Application of non-destructive testing for seamless steel pipes

Surface defects mainly include scratches, cracks, folds, warping, transverse cracks, non-metallic inclusions, etc; Internal defects mainly include cracks, inclusions, delamination, etc; In addition, there are size defects such as bamboo knots, eccentricity, and uneven wall thickness. The characteristic of scratching is that it forms longitudinal straight scratches on the surface of the steel pipe, mostly in the form of grooves; Cracks are a type of defect in which the continuity of metal has been disrupted, characterized by metal cracking along the longitudinal or spiral direction of the pipe, with varying lengths and depths, often appearing on the outer surface; Folding is also a defect in metal continuity that has been disrupted, with two shapes: linear and block like. The former appears similar to cracks on the surface, but its depth extension direction is different from that of cracks. Cracks are mostly radial, while folding has a certain tendency. Folding is generally a metal folding that appears on the inner and outer surfaces of steel pipes, with varying lengths and depths, often distributed in a longitudinal or spiral direction; Scrap skin is a thin sheet that appears locally on the inner and outer surfaces of a steel pipe and separates from the metal substrate. It is often distributed in blocks and may or may not take root on the pipe wall, but it should not peel off; Horizontal cracking refers to the transverse rupture of the inner and outer surfaces of a steel pipe, with varying depths but generally short lengths, often in the shape of a human; Inclusions are non-metallic inclusions embedded on the surface of steel pipes or within the metal, often distributed in sheet or block form; Layered cracking can be observed on the cross-section of steel pipes, and in severe cases, it can sometimes occur. Its characteristic is that it often appears at a distance of 1/3 of the wall thickness from the inner surface of the pipe, with a circumferential length generally ranging from 1/3 to 1/2 of the circumference and varying axial lengths, with severe cases reaching several meters.
Therefore, various methods for non-destructive testing of seamless steel pipes have emerged. The main methods for non-destructive testing of seamless steel pipes include ultrasonic testing, eddy current testing, magnetic flux leakage testing, and magnetic particle testing. Among them, the latter three methods can be combined into electromagnetic testing methods.
1、 Characteristics and Application of Ultrasonic Testing Method for Seamless Steel Pipe
Seamless steel pipes are generally inspected using transverse wave reflection method due to their shape and geometric dimensions. According to the coupling method between the probe and the surface of the pipe, it can be divided into two basic methods: contact method and water immersion method. The following are brief introductions:
（1） Contact method flaw detection
The contact method refers to the method of directly contacting the probe with the surface of the pipe for non-destructive testing. It is generally operated manually and has a slow testing speed. However, this method is simple in equipment, easy to operate, and convenient for debugging when changing specifications. Therefore, it is still often used in small batches with multiple specifications.
（2） Water immersion method for non-destructive testing
The water immersion method refers to the method of using water as a medium for non-destructive testing between the probe and the surface of the pipe without contact. The water immersion method has the following advantages: it can use a focused sound beam, which can significantly improve the sensitivity, resolution, and ability to detect small-diameter pipes, making it easy to achieve mechanization and automation; Due to the solution of probe wear and the use of inexpensive coupling media, production costs can be reduced. Therefore, the water immersion method has been widely used in the inspection of large quantities of pipes.
2、 Characteristics and application of eddy current testing method for seamless steel pipes
Eddy current testing has been used for quality inspection of steel pipes for decades. Through continuous improvement and development, eddy current testing has become one of the recognized inspection methods that can replace hydrostatic testing at home and abroad. It is widely used and applied to conductive materials. Almost all steel pipes, whether ferromagnetic or non-magnetic, with a diameter greater than 2mm and a wall thickness of not less than 0.1mm, can be inspected using eddy current testing. Currently, it is particularly used in online or offline automatic testing of pipes and bars, with the characteristic of high detection speed. According to the requirements of the variety and other aspects of steel pipes, there are generally several forms of eddy current testing for steel pipes:
（1） Penetrating eddy current testing
Penetrating eddy current testing is a widely used method in eddy current testing of steel pipes. Generally, steel pipes with a diameter less than 180mm mostly use this testing method, which is characterized by fast detection speed, the ability to inspect the entire surface of the steel pipe, and relatively simple equipment adjustment and operation.
The commonly used through type eddy current flaw detection equipment consists of a detection part, including an eddy current flaw detector, a detection coil, and a magnetic saturation device; The auxiliary part, including the mechanical part, consists of the upper and lower material racks, conveyor rollers, detection host, control system, demagnetization and marking devices, etc.
（2） The rotating point probe is used for eddy current testing
Rotating point probe eddy current testing is an automated testing method in which the point probe mesh rotates around the steel pipe while the steel pipe advances in a straight line. The point probe is placed in the rotating head, and the rotation of the probe completes the scanning of the surface of the steel pipe.
The detection sensitivity of point probes is high, so they are generally used when high quality requirements are placed on steel pipes. Rotating point probes are often combined with through type probes because they are more sensitive to cracks distributed along the axis of the steel pipe, while through type coils are more sensitive to defects or cracks distributed around the steel pipe. Both can complement each other.
The rotating point probe method is more suitable for the detection of medium and small diameter steel pipes, and the sensitivity of the point probe is high, so it is suitable for the detection of steel pipes with smooth surfaces. For steel pipes with rough surfaces and oxide scales such as hot-rolled steel pipes, the rotating point probe eddy current testing is not suitable.
The composition of the rotating point probe in eddy current testing equipment is basically the same as that of the through type eddy current testing equipment, with the only difference being the use of a preferred head instead of a magnetic saturation device installed on the testing mechanism.
3、 Characteristics and Applications of Magnetic Particle/Leakage Magnetic Testing Methods for Seamless Steel Pipes
Magnetic particle testing and magnetic leakage testing are collectively referred to as magnetic leakage testing. Physically speaking, they belong to the same testing method used for detecting defects in ferromagnetic materials through leakage magnetic field detection.
Magnetic particle inspection room is a widely used method of magnetic flux leakage testing, and the use of detection components to determine the presence of magnetic flux leakage is another type of magnetic flux leakage testing method. In recent years, it has been increasingly used in the inspection of pipes and bars in the metallurgical industry, and is commonly referred to as magnetic flux leakage testing.
（1） Magnetic particle inspection
Seamless steel pipe magnetic particle inspection is generally used for automatic inspection equipment to detect the undetectable area of the pipe end, that is, pipe end inspection. Fluorescent magnetic particle inspection method is generally used to magnetize the longitudinal and axial directions within about 300mm of the pipe end. Magnetic coils are generally used to magnetize the longitudinal direction of the steel pipe, and the core rod is energized to magnetize the axial direction of the steel pipe, forming a composite magnetic field to inspect defects in various directions on the inner and outer surfaces and near surfaces. The magnetization and demagnetization of the equipment are controlled automatically, and the magnetic trace display on the inner and outer surfaces is observed manually under the irradiation of ultraviolet light source. This method adopts the fluorescent magnetic particle inspection method, which has high detection sensitivity, simple operation, convenience, intuitive detection results, no blind spots within the detection range, high degree of automation, fast inspection speed, and can generally achieve simultaneous inspection of multiple steel pipes, improving inspection capacity. It can match the production capacity of the previous automatic inspection equipment. However, due to the need to observe magnetic traces with human eyes, the quality and reliability of inspection depend entirely on human factors.
（2） Magnetic flux leakage testing
Seamless steel pipe magnetic flux leakage testing detects defects by using sensors to detect the magnetic field leakage. The sensor and the detected steel pipe move relative to each other, cutting magnetic field lines, and processing them with electronic instruments to obtain defect signals.
Magnetic flux leakage testing equipment generally consists of a magnetizing power supply, magnetizing coil (magnetic yoke), detection probe, probe rotating device (or steel pipe rotating device), demagnetization (transverse magnetic flux leakage requires demagnetization), marking and other devices.
Magnetic flux leakage testing is generally divided into two types: longitudinal magnetic flux leakage testing equipment and transverse magnetic flux leakage testing equipment. Longitudinal magnetic flux leakage testing equipment applies circumferential magnetization to steel pipes to detect longitudinal defects on the inner and outer surfaces and near surfaces of steel pipes; The transverse magnetic flux leakage testing equipment applies transverse magnetization to the steel pipe to detect transverse defects on the inner and outer surfaces and near surfaces of the steel pipe.
Magnetic flux leakage testing has the following advantages:
Easy to achieve automation
Has high detection reliability
Can achieve preliminary assessment of defects
Characterized by high efficiency and no pollution
At the same time, magnetic flux leakage testing also has the effect of improving electromagnetic testing, which affects the sensitivity of testing. When using contact method for testing, probe wear can affect the sensitivity of detection.