Testing method for surface and internal quality of

Surface and internal quality inspection methods of castings

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 prone to 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 Radiographic testing, ultrasonic testing, vibration testing, etc

1 detection of surface and near surface defects of castings

1.1 liquid penetrant 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 by naked eyes. The commonly used penetrant testing is dye detection, which is to wet or spray the colored (generally red) liquid (penetrant) with high permeability on the surface of the casting, the penetrant infiltrates into the opening defects, quickly wipe off the surface penetrant layer, and then spray the easy to dry indicator (also known as developer) on the surface of the casting. After the penetrant remaining in the opening defects is sucked out, the indicator will be dyed, Thus, the shape, size and distribution of defects can be reflected. It should be pointed out that the accuracy of penetrant testing decreases with the increase of the surface roughness of the tested material, that is, the brighter the surface, the better the detection effect. The surface polished by the grinder has the highest detection accuracy, and even intergranular cracks can be detected. In addition to dye detection, fluorescent penetrant detection is also a commonly used liquid penetrant detection method. It needs to be equipped with ultraviolet light for irradiation observation, and the detection sensitivity is higher than that of dye detection

1.2 eddy current testing

eddy current testing is applicable to the inspection of defects less than 6 ~ 7mm deep below the surface. Eddy current testing can be divided into two types: the placed coil method and the 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 eddy current (eddy current) flowing in the direction perpendicular to the excitation magnetic field in the test piece. The eddy current will produce a magnetic field opposite to the direction of the excitation magnetic field, which will partially reduce the original magnetic field in the coil, thus causing the change of coil impedance. If there are defects on the surface of the casting, 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 cannot visually display the size and shape of the detected defects. Generally, it can only determine the location and depth of the surface where the defects are located. In addition, its detection sensitivity to small opening defects on the surface of the workpiece is not as sensitive as penetrant testing

1.3 magnetic particle testing

magnetic particle testing is suitable for detecting surface defects and defects several millimeters deep below the surface. It requires DC (or AC) magnetizing equipment and magnetic particle (or magnetic levitation fluid) to carry out testing operations. Magnetizing equipment is used to generate magnetic field on the inner and outer surfaces of castings, and magnetic particle or magnetic suspension fluid is used to show defects. When a magnetic field is generated within a certain range of the casting, the defects in the magnetized area will produce a magnetic leakage field. When the magnetic particle or suspension is sprinkled, the magnetic particle is absorbed, so that the defects can be displayed. The defects shown in this way basically have problems with any accessories of the experimental machine. More than 3/4 of them are the defects that the hot-air plastic granulator is crosscutting the magnetic line of force, and the long strip defects parallel to the magnetic line of force cannot be displayed. Therefore, it is necessary to constantly change the magnetization direction during operation to ensure that all defects in the unknown direction can be detected

2 detection 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 testing is the best. It can get an intuitive 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

2.1 radiographic testing (micro focus Xray)

radiographic testing, generally X-ray or γ As the ray source, the ray generating equipment and other auxiliary facilities are needed. When the workpiece is exposed to the ray field, the radiation intensity of the ray 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 by radiographic film, or detected and observed in real time by fluorescent screen, or detected by radiation counter. Among them, the method of recording through radiographic film imaging is the most commonly used method, which is commonly referred to as radiographic testing. The defect image reflected by radiography is intuitive, and the shape, size, quantity, plane position and distribution range of defects can be presented. Only the defect depth can not be reflected generally, so special measures and calculations need to be taken to determine it. Now there is the application of radiographic computer layer, so the precision analysis and photography method, because the equipment is relatively expensive and the use cost is high, it can not be popularized at present, but this new technology represents the future development direction of high-definition radiographic testing technology. In addition, the micro focus X-ray system using an approximate point source can actually eliminate the fuzzy edges generated by the larger focus equipment, so that the image outline is clear. The use of digital image system can improve the signal-to-noise ratio of the image and further improve the image definition

2.2 ultrasonic testing

ultrasonic testing can also be used to inspect internal defects. It uses the sound beam with high-frequency sound energy to spread inside the casting and reflect when it encounters the internal surface or defects to find defects. The magnitude of the reflected sound energy is a function of the directivity and nature of the inner surface or defect and the acoustic impedance of this reflection to the emitter. Therefore, the sound energy reflected by various defects or inner surfaces can be used to detect the location, wall thickness or depth of defects under the surface. As a widely used nondestructive testing method, ultrasonic testing has the following main advantages: high detection sensitivity, which can detect small cracks; It has large penetration ability and can detect thick section castings. Its main limitations are: it is difficult to interpret the reflection waveform of the disconnection defect with complex outline size and poor directivity; Undesirable internal structures, such as grain size, microstructure, porosity, inclusion content or fine dispersed precipitates, also hinder waveform interpretation; In addition, reference to standard test blocks is required for testing