The reliability of several non-destructive testing methods, such as magnetic flux leakage, eddy current, ultrasonic and electromagnetic ultrasonic, is discussed. By comparing the reliability of the above methods, it can be seen that the electromagnetic ultrasonic method has the advantages of other non-destructive testing methods. And broad application prospects.
Steel is the most widely used steel variety. Its quality directly affects the economic benefits and the safety of personnel. Countries around the world have paid great attention to the quality of steel pipe inspection, using a variety of non-destructive testing (NDT) method of steel pipe for a rigorous testing. For example, Mannesmann of Germany and Sumitomo Metals of Japan have adopted the field (UT) and magnetic flux leakage (MFL) methods in the detection of large diameter steel tubes. The detection of small diameter steel tubes by means of overtravel and eddy current (ET) A more mature detection program. China’s steel pipe inspection using a large number of ultrasonic and eddy current methods, more and more use of magnetic flux leakage method. However, due to the natural defects in the production of steel pipe produced by NDT method can not detect the phenomenon. Therefore, how to improve the NDT detection reliability, it becomes an increasingly urgent issue.
In this paper, combined with some defects in the detection of natural defects in steel pipe inspection, in-depth analysis of the principle and the performance of the testing equipment provides a reference for the development of the best detection scheme.
2. MFL (MagneticFluxLeakage) method reliability
MFL because of its surface condition on the pipe is not high, the detection depth is also large, in the foreign steel pipe detection in a large number of domestic and more and more use, especially in the steel pipe inspection has been widely used The United States manufactured by Tubescope MFL flaw detection equipment.
In the use of MFL, due to management and technical reasons, there have been some missed problems. One of them is a defect that is at an angle of 45 degrees to the tube axis. Such as the MFL device in the longitudinal probe and horizontal probe at the same time, may reduce the missed rate, otherwise it is difficult to ensure reliable detection of such inclinations.
Another important factor influencing the reliability in the MFL method is the angle between the natural defect and the tube surface. Theoretical calculations and experimental studies have shown that when the orientation of the artificial groove along the wall thickness direction is 30 degrees from the outer surface of the tube, it is impossible to detect the defects parallel to the surface, such as delamination defects, by MFL.
Some, such as steel pipe in the process of forming a larger deformation, and sometimes the natural defects will be rolled. At this time, the defect generated by the magnetic flux leakage is very small, resulting in difficult to use MFL method out.
In the production of detection, there have been used MFL method can not detect the phenomenon of large holes in the pipe through the wall. Aside management and personnel factors, the technology should also be into the sub-board and to guard against. For the MFL method can detect the surface of the deep defects, there has been no clear conclusion. This is related to instrument and probe performance and defect size and so on.
3. ET (EddyCurrentTesting) method of reliability
Due to the high detection speed, the through-coil ET method has been widely used for testing the quality of steel tubes over the years, especially its compactness. In use, it is difficult to detect cracks in the ferromagnetic steel pipe defects, so in the high standard ET, the use of probe coil ET method. In addition, for the ET method can be pulled out of the surface of the deep lack of such a simple question, it seems that has not yet formed a clear consensus.
For the steel pipe often produce “outside the fold” class defects, many eddy current instruments and probes are often found. Often appear outside the eye is clearly visible, but can not use the instrument with the alarm of the embarrassing phenomenon. This has been fundamentally changed since the vortex device with sector-type phase alarms has emerged. But choosing the right instrument and probe and adjusting it properly is still an important issue that can not be ignored
4. UT (UltrasonicTesting) method of reliability
UT method in the steel pipe inspection and thickness measurement in the most widely used. However, as a detection method, its reliability is affected by various factors. Such as the analysis of the study is not enough, or even a serious missed, false positive phenomenon. Here are just a few of the possible factors that may exist in UT to reduce the reliability of UT.
(1) the effect of natural defect orientation on UT reliability
In the steel pipe rolling process, the occurrence of high frequency is the axial (longitudinal) defects. However, with the steel pipe axis is an extension of the angle of the defect is not uncommon. Vertical (transverse) defects perpendicular to the tube axis also occur from time to time. NDT’s task is to these different orientations of the defects are stuck out.
Similarly, in order to reliably detect the folding defects with the steel pipe surface, two sets of probes which are incident in the opposite direction of the tube circumference must be provided.
(2) the impact of acoustic coupling on UT reliability
Water intrusion UT, the sound waves in the wall of the attenuation is very serious. Because the acoustic characteristic impedance of the water is much smaller than that of the steel, the reciprocating transmittance of the sound wave from the water to the water from the steel to the water is very small. Second, the sound waves in the wall of each reflection are accompanied by wave conversion. And the transverse wave to the water transmission when it is completely absorbed by the water, it is significantly increased water intrusion UT ultrasonic attenuation. This leads to the propagation of ultrasound along the wall is very small, even a quarter of a week can not reach. The calculated value T (LS) of the reciprocating transmittance of the sound pressure of the steel pipe is measured by the oblique contact method of the plexiglass wedge when the transverse wave refraction angle is 45 degrees without considering the wave mode conversion during the acoustic propagation process, About equal to 25%. And T (LS) is about 15% when immersed in water on the outside of the pipe and the inside is still air. The The latter 4dB lower than the former. If the bore of the pipe is also filled with water (for example, water is immersed in the hole from the hole), the defect echo signal is reduced by at least 6 dB compared to the contact method or the water film method. If the defective echo signal is not strong due to the influence of unfavorable factors such as defect shape or orientation, the defect is likely to be missed. A better solution to this problem is to water film method instead of immersion method for coupling.
5. Electromagnetic Magnetic EMAT (ElectronmagneticAcousticTransducer) Flaw Detection Technology
In order to solve the various difficulties brought by the acoustic coupling to the UT, there was an electromagnetic ultrasonic transducer (EMAT) that did not need the acoustic coupling in the 20th century to stimulate and receive the sound waves in the metal. After 30 years of research, Development, now has entered the engineering, commercialization stage. The United States, Germany, Russia, Japan and other countries have been selling goods and equipment. I began from the early 70s MEAT research and development, at present, the development of EMAT equipment has a number of domestic steel pipe, steel production and use of units should be successful.
EMAT is produced by three ways to produce Lorentz force, magnetostrictive force and magnetic force to stimulate and receive ultrasound, directly in the metal to stimulate and receive ultrasound, no acoustic coupling, so the rough surface of the workpiece and high temperature, high speed Motion of the workpiece for ultrasonic testing. It can easily select the desired ultrasonic mode, in particular, can be very simply excited with the receiving SH-wave, which is difficult to do in piezoelectric ultrasonic transducers. It is excited in the steel tube of the ultrasound, can spread around the workpiece for several weeks or even a dozen hit, which is used to detect the defect by the wave size to lay a good foundation. As mentioned earlier, if the defect is not completely perpendicular to the sound beam in the UT, the reflected wave is greatly reduced if it is more than 10 degrees. Skewed 45 degrees defects will be missed.
6. MEL, ET, UT and EMAT method reliability comparison
MEL, ET, UT and EMAT methods were compared with a large number of natural defects. The measurement of the depth of defects of each method is calibrated with various artificial standard defects of a given depth. And then scan these natural defects. When the depth of the defect is large, the dispersion of the measurement result is the smallest, but when the depth of the defect is 2mm, the dispersion is quite large. The MFL and ET method of the greater dispersion, measured in many parts of the depth of 0, can not detect. this might be. As a result of the defects in the site was rolled. EMAT does not have this missed test, in our EMAT equipment also confirmed the use of this advantage, regardless of artificial defects or natural defects, EMAT equipment has a high detection rate.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)