Casting Quality Inspection Method

(1) Detection of casting surface and near surface defects

1.1 liquid penetrant testing

Liquid penetrant testing is used to check various opening defects on the casting surface, such as surface cracks, surface pinholes and other defects that are difficult to be found by the naked eye. The commonly used penetrant testing is dye testing. It is to wet or spray the colored (generally red) liquid (penetrant) with high penetrability on the surface of the casting. The penetrant infiltrates into the opening defect, quickly wipe off the surface penetrant layer, and then spray the easy to dry display agent (also called developer) on the surface of the casting. After the penetrant remaining in the opening defect is sucked out, the display agent is stained, So that 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 grinding machine has the highest detection accuracy, and even the 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 lamp 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 below the surface that are generally no more than 6-7mm deep. Eddy current testing is divided into two types: the placement 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 form of eddy current in the test piece in the direction perpendicular to the excitation magnetic field. The eddy current will generate a magnetic field in the direction opposite to the excitation magnetic field, so that the original magnetic field in the coil is partially reduced, thereby causing the change of the coil impedance. If there are defects on the surface of the casting, the electrical characteristics of eddy current will be distorted to detect the presence of defects. The main disadvantage of eddy current testing is that it can not visually display the size and shape of the detected defects. Generally, it can only determine the surface position and depth of the defects. In addition, it is less sensitive to detect small opening defects on the surface of the workpiece than 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) magnetization equipment and magnetic particle (or magnetic levitation liquid) to conduct testing. Magnetization equipment is used to generate magnetic field on the inner and outer surfaces of castings, and magnetic powder or magnetic suspension liquid is used to display defects. When a magnetic field is generated within a certain range of the casting, the defects in the magnetized area will generate leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic powder will be absorbed, so that the defects can be displayed. The defects displayed in this way are basically those that cross the magnetic lines of force, but the long defects that are parallel to the magnetic lines of force cannot be displayed. Therefore, the magnetization direction needs to be constantly changed 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 a visual image reflecting the type, shape, size and distribution of internal defects. However, for large-scale castings with large thickness, ultrasonic testing is very effective and can accurately measure the position, equivalent size and distribution of internal defects.

2.1 Radiographic testing (micro focus Xray)

X-ray testing, generally using X-ray or γ As the ray source, the ray generating equipment and other auxiliary facilities are required. 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 recorded by radiographic film, or real-time detected by fluorescent screen, or detected by radiation counter. Among them, the method of recording by radiographic film is the most commonly used method, which is commonly known as radiographic inspection. The defect image reflected by radiography is intuitive, and the shape, size, quantity, plane position and distribution range of defects can be presented. However, the defect depth can not be reflected generally, so special measures and calculations are required to determine. The international casting network appears to apply the radiographic computer tomography method. Because the equipment is expensive and the use cost is high, it cannot be popularized. However, 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 blurred edges generated by the larger focus equipment, and make the image outline clear. The digital image system can improve the signal-to-noise ratio of the image and further improve the image clarity.

2.2 Ultrasonic testing

Ultrasonic testing can also be used to check internal defects. It is to use the sound beam with high-frequency sound energy to transmit in the casting and generate reflection when it meets the internal surface or defect to find the defect. The magnitude of the reflected acoustic energy is a function of the directivity and nature of the inner surface or the defect and the acoustic impedance of such a reflector. Therefore, the acoustic energy reflected by various defects or the inner surface can be applied to detect the presence position, wall thickness or depth of the defect under the surface. Ultrasonic testing is a widely used non-destructive testing method. Its main advantages are as follows: high detection sensitivity, 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 broken 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 standard test blocks are required for testing.