For the defects of internal marine castings, the commonly used nondestructive testing methods are radiographic testing and ultrasonic testing. Among them, the effect of radiographic testing is good, and 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, and it can accurately measure the position, equivalent size and distribution of internal defects.

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 radiographic film imaging and recording is a common method, which is commonly called radiographic detection. 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, and special measures and calculations are required to determine it.

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.

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;