There is practically no industrywherever welding work is carried out. The vast majority of steel structures are mounted and interconnected by means of welding seams. Of course, not only the reliability of the building, structure, car or any assembly, but also the safety of people who will somehow interact with these structures depends on the quality of such work. Therefore, to ensure the proper level of performance of such operations, ultrasonic testing of welding seams is used, thanks to which it is possible to detect the presence or absence of various defects at the junction of metal products. About this advanced method of control and will be discussed in our article.
Ultrasonic flaw detection as such wasdeveloped in the 30s. However, the first really working device was born only in 1945 thanks to the company Sperry Products. Over the next two decades, the latest control technology has gained worldwide recognition, and the number of manufacturers of such equipment has increased dramatically.
Ultrasonic flaw detector whose price is onToday begins from 100,000-130,000 thousand rubles, initially it was based on vacuum tubes. Such devices differed bulkiness and great weight. They worked exclusively on AC power supplies. But already in the 60s, with the advent of semiconductor circuits, flaw detectors significantly decreased in size and were able to work on batteries, which made it possible in the end to use devices even in field conditions.
In the early stages of the described devices usedanalog signal processing, due to which, like many other similar devices, were subject to drift at the time of calibration. But already in 1984, Panametrics gave a start to life to the first portable digital flaw detector, called EPOCH 2002. Since then, digital units have become highly reliable equipment that ideally provides the necessary stability of calibration and measurement. The ultrasonic flaw detector, the price of which directly depends on its technical characteristics and the manufacturer’s brand, also received the data recording function and the ability to transfer readings to a personal computer.
In modern conditions, more and moreOf interest are systems with phased arrays that use sophisticated technology based on multi-element piezoelectric elements that generate directional rays and create transverse images similar to medical ultrasound imaging.
Ultrasonic control method is used in anyindustry direction. Its application has shown that it can be equally effectively used to test almost all types of welded joints in construction, which have a base metal of more than 4 millimeters thick. In addition, the method is actively used to check the connection of the joints of gas and oil pipelines, various hydraulic and water supply systems. And in such cases, as the control of thick seams obtained as a result of electroslag welding, ultrasonic flaw detection is the only acceptable method of control.
The final decision on whether a part or a welding seam is suitable for operation is made on the basis of three fundamental indicators (criteria) - amplitude, coordinates, conditional dimensions.
In general, ultrasound testing is the method that is most fruitful from the point of view of imaging in the process of studying the seam (detail).
Описываемый метод контроля с применением Ultrasound is good because it has a much higher sensitivity and reliability of indications in the process of detecting defects in the form of cracks, lower cost and high safety in the process of use compared with the classical methods of radiographic control. To date, ultrasonic testing of welded joints is used in 70-80% of inspections.
Without the use of these devices, non-destructive ultrasonic testing is simply unthinkable. Devices are used to form the excitation, as well as receiving ultrasonic vibrations.
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In the world of technology and industry, everything is headedGOST. Ultrasonic control (GOST 14782-86) in this matter is also no exception. The standard specifies that defects are measured by the following parameters:
Non-destructive testing, which isultrasonic, has its own method of use, which states that the main measured parameter is the amplitude of the echo signal obtained directly from the defect. To differentiate the echo signals by the magnitude of the amplitude, the so-called rejection level of sensitivity is fixed. It, in turn, is configured using a standard enterprise sample (SOP).
The start of operation of the flaw detector is accompanied byits setting. For this expose defective sensitivity. After that, in the course of the ultrasound studies carried out, the obtained echo signal from the detected defect is compared with the recorded rejection level. If the measured amplitude exceeds the rejection level, the specialists decide that such a defect is unacceptable. Then the seam or product is rejected and sent back for revision.
Most common defectsthe welded surfaces are: incomplete penetration, incomplete penetration, cracking, porosity, slag inclusions. It is these violations effectively detects the use of ultrasound.
Over time, the verification process gotseveral effective methods for studying welds. Ultrasonic control provides for a fairly large number of options for acoustic research of the metal structures in question, however, the most popular were:
Most often in industry and railwaytransport applies the echo-pulse method. It is thanks to him that more than 90% of all defects are diagnosed, which becomes possible due to the registration and analysis of almost all signals reflected from the surface of the defect.
By itself, this method is based on the sounding of a metal product by pulses of ultrasonic vibrations with their subsequent registration.
Advantages of the method are:
- the possibility of one-way access to the product;
- quite high sensitivity to internal defects;
- the highest accuracy of determining the coordinates of the detected defect.
However, there are also disadvantages, including:
- low resistance to interference from surface reflectors;
- a strong dependence of the signal amplitude on the location of the defect.
The described flaw means asending ultrasound pulses into the product. Reception of the response signal occurs by him or by the second searcher. In this case, the signal can be reflected both directly from defects and from the opposite surface of the part, product (seam).
It is based on a detailed analysis of the amplitude.ultrasonic vibrations transmitted from the emitter to the receiver. In the case when this indicator decreases, this indicates the presence of a defect. Moreover, the larger the size of the defect itself, the smaller the amplitude of the signal received by the receiver. To obtain reliable information, the emitter and receiver should be located coaxially on opposite sides of the object under study. The disadvantages of this technology can be considered low sensitivity in comparison with the echo method and the complexity of the orientation of the probe (piezoelectric transducers) relative to the central rays of the radiation pattern. However, there are advantages that consist in high immunity to interference, low dependence of the signal amplitude on the location of the defect, and the absence of a dead zone.
This ultrasonic quality control is more common.In total it is used to control the welded joints of reinforcement. The main sign that a defect is detected is the weakening of the amplitude of the signal, which is reflected from the opposite surface (most often it is called bottom). The main advantage of the method is a clear detection of various defects, the dislocation of which is the root of the seam. Also, the method is characterized by the possibility of unilateral access to the seam or part.
The most effective detection option is verticallocated defects. Inspection is carried out using two transducers, which move along the surface near the seam on one side of it. At the same time, their movement is produced in such a way as to fix one PEP signal emitted from another PEP and twice reflected from an existing defect.
Главное преимущество метода:it can be used to estimate the shape of defects whose magnitude exceeds 3 mm and which deviate in the vertical plane by more than 10 degrees. The most important thing is to use the probe with the same sensitivity. This option of ultrasound is actively used to test thick-walled products and their welds.
Specified ultrasonic inspection of weldsuses ultrasonic energy reradiated by a defect. The transverse wave, which falls on the defect, is partially reflected, partially transformed into a longitudinal wave, and also re-radiates the diffracted wave. As a result, the required PEP waves are captured. The disadvantage of the method is the cleaning of the seam, rather high complexity of decoding the received signals while monitoring welded joints up to 15 millimeters thick.
Investigation of welded joints using soundhigh frequency is, in fact, non-destructive testing, because such a method is not capable of causing any damage to the investigated area of the product, but it rather accurately determines the presence of defects. Also of particular note is the low cost of the work and their high speed of execution. It is also important that the method is absolutely safe for human health. All studies of metals and welds based on ultrasound are conducted in the range from 0.5 MHz to 10 MHz. In some cases, it is possible to work using ultrasonic waves having a frequency of 20 MHz.
Weld analysis byultrasound must necessarily be accompanied by a whole set of preparatory measures, such as cleaning the weld under test or the surface, applying specific contact liquids (special gels, glycerin, machine oil) to the controlled area. All this is done to ensure proper stable acoustic contact, which ultimately provides the necessary image on the instrument.
Ultrasonic control is absolutely irrationalused for inspection of welding compounds of metals with coarse-grained structure (for example, cast iron or austenitic weld with a thickness of more than 60 millimeters). And all because in such cases there is a sufficiently large scattering and strong attenuation of ultrasound.
It is also not possible to unambiguously fully characterize the defect found (tungsten inclusion, slag inclusion, etc.).
