전자기적 방법을 이용한 배관 용접부의 대체 표면 비파괴검사 기술 개발

Abstract

Pipes for transporting fluid are widely used in industry, and their lengths extend from several kilometers to hundreds of kilometers. Welding is widely used as a means of applying a relatively short length of factory produced pipe to such a long-distance pipe installation. Welding has the advantage of having relatively few restrictions on the thickness and shape of the target material, and of reducing the weight of the equipment. On the other hand, changes of physical properties, deformation, and residual stress due to local heating and cooling accompanying welding may exist and may cause defects. In particular, the possibility of defects is greatly increased during long-term operation in high temperature, high pressure and corrosive environments such as petrochemical plants, heat transport pipes, nuclear power plants, ect. For pressure piping, carbon steel piping is used in most major sections, with the exception of sections requiring particularly high corrosion resistance. It has good weldability, low cost and excellent heat resistance compared to stainless steel and other alloy steels. The non-destructive testing method applied to assess the strength of the carbon steel pipe welds is divided into a surface volumetric testing. Ultrasonic testing (UT) and radiographic testing (RT) are applied as volumetric testing methods. For surface inspection of pipe welds, which is the subject of this paper, liquid penetrant examination (LPE) and magnetic particle examination (MPE) are applied. However, LPE and MPE have a complex inspection process of pre-treatment (cleaning), application of penetrant and magnetic particle, residual penetrant removal, developer application, inspection and post-treatment (cleaning and demagnetization). As a result, the inspection time increases, and the use of penetrants, developers, and magnetic particles, which are chemical substances harmful to the human body, is essential. However, when magnetic powder and penetrant in the form of powder and liquid are applied to the lower part of the pipe, which is a position against gravity, they are not applied evenly to the surface of the welded part and fall off by gravity. In addition, when inspecting a welded part, a defect signal and a pseudo-defect signal are mixed and appear due to material changes, residual stress, etc. occurring during the welding process. The distinction between the defect signal and the pseudo-defect signal is made by subjective judgment based on the inspector’s experience, which leads to a problem of reliability of the inspection result. Therefore, it does not use chemicals harmful to the human body, is not affected by the inspection location, can scan the entire welding line in a short time and can improve the reliability of inspection results Alternative Surface Examination (ASE) method needs to be developed.

Therefore, this thesis proposes an alternative surface inspection method applied to pipe welding that can be converted in order to:

1. No use of chemicals. 2. Shortens the inspection speed by a simple inspection process. 3. Has a sensitivity equal to or higher than that of the existing surface inspection method. 4. Not affected by inspection position. 5. Use of artificial intelligence techniques for measurement results to improve the reliability of defect detection capability. 6. Propose an alternative surface inspection method for pipe welds that can create a database of inspection results.

In order to achieve the proposed goal, a non-destructive testing system using an electromagnetic method was developed. When magnetic sensors are arranged and used, the residual magnetic flux density, leakage magnetic flux density, and time-varying magnetic field distribution in a certain area can be measured and imaged. It has the advantage that it can be measured and imaged. Therefore, using a magnetic sensor and an excitation coil, the method of applying an electromagnetic field and the defect detection capability depending on the type of magnetic sensor were investigated. As a result, we developed the optimal arrangement of the magnetic sensor and the signal processing circuit that can measure the selected electromagnetic field application method and the distribution of the generated magnetic field around the defects in the welded part, and that can move and measure automatically along the circumferential welded part of the pipe. We have developed an ASE system for pipe welds consisting of static balance scanners. In addition, the possibility was confirmed by analyzing the defect signal using various signal processing techniques and artificial intelligence techniques to improve the defect detection capability and reliability. Moreover, the usefulness of the inspection system was verified using a test piece simulating the welding of a carbon steel pipe, which is widely used for piping in various industries such as petrochemical plants, heat transport pipes and nuclear and thermal power generation facilities.