Detailed Explanation of Magnetic Flux Leakage (MFL) Method

Detailed Explanation of Magnetic Flux Leakage (MFL) Method
1. Magnetic flux leakage testing (MFL) refers to a non-destructive testing technique in which surface and near surface defects of ferromagnetic materials form a leakage magnetic field on the material surface after magnetization, and defects are detected by detecting the leakage magnetic field.
When magnetizing the ferromagnetic material being tested with a magnetic saturator, if the material is continuous and uniform, the magnetic induction lines in the material will be constrained within the material, and the magnetic flux will be parallel to the surface of the material, with almost no magnetic induction lines passing through the surface and no magnetic field on the tested surface. But when there are defects that cut magnetic field lines in the material, the surface defects or changes in tissue state of the material will cause changes in magnetic permeability. Due to the small magnetic permeability at the defect site and the large magnetic resistance, the magnetic flux in the magnetic circuit will be distorted, and the magnetic induction lines will change their path. In addition to a part of the magnetic flux directly passing through the defect or bypassing the defect inside the material, some of the magnetic flux will leave the surface of the material, bypass the defect through air, and re-enter the material, forming a leakage magnetic field at the defect site on the material surface. We can achieve non-destructive testing by detecting the distribution and magnitude of the leakage magnetic field through magnetic sensitive sensors.

2. The magnetization method of the magnetic flux leakage detection system plays an important role in magnetic flux leakage detection, which affects the magnetic field signal of the detected object.
From the perspective of the scope of magnetization, it can be divided into local magnetization and overall magnetization; from the perspective of the excitation magnetic source used for magnetization, it can be divided into alternating magnetic field magnetization method, DC magnetic field magnetization method and permanent magnet magnetization method. The alternating magnetic field magnetization method uses alternating current to excite the electromagnet for magnetization. The increase in the current frequency reduces the depth of magnetization. After magnetization, the ferromagnetic material will not produce residual magnetism and does not need demagnetization; the DC magnetic field magnetization method uses DC current to excite the electromagnet to generate a magnetic field for magnetization. The intensity of magnetization can be achieved by controlling the current: the permanent magnet magnetization method uses permanent magnets as the excitation magnetic source, and its effect is equivalent to fixed DC magnetization. Permanent magnets can be rare earth permanent magnets, aluminum nickel cobalt permanent magnets, etc. Rare earth permanent magnets are generally used, which have high magnetic energy and small size. Both DC magnetization and permanent magnetization will produce residual magnetism. Whether to demagnetize depends on specific requirements, and there is no specific requirement for the detection speed parameter. The selection of magnetization intensity is generally aimed at ensuring detection sensitivity and mitigating the magnetizer so that the magnetic field generated by defects or structural features can be detected.
3、 Due to the use of magnetic sensors to detect defects in magnetic flux leakage testing, it has the following advantages compared to methods such as magnetic powder and penetration testing:
1. Magnetic flux leakage detection mainly involves sensors obtaining signals and computers processing and judging them, making it easy to achieve automation.
2. Compared to visual observation of magnetic powder and penetration, it has higher detection reliability.
3. Preliminary quantification of defects can be achieved.
4. Efficient, pollution-free, and automated testing can achieve high detection efficiency.
5. In pipeline inspection, defects on both the inner and outer walls can be detected simultaneously within the Cangzhou Oupu range with a wall thickness of up to 30mm.
4、 The main application areas of magnetic leakage detection are:
1. The steel metallurgy industry deals with steel structural components, billets, round steel, steel cables, metal bars, pipes, wires, welds, and more.
2. The petroleum and petrochemical industry, such as oil and gas pipelines, oil storage tank bottoms, high-pressure vessels, etc.
3. In the field of railways, rails and wheels.
The biggest advantage of magnetic flux leakage testing is that it can detect defects such as corrosion pits inside the tank from the outside, in addition to detecting surface and near surface cracks. It does not require polishing of the tank surface and is suitable for online inspection of tanks. Online inspection of tanks is currently the most urgently needed method for users, as it can reduce unnecessary downtime and lower inspection costs. In addition, magnetic flux leakage testing can also determine certain characteristic dimensions of defects, such as depth and length. Magnetic flux leakage detection is commonly used for online inspection of tank bodies. Using magnetic flux leakage detection technology on the outer wall of the container to reflect the defect status of the inner wall of the container, especially the severity and distribution of corrosion on the inner and outer surfaces. In addition, magnetic flux leakage testing can also be used for defect detection of mobile pressure vessels that cannot be inspected internally during parking inspections such as unmanned holes. At present, there is no good inspection method for equipment that cannot undergo internal inspection except for endoscopic observation, and due to the inability to remove the cover of the inner and outer surface media, it may not be possible to effectively observe the condition of the inner and outer surfaces.
Magnetic flux leakage detection can be used for scanning detection under the condition of paint layer on the outer surface, and the size and depth of corrosion pits on the inner wall can be detected from the outside, thereby reducing the inspection cost caused by unnecessary parking and polishing, and also reducing labor intensity. Online magnetic flux leakage testing can provide an overall understanding of the surface condition and defect distribution inside and outside the tank body. The magnetic flux leakage testing results of key equipment and important parts can be archived as a reference for regular spot checks to predict the rate of thinning of the tank truck wall thickness and ensure that the equipment can be safely transported to the next inspection cycle.