Machining refers to workpieces processed using mechanical processing techniques, such as sheet metal processing: shearing, punching, folding, pressing, and bending. Or cutting processing: turning, milling, planing, grinding, drilling, sheet metal processing does not change the thickness of the material, cutting removes material and changes the thickness of the workpiece. Mainly refers to the processing method in which no chemical reaction occurs (or the reaction is very small). If it is to be detected for internal defects, a flaw detector needs to be used.
Flaw detectors can be divided into: ultrasonic flaw detectors, magnetic particle flaw detectors, eddy current flaw detectors, radiographic flaw detectors and fluorescence flaw detectors based on different measurement principles. They are mainly used to detect internal defects (cracks) in machined parts. , blisters, pores, white spots, inclusions, etc.), check whether the weld is qualified, and look for hidden damage to determine whether the workpiece is qualified or not. Flaw detectors can be divided into: ultrasonic flaw detectors, magnetic particle flaw detectors, eddy current flaw detectors, radiographic flaw detectors and fluorescence flaw detectors based on different measurement principles. Among them, magnetic particle flaw detectors, eddy current flaw detectors, and radiographic flaw detectors mainly detect defects near the surface of the workpiece. It is large and inconvenient to carry, and its rays pollute the environment. With the development of science and technology, ultrasonic flaw detectors are more and more widely used. They are small in size, light in weight, easy to operate, and have strong practicability. High-end development will definitely happen in the future. There is a detection method that uses scanning images instead of sound waveforms, which is similar to B-ultrasound, but it is expensive.
Flaw detectors have a wide range of applications, such as using ultrasonic reflection to measure distances, using high-power ultrasonic vibration to remove scale attached to boilers, and using high-energy ultrasound to make an "ultrasonic knife" to eliminate it. , crushing cancer, stones, etc. in the human body. Flaw detectors that use the reflection effects of ultrasound and the characteristics of strong penetration and linear propagation are also one of the big application fields. The detection applications of flaw detectors mainly include the detection of various materials in industry and the detection and diagnosis of human body in medical treatment. Through it, people can detect whether there are bubbles, scars, cracks and other defects in metal and other industrial materials, and can detect Whether the soft tissues and blood flow of people's bodies are normal.
The five conventional methods refer to radiographic flaw detection, ultrasonic flaw detection, magnetic particle flaw detection, eddy current flaw detection and penetrant flaw detection.
1. Radiographic flaw detection method
Raygraphic flaw detection is a method that uses the penetrability and linearity of rays to detect flaws. Although these rays cannot be directly detected by the naked eye like visible light, they can sensitize photographic films and can also be received by special receivers. Rays commonly used for flaw detection include x-rays and gamma rays emitted by isotopes, which are called x-ray flaw detection and gamma ray flaw detection respectively. When these rays pass through (irradiate) a material, the greater the density of the material, the more the intensity of the rays is weakened, that is, the less intense the rays can penetrate through the material. At this time, if a photographic film is used to receive the signal, the light sensitivity of the film will be small; if an instrument is used to receive the signal, the signal obtained will be weak. Therefore, when using rays to irradiate the parts to be inspected, if there are defects such as pores and slag inclusions inside, the density of the material passed by the rays passing through the defective path is much smaller than that of the path without defects, and its intensity will be The less it is weakened, that is, the transmitted intensity is greater. If a film is used to receive it, the light sensitivity will be greater, and the plane projection of the defect perpendicular to the direction of the ray can be reflected on the film; the same can be done if other receivers are used. Instruments are used to reflect the plane projection of the defect perpendicular to the ray direction and the amount of ray transmission. It can be seen that under normal circumstances, it is not easy to find cracks by radiographic inspection, or in other words, radiographic inspection is not sensitive to cracks. Therefore, radiographic flaw detection is most sensitive to volumetric defects such as pores, slag inclusions, and incomplete welding. That is to say, radiographic flaw detection is suitable for volumetric flaw detection, but not suitable for area flaw detection.
2. Ultrasonic flaw detection method
The frequency range of sound waves that people’s ears can directly receive is usually 20Hz to 20kHz, that is, sound (sound) frequency. Frequencies below 20 Hz are called infrasound waves, and frequencies above 20 kHz are called ultrasonic waves. Several megahertz ultrasonic waves are commonly used in industry for flaw detection. Ultrasonic waves with high frequency have strong linear propagation, are easy to propagate in solids, and are easy to reflect when encountering the interface formed by two different media, so it can be used for flaw detection. Usually, an ultrasonic probe is used to make good contact with the surface of the workpiece to be probed. The probe can effectively emit ultrasonic waves to the workpiece, and can receive the ultrasonic waves reflected from the (defect) interface, and at the same time convert them into electrical signals, and then transmit them to the instrument for processing. According to the speed of ultrasonic wave propagation in the medium (often called the speed of sound) and the propagation time, the location of the defect can be known.
When the defect is larger, the reflective surface is larger, and the energy it reflects is greater. Therefore, the size of each defect (equivalent) can be determined based on the size of the reflected energy. Commonly used waveforms for flaw detection include longitudinal waves, transverse waves, surface waves, etc. The first two are suitable for detecting internal defects, while the latter are suitable for detecting surface defects, but have high requirements on surface conditions.
3. Magnetic particle inspection method
Magnetic particle inspection is a magnetic inspection method based on the principle of magnetic flux leakage. When magnetic lines of force pass through ferromagnetic materials and their products, a leakage magnetic field will be generated at their (magnetic) discontinuities to form magnetic poles. At this time, sprinkle dry magnetic powder or pour magnetic suspension, and the magnetic poles will absorb the magnetic powder, producing obvious magnetic marks that can be directly observed with the naked eye. Therefore, the magnetic traces can be used to reveal defects in ferromagnetic materials and their products. Magnetic particle inspection can detect tiny defects on the exposed surface that cannot be directly observed with the naked eye or with the help of a magnifying glass. It can also detect near-surface defects that are not exposed on the surface but are buried a few millimeters below the surface. Although this method can also detect volume defects such as pores, inclusions, and incomplete welding, it is more sensitive to area defects and is more suitable for inspecting defects caused by quenching, rolling, forging, casting, welding, electroplating, grinding, and fatigue. cracks caused by etc.
There are many ways to display defects in magnetic flaw detection, some with magnetic particle display, and some without magnetic particle display. The use of magnetic particle display is called magnetic particle flaw detection. It is one of the most commonly used methods because of its intuitive display, simple operation, and people are happy to use it. Those that do not use magnetic particle inspection are customarily called magnetic flux leakage inspection. It often uses induction coils, magnetic tubes, Hall elements, etc. to reflect defects. It is more hygienic than magnetic particle inspection, but not as intuitive as the former. Since current magnetic flaw detection mainly uses magnetic particles to reveal defects, people sometimes refer to magnetic particle flaw detection directly as magnetic flaw detection, and its equipment is called magnetic flaw detection equipment.
4. Eddy current flaw detection method
Eddy current flaw detection is an alternating magnetic field generated by an alternating current that acts on the conductive material to be detected, inducing eddy currents. If there are defects in the material, it will interfere with the generated eddy currents, creating an interference signal. The status of the defect can be known by detecting its interference signal with an eddy current flaw detector. There are many factors that affect eddy currents, which means that eddy currents carry a wealth of signals. These signals are related to many factors of the material. How to separate the useful signals from the many signals one by one is a difficult problem for current eddy current researchers. , some progress has been made over the years, and some problems can be solved under certain conditions, but it is still far from meeting the requirements of the site and needs to be vigorously developed.
5. Penetrant flaw detection method
Penetrant flaw detection is a method of flaw detection that uses capillary phenomena. For parts with smooth and clean surfaces, use a colored (usually red) or fluorescent, highly permeable liquid to coat the surface of the parts to be tested. If there are microcracks on the surface that cannot be directly detected by the naked eye, due to the strong permeability of the liquid, it will penetrate along the cracks to its roots. Then wash away the penetrating liquid on the surface, and then apply a display liquid with a higher contrast (usually white). After leaving it for a while, because the crack is very narrow and the capillary phenomenon is significant, the penetrant that originally penetrated into the crack will rise to the surface and spread, showing a thick red line on the white substrate, thus showing that the crack is exposed on the surface. The shape is therefore often called colored flaw detection. If the penetrant is a fluorescent liquid, the liquid that rises to the surface due to capillary action will fluoresce under the irradiation of ultraviolet light, which can better show the shape of the crack exposed on the surface. Therefore, the penetrating liquid at this time is often regarded as Flaw detection is directly called fluorescence flaw detection. This flaw detection method can also be used to detect flaw on metal and non-metal surfaces. The flaw detection liquid used has a strong smell and is often toxic.