Introduction
Magnetic flux leakage testing (MFL) is a non-destructive testing technique that detects defects by detecting magnetic leakage fields. When magnetizing the measured ferromagnetic material with a magnetic saturator, if the material is continuous and uniform, the magnetic induction lines in the material will be constrained within the material.
Details
- 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 dry 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.
- The magnetization method of the leakage magnetic detection system plays an important role in leakage magnetic detection, as it affects the magnetic field signal of the detected object. From the perspective of magnetization range, it can be divided into local magnetization and global magnetization; From the perspective of the excitation magnetic source used for magnetization, it can be divided into alternating magnetic field magnetization method, direct current magnetic field magnetization method, and permanent magnet magnetization method. The alternating magnetic field magnetization method uses alternating current to excite the electromagnet for magnetization. As the current frequency increases, the depth of magnetization decreases, and the ferromagnetic material does not produce residual magnetism after magnetization, so demagnetization is not required. The direct current magnetic field magnetization method uses direct current to excite the electromagnet to generate a magnetic field for magnetization, and the strength of magnetization can be achieved by controlling the current; The permanent magnet magnetization method uses a permanent magnet as the excitation magnetic source, which is equivalent to fixed DC magnetization. Permanent magnets can use rare earth permanent magnets, aluminum nickel cobalt permanent magnets, etc. Generally, rare earth permanent magnets are used, which have high magnetic energy and small volume. Both direct current magnetization and permanent magnetization will produce remanence, and demagnetization depends on specific requirements. There are no specific requirements for the detection speed parameters. The selection of magnetization intensity is generally aimed at ensuring detection sensitivity and reducing the magnetic field generated by the magnetizer to detect defects or structural features.
- 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, steel damage, round steel, steel cables, metal bars, pipes, wires, welds, and so on.
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.
- In addition to being able to detect surface and near-surface crack defects, the biggest advantage of magnetic flux leakage testing is that it can detect defects such as corrosion pits inside the tank from the outside, and there is no need to polish the surface of the tank. It is suitable for online inspection of tanks, which is the most urgent method for users at present. It can reduce unnecessary downtime and reduce inspection costs. In addition, magnetic flux leakage testing can also determine certain characteristic dimensions of defects, such as depth and length. The application of magnetic flux leakage testing to dry tanks is mostly online inspection. The magnetic flux leakage detection technology of the outer wall of the container reflects 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 be inspected internally except for endoscope observation, and the conditions of the inner and outer surfaces may not be effectively observed due to reasons such as the inability to remove the covering of the inner and outer surfaces. Ultrasonic flaw detector magnetic flux leakage detection can be used for scanning detection of the paint layer on the surface, and the size and depth of the corrosion pits on the inner wall can be detected from the outside, thereby reducing the inspection cost increased by unnecessary parking and grinding, and also reducing labor intensity. Online magnetic flux leakage detection can provide an overall understanding of the internal and external surface conditions and defect distribution of the tank body. The magnetic flux leakage detection 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 car wall thickness and ensure that the equipment can operate safely until the next inspection cycle.
