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Magnetic Flux Leakage Detection Systems for Seamless Steel Pipes

Posted: 12/09/2021 09:29:56 Hits: 25

Seamless steel pipes have been widely used in the delivery, construction, machinery and other fields, and play a pivotal role in the industry. They are called the vessel of modern industry. The defects of seamless steel pipes are mainly formed by the slow extension of metallurgical defects. In the subsequent processing of the steel pipe, if the operation is improper, or the design of the roll or drawing is not appropriate, defects will also be formed on the steel pipe. These defects include surface or internal defects such as cracks, scratches, peeling, and defects caused by strains. Any potential defects on seamless steel pipes may cause accidents and cause losses to social property and personal safety. Therefore, the seamless steel pipe must be tested after it is produced to ensure that it can be delivered only after it is qualified. The seamless steel pipe factory will choose different non-destructive testing methods to test the seamless steel pipe according to the different quality requirements of the factory. At present, non-destructive testing methods mainly include ultrasonic testing, radiographic testing, magnetic flux leakage testing and eddy current testing. In the inspection of seamless steel pipes, ultrasonic testing technology is widely used, but ultrasonic testing has relatively high requirements on the shape and surface cleanliness of seamless steel pipes, and the testing speed is not high. It is difficult to form a high-speed online testing system. The radiographic inspection method has no restrictions on the structure and surface of the tested piece, and is suitable for detecting some abrupt defects. However, when the crack kind of defect is detected, there are requirements for directionality, and the problem of radiation protection needs to be considered; the inspection cost is relatively high. Eddy current testing will produce the skin effect on the surface of ferromagnetic specimens, so this method can only detect defects on or near the surface of seamless steel pipes. Table 1 is a comparison of various aspects of commonly used non-destructive testing methods.

Table 1 The comparison of several conventional non-destructive testing methods

Testing methods	Defects	Fastest testing speeds	Testing accuracy	Testing conditions	Reliability
Ultrasonic testing	Longitudinal defects	20m/min	1.2	Oil and water coupling	Easily missed inspection
Eddy current testing	Outer surface defects	50m/min	1.2	Magnetic coupling	Easily missed inspection
Radiographic testing	Various defects on the inner and outer walls	The slowest	1.2	Cleaning and good protection	Easily missed inspection
Magnetic flux leakage testing	Various defects on the inner and outer walls	120m/min	1.2	Magnetic coupling	The most reliable

It can be seen from Table 1 that the magnetic flux leakage nondestructive testing technology has obvious advantages. However, the current magnetic flux leakage detection equipment for seamless steel pipes is imported. The equipment is expensive, and it is difficult to maintain. As time goes on, the previous magnetic flux leakage detection equipment has been unable to meet the higher accuracy requirements at this stage. Therefore, independent research and development of domestic magnetic flux leakage detection systems for seamless steel tubes is very urgent.

Development and application of magnetic flux leakage testing technologies
The basic principle of magnetic flux leakage detection is shown in Figure 1. Take seamless steel pipes as an example. After magnetization, if the seamless steel pipe has no defects, most of the magnetic field lines induced by the seamless steel pipe pass through the pipe wall in parallel; if the seamless steel pipe is defective, because the magnetoconductivity of the ferromagnetic materials and the air at the defect is quite different, the magnetic line vector will bend, causing part of the magnetic line vector to leak from the tube wall. At this time, a magnetic sensor is used to pick up the leakage magnetic field on the surface of the tested piece and convert the leakage magnetic field signal into a corresponding electrical signal. The specific information of the defect can be determined through the processing and analysis of the electrical signal.

Figure 1 The schematic diagram of magnetic flux leakage detection

Magnetic flux leakage detection technology is divided into magnetic particle flux leakage detection and sensor magnetic flux leakage detection. Nowadays, the technology that uses a magnetic sensor to measure magnetic flux leakage is called magnetic flux leakage detection technology.

In 1933, Zuschlug from Germany first got the idea of using sensors to pick up magnetic field leakages. This idea has a history of more than 80 years till now. In the 1940s, Hasting S successfully developed the world's first magnetic flux leakage detection device. After that, developed countries began to gradually realize their unique advantages. Since then, various magnetic flux leakage testing instruments have been developed one after another, and various countries have successively formulated relevant standards to regulate this technology. Japan adopted the JIS standard, Germany the DIN standard, and the United States ASTM and API standards. Among these companies, the American TUBOSCOPE company is well-known. Its large-scale magnetic flux leakage detection equipment has the advantages of high accuracy and fast detection speeds. The detection device is composed of a longitudinal flaw detection system and a horizontal flaw detection system, which can detect various defects for steel pipes. In 1965, Tube Cope Vetco International Co., Ltd. successfully developed the Well check magnetic flux leakage detection systems, which could accurately detect various defects on the inner and outer surfaces of the pipeline. For the first time, the magnetic flux leakage detection technology was used for inspecting the inside of the pipe. In the magnetic flux leakage detection technology, the quantitative detection of defects has always been a pain in the ass. In 1973, the British Natural Gas company conducted on-line magnetic flux leakage detection for its natural gas pipelines and realized the quantitative detection of pipeline defects. After entering the 21st century, the magnetic flux leakage detection technology has been relatively mature in developed countries. For the first time, the American MAC company integrated three non-destructive testing technologies including eddy current, ultrasonic, and magnetic flux leakage technologies into a testing system. This non-destructive testing system could detect various cylindrical shapes such as seamless steel pipes, welded pipes, etc., and detect various defects outside and inside the pipe with high detection accuracy and fast speeds.

In China, the research on magnetic flux leakage detection technology was carried out on a large scale around 1980. At that time, the magnetic flux leakage detection equipment was imported. In May 1991, China formulated the first standard GB/T12606-199 for magnetic flux leakage testing. After that, some achievements have been made in the research of magnetic flux leakage detection technology in China, such as relatively rich research results in the numerical calculation of leakage magnetic fields, magnetization technologies and magnetic flux leakage signal processing.

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