Is the finer the particle size the better?
Magnetic particle testing, as one of the five conventional nondestructive testing methods, develops rapidly and is widely used.
Magnetic particle testing is to show defects by forming magnetic marks on the leakage magnetic field. The degree of magnetic mark display is not only related to the defect property, magnetization method, magnetization specification, application method of magnetic particles and other conditions, but also related to the performance of magnetic particles themselves. Therefore, it is very important to know and select the magnetic particles with good performance. Today, we will learn about the particle size selection of the following magnetic particles.
The particle size of magnetic particle is also the size of magnetic particle. The particle size has a great influence on the suspension and magnetic leakage of magnetic particles, so the finer the particle size is, the better?
When we choose the appropriate particle size, we should consider the nature, size, burial depth of the defect and the application mode of the magnetic powder.
Magnetic powder for wet process includes fluorescent magnetic powder and non fluorescent magnetic powder. When detecting the micro defects on the surface of the workpiece, the wet magnetic powder with small particle size should be selected. Because the fine magnetic powder has good suspension, it is easy to be magnetized and adsorbed by the weak magnetic leakage field generated by the small defects, and the lines of the magnetic traces will be clear and the positioning will be accurate. Generally, non fluorescent magnetic powder is used for dry method magnetic powder. When non fluorescent magnetic powder is used for detecting surface and near surface defects of workpieces, dry method magnetic powder with coarser particle size should be selected. Because the coarse magnetic powder is easy to disperse in the air, it is also easy to overlap defects with large span, and the permeability is higher than that of fine magnetic powder, so it is easy to magnetize and form magnetic trace display when lapping, and reduce the impact of dust.
In practical application, it is required to find various defects of different sizes, as well as defects on the surface and near the surface of the workpiece. Therefore, magnetic powder with various particle sizes should be used, so as to obtain a more balanced sensitivity for various defects. Fine particle size can be used to improve the sensitivity of detecting fine defects, but it can not be all composed of fine particle size. This is because: first, the small magnetic particles are easy to stick to the greasy, wet, fingerprint and uneven places to form excessive background; second, the flying dust in the detection environment is extremely unsafe for the detection personnel. For dry process magnetic powder, the particle size range is 10-50 μ m, and the maximum is not more than 150 μ M. For the black magnetic powder and red magnetic powder for wet process, the particle size should be 5-10 μ M. The magnetic powder with particle size greater than 50 μ m cannot be used for wet inspection because it is difficult for it to suspend in the magnetic suspension. During the flow process of the magnetic suspension, the coarse magnetic powder will stay on the surface of the workpiece to interfere with the relevant display. And the particle size is too fine in use, they will gather together. Because of the coating layer on the surface, the particle size of the fluorescent magnetic particle can not be too small, generally between 5-25 μ m, but this does not mean that the detection sensitivity is reduced, because the visibility, contrast and resolution of the fluorescent magnetic particle are high, so it can still obtain high sensitivity.
In magnetic particle testing, it is generally recommended to use 80-160 mesh of coarse magnetic particle for dry method and 300-400 mesh of fine magnetic particle for wet method.
Therefore, it is not that the finer the particle size is, the better the particle size is. Both the coarse and fine particles have their own scope of application. We need to select the appropriate particle size according to the nature, size, burial depth of the defect and the application mode of the particle in the detection.