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The Most Easily Overlooked Details in Magnetic Particle Testing
Release Date:
2023/03/25
We know that magnetic particle testing is relatively simpler than radiographic testing and ultrasonic testing, and the defects it detects are generally of low severity. Consequently, magnetic particle testing operators often overlook certain detailed issues. However, as a quality-control inspector, one must possess a high level of professional integrity and meticulously control every step of the process. The following are several points that the author has identified as being most easily overlooked by operators:
1. During inspection, the current magnitude is simply ensured to be no less than 1050 A (this value is specific to the workpieces used by the author’s organization; the appropriate current for other types of workpieces will undoubtedly vary), without first determining the optimal current through testing with standard test specimens.
Solution: Defects vary from one workpiece to another, so the optimal current setting may also differ depending on the defect detected. Therefore, when inspecting different types of workpieces, the optimal current magnitude and magnetization direction should be determined using standard test pieces.
2. Prior to testing, the intensity of the blacklight and white light was not verified, nor was the darkness level of the darkroom measured.
Testing specifications require that, at a distance of 380 mm, the blacklight intensity be ≥1000 μW/cm², while the visible-light illuminance shall not exceed 20 lux. At an appropriate distance, the white-light intensity shall be ≥1000 lux, and the illuminance on the surface of the test workpiece shall be no less than 500 lux. Yet these fundamental details are often overlooked.
3. The prepared magnetic suspension was not subjected to a wetting performance test (water break test).
Solution: Prior to each inspection, the prepared magnetic suspension should be subjected to a wetting-performance test. The procedure involves applying the water-based magnetic suspension to the workpiece surface; if the suspension is applied continuously and forms a uniform, uninterrupted film across the entire surface, this indicates good wetting performance. Conversely, if the film breaks up and exposed areas of the workpiece surface remain (i.e., “dry spots” appear), the wetting performance of the magnetic suspension is deemed inadequate. In such cases, a wetting agent should be added or the workpiece surface should be cleaned to ensure complete wetting.
4. The concentration of the magnetic suspension is improperly prepared.
Some personnel are not very sensitive to the concentration of the magnetic suspension; although defects can still be detected at non-optimal concentrations, the concentrations specified in the testing standards are the optimal levels, which correspondingly result in a higher defect detection rate.
Solution: The magnetic suspension concentration should be configured as required; the standard magnetic suspension concentration is:
Fluorescent magnetic powder: 0.1–0.4 mL/100 mL
Non-fluorescent magnetic powder: 1.2–1.4 mL/100 mL
5. The influence of testing personnel.
Some operators wear myopic glasses, which impairs their ability to detect defects; others do not wear protective eyewear.
Solution: Currently, most myopic eyeglasses in use are photochromic lenses, which darken in response to black-light radiation in proportion to the intensity of the incident radiation. This darkening can interfere with the observation and identification of fluorescent magnetic-particle indications, so such eyeglasses are not permitted during inspection. Since ultraviolet radiation must be prevented from entering the eyes—either directly or indirectly—inspection personnel should wear UV-absorbing safety goggles that block ultraviolet light as well as most violet and blue light. At the same time, it is essential to ensure that these goggles do not compromise the ability to detect yellow-green fluorescent magnetic-particle indications. Wearing safety goggles also serves as a protective measure for inspection personnel.
6. The workpiece is divided into standard zones, each measuring 150 mm × 150 mm as per the standard requirements; however, some zone divisions exhibit significant deviations, and the overlap during magnetization is less than 25%.
Solution: The inspection area should be as close as possible to 150 mm × 150 mm, since the minimum contact spacing must not be less than 75 mm. This is because, within a 25-mm zone near the contacts, the current density becomes excessively high, leading to excessive background noise that may obscure relevant indications. Conversely, the contact spacing should not be too large, as excessive spacing weakens the magnetic field and thereby impairs the visibility of defects.
China’s manufacturing sector has transitioned from an era of rapid growth to one focused on quality; therefore, as inspection personnel, we must start with the details to ensure both quality and quantity.
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Source: Materials and Testing; Author: Galaxy’s Gaze
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