stability of the coupling layer on the impact of testing often become the most intractable problems. In automatic flaw detection, the lift-off effect and the stable coupling layer are the main causes of missed inspection and false positives. Whether it is missing or false positives, all affect the reliability of the test. For a long time, in the practical application of automatic flaw detection, the problem of decrease of detection reliability caused by lift-off fluctuation or the change of thickness of water-coupled layer has always been a "bottleneck" that plagued the normal use of this technology.
Usually, the solution to the lift-off effect mainly includes: the mechanical tracking method of the probe, the bridge connection of the probe coil, the change of the capacitance of the detection coil LC circuit and the use of the multi-frequency detection technique. In addition to the mechanical tracking method, several other solutions are achieved by improving the probe and the instrument, but mechanical tracking can only improve the probe holder to prevent lift-off variations.
In practical industrial applications, the probe mechanical tracking method is the most commonly used method to overcome the effect of lift-off effect. Common probe mechanical tracking mode, there are two: one is the use of roller limit and the cylinder or spring push combination of methods, the test probe and the workpiece surface to be maintained constant distance. Although this method can inhibit the lift-off effect can play a good role, at the same time will increase the vibration and noise. Another way, the use of probe mechanical tracking approach is the use of distance measuring probe to detect the test probe lift off the gap between the fluctuations and distance measurement signal to control and drive stepper motor and other power devices to drive the detection probe action to ensure that the gap between the probe and the workpiece is constant. This method is suitable for flat surface inspection such as sheet or billet flaw, the disadvantage is due to mechanical reaction more slowly, but also more complex.
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