Factors affecting the sensitivity of magnetic particle testing

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1. Sensitivity of magnetic particle testing: Quantitatively speaking, it refers to the ability to effectively detect defects of a specified size on or near the surface of a workpiece； In qualitative terms, it refers to the ability to detect the smallest defects; the smaller the defects that can be detected, the higher the detection sensitivity. II. Factors affecting the sensitivity of magnetic particle testing 1. External magnetic field strength. The sensitivity of magnetic particle testing is closely related to the degree of magnetization of the workpiece. Generally speaking, the external magnetic field strength must be greater than Hμm; that is, a magnetic field strength value to the right of the Hμm point corresponding to the maximum permeability μm should be selected. At this point, the permeability decreases, the magnetic resistance increases, and the leakage magnetic field increases. When the magnetic induction intensity of ferromagnetic materials reaches about 80% of its saturation value, the leakage magnetic field increases rapidly. 2. Magnetization method: To detect defects in all directions, it is usually necessary to magnetize the same area in two mutually perpendicular directions. Different magnetization methods have varying capabilities in detecting defects oriented in different directions. Circumferential magnetization exhibits higher sensitivity in detecting longitudinal defects, while longitudinal magnetization has higher sensitivity in detecting transverse defects. 3. Types of magnetizing current: Different types of magnetizing current exhibit different penetrability and pulsation characteristics; therefore, the type of magnetizing current affects the sensitivity of magnetic particle testing. AC current has high sensitivity to surface defects, but its detection sensitivity for near-surface defects **decreases**. Direct current has the greatest penetrability, which is beneficial for detecting buried defects at great depths; however, its sensitivity to surface defects is lower than that of alternating current. 4. Properties of magnetic particles: Magnetic particle inspection relies on magnetic particles gathering at the leakage magnetic fields to form magnetic traces that reveal defects; therefore, the sensitivity of this inspection method is related to the properties of the magnetic particles themselves, such as their magnetic characteristics, particle size, shape, fluidity, density, and distinguishability. 5. Types and concentrations of magnetic suspensions: The commonly used magnetic suspensions are water-based and oil-based types. These two types have different viscosity values and flow properties, resulting in variations in detection sensitivity. The magnetic suspension concentration has a significant impact on the sensitivity of magnetic particle testing. The concentration is too low; consequently, the leakage magnetic field has a minimal effect on the adsorption of magnetic particles. As a result, the magnetic traces are not distinct, leading to missed detection of defects ； If the concentration is too high, a large amount of magnetic particles will remain on the surface of the workpiece, creating an excessive background that may even obscure the relevant indications. 6. Equipment performance: It is necessary to ensure that the magnetic particle inspection equipment is used in good condition. If a certain function of the equipment is missing, it not only reduces the detection sensitivity but can also lead to the failure of the entire inspection process in severe cases. 7. Workpiece material, shape, dimensions, and surface condition. The effect of the workpiece material on detection sensitivity is mainly reflected in the influence of the workpiece’s magnetic properties on the sensitivity. The shape and size of the workpiece affect the selection of magnetization methods and the detection sensitivity of ring gaps. The surface roughness of the workpiece, oxide scale, oil stains, rust, etc., all have a certain impact on the sensitivity of magnetic particle testing. 8. Orientation, nature, shape, and burial depth of defects: The sensitivity of magnetic particle testing depends on the angle between the orientation of the defect and the direction of the magnetic field. When the defect is perpendicular to the magnetic field direction, the leakage magnetic field is at its maximum, resulting in the greatest amount of magnetic particles being attracted; this condition is most favorable for detecting defects, thereby achieving the highest detection sensitivity. The detection sensitivity for planar defects is higher than that for point defects. For defects of the same size, when located on the surface of the workpiece, they generate a larger leakage magnetic field, resulting in higher sensitivity ； When located near the surface of the workpiece, the resulting leakage magnetic field will be significantly reduced, resulting in a lower detection sensitivity. 9. Process operations: The process operations involved in magnetic particle testing mainly include cleaning the surface of the workpiece, applying the magnetic suspension, and observing/analyzing the results. Improper execution of any of these steps—such as mishandling of the magnetized workpiece or incorrect application of magnetic particles—can affect the detection of defects.
10. Qualifications of inspectors: The vision and color discrimination abilities of inspectors directly influence their ability to detect defects. 11. Conditions of the inspection environment: When using non-fluorescent magnetic particle inspection, the inspection area should have sufficient natural or white light. Insufficient lighting reduces the human eye’s ability to distinguish colors and contrasts, which in turn leads to a decrease in detection sensitivity. When using fluorescent magnetic particle inspection, a suitable dark area or dark room is necessary. Excessive light intensity hinders the human eye from detecting the yellow-green light emitted by defects under blacklight illumination, which in turn leads to a decrease in detection sensitivity. III. Summary 1. The sensitivity of magnetic particle testing refers to the ability to effectively detect defects of a specified size near the surface of a workpiece ； 2. The factors that affect the sensitivity of magnetic particle testing include: the intensity of the external magnetic field ； Magnetization methods ； Magnetization current type ； Magnetic powder properties ； Types and concentrations of magnetic suspensions ； Equipment performance ； Workpiece material, shape and size, and surface condition ； Direction, nature, shape, and depth of burial of the defect ； Process operation ； Quality of inspectors ； Conditions of the testing environment, etc.