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The inspection of finished castings manufactured by Chinese foundries mainly includes appearance, size inspection and surface visual inspection, chemical composition analysis and mechanical performance test. For castings that require more important requirements or are prone to problems in the casting process, they also need to be tested. Non-destructive testing. Non-destructive testing techniques that can be used for quality testing of ductile iron parts include liquid penetration testing, magnetic particle testing, eddy current testing, radiographic testing, ultrasonic testing and vibration testing. Installation and design rules of ductile iron pipes.
For problems that tend to occur inside Chinese foundries, the commonly used non-destructive testing methods are ultrasonic testing and radiographic testing. Among them, the radiographic detection effect is the best. It can obtain intuitive images that reflect the type, shape, size and distribution of the internal deficiencies, but for large-thickness large-scale
For castings, ultrasonic testing is very effective, and the location, equivalent size and distribution of internal deficiencies can be measured more accurately.
Ultrasonic testing can also be used to check the internal deficiencies of Chinese foundries. It uses the sound beam propagation of high-frequency sound energy inside the castings to inspect. When it encounters internal surfaces or deficiencies, it reflects and finds deficiencies. The magnitude of the reflected sound energy is the direction of the inner surface or deficiencies
It is a function of the characteristics and properties of the reflector and the acoustic impedance of the reflector. Therefore, various deficiencies or the sound energy reflected from the inner surface can be used to detect the location of the deficiencies, the wall thickness, or the depth of the deficiencies under the surface. Ultrasonic testing is a widely used non-destructive testing tool
Section, its main advantages are: high detection sensitivity, can detect small cracks; with large penetrating ability, can detect thick section castings. Its main limitation lies in the difficulty in explaining the reflection waveform of the insufficiency of the disconnection due to the complicated outline size and poor directivity;
Undesirable internal structures, such as grain size, structure, porosity, inclusion content, or fine dispersed precipitates, also hinder waveform interpretation; in addition, reference standard test blocks are required for testing.
Chinese foundries generally use X-rays or γ-rays as the radiation source. Therefore, equipment and other ancillary facilities are required to generate radiation. When the workpiece is exposed to the radiation field, the radiation intensity of the radiation will be affected by the inadequacy of the casting influences. Visualization and recording by radiographic film, or real-time detection and observation by fluorescent screen, or detection by radiation counter. The intensity of radiation emitted through the casting varies locally depending on the size and nature of the deficiency, forming a deficiency. Ray image. 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 image of the shortcomings reflected by the radiography is intuitive, and the shape, size, number, and plane position of the shortcomings Both the distribution range and the distribution range can be presented, but the depth of the shortcomings is generally not reflected, and special measures and calculations are needed to determine. Nowadays, the application of radiographic computer tomography methods has emerged. 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-definition radiographic detection technology. In addition, the use of a micro-focus X-ray system with a similar point source can actually eliminate the fuzzy edges produced by larger focus devices and make the image outline clear. Using a digital image system can improve the signal-to-noise ratio of the image and further improve the image clarity.Detection of defects on the surface and near the surface of Chinese foundries
Eddy current testing in Chinese foundries
Eddy current testing is suitable for checking the deficiencies below the surface that are generally not more than 6 to 7 mm deep. Eddy current testing is divided into two types: the placement coil method and the through coil method. If there are deficiencies on the surface of the casting, the electrical characteristics of the eddy current
Distortion will occur, thereby detecting the existence of deficiencies. The main disadvantage of eddy current testing is that the size and shape of the detected deficiencies cannot be displayed intuitively. Generally, only the surface position and depth of the deficiencies can be determined. Inadequate small openings on the surface
The detection sensitivity is not as good as penetrant detection. When the test piece is placed near a coil with alternating current, the alternating magnetic field entering the test piece can induce the direction and direction in the test piece.
The current (eddy current) flowing in the shape of an eddy current perpendicular to the excitation magnetic field. The eddy current generates a magnetic field opposite to the direction of the excitation magnetic field, which partially reduces the original magnetic field in the coil, thereby causing a change in the impedance of the coil.
Magnetic particle testing in Chinese foundries
Magnetic particle detection is suitable for detecting defects on the surface and several millimeters deep below the surface. It requires DC (or AC) magnetization equipment and magnetic powder (or magnetic suspension) to perform detection operations. Magnetization equipment is used to generate magnetic field, magnetic powder or magnetic suspension on the inner and outer surfaces of the casting
Used to show deficiencies. When a magnetic field is generated within a certain range of the casting, the deficiencies in the magnetization area will generate a leakage field. When the magnetic powder or suspension is sprinkled, the magnetic powder is attracted, so that the deficiencies can be displayed. The shortcomings shown in this way are basically
The shortcomings of the transverse magnetic line of force cannot be displayed for the long-shaped shortcomings parallel to the magnetic line of force. For this reason, the magnetization direction needs to be constantly changed during operation to ensure that the various shortcomings in the unknown direction can be detected.
Liquid penetration testing is used to check the deficiencies of various openings on the surface of the casting, such as surface cracks, surface pinholes and other deficiencies that are difficult to find with the naked eye. Commonly used penetrant detection is color detection, which is a colored (usually red) liquid with high penetrating ability (penetrant)
Soak or spray on the surface of the casting, the penetrant penetrates into the insufficiency of the opening, quickly wipe off the surface penetrating liquid layer, and then spray the easy-drying display agent (also called the developer) on the surface of the casting, and wait for the residue After the penetrant in the insufficiency of the opening is sucked out, the revealing agent is Dyeing, which can reflect the shape, size and distribution of the deficiencies. 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, and the surface detection accuracy of the grinding machine is the highest, even
Intercrystalline cracks were detected. In addition to color detection, fluorescent penetrant detection is also a commonly used liquid penetrant detection method. It needs to be equipped with an ultraviolet lamp for irradiation observation, and the detection sensitivity is higher than that of color detection.