When the test piece is placed near a coil with alternating current
When the test piece is placed near a coil with alternating current, the alternating magnetic field entering the test piece can induce an eddy current (eddy current) flowing in the direction perpendicular to the excitation magnetic field in the test piece. A magnetic field opposite to the direction of the excitation magnetic field will be generated
The inspection of castings mainly includes size inspection, visual inspection of appearance and surface, chemical composition analysis and mechanical performance test. For castings that require more important requirements or are prone to problems in the casting process, non-destructive testing is also required. It can be used for ductile iron castings. Non-destructive testing techniques for quality testing include liquid penetration testing, magnetic particle testing, eddy current testing, radiographic testing, ultrasonic testing, and vibration testing.
1. Detection of surface and near surface defects of castings
1. Liquid penetration testing
Liquid penetration testing is used to check various opening defects on the surface of castings, such as surface cracks, surface pinholes and other defects that are difficult to find with the naked eye. The commonly used penetrant inspection is color inspection, which is to wet or spray a colored (usually red) liquid (penetrant) with high penetrating ability on the surface of the casting. The penetrant penetrates into the opening defect and quickly wipes off the surface penetrant. Then spray the easy-drying display agent (also called developer) on the surface of the casting. After the penetrant remaining in the opening defect is sucked out, the display agent is dyed, which can reflect the shape of the defect. Size and distribution. It should be pointed out that the accuracy of penetrant detection decreases with the increase of the surface roughness of the tested material, that is, the more light the surface, the better the detection effect. The surface detection accuracy of the grinding machine is the highest, and even intercrystalline cracks can be detected. In addition to color detection, fluorescent penetrant detection is also a commonly used liquid penetrant detection method. It needs to be equipped with ultraviolet light for irradiation observation, and the detection sensitivity is higher than that of color detection.
2. Eddy current testing
Eddy current testing is suitable for inspecting defects that are generally not more than 6mm to 7mm deep below the surface. There are two types of eddy current testing: the placement coil method and the through coil method. : When the test piece is placed near a coil with alternating current, the alternating magnetic field entering the test piece can induce an eddy current (eddy current) flowing in the direction perpendicular to the excitation magnetic field in the test piece. A magnetic field opposite to the direction of the excitation magnetic field will be generated, which will partially reduce the original magnetic field in the coil, thereby causing the change of the coil impedance. If there are defects on the surface of the casting, the electrical characteristics of the eddy current will be distorted, thereby detecting the existence of the defect. The main disadvantage of eddy current testing is that the size and shape of the detected defect cannot be displayed visually. Generally, only the surface position and depth of the defect can be determined. In addition, it is not as sensitive as penetrant detection for detecting small opening defects on the surface of the workpiece.
3. Magnetic particle inspection
Magnetic particle inspection is suitable for detecting surface defects and defects several millimeters deep below the surface. It requires DC (or AC) magnetization equipment and magnetic powder (or magnetic suspension) to perform detection operations. Magnetizing equipment is used to generate a magnetic field on the inner and outer surfaces of the casting, and magnetic powder or magnetic suspension is used to display defects. When a magnetic field is generated within a certain range of the casting, the defects in the magnetized area will produce a leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic powder is attracted, so that defects can be displayed. The defects displayed in this way are basically defects that cross the lines of magnetic force, but they cannot be displayed for long-length defects parallel to the lines of magnetic force. For this reason, the magnetization direction needs to be constantly changed during operation to ensure that various defects in unknown directions can be detected. .
2. Detection of internal defects in castings
For internal defects, the commonly used non-destructive testing methods are radiographic testing and ultrasonic testing. Among them, radiographic inspection has the best effect. It can obtain intuitive images reflecting the types, shapes, sizes and distribution of internal defects. However, for large-thickness large castings, ultrasonic inspection is very effective and can accurately detect the location of internal defects. , Equivalent size and distribution.
1. Radiographic inspection (microfocus XRAY)
Radiographic testing generally uses X-rays or gamma; rays as the ray source, so equipment and other ancillary facilities are required to generate rays. When the workpiece is exposed to the ray field, the radiation intensity of the rays will be affected by the internal defects of the casting. The intensity of the radiation emitted through the casting varies locally with the size and nature of the defect, forming a radiographic image of the defect, which is developed and recorded by radiographic film, or real-time detection and observation through a fluorescent screen, or detection by a radiation counter. Among them, the method of developing and recording by radiographic film is the most commonly used method, which is commonly referred to as radiographic inspection. The defect image reflected by radiography is intuitive, and the defect shape, size, number, plane position and distribution range are all It can be shown, but the depth of the defect generally cannot be reflected, and special measures and calculations are needed to determine it. Nowadays, the application of radiographic computer tomography methods appears. Due to the relatively expensive equipment and high cost of use, it is currently not popular, but this new technology represents the future development direction of high-resolution radiographic detection technology. In addition, the use of a micro-focus X-ray system that is similar to a point source can actually eliminate the fuzzy edges produced by larger focus devices and make the image contour clear. Using a digital image system can improve the signal-to-noise ratio of the image and further improve the image clarity.
2. Ultrasonic testing
Ultrasonic testing can also be used to check internal defects. It uses sound beams with high-frequency sound energy to propagate inside the casting, and when they encounter internal surfaces or defects, they reflect and find defects. The magnitude of the reflected sound energy is a function of the directivity and properties of the inner surface or defect and the acoustic impedance of this reflector. Therefore, various defects or the sound energy reflected by the inner surface can be used to detect the location, wall thickness or surface of the defect The depth of the next defect. Ultrasonic testing, as a widely used non-destructive testing method, has its main advantages in: high detection sensitivity and can detect small cracks; it has large penetration capabilities and can detect thick section castings. Its main limitation lies in: it is difficult to explain the reflection waveform of the disconnected defect with complex outline size and poor directivity; for undesirable internal structure, such as grain size, structure, porosity, inclusion content or fine dispersion Precipitates, etc., also hinder the interpretation of the waveform; in addition, reference standard test blocks are required for testing.