Introduction to Magnetic Particle Testing for Pressure Vessels

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Basic Overview (1) Working Principle: Defect detection is primarily achieved through the interaction between the leakage magnetic field and magnetic particles. When an iron material is magnetized, the continuity on its surface becomes uneven due to magnetization, and this uneven continuity in turn causes changes in the magnetic field lines. The altered magnetic field lines generate a leakage magnetic field in certain areas of the material; this leakage magnetic field has a certain degree of attracting force. To ensure the integrity and correctness of the magnetic field lines, the magnetic powder on the surface of the absorbing components can be detected using light, allowing clear visualization of the magnetic traces formed by the absorbed magnetic powder. In this way, it is possible to identify exactly which part of the material’s surface has a discontinuity, as well as to determine the extent of that discontinuity. This is a very effective detection method. (2) Scope of application The principle of magnetic particle inspection technology determines that its scope of application is limited, as it can only be used on magnetic materials. When defects appear on the surface of magnetic materials, magnetic particle inspection technology can accurately and effectively detect the locations of these defects, such as in iron-based materials, martensitic stainless steels, and precipitation-hardening stainless steels. These materials are all magnetic, so magnetic particle testing can be used for defect detection. Methods (1) Yoke method: In non-destructive testing, the yoke method is very common. This method enables rapid and accurate detection of defects in fillet welds. At the same time, this method is simple and efficient, and can be effectively applied to quality inspection. Identify each of the device’s various inspection directions one by one, then subject the inspection areas of the device to vertical inspections at least twice, and enable the welds to be divided into multiple sections. Of course, such inspection tasks are carried out at a low efficiency, and any improper operation can result in uninspected areas, thereby affecting the quality of the inspection. (2) Cross-yoke method: The cross-yoke method is also a very effective defect detection technique, especially for the inspection of boiler pressure vessels. The key advantage of the cross-yoke method is its high sensitivity and ease of use. Most importantly, it enables the creation of a rotating magnetic field; by utilizing this rotating magnetic field for defect detection, it is possible to inspect multiple areas simultaneously, allowing for the detection of defects in different locations at the same time, which makes the process convenient and efficient. (3) Contact method: The contact method is the same as the cross-yoke method; both are only suitable for vertical testing, and two cross-vertical operations are required to ensure the accuracy of the test results. The difference is that the contact method is a unidirectional magnetization method; it allows the distance between the electrodes to be adjusted based on its sensitivity and the location of the defect, thereby ensuring flexibility in detection. The contact method and the cross-yoke method are widely used for the ring welding and longitudinal welding inspection of boiler pressure vessels, as they enable rapid detection, offer high sensitivity to ensure accuracy, are simple to operate, and are therefore more practical than other methods. (4) Coil method: As a detection method for longitudinal magnetization, the coil method can effectively detect defects in the pipe corner welds and the circumference of the pipeline. The cable is wrapped around the test object, and defect detection is carried out by utilizing the heat-affected zone as well as longitudinal cracks in the welds; it is also possible to detect other potential safety hazards in the equipment. Operation method: Magnetic particle inspection technology, as the name suggests, is a technique that uses magnetic particles as a visible medium to detect defects in the components being inspected. By taking the types of magnetic particles used in the practical application of magnetic particle inspection as the object of study and the basis for classification, it is possible to divide the inspection methods of this technique into dry inspection and wet inspection ； Using the actual operation time of magnetic particle inspection technology as a criterion, it can also be divided into the continuous inspection method and the residual magnetism inspection method; the use of the continuous method requires the application of an external magnetic field. The testing methods for dry testing and wet testing are roughly the same. First, the magnetic particles are suspended in a certain medium; for example, they can be suspended in water, oil, or other liquid media ； Next, the prepared suspension containing magnetic particles is evenly applied to the surface of the component to be inspected; under the interaction between the leakage magnetic field and the magnetic particles, and thanks to the fluidity of the liquid, these particles gather at the areas with defects on the surface of the component. Precautions: When using magnetic particle inspection technology for defect detection, certain precautions must also be taken. Looking at the practical application of this technology, the following points deserve attention: First, when the lighting in the inspection environment is insufficient and it is quite dark, magnetic particle inspection technicians must work closely with those who are inside the structure in question to ensure the safety of the inspection process and prevent any potential accidents that could harm people ； Secondly, in order to address the issue of aging detection equipment, it is necessary to ensure that old equipment is replaced in a timely manner. As for new detection equipment, regular maintenance and repair should be carried out to guarantee its proper operation and the accuracy of the test results; this is also the foundation and guarantee for the smooth progress of industrial production.