The inspection of castings mainly includes dimension inspection, visual inspection of appearance and surface, chemical composition analysis and mechanical properties test. For castings that require more important requirements or are prone to problems in casting process, non-destructive testing is also needed. Non-destructive testing techniques for quality inspection of nodular iron castings include liquid penetration testing, magnetic powder testing, eddy current testing and radiographic testing. Measurement, ultrasonic detection and vibration detection, etc.

DETECTION OF SURFACE AND NEAR SURFACE DEFECTS OF A CAST

1. Liquid penetration detection

Liquid penetration testing is used to inspect various open defects on the surface of castings, such as surface cracks, surface pinholes and other defects that are difficult to detect by naked eyes. The common penetration detection is coloring detection. It is to wet or spray colored (usually red) liquid (penetrating agent) with high permeability on the surface of castings. The penetrating agent penetrates into the opening defect, wipes off the surface penetrating liquid layer quickly, and sprays the easy-to-dry display agent (also called imaging agent) on the surface of castings. The penetrating agent remaining in the opening defect is to be sucked out. Later, the displaying agent is dyed to reflect the shape, size and distribution of the defect. It should be pointed out that the accuracy of penetration detection decreases with the increase of the surface roughness of the tested material, that is, the better the surface detection effect is, the surface detection accuracy of grinder polishing is the highest, and even intergranular cracks can be detected. In addition to coloring detection, fluorescence penetration detection is also a commonly used method for liquid penetration detection. It needs to be equipped with ultraviolet lamp for irradiation observation. The detection sensitivity is higher than that of coloring detection.

2. Eddy Current Detection

Eddy current testing is suitable for inspecting defects below the surface which are generally not more than 6-7MM deep. Eddy current detection can be divided into two types: placement coil method and 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 current (eddy current) flowing in the direction perpendicular to the exciting magnetic field. The eddy current will produce a magnetic field opposite to the direction of the exciting magnetic field, which will reduce the original magnetic field in the coil and cause the change of the coil impedance. If there are defects on the surface of castings, the electrical characteristics of eddy current will be distorted to detect the defects. The main disadvantage of eddy current testing is that it can not visually display the size and shape of defects detected. Generally, it can only determine the location and depth of defects on the surface. Moreover, it is less sensitive to detect small openings on the surface of workpieces than penetration testing.

3. Magnetic Particle Detection

Magnetic particle detection is suitable for detecting surface defects and defects several millimeters 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 on the inner and outer surface of castings, and magnetic powder or magnetic suspension is used to show defects. When a magnetic field is generated in a certain range of castings, the defects in the magnetized area will produce leakage magnetic field. When magnetic powder or suspension is sprayed, the magnetic powder is absorbed, so that defects can be displayed. The defects shown in this way are basically the defects of the transverse magnetic line, but the long strip defects parallel to the magnetic line can not be shown. Therefore, the direction of magnetization needs to be constantly changed during operation to ensure that the defects in the unknown direction can be detected.

Inspection of Internal Defects in Second Castings

For internal defects, the common non-destructive testing methods are ray detection and ultrasonic detection. Among them, ray detection has the best effect. It can get intuitive images reflecting the type, shape, size and distribution of internal defects. But for large castings with large thickness, ultrasonic detection is very effective. It can measure the location, equivalent size and distribution of internal defects more accurately.

1. X-ray detection (micro-focus XRAY)

X-ray detection generally uses X-ray or gamma-ray as the source of radiation, so it needs the equipment and other auxiliary facilities to produce radiation. When the workpiece is irradiated by the radiation field, the radiation intensity of the radiation will be affected by the internal defects of the casting. The radiation intensity emitted through castings varies locally with the size and nature of defects, forming a defect radiographic image, which is recorded by radiographic film, or real-time detection and observation by fluorescent screen, or detection by radiometer. Among them, the method of recording by radiographic film is the most commonly used method, which is commonly called radiographic detection. The defect image reflected by radiography is intuitive. The shape, size, quantity, plane position and distribution range of the defect can be presented, but the depth of the defect can not be generally reflected. Special measures and calculation are needed to determine the defect. At present, the application of computerized tomography (CT) is not popular because of its expensive equipment and high cost. However, this new technology represents the direction of future development of high-definition X-ray detection technology. In addition, the micro-focus X-ray system using approximate point sources can actually eliminate the blurred edges generated by larger focus devices and make the image contour clear. The use of digital image system can improve the signal-to-noise ratio of the image and further improve the image clarity.

2. Ultrasound detection

Ultrasound detection can also be used to check internal defects. It uses high-frequency sound energy to detect defects by reflecting on the inner surface or defects during the propagation of sound beams in castings. The magnitude of the reflected acoustic energy is a function of the directivity and property of the inner surface or defect and the acoustic impedance of the reflector. Therefore, the acoustic energy reflected from various defects or inner surfaces can be used to detect the location, wall thickness or depth of the defect under the surface. As a widely used nondestructive testing method, the main advantages of ultrasonic testing are: high sensitivity, small cracks can be detected; large penetration capacity, can detect thick section castings. Its main limitations are that it is difficult to interpret the reflected waveforms of disconnected defects with complex outlines and poor directivity; it is also difficult to interpret the waveforms for unsatisfactory internal structures, such as grain size, structure, porosity, inclusion content or fine dispersed precipitates; in addition, reference to standard samples is needed for testing.