Ultrasonic flaw detection function: a method to check the defects of parts. Main advantages: strong penetration ability and detection depth of several meters. Main Disadvantages: Experienced personnel are required to operate the instrument carefully. Brief introduction: basic principle, main features, main advantages, main disadvantages, band introduction, instrument introduction, application, steel gate detection, defect prevention measures, advantages, limitations and disadvantages, and basic principle of ultrasonic wave in medium. Longitudinal wave can be used to detect defects such as inclusions, cracks, shrinkage cavities, white spots and delamination in metal ingots, blanks, plates, large forgings and simple parts. Shear waves can be used to detect circumferential and axial cracks, scratches, pores, slag inclusions, cracks, incomplete penetration and other defects in pipelines; Surface defects of simple castings can be detected by surface waves. Defects in thin plates can be detected by plate waves. The main feature of ultrasonic flaw detector is a portable industrial nondestructive testing instrument, which can detect, locate, evaluate and diagnose various defects (cracks, inclusions, folds, pores, sand holes, etc.). ) quickly, conveniently and accurately installed on the workpiece. It can be used in laboratory and engineering fields. The instrument can be widely used in manufacturing, steel metallurgy, metal processing, chemical industry and other fields that need defect detection and quality control, and can also be widely used in in-service safety detection and life evaluation in aerospace, railway transportation, boilers and pressure vessels and other fields. This is necessary for nondestructive testing industry. (1) When the ultrasonic wave propagates in the medium, it has the characteristics of reflection on different interfaces. If a defect is encountered, and the size of the defect is equal to or greater than the wavelength of the ultrasonic wave, the ultrasonic wave will reflect back on the defect, and the flaw detector can display the reflected wave. If the size of the defect is even smaller than the wavelength, the sound wave will bypass the defect and cannot be reflected; (2) The directivity of wave sound is good, and the higher the frequency, the better the directivity. It radiates into the medium with a narrow beam, and it is easy to locate the defect. (3) The propagation energy of ultrasonic wave is relatively large, for example, the propagation energy of ultrasonic wave with frequency of1MHz is equivalent to that of ultrasonic wave with the same amplitude and frequency of1000 Hz. The main advantages are: 1) strong penetration ability, and the detection depth can reach several meters; (2) high sensitivity, and it can be found that the reflection ability is equivalent to an air gap with a diameter of about several tenths of a millimeter; The size of detectable defects can generally be considered as 1/2 of wavelength. ③ It is more accurate in determining the orientation, size and shape of the internal reflector; ④ The inspected object can only be approached from one side; ⑤ The defect inspection results can be provided immediately; ⑥ Safe operation and light equipment. The main disadvantage is (1) careful operation by experienced personnel; ② It is difficult to check rough, irregular, small, thin or uneven materials; It is still difficult for ultrasonic flaw detection vehicles to characterize the defects found accurately qualitatively and quantitatively. ④ Not suitable for structures with holes; ⑤ Unless photographs are taken, there are generally few traceable materials left. The human ear can feel elastic waves with frequencies higher than 20 Hz and lower than 20000Hz, so the elastic waves in this frequency range are also called sound waves. Elastic waves with frequencies below 20 Hz are also called infrasound waves, and elastic waves with frequencies above 20000Hz are called ultrasonic waves. Infrasound waves and ultrasonic waves are imperceptible to human ears. A nondestructive testing method for detecting internal defects of materials by using the influence of acoustic performance differences between materials and their defects on ultrasonic propagation. Ultrasonic pulse reflection method is widely used to observe the reflection of sound pulses in materials, and there are also penetration methods to observe the amplitude changes of incident sound waves after passing through materials. The common frequency is between 0.5 and 5 MHz. The commonly used testing instrument is the A-type display pulse reflection ultrasonic flaw detector. According to the presence or absence of the reflected signal on the oscilloscope screen of the instrument, the time between the reflected signal and the incident signal and the height of the reflected signal, the presence or absence, position and relative size of the reflecting surface can be determined. The basic structure and principle of the instrument are shown in figure 1. The instrument introduces B-type and C-type flaw detectors developed on the basis of A-type flaw detectors, which can obtain the signals of reflecting surfaces in different directions, and can also combine B-type and C-type display to obtain the three-dimensional display diagram of internal reflecting surfaces of materials. The above flaw detectors all use pulsed electrical signals to excite piezoelectric transducers to emit ultrasonic waves, but eddy current acoustic transducers can also be used to detect conductive materials. The energy conversion process of this transducer is carried out on the surface of the tested part, which can detect metal materials with rough surface and temperature above 500℃ without contacting with materials or coupling agents, and is widely used in metallurgical industry. When ultrasonic wave propagates in materials, its intensity will be attenuated by absorption and scattering. Therefore, by measuring the attenuation in alloy materials such as vacuum consumable furnace, the uniformity of material structure can be understood without loss. Compared with other nondestructive testing methods, pulse reflection ultrasonic method is widely used in the detection of steel gates in water conservancy projects. Mainly welded by high-quality steel plates. The surface is sealed with rubber, and rust is removed by sandblasting and zinc spraying. Widely used in hydropower stations, reservoirs, irrigation and drainage, rivers, environmental protection, sewage treatment, aquaculture and other water conservancy projects. The welding quality of steel gate is directly related to the safety of people's lives and property downstream of the gate, so the welding quality and welding detection method of steel gate are very important. As one of the nondestructive testing methods, ultrasonic flaw detection can not only detect the internal defects of the workpiece that can not be detected by naked eyes, but also greatly improve the accuracy and reliability of detection by using ultrasonic instruments or equipment without damaging the machined surface of the workpiece.
Ultrasonic wave is a kind of mechanical wave, its frequency is very high, the frequency ratio exceeds 20 kHz, and its energy is much greater than that of audible sound wave with the same amplitude, so it has strong penetration ability. Ultrasonic waves for flaw detection have a frequency of 0.4- 25 MHz, of which 1- 5 MHz is the most commonly used. Ultrasonic flaw detection can quickly, conveniently, non-destructively and accurately detect and locate all kinds of defects in the workpiece, and it has the characteristics of long detection distance, small size, light weight, easy to carry to the site for flaw detection, fast detection speed, only consuming coupling agent and wearing probe, and low total detection cost, so its application is more and more extensive.
Using ultrasonic flaw detection, there are mainly two ways: penetrating flaw detection and reflection flaw detection. Penetrant inspection uses two probes, one emitting ultrasonic waves and the other receiving ultrasonic waves. When testing, two probes are placed on both sides of the workpiece, and the internal quality of the workpiece is judged according to the energy change after the ultrasonic wave penetrates the workpiece. The high-frequency pulse excitation signal generated by the reflection flaw detection high-frequency generator acts on the probe, and the generated wave propagates to the workpiece. If there are defects in the workpiece, some waves will be reflected back as defect waves, and the rest of the emitted waves will also be reflected back as bottom waves. According to the positions of emission wave, defect wave and bottom wave relative to the scanning baseline, the defect position can be determined. According to the amplitude of defect wave, the size of defect can be determined; According to the defect waveform, the properties of defects can be analyzed. If there are no defects inside the workpiece, there are only emission waves and bottom waves.
In the process of flaw detection, we must first understand the technical requirements of drawings for welding quality. The acceptance standard of steel structure shall be in accordance with GB50205- 200 1 Code for Acceptance of Construction Quality of Steel Structure Engineering. The standard stipulates that: when the welding quality grade of the weld is required to be Grade I in the drawing and the evaluation grade is Grade II, the specification requires 100% ultrasonic flaw detection; For drawings, when the welding quality grade of the weld is Grade II and the evaluation grade is Grade III, the specification requires 20% ultrasonic flaw detection; When the welding quality grade of the weld is Grade III as required by the drawing, it is unnecessary to carry out ultrasonic inspection of internal defects.
It is worth noting that ultrasonic flaw detection is used for full penetration welds, and its flaw detection ratio is calculated as a percentage of the length of each weld, and it is not less than 200 mm For welds with local flaw detection
If an impermissible defect is found, the flaw detection length should be increased at the extension of both ends of the defect, and the increased length should not be less than 10% and 200mm of the weld length. When there are still impermissible defects, the welds shall be 100% inspected. In addition, it is necessary to know the thickness of base metal, joint type and groove type of the workpiece to be measured. Generally, the thickness of parent metal is between 8- 16mm, and the groove types are I-type, single V-type and X-type. The preparation before flaw detection can only be carried out if the above things are clearly understood.
Before each flaw detection operation, the comprehensive performance and panel curve of the instrument must be calibrated with standard test blocks (CSK- IA, CSK-Ⅲ A) to ensure the accuracy of flaw detection results.
(1) Inspection surface finishing: Splash, scale, pits and rust on the welding working surface should be removed, and the smoothness is generally lower than ▽4. Generally, the trimming width of flaw detection surfaces on both sides of weld is 2KT+50mm (k: probe k value, t: workpiece thickness). Generally, a probe with a K value of 2.5 is selected according to the parent metal of the weldment. For example, if the parent metal thickness of the workpiece to be tested is 10mm, then both sides of the weld should be polished100 mm.
(2) The selection of coupling agent should consider viscosity, fluidity, adhesion, no corrosion to the workpiece surface, easy cleaning and economy. Based on the above factors, paste was selected as coupling agent.
(3) Because the thickness of the substrate is relatively thin, the detection direction is single-sided and double-sided.
(4) Because the thickness of the plate is less than 20mm, the scanning speed of the instrument is adjusted by horizontal positioning.
(5) In the process of flaw detection, coarse flaw detection and fine flaw detection are adopted. In order to roughly understand the existence and distribution of defects, quantification and location are fine flaw detection. Several scanning methods, such as zigzag scanning, left-right scanning, front-back scanning, corner scanning and surrounding scanning, are used to find various defects and judge their properties.
(6) Record the test results and evaluate and analyze the internal defects found. The classification of internal defects of welded butt joints shall conform to the current national standard GB/T11345-2013 "Ultrasonic Testing Technology, Testing Grade and Nondestructive Testing and Evaluation of Welds" to judge whether welding is qualified or not. If defects exceeding the standard are found, a rectification notice shall be issued to the workshop, and re-inspection shall be conducted after rectification until they are qualified. The common defects of general welds are porosity, slag inclusion, incomplete penetration, incomplete fusion and cracks. Up to now, there is not a mature method to accurately judge the nature of defects, and the defects can only be comprehensively judged according to the shape of defect wave and the change of reflected wave height obtained on the fluorescent screen, combined with the position of defects and welding process. Defect prevention measures 1) blowhole. The echo height of a single pore is low, and the waveform is single peak, which is relatively stable. From all directions, the reflected waves are almost the same, but the probe disappears when it moves slightly, and a cluster of reflected waves will appear in dense pores. The wave height varies with the pore size, and will appear one after another when the probe rotates at a fixed point. The main reasons for this kind of defects are: the welding material is not dried according to the specified temperature, the covered covered electrode coating is deteriorated and peeled off, the welding core is corroded, the welding wire is not cleaned cleanly, the current is too large and the arc is too long during manual welding. The measures to prevent such defects are: covered electrode with cracked coating, peeling, deterioration and core corrosion should not be used, and rusty welding wire must be derusted before use. The welding materials used shall be dried at the specified temperature, the groove and its two sides shall be cleaned, and appropriate welding current, arc voltage and welding speed shall be selected.
2) Slag inclusion. The echo signal of dotted slag inclusion is similar to that of dotted blowhole. The echo signals of strip slag inclusion are mostly serrated, with low amplitude and dendritic waveform. There is a small peak on one side of the main peak, and the translation amplitude of the probe changes, and the reflection amplitude is different when detecting in all directions. The reasons for this kind of defects are: the welding current is too small, the welding speed is too fast, the slag can't float, the welding edge and all layers of welds are not cleaned cleanly, the chemical composition of the base metal and welding materials is improper, and there are more sulfur and phosphorus. The preventive measures are as follows: choose the welding current correctly, the groove angle of the welding part should not be too small, the groove must be cleaned before welding, and the welding slag must be removed layer by layer during multi-layer welding; Reasonable selection of welding speed of steel bar conveying angle, etc.
3) Incomplete penetration. High reflectivity and high amplitude. When the probe is translated, the waveform is stable, and almost the same reflection amplitude can be obtained when detecting both sides of the weld. The general reasons are: the pure edge gap of groove is too small, the welding current is too small or the rod feeding speed is too fast, the groove angle is small, the rod feeding angle is wrong, and the arc is blown off. Preventive measures include: reasonable selection of groove type, assembly clearance and correct welding process.
4) Not fused. The waveform is stable when the probe is translated, and the reflection amplitude is different when the two sides are detected, and sometimes it can only be detected from one side. The reasons are: the groove is not clean, the welding speed is too fast, the current is too small or too large, the covered electrode angle is wrong, and the arc is biased. Preventive measures: correct selection of groove and current, cleaning groove, correct operation to prevent welding deviation, etc.
5) cracks. The echo is high in height and wide in amplitude, and there will be multiple peaks. When the probe moves, the amplitude of the reflected wave changes constantly, and when the probe rotates, the peak moves up and down. The causes of thermal cracks are: the cooling rate of molten pool is very fast during welding, resulting in segregation; Uneven heating of weld will produce tensile stress. Preventive measures: limit the contents of easily segregating elements and harmful impurities in base metal and welding materials, mainly limit the sulfur content and improve the manganese content; Improve the alkalinity of covered electrode or flux to reduce impurity content and improve segregation degree; Improve the welding structure and adopt reasonable welding sequence to improve the freedom of weld shrinkage. Advantages: (1) The ultrasonic beam can be concentrated in a specific direction, propagate in a straight line in the medium, and has good directivity; (2) Ultrasonic waves will be attenuated and scattered when propagating in the medium; (3) Ultrasonic wave will be reflected, refracted and mode converted at the interface of inhomogeneous medium. Using these characteristics, the reflected wave reflected by the defect interface can be obtained, so as to achieve the purpose of detecting defects; (4) The energy of ultrasonic wave is much greater than that of sound wave; (5) The propagation loss of ultrasonic wave in solid is small, and the detection depth is large. Ultrasonic waves can be reflected and refracted on heterogeneous interfaces, especially on the interface between gas and solid. If there are defects such as pores, cracks and delamination in the metal (gas in the defect) or slag inclusion (foreign medium in the defect), the ultrasonic wave will be totally or partially reflected when it propagates to the interface between the metal and the defect. The reflected ultrasonic waves are received by the probe and processed by the internal circuit of the instrument, and waveforms with different heights and certain intervals are displayed on the fluorescent screen of the instrument. The flaw detector judges the depth, size and type of defects in the workpiece according to the changing characteristics of waveform. Limited ultrasonic flaw detection has the advantages of large detection thickness, high sensitivity, high speed, low cost and no harm to human body, and can locate and quantify defects. However, ultrasonic flaw detection has its limitations due to the non-intuitive display of defects, difficult flaw detection technology, easy to be influenced by subjective and objective factors, and inconvenient storage of flaw detection results. What are the ultrasonic testing methods? There are usually penetration method, pulse reflection method, series method, penetrating reflection method and so on. What are the applications of ultrasonic testing? Detecting steel pipes and forgings by water immersion (water spraying) method; Single (double) probe for detecting weld; Multi-probe inspection of large pipelines: ultrasonic inspection of plates; ultrasonic inspection of composite materials; testing of nonmetallic materials, etc.