The inspection of castings mainly includes dimensional inspection, visual inspection of appearance and surface, chemical composition analysis, and mechanical property test. For castings that are more important or prone to problems in the casting process, non-destructive testing is also required, which can be used for casting quality inspection. Our non-destructive testing technologies include liquid penetrant testing, magnetic particle testing, eddy current testing, radiographic testing, ultrasonic testing, etc.
Detection of casting surface and near-surface defects
- Liquid Penetration Testing
Liquid penetrant testing is used to inspect 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 coloring inspection, which is to soak or spray a colored (generally red) liquid (penetrant) with high penetrability on the surface of the casting, and the penetrant penetrates the opening defect and quickly wipes off the surface penetrant. layer, and then spray the easy-drying 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.
- Eddy current testing
Eddy current testing is suitable for inspecting defects that are generally no more than 6-7mm deep below the surface. Eddy current testing is divided into two types: the placed coil method and the through-type coil method. When the test piece is placed near the coil with the alternating current flowing, the alternating magnetic field entering the test piece can induce in the test piece an eddy current (eddy current) flowing in the direction perpendicular to the excitation magnetic field, and the eddy current will A magnetic field opposite to the excitation magnetic field is generated, so that the original magnetic field in the coil is partially reduced, thereby causing the coil impedance to change. If there is a defect on the surface of the casting, the electrical characteristics of the eddy current will be distorted, and the existence of the defect will be detected. The main disadvantage of the eddy current inspection is that the size and shape of the detected defect cannot be visually displayed. Generally, only the surface position and depth of the defect can be determined. , and it is not as sensitive as penetrant detection for detecting small open defects on the workpiece surface.
- Magnetic particle testing
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) for inspection 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 reveal defects. When a magnetic field is generated within a certain range of the casting, the defects in the magnetized area will generate a leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic powder is attracted, so that the defects can be displayed. The defects displayed in this way are defects transverse to the magnetic field lines, and the long-shaped defects parallel to the magnetic field lines cannot be displayed. Therefore, the magnetization direction needs to be changed continuously during operation to ensure that each defect in an unknown direction can be detected.
Detection of internal defects of castings
For internal defects, the commonly used non-destructive testing methods are radiographic testing and ultrasonic testing. Among them, the radiographic inspection effect is the best, it can obtain an intuitive image reflecting the type, shape, size, and distribution of internal defects, but for large-thickness large castings, ultrasonic inspection is very effective, and the position of internal defects can be measured more accurately. , equivalent size and distribution.
- Radiographic inspection
X-rays or γ-rays are generally used as radiation sources for ray detection, so equipment and other auxiliary facilities for generating rays are required. When the workpiece is placed in the ray field, the radiation intensity of the rays will be affected by the internal defects of the casting. The radiation intensity emitted through the casting varies locally with the size and nature of the defect, forming a radiographic image of the defect, which is imaged and recorded through a radiographic film, detected and observed in real-time through a fluorescent screen, or detected by a radiation counter. Among them, the method of imaging and recording by the 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 shape, size, number, plane position and distribution range of defects are all It can be shown, but the depth of the defect cannot be reflected in general, and special measures and calculations are needed to determine it.
- Ultrasonic testing
Ultrasonic testing can also be used to inspect internal defects. It uses the sound beam with high-frequency sound energy to propagate inside the casting and finds defects when it encounters internal surfaces or defects and reflects. The magnitude of the reflected acoustic energy is a function of the directivity and nature of the interior surface or defect and the acoustic impedance of such a reflector, so the acoustic energy reflected from various defects or interior surfaces can be applied to detect the presence of defects, wall thickness or surface the depth of the defect. As a widely used non-destructive testing method, ultrasonic testing has the main advantages of high detection sensitivity, which can detect small cracks; large penetration ability, which can detect thick section castings. Its main limitations are: difficult to interpret reflection waveforms for discontinuous defects with complex contour dimensions and poor directivity; for undesired internal structures such as grain size, microstructure, porosity, inclusion content or fine dispersion Precipitates, etc., also hinder waveform interpretation; in addition, it is necessary to refer to standard test blocks during detection.
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