What is non-destructive testing of seamless steel pipes (SMLS)? What kinds of flaw detection methods are included, and what are the requirements for pipes?
Non-destructive testing (NDT) is a modern inspection technology that detects the shape, position, size and development trend of internal or external defects without damaging or affecting the object being inspected. It has been widely used in steel pipe production in recent years. The non-destructive testing methods used in the production of seamless steel pipes mainly include ultrasonic testing, magnetic particle testing, eddy current testing, penetrant testing, radiographic testing, magnetic flux leakage testing, etc. Various testing methods have a certain range of application.
1. Ultrasonic flaw detection (UT)
Ultrasonic testing is the use of ultrasonic technology for testing, and it is one of the five conventional nondestructive testing methods. It mainly uses the strong penetrability and good directionality of ultrasonic waves to collect the reflection of ultrasonic waves in different media, and convert the interference waves into electronic digital signals on the screen to realize non-destructive flaw detection.
Advantages: no damage, no impact on the performance of the inspected object, accurate imaging of the internal structure of opaque materials, a wide range of detection applications, suitable for metals, non-metals, composite materials and other materials; more accurate defect positioning; sensitive to area defects, High sensitivity, low cost, fast speed, harmless to human body and environment.
Limitations: Ultrasonic waves must rely on media and cannot propagate in a vacuum. Ultrasonic waves are easily lost and scattered in the air. Generally, detection requires the use of couplants that connect the detection objects, and media such as (deionized water) are common.
2. Magnetic Particle Testing (MT)
Apply a magnetic field to the inside of the test material to make it magnetized, and then sprinkle magnetic powder on the surface of the workpiece to observe the distribution of magnetic powder to achieve the method of analyzing and judging material defects.
Advantages: simple and intuitive, low cost.
Limitations: The detection material is required to be ferromagnetic and the surface is smooth, and it can only detect the surface of the object, and the detection near the surface has a small detection range and slow speed, and it is impossible to accurately judge the internal defects of the material. Penetration testing Some materials are somewhat toxic and harmful to the human body.
3. Radiographic testing (RT)
It is often referred to as one of the five commonly used non-destructive testing methods of X-ray in the industry. X-ray is an electromagnetic wave with extremely high frequency, extremely short wavelength, and high energy. It can penetrate objects that visible light cannot penetrate. At the same time, the object will have complex physical and chemical interactions with the substance, which can cause the atoms to generate ionizing radiation and cause some substances to react. If there is a defect in the local area of the workpiece, it will change the attenuation of the object to the ray and cause a change in the intensity of the transmitted ray. , In this way, using a certain detection method, such as using a film to detect the intensity of the transmitted ray, can determine whether there is a defect in the workpiece and the location and size of the defect.
Advantages: Accurate imaging, intuitive overhead perspective view, fast detection and imaging, non-destructive inspection imaging can be performed inside the workpiece, rays can penetrate thinner workpieces for detection, and local differences in images can be observed through the attenuation of the penetrated rays.
Limitations: subject to the density difference of the material itself, the analysis and imaging of materials with insignificant density differences are inaccurate, such as some composite materials such as diamond composite sheets, etc., and are also limited by the overlapping and hiding of internal images, sometimes requiring multiple times Multi-angle shooting and professional analysis. The detection cost is high, and there is ionizing radiation hazard to the human body.
4. Eddy current testing (ET)
Using the principle of electromagnetic induction to detect defects on the surface and near the surface of conductive materials. The principle is to use the excitation coil to generate eddy current inside the conductive material structure, and use the detection coil to measure the change of eddy current to obtain material defect information.
Advantages: No medium is required, the detection coil does not need to contact the detection material itself, the detection of the surface of the metal workpiece has fast response, high sensitivity, low detection environment requirements,
Limitations: The material itself must be conductive, and it is only suitable for detecting metal surface defects. It requires professional analysis and judgment, customized testing schemes, and high testing costs.
5. Penetrant testing (PT)
The principle of penetrant testing is based on the capillary phenomenon of liquid and the luminescence phenomenon of solid dye under certain conditions, and then analyzes and judges the surface defects of the tested workpiece. Under capillary action, after a certain period of time, the penetrant can penetrate into the surface opening defect; remove the excess penetrant on the surface of the workpiece, and after drying, apply an adsorption medium-imaging agent on the surface of the workpiece to measure the morphology of the defect and distribution status.
Advantages: convenient and cheap, low requirements on the detection environment, no requirements on the physical and chemical properties of materials, and sensitive to surface defects of materials.
Disadvantages: The imaging is not intuitive, and the internal defects of the material cannot be judged.
6. Magnetic flux leakage (MFL)
Magnetic flux leakage detection uses magnetic sensors to detect defects. Compared with penetration, magnetic powder and other methods, it has the following advantages:
1) Easy to automate. The signal is received by the sensor, and the software judges whether there is a defect, which is suitable for forming an automatic detection system.
2) It has high reliability. From the sensor to the computer processing, the error caused by the influence of human factors is reduced, and it has high detection reliability.
3) Preliminary quantification of defects can be achieved. This quantification can not only realize the judgment of the existence of defects, but also make a preliminary assessment of the degree of harm of defects.
4) For pipes with a wall thickness of less than 30mm, it can detect inner and outer wall defects at the same time.
5) Because it is easy to automate, it can obtain high detection efficiency and no pollution.
Magnetic flux leakage detection technology is not a panacea and has its limitations:
1) Only applicable to ferromagnetic materials. Because the first step of magnetic flux leakage detection is magnetization, the magnetic permeability of non-ferromagnetic materials is close to 1, the magnetic field around the defect will not change due to the different magnetic permeability, and no leakage magnetic field will be generated.
2) Strictly speaking, MFL testing cannot detect defects inside ferromagnetic materials. If the distance between the defect and the surface is large, the magnetic field distortion around the defect will mainly appear around the defect, and the leakage magnetic field may not appear on the surface of the workpiece.
3) Magnetic flux leakage testing is not suitable for testing specimens with coatings or covering layers on the surface.
4) Magnetic flux leakage testing is not suitable for specimens with complex shapes. Magnetic flux leakage detection uses sensors to collect magnetic flux leakage signals, and a slightly complicated shape of the specimen is not conducive to detection.
5) Magnetic leakage testing is not suitable for detecting cracks with very narrow cracks, especially closed cracks.
