Application of ultrasonic flaw detection

Application of ultrasonic flaw detection in steel gate detection

Steel gates are widely used in water conservancy projects, mainly welded by high-quality steel plates. The surface is made of rubber to stop water and prevent corrosion, and the surface is sandblasted to remove rust and hot zinc, which is widely used in water conservancy projects such as hydropower stations, reservoirs, irrigation and drainage, rivers, environmental protection, sewage treatment and aquaculture. 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. Ultrasonic flaw detection, as one of nondestructive testing methods, is based on the application of ultrasonic instruments or equipment without destroying the machined surface, which can not only detect the internal defects of the workpiece that can not be inspected by naked eyes, but also greatly improve the accuracy and reliability of detection. 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 inside 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 it has been widely used. 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- 95 Code for Construction and Acceptance of Steel Structure Engineering. The standard stipulates that 100% ultrasonic flaw detection is required in the specification when the welding quality grade of the weld is required to be Grade I and the evaluation grade is Grade II; 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 not necessary to carry out ultrasonic internal defect inspection. It is worth noting that ultrasonic flaw detection is used for full penetration welds, and the flaw detection ratio is calculated as a percentage of the length of each weld, and it should not be less than 200 mm. If local flaw detection is found in the weld, the flaw detection length should be increased at the extension parts at both ends of the defect, and the increased length should not be less than 10% and 200 mm. When there are still unacceptable defects, the weld should be 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: spatter, 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 thickness of the base metal of the workpiece to be tested is 10mm, then both sides of the weld should be polished 100 mm. 2) The selection of coupling agent should consider viscosity, fluidity, adhesion, no corrosion to the surface of the workpiece, easy cleaning and economy. Based on the above factors, paste is selected as the coupling agent. 3) Due to the thin thickness of the parent material, 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 1 1345- 89 "Manual ultrasonic flaw detection method and flaw detection result classification of steel welds" to judge whether the 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. The nature of internal defects, the causes of defects and the evaluation of preventive measures are as follows: 1) blowhole. The echo height of a single air hole is low, and the waveform is single slit, 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 is not 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 the echo signal of dotted blowhole, and it is strip-shaped. The echo signals of slag are mostly serrated, with low amplitude, and the waveform is mostly dendritic, with a small peak on one side of the main peak. The translation amplitude of the probe changes, and the reflection amplitude is different when detecting from 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.