The inspection of castings mainly includes dimensional inspection, visual inspection of appearance and surface, chemical composition analysis and mechanical property test. For castings with important requirements or easy to produce problems in casting process, nondestructive testing is also required. The nondestructive testing technologies that can be used for the quality inspection of ductile iron castings include liquid penetrant testing, magnetic particle testing, eddy current testing and radiographic testing Testing, ultrasonic testing and vibration testing.

Inspection of surface and near surface defects of castings

1. Liquid penetrant testing

Liquid penetrant testing (LPI) is used to inspect various kinds of opening defects on the surface of castings, such as surface cracks, surface pinholes, etc. The commonly used penetrant testing is dye detection. It is to wet or spray the colored (generally red) liquid (penetrant) with high permeability on the surface of the casting. The penetrant penetrates into the open defects, quickly wipe off the surface penetrant layer, and then spray the driable indicator (also known as imaging agent) on the casting surface to suck out the residual penetrant in the opening defect The indicator is then dyed to reflect the shape, size and distribution of defects. It should be pointed out that the accuracy of penetrant testing decreases with the increase of surface roughness of the tested material, that is, the smoother the surface is, the better the detection effect is; the surface polished by grinding machine has the highest detection accuracy, and even intergranular cracks can be detected. In addition to dye detection, fluorescent penetrant testing is also a commonly used liquid penetrant testing method. It needs to be equipped with ultraviolet lamp for observation, and the detection sensitivity is higher than that of dye detection.

2. Eddy current testing

Eddy current testing is applicable to the defects below the surface which are generally no more than 6-7mm deep. Eddy current testing can be divided into two types: placed 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 the current (eddy current) flowing in the form of eddy current perpendicular to the exciting magnetic field in the test piece. The eddy current will produce a magnetic field opposite to the exciting magnetic field, which will reduce part of the original magnetic field in the coil and cause the change of coil impedance. If there are defects on the surface of castings, the electrical characteristics of eddy current will be distorted, so as to detect the existence of defects. The main disadvantage of eddy current testing is that it can not directly display the size and shape of the detected defects. Generally, it can only determine the location and depth of defects on the surface of castings. In addition, the sensitivity of eddy current testing for small opening defects on the workpiece surface is not as good as that of penetrant testing.

3. Magnetic particle testing

Magnetic particle testing is suitable for testing surface defects and defects several millimeters deep below the surface. It requires DC (or AC) magnetizing equipment and magnetic powder (or magnetic suspension liquid) to carry out the detection operation. Magnetic equipment is used to generate magnetic field on the inner and outer surface of castings, and magnetic powder or magnetic suspension liquid is used to show defects. When the magnetic field is generated in a certain range of castings, the defects in the magnetized area will produce magnetic leakage field. When the magnetic powder or suspension is sprinkled, the magnetic powder is absorbed, so that the defects can be displayed. The defects shown in this way are basically the defects that cross the magnetic line of force, while the long strip defects parallel to the magnetic line of force can not be displayed. Therefore, the magnetization direction needs to be changed continuously during operation to ensure that all defects in the unknown direction can be detected.

Inspection of internal defects of castings

For internal defects, the commonly used nondestructive testing methods are radiographic testing and ultrasonic testing. Among them, the effect of radiographic inspection is the best, which can obtain the visual image reflecting the type, shape, size and distribution of internal defects. However, for large castings with large thickness, ultrasonic testing is very effective, which can accurately measure the location, equivalent size and distribution of internal defects.

1. Radiographic testing (microfocus Xray)

X-ray or gamma ray is generally used as the ray source for radiographic testing. Therefore, equipment and other ancillary facilities are needed to generate radiation. When the workpiece is exposed to the radiation field, the radiation intensity of the ray will be affected by the internal defects of the casting. The radiation intensity emitted from the casting varies with the size and nature of the defects, forming a radiographic image of the defect, which can be visualized and recorded by a radiographic film, real-time inspection and observation by a fluorescent screen, or detected by a radiation counter. Among them, the method of radiographic film imaging and recording is the most commonly used method, which is commonly referred to as radiographic testing. The defect image reflected by radiography is intuitive, and the defect shape, size, quantity, plane position and distribution range can be presented. Only the defect depth can not be reflected generally, and special measures and calculation are needed to determine. At present, the application of computerized radiography has not been popularized due to its high cost and high equipment cost. However, this new technology represents the future development direction of high-definition radiographic testing technology. In addition, the micro focus X-ray system using the approximate point source can also eliminate the fuzzy edges produced by the larger focus devices and make the image outline clear. The use of 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 the internal defects. It is the use of high-frequency sound energy in the transmission of sound beam in the casting, when it encounters the internal surface or defect, it will produce reflection and find the defect. The reflected sound energy is a function of the directivity and properties of the inner surface or defect and the acoustic impedance of the reflector. Therefore, the acoustic energy reflected by various defects or inner surfaces can be used to detect the defect location, wall thickness or depth of defects under the surface.