- Jan 08, 2018 -
Detection of internal defects in castings
For internal defects, commonly used non-destructive testing methods are radiographic testing and ultrasonic testing. Among them, the ray detection has the best effect, and it can obtain the direct image which reflects the type, shape, size and distribution of internal defects. However, for large thick castings, the ultrasonic inspection is very effective and the position of the internal defect can be measured more accurately , Equivalent size and distribution.
1) ray detection (micro-focus XRAY)
Radiographic testing, generally using X-rays or gamma rays as the source of radiation, requires the generation of radiation equipment and other ancillary facilities. When the workpiece is exposed to a radiation field, the radiation intensity of the radiation is affected by internal defects in the casting. The intensity of the radiation emitted through the casting varies locally depending on the size and nature of the defect. A radiographic image of the defect is recorded on a radiographic film, either real-time through a fluorescent screen or through a radiation counter. Among them, the method of recording by radiographic film imaging is the most commonly used method, that is, the so-called radiographic test. The defect images reflected by the radiographs are intuitive. The defect shape, size, number, plane position and distribution range Can be presented, but the depth of defects generally can not be reflected, need to take special measures and calculations to determine. The emergence of international casting network using computer tomography method, due to the equipment is more expensive, high cost, can not be universal, but this new technology represents the high-definition ray detection technology in the future direction. In addition, microfocus X-ray systems that use an approximate point source virtually eliminate blurred edges from larger focal devices, making the image contours clear. Use of digital image system can improve the signal to noise ratio of the image, to further improve the image clarity.
2) Ultrasound testing
Ultrasound testing can also be used to detect internal defects, which are the use of sound waves with high-frequency sound within the casting propagation, hit the internal surface or defect reflection and found defects. The magnitude of the reflected acoustic energy is a function of the directivity and nature of the inner surface or defect and of the acoustic impedance of such a reflector so that various defects or acoustic energy reflected by the inner surface can be used to detect the presence of defects, wall thickness, or surface The depth of defects. As a widely used non-destructive testing method, the main advantages of ultrasonic testing are as follows: high detection sensitivity and small crack detection; large penetrating ability to detect thick section castings. Its main limitation is that it is difficult to interpret the reflected waveforms of the open defects with complex contour and poor directivity. For unwanted internal structures such as grain size, structure, porosity, inclusion content or fine dispersion Precipitates, also hinder the waveform interpretation; In addition, the test requires reference standard test block.