Basic Knowledge Of Flaw Detection Technology For Cold Drawn Seamless Steel Pipes

In the field of flaw detection technology, cold drawn seamless steel pipe refers to a steel pipe with an outer diameter greater than φ80 mm. Cold drawn seamless steel pipe is an important material for petroleum, chemical, heat, boiler, mechanical hydraulics and other industries.

With the development of the national economy, during the “Eleventh Five-Year Plan” period, the demand for cold-drawn seamless steel pipes has increased significantly, far exceeding the resistance welded pipes, and it has clearly shown a trend of large-diameter development. Especially for oil well pipes and large-diameter high-pressure boiler pipes requiring high corrosion resistance and anti-extrusion, high-quality petroleum cracking pipes, petroleum and petrochemical pipelines, etc., will become demand as the country invests in energy infrastructure. Hot spot. Therefore, the non-destructive testing of the quality of the products is proposed to propose new methods and techniques.

The sink type ultrasonic inspection adopts a steel pipe spiral advance type, and the ultrasonic probe is fixed. The thickness of the coupling layer is ensured by the characteristics of sufficient water coupling between the water tank and the bottom of the steel pipe to be inspected. However, because the ultrasonic mainly detects the internal defects and the blind areas of the surface and subsurface defects, which makes it impossible to detect, and the use of the spiral advance type, it is necessary for the 12m long steel pipe to occupy a space of 30m, which has always affected the selection of the steel pipe detection method and Promotion.

Therefore, at home and abroad, for the detection of cold drawn seamless steel pipes, magnetic flux leakage or water pressure experiments are generally used. In China, there is no performance of magnetic flux leakage testing equipment suitable for cold drawn seamless steel pipes. Once used, it needs to be imported. Imported magnetic flux leakage testing equipment is expensive, and it is difficult for most domestic enterprises to accept; and the hydraulic pressure test has low efficiency and high labor intensity, especially when the operator’s responsibility is not high, the water pressure test is ineffective. It can be seen that the detection of cold drawn seamless steel pipes has become an urgent problem to be solved in the metallurgical steel pipe industry.

The cold drawn seamless steel pipe is characterized by a large diameter and a relatively thick wall thickness. Therefore, according to this feature, the combination of ultrasonic detection inside and the characteristics of the eddy current detecting surface and the subsurface can be used to realize the “no blind zone” flaw detection. By adopting the “combination method of in-situ rotation of the steel pipe and detecting the advancement of the probe”, not only the detection problem is solved, but also the space for occupying the occupied space is solved.

In automatic flaw detection, the effect of lift-off and stable coupling layer on flaw detection is often the most difficult problem. In automatic flaw detection, the lift-off effect and stable coupling layer are the main causes of missed detection and false positives. Whether it is missed or false positive, it affects the reliability of the test. For a long time, in the practical application of automatic flaw detection, the problem of reduced detection reliability due to lift-off fluctuations or the thickness variation of the water-coupling layer has been a “bottleneck” that plagues the normal use of this technology.

Generally, the methods for solving the lift-off effect mainly include: the mechanical tracking method of the probe, the bridge connection of the probe coil, the change of the capacitance value of the detection coil LC loop, and the use of multi-frequency detection technology. In addition to the mechanical tracking method, several other solutions can be realized by improving the probe and the instrument, but the mechanical tracking can only improve the probe holder to prevent the change of the lift-off gap. In practical industrial applications, the probe mechanical tracking method is the most commonly used method to overcome the effects of lift-off effects.

There are two common mechanical tracking modes for probes: one is to use a combination of roller limit and cylinder or spring push to maintain a constant distance between the test probe and the surface of the workpiece being inspected. Although this method can play a better role in suppressing the lift-off effect, it also increases the vibration noise.

Another method of using the mechanical tracking of the probe is to measure the fluctuation of the detection probe’s lift-off gap in time by using the distance measuring probe, and use the ranging signal to control and drive the power device such as the stepping motor to drive the detection probe to ensure the action. The gap between the probe and the workpiece being inspected is constant. This method is suitable for plane scanning inspection of plates or billets. The disadvantage is that the reaction speed of mechanical actions is relatively slow and complicated.

The probe is loaded into a probe car and a secondary spring is used to push the probe to a certain distance from the surface of the workpiece to be inspected. From the experimental results, the follow-up of the probe is relatively strong, which basically ensures that the distance between the probe and the surface of the tested steel tube is constant, and the flaw detection also achieves a good effect. Generally, the methods for solving the water coupling layer mainly include: fixing the tank box and stabilizing the water spray device. Due to the advancement of the steel pipe rotating probe, the length of the cold drawn seamless steel pipe is generally about 10 m.

Therefore, it is necessary to consider the use of a stable water spray device, such as increasing the diameter of the flow port, reducing the height of the flow port and the steel pipe, and reducing the water splash. The current conventional solution can only be like this, but the effect of the solution is within acceptable limits.

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