The most easily overlooked details in magnetic particle testing

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We know that magnetic particle testing is relatively simpler compared to radiographic testing and ultrasonic testing, and the defects detected are those with relatively low severity; as a result, operators using magnetic particle testing often overlook some detailed issues. However, as a quality inspector responsible for ensuring product quality, one must possess a high level of professional competence; therefore, every aspect needs to be carefully monitored. The following are the points that the author identified as those most easily overlooked by operators: 1. During testing, only it was ensured that the current level was not less than 1050 A (this value applies to the workpieces in the author’s own facility; the current required for different workpieces will certainly vary), without using standard test pieces to determine the optimal current level. Solution: The defects in each workpiece vary, so the optimal current level for detecting defects may also differ. When testing different types of workpieces, we should use standard test pieces to determine the optimal current level as well as the direction of magnetization. 2. Before the testing, the intensity of the black light or the white light was not measured, nor was the darkness level in the dark room assessed. The testing specifications require that the intensity of the black light at a distance of 380 mm be ≥1000 UW/cm2, while the illuminance from visible light should not exceed 20 LX. At an appropriate distance, the intensity of white light should be ≥1000 LX, and the illuminance on the surface of the workpiece under inspection should be no less than 500 LX. And these basic details are often overlooked. 3. The prepared magnetic suspension was not subjected to a wetting performance test (water break test). Solution: The prepared magnetic suspension should be tested for its wetting properties before each measurement. The testing method involves applying a water-magnetic suspension to the surface of the workpiece. If the application of this suspension is continuous and forms a uniform layer over the entire surface of the workpiece, it indicates good wetting properties ； If the thin film of the water-magnetic suspension on the surface of the workpiece breaks, exposing a bare surface on the workpiece (i.e., a surface without the suspension), it indicates that the wetting property of the water-magnetic suspension is inadequate. At this point, a wetting agent should be added or the workpiece surface cleaned to achieve complete wetting. 4. The concentration of the magnetic suspension was not prepared properly. In my work, I have found that some personnel are not very sensitive to the concentration of the magnetic suspension. Although defects can still be detected at unreasonable concentrations, the concentrations specified in the testing standards represent the optimal levels, which in turn leads to an improved defect detection rate. Solution: The concentration of the magnetic suspension should be adjusted as required. The standard concentrations are as follows: for fluorescent magnetic particles, it is 0.1–0.4 mL/100 mL; for non-fluorescent magnetic particles, it is 1.2–1.4 mL/100 mL. 5. Influence of the inspectors. Some operators wear glasses for nearsightedness, which affects their ability to detect defects ； Some operators also did not wear protective glasses. Solution: The myopia glasses worn by people today are generally photo-sensitive lenses, which darken when exposed to black light radiation. The degree of darkening is proportional to the amount of radiation incident, which hinders the observation and identification of magnetic traces left by fluorescent magnetic powder. Therefore, such glasses are not allowed to be used in inspections. Since ultraviolet rays must not be allowed to enter the human eyes, directly or indirectly, to prevent the eyes from being exposed to such radiation, inspectors should wear UV-absorbing goggles that can block ultraviolet rays as well as most purple and blue light. However, it is necessary to ensure that the goggles do not reduce the ability to detect yellow-green fluorescent magnetic particle marks. Wearing goggles is also a protective measure for the inspectors. 6. The workpiece is divided into standard sections; according to the standards, these sections should be 150mm×150mm in size. However, in some cases the differences between the sections are too large, and the overlap during magnetization is less than 25%. Solution: The partition should be as close as possible to 150mm×150mm, as the minimum distance between contacts must be at least 75 mm. This is because within a range of 25 mm near the contact, the current density is too high, resulting in excessive background that may obscure the relevant display. The distance between the contacts should not be too large either, as a large distance will weaken the magnetic field and affect the detection of defects. Chinese manufacturing has evolved from an era focused on speed to one focused on quality; therefore, as inspectors, we must pay attention to the details in order to ensure both quality and quantity.