The inspection of castings mainly includes dimensional inspection, visual inspection of appearance and surface, chemical composition analysis and mechanical performance test. It is also necessary to carry out non-destructive testing for castings which are more important or have problems in casting process and can be used for ductile iron Non-destructive testing of quality testing technology, including liquid penetration testing, magnetic particle testing, eddy current testing, ray detection, ultrasonic testing and vibration detection.
First, the casting surface and near surface defects detection
1, liquid penetration testing
Liquid penetration testing is used to check the various imperfections on the surface of a casting, such as surface cracks, surface pinholes, and the like. Commonly used penetration testing is coloring detection, it is a high permeability of the colored (usually red) liquid (penetrant) soaked or sprayed on the casting surface, penetration agent into the opening defects inside, quickly wipe the surface permeate Layer, and then easy to dry display agent (also known as imaging agent) sprayed onto the casting surface, to be left in the opening defects in the penetrant suction, the display agent was dyed, which can reflect the shape of the defect, Size and distribution.
2, eddy current testing
Eddy current testing is suitable for inspection of the surface below the general is not greater than 6 ~ 7MM deep defects. Eddy current detection points placed coil method and through the coil method 2 kinds. When the specimen is placed in the vicinity of the coil with alternating current, the alternating magnetic field entering the specimen can induce the current (eddy current) which is perpendicular to the excitation magnetic field in the specimen, and the eddy current Generating a magnetic field opposite to the direction of the excitation field, causing the primary magnetic field in the coil to be partially reduced, thereby causing a change in the coil impedance. If the surface of the casting is defective, the electrical characteristics of the eddy current will be distorted to detect the presence of defects. The main drawback of eddy current testing is that the detected defect size and shape can not be visually displayed. Generally, only the surface position and depth , In addition to the workpiece on the surface of the small opening defects detection sensitivity than penetration testing.
3, magnetic particle testing
Magnetic particle testing is suitable for the detection of surface defects and depth of several millimeters below the surface of the defects, it requires DC (or AC) magnetization equipment and magnetic powder (or magnetic suspension) to carry out detection operations. The magnetization device is used to create a magnetic field on the inner and outer surfaces of the casting, magnetic powder or magnetic suspension 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 leakage magnetic field. When the magnetic powder or suspension is sprinkled, the magnetic powder is sucked, so that a defect can be exhibited. This shows that the defects are basically transverse to the magnetic field lines defects, parallel to the magnetic field lines for the long-type defects are not shown, for this time, the operation need to constantly change the direction of magnetization to ensure that the unknown direction to check the various defects The
Second, the detection of internal defects in the casting
For internal defects, commonly used non-destructive testing methods are ray detection and ultrasonic testing. Which is the best radiation detection effect, it can be reflected to reflect the internal defects of the type, shape, size and distribution of the visual image, but for large thickness of large castings, ultrasonic testing is very effective, you can more accurately detect the location of internal defects , Equivalent size and distribution.
1, ray detection (micro-focus XRAY)
Ray detection, generally used X-ray or γ-ray as a radiation source, so the need for radiation equipment and other ancillary facilities, when the workpiece placed in the radiation field irradiation, the radiation intensity of the radiation will be affected by the internal defects of the casting. The intensity of the radiation emitted through the casting varies with the size and nature of the defect, resulting in a defective radiographic image, which is recorded by radiographic film, or detected in real time by a phosphor screen, or detected by a radiation counter. The method of recording by radiography is the most commonly used method, which is commonly referred to as radiographic detection. The defective images reflected by radiography are intuitive, and the defect shape, size, number, plane position and distribution range are Can be presented, but the depth of defects generally can not be reflected, the need to take special measures and calculations to determine.
2, ultrasonic testing
Ultrasonic testing can also be used to check for internal defects by using a sound beam with high frequency acoustic energy to propagate inside the casting, causing reflections when the internal surface or defect is encountered and finding defects. The magnitude of the reflected acoustic energy is a function of the directivity and nature of the inner surface or defect and the acoustic impedance of the reflector, so that the presence of the defect, the wall thickness or the surface can be detected by applying various acoustic or acoustic energy The depth of the defect. Ultrasonic testing as a widely used non-destructive testing means, the main advantages of performance: detection sensitivity, can detect small cracks; with a large penetration ability, can detect thick section castings. The main limitation is that it is difficult to explain the reflection waveforms with complex contours and poor disconnections. For unsatisfactory internal structures such as grain size, microstructure, porosity, inclusions or fine dispersion Precipitates, etc., also hinder the interpretation of the waveform; In addition, the need to refer to the standard test block test.