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 easy to cause problems in the casting process, non-destructive testing is required, which can be used for ductile iron castings. Non-destructive testing techniques for quality testing include liquid penetrant testing, magnetic particle testing, eddy current testing, radiation testing, ultrasonic testing, and vibration testing.
1 Detection of casting surface and near surface defects
1.1 Liquid penetration test
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 hard to find by the naked eye. The commonly used penetration test is coloring detection, which wets or sprays a colored (usually red) liquid (penetrant) with high permeability on the surface of the casting, and the penetrant penetrates into the opening defect to quickly wipe off the surface permeate. a layer, and then spraying an easy-to-dry display agent (also called an imaging agent) onto the surface of the casting. After the penetrating agent remaining in the opening defect is sucked out, the display agent is dyed, thereby reflecting the shape of the defect, Size and distribution. It should be pointed out that the accuracy of the penetration detection decreases as the surface roughness of the material to be inspected increases, that is, the better the surface light detection effect, the surface precision of the grinding of the grinding machine is the highest, and even the intergranular crack can be detected. In addition to color detection, fluorescence permeation detection is also a commonly used method for liquid permeation detection. It requires an ultraviolet lamp to be used for illumination observation, and the detection sensitivity is higher than that of color detection.
1.2 Eddy current testing
Eddy current testing is suitable for inspecting defects below the surface that are generally not more than 6 to 7 MM deep. The eddy current detection is divided into two types: a placement coil method and a through coil method. When the test piece is placed near the coil with alternating current, the alternating magnetic field entering the test piece can induce a eddy current (eddy current) perpendicular to the excitation magnetic field in the test piece, and the eddy current will be generated. A magnetic field opposite to the direction of the excitation magnetic field is generated to partially reduce the original magnetic field in the coil, thereby causing a change in the impedance of the coil. If there is a defect 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 the eddy current detection is that the size and shape of the detected defect cannot be visually displayed, and generally the surface position and depth of the defect can be determined. In addition, it is less sensitive to the detection of small open defects on the surface of the workpiece than the penetration test.
1.3 Magnetic particle testing
Magnetic particle testing is suitable for detecting surface defects and defects a few millimeters 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 display defects. When a magnetic field is generated within a certain range of the casting, a defect in the magnetized region generates a leakage magnetic field, and when the magnetic powder or the suspension is sprinkled, the magnetic powder is sucked, so that the defect can be revealed. The defects thus displayed are basically defects that cross the magnetic lines of force, and the long-type defects parallel to the magnetic lines of force are not displayed. For this reason, the magnetization direction needs to be constantly changed during operation to ensure that various defects in an unknown direction can be detected. .
2 Detection of internal defects of castings
For internal defects, common non-destructive testing methods are radiation detection and ultrasonic testing. Among them, the radiation detection effect is the best, it can obtain an intuitive image reflecting the type, shape, size and distribution of internal defects, but for large castings with large thickness, ultrasonic testing is very effective, and the position of internal defects can be accurately measured. , equivalent size and distribution.
2.1 Radiation detection (microfocus XRAY)
Radiation detection, generally using X-rays or gamma rays as a source of radiation, requires equipment and other ancillary facilities that generate radiation. When the workpiece is placed in a ray field, the radiation intensity of the radiation is affected by internal defects of the casting. The intensity of the radiation emitted through the casting varies locally with the size and nature of the defect. The image of the defect is formed by radiographic recording, or detected by real-time observation through a fluorescent screen, or by a radiation counter. Among them, the method of 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 shape, size, number, plane position and distribution range of the defect are all Can be presented, but the depth of the defect is generally not reflected, and special measures and calculations are needed to determine. At present, the international casting industry network [1] has applied the application of computerized tomography. Due to the relatively expensive equipment and high cost of use, it is still not popular, but this new technology represents the future development of high-definition radiography. In addition, the use of a microfocus X-ray system that approximates the point source actually eliminates the blurring edges produced by larger focus devices, making the image outline clear. Using a digital image system can increase the signal-to-noise ratio of the image and further improve image clarity.
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