Flaw detection and reliability analysis of thick-walled seamless steel tubes

Thick-walled seamless steel pipeStructural tube GB />The reliability of several non-destructive testing methods for magnetic flux leakage, eddy current, ultrasonic and electromagnetic ultrasonic used in flaw detection is discussed. It can be seen from the comparison of the reliability of the above methods that electromagnetic ultrasonic method has advantages that are incomparable to other non-destructive testing methods. And broad application prospects. 
1 16Mn seamless steel pipe Foreword 
Steel pipe is the most widely used steel product. Its quality directly affects economic benefits and the safety of personnel. All countries in the world have paid great attention to the quality inspection of steel pipes, and various non-destructive testing (NDT) methods have been used to strictly test steel pipes. For example, Mannesmann of Germany and Sumitomo Metals of Japan use the method of super-field (UT) and magnetic flux leakage (MFL) when testing large-diameter steel pipes. When detecting small-diameter steel pipes, super-field and eddy current (ET) methods have been used. A more mature detection program. Ultrasonic and eddy current methods have been widely used in the detection of steel pipes in China, and magnetic flux leakage methods have been increasingly used. However, the natural defects generated in the production of steel pipes cannot be detected by the NDT method. Therefore, how to improve the reliability of NDT detection becomes an increasingly urgent issue.


This article combinesSome natural defects have been detected in the inspection of oil casings , and the in-depth analysis of the principle and the performance of the testing equipment provides a reference for the development of the best testing program. 
2 MFL (Magnetic Flux Leakage) method reliability
MFL method is not high in the surface condition of the pipe, and the detection depth is also large. It is widely used in the inspection of foreign steel pipes, and it is increasingly used in China, especially MFL flaw detection equipment manufactured by Tubescope Corporation of the United States has been widely used in the inspection of petroleum steel pipes.


In the MFL company use, because of the management and technical reasons, there had been a number of undetected problems. One of them is a defect that is at a 45 degree angle to the tube axis and is often missed. If the longitudinal probe in the MFL device is used simultaneously with the lateral probe, the missed detection rate may be reduced, otherwise it is difficult to ensure reliable detection of such a tilt injury.


Another important factor affecting 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 defects parallel to the surface with the MFL, such as layered defects.


In addition, if the steel pipe is deformed greatly during the process of forming, the natural defects are sometimes rolled. At this time, the leakage magnetic flux generated by the defect is small, which makes it difficult to find out by the MFL method. 
　　In the production test, there has been a phenomenon in which the large hole in the steel pipe is not detected by the MFL method. Aside from the management and personnel factors, technically, it should also explore the board and prevent it. There has been no clear conclusion about the extent to which the MFL method can detect the depth of the surface. This is related to instrument and probe performance and defect size and shape. 
3. Reliability of the ET (Eddy Current Testing) method 
　　 Due to the high detection speed, the through-coil ET method has been widely used for many years to detect the quality of steel pipes, especially their compactness. It has been proved in use that it is difficult to detect crack-like defects in ferromagnetic steel pipes, so in the high standard ET, the probe coil ET method is employed. In addition, as for the simple question of how deep the ET method can reveal the deep surface, it seems that there has not been a clear consensus. 
　　 For the "outer fold" defects often found in steel pipes, many eddy current instruments and probes are often not found. Frequently, the external fold is visible to the naked eye, but it cannot be used to report the embarrassing phenomenon of the ET meter. This situation has changed radically since the emergence of sectorized phase alarm eddy current devices. However, choosing the right instrument and probe and correcting it correctly is still an important issue that cannot be ignored. 
4 UT (Ultrasonic Testing) reliability 
UT method is the most widely used in steel pipe flaw detection and thickness measurement. However, as a detection method, its reliability is affected by various factors. If there is not enough analysis and research, there will even be serious missed detection and false detection. Only a few of the factors that may exist in the UT to reduce the reliability of the UT are discussed below. 
(1) Influence of natural defect orientation on UT reliability 
In the process of steel tube rolling, axial (longitudinal) defects occur frequently. However, defects that extend at an angle to the axis of the steel pipe are not uncommon. A circumferential (lateral) defect perpendicular to the tube axis also occurs from time to time. The mission of NDT is to find out the flaws with different orientations.
Similarly, in order to reliably detect folding defects that are inclined to the surface of the steel pipe, it is necessary to provide two sets of probes that are incident in the opposite direction of the circumferential direction of the pipe. 
(2) Influence of acoustic coupling on UT reliability In 
water intrusion UT, the attenuation of sound waves in the pipe wall is very serious. Since the acoustic characteristic impedance of water is much smaller than that of steel, the reciprocal transmission of sound waves from water to water and from steel to water is small. Second, each reflection of sound waves in the wall of the tube is accompanied by a wave pattern transition. When the transverse wave is transmitted to the water, it is completely absorbed by the water, so the ultrasonic attenuation of the water intrusion UT is significantly increased. This causes the ultrasonic wave to travel a small distance along the pipe wall, even 1/4 of the length. Regardless of the mode conversion during sound propagation, that is, when the transverse wave refraction angle is 45 degrees, the oblique contact probe made of plexiglass wedges, the calculated value of the sound pressure reciprocal transmittance of the steel pipe T(LS) About equal to 25%. When the outside of the steel pipe is immersed in water, the inner side is still air, and T (LS) is about 15%. . The latter is 4 dB lower than the former. If the inner hole of the steel 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 with the contact method or the water film method. If the defect echo signal is not strong due to adverse factors such as defect shape or orientation, the defect is likely to be missed. A better solution to this problem is to use water film instead of water immersion for coupling. 
5 Electromagnetic ultrasonic EMAT (Electronmagnetic Acoustic Transducer) flaw detection technology 
to solve the various difficulties brought by acoustic coupling to the UT. In the late 1960s, there was an electromagnetic ultrasonic transducer that did not require acoustic coupling and directly adhered to the excitation and reception of sound waves in the metal. (EMAT), after 30 years of research and development, has entered the stage of engineering and commercialization. In the United States, Germany, Russia, Japan and other countries, there are already commercial instruments and equipment for sale. The author also started the research and development of MEAT from the early 1970s. At present, the EMAT equipment developed by the company has been successfully applied in the production and use of many steel pipes and steel plates in China.EMAT generates Lorentz force, magnetostrictive force and magnetic force to excite and receive ultrasonic waves in three ways. It directly excites and receives ultrasonic waves in the metal. It does not require acoustic coupling, so it can be used in rough surface workpieces and high temperature and high speed. Ultrasonic testing is performed in moving workpieces. It makes it easy to select the desired ultrasonic mode, especially for exciting and receiving SH waves, which is difficult to achieve in piezoelectric ultrasonic transducers. Ultrasonic excitation in the steel tube can propagate around the workpiece for several weeks or even dozens of shots, which lays a good foundation for detecting the defect size with the transmitted wave. As mentioned above, if the defect is not completely perpendicular to the sound beam in the UT, if it is inclined more than 10 degrees, the reflected wave is greatly reduced. Defects that are skewed at 45 degrees will be missed. 
6. Reliability comparison 
of MEL, ET, UT and EMAT methods The MEL, ET, UT and EMAT methods were compared using a large number of natural defects. The measurement of the defect depth of each method is calibrated using artificial standard defects of various established depths. These natural defects are then scanned for detection. When the folding type defect depth is large, the dispersion of the measurement result is the smallest, but when the defect depth is 2 mm, the dispersion is quite large. The MFL and ET methods are more discrete, and the depth of measurement is 0 in many parts, which cannot be detected. this might be. The defects are rolled up at this location. EMAT does not have this type of missed detection. This advantage has been confirmed in the use of our EMAT equipment. EMAT equipment has a high detection rate regardless of manual or natural defects. 
7 Conclusions 
(1) The EMAT method can detect artificial defects specified by various standards, and can detect a variety of natural defects, and its reliability is unmatched by other NDT methods. 
(2) In the EMAT test, probes with probing and lateral defects and high- and low-pass filters should be used to prevent defects from being missed. If possible, probes that detect horizontal component leakage fields should be developed to ensure reliability. 
(3) In the ET method, the phase alarm method should be adopted to prevent the missing defects from being folded. When the reliability of the ET method is high, ET instruments and equipment with point probe coils should be used to reliably detect crack defects.
(4) In the UT method of steel pipe flaw detection, it is not advisable to use a full water immersion probe, but a water film probe should be used to prevent dangerous defects from being missed.