There is practically no industry wherever welding is carried out. The vast majority of metal structures are mounted and interconnected by means of welds. Of course, in the future, not only the reliability of the building, structure, machine or any unit that is being built depends on the quality of this kind of work, but also the safety of people who will somehow interact with these structures. Therefore, to ensure the proper level of performance of such operations, ultrasonic testing of welds is used, due to which it is possible to detect the presence or absence of various defects at the junction of metal products. This advanced control method will be discussed in our article.
History of occurrence
Ultrasonic flaw detection as such was developed 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, the number of manufacturers of such equipment has increased dramatically.
An ultrasonic flaw detector, the price of which starts today from 100,000-130,000 thousand rubles, initially initially contained vacuum tubes. Such devices were bulky and heavy. They worked exclusively from AC power sources. But already in the 60s, with the advent of semiconductor circuits, flaw detectors significantly decreased in size and got the opportunity to work on batteries, which made it possible to use the devices even in the field.
Step into Digital Reality
In the early stages, the described devices used analog signal processing, due to which, like many other similar devices, they were subject to drift at the time of calibration. But already in 1984, Panametrics gave a start to the life of the first portable digital flaw detector called EPOCH 2002. Since then, digital units have become highly reliable equipment, ideally providing the necessary stability of calibration and measurements. An ultrasonic flaw detector, the price of which directly depends on its technical characteristics and the brand of the manufacturer, also received a data recording function and the ability to transmit readings to a personal computer.
In modern conditions, phased array systems that use sophisticated technology based on multi-element piezoelectric elements that generate directional rays and create transverse images similar to medical ultrasound imaging are becoming more and more interesting.
Scope of application
The ultrasonic control method is applied in any direction of the industry. Its application showed that it can be equally effectively used to test almost all types of welded joints in construction, which have a thickness of the welded base metal of more than 4 millimeters. In addition, the method is actively used to verify the joints of the joints of gas and oil pipelines, various hydraulic and plumbing systems. And in such cases, as the control of large thickness seams obtained as a result of electroslag welding, ultrasonic inspection is the only acceptable method of monitoring.
The final decision on whether a part or a weld is suitable for use is made on the basis of three fundamental indicators (criteria) - amplitude, coordinates, relative sizes.
In general, ultrasonic testing is precisely the method that is most fruitful from the point of view of image formation in the process of studying a seam (detail).
Reasons for demand
The described control method with the use of ultrasound is good in that it has a much higher sensitivity and reliability of the readings in the process of detecting defects in the form of cracks, lower cost and high safety during use compared with classical methods of radiographic control. Today, ultrasonic testing of welded joints is used in 70-80% of inspections.
Ultrasonic transducers
Without the use of these devices, non-destructive ultrasonic testing is simply unthinkable. Devices serve to form excitement, as well as receive ultrasonic vibrations.
Units are different and are subject to classification by:
- The method of creating contact with the test product.
- The method of connecting the piezoelectric elements in the electrical circuit of the flaw detector itself and the electrode dislocation relative to the piezoelectric element.
- The orientation of the acoustic relative to the surface.
- The number of piezoelectric elements (single, double, multi-element).
- The width of the operating frequency band (narrow-band - band less than one octave, broad-band - bandwidth greater than one octave).
Measured defect characteristics
In the world of technology and industry, GOST rules everything. Ultrasonic testing (GOST 14782-86) in this issue is no exception. The standard regulates that defects are measured by the following parameters:
- Equivalent area of the defect.
- The amplitude of the echo signal, which is determined taking into account the distance to the defect.
- The coordinates of the defect at the weld point.
- Conditional sizes.
- Conditional distance between defects.
- The number of defects on the selected length of the weld or joint.
Flaw Detector Operation
Non-destructive testing, which is ultrasound, 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 sensitivity level is fixed. It, in turn, is configured using a standard enterprise standard (SOP).
The start of operation of the flaw detector is accompanied by its configuration. For this, rejection sensitivity is set. After that, in the process of ultrasound studies, the received echo signal from the detected defect is compared with a fixed rejection level. In the event that the measured amplitude exceeds the rejection level, specialists decide that such a defect is unacceptable. Then the seam or product is rejected and sent for revision.
The most common defects of welded surfaces are: lack of penetration, incomplete penetration, cracking, porosity, slag inclusions. It is these disorders that are effectively detected by ultrasonic inspection.
Ultrasound Research Options
Over time, the verification process has received several effective methods for studying welding joints. Ultrasonic testing provides a fairly large number of options for acoustic research of the considered metal structures, however, the most popular were:
- Echo method.
- Shadow.
- Mirror-shadow method.
- Echo Mirror.
- Delta method.
Method number one
Most often, an echo-pulse method is used in industry and railway transport. Thanks to him, more than 90% of all defects are diagnosed, which becomes possible due to 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.
The advantages of the method are:
- the possibility of one-way access to the product;
- fairly high sensitivity to internal defects;
- the highest accuracy in determining the coordinates of a detected defect.
However, there are disadvantages, including:
- low resistance to interference of surface reflectors;
- a strong dependence of the signal amplitude on the location of the defect.
The described flaw detection involves sending an ultrasonic pulse finder into the product. A response signal is received by it 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).
Shadow method
It is based on a detailed analysis of the amplitude of ultrasonic vibrations transmitted from the emitter to the receiver. In the case when there is a decrease in this indicator, this signals 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 aligned 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 difficulty of orienting the probe (piezoelectric transducers) relative to the central rays of the radiation pattern. However, there are advantages that are high immunity to interference, a small dependence of the signal amplitude on the location of the defect, and the absence of a dead zone.
Mirror shadow method
This ultrasonic quality control is most often used to control 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 the bottom). The main advantage of the method is the clear detection of various defects, the dislocation of which is the root of the seam. Also, the method is characterized by the possibility of one-way access to the seam or part.
Echo Mirror Method
The most effective option for detecting vertically located defects. Verification is carried out using two probes, which are moved on the surface near the seam on one side of it. At the same time, their movement is performed in such a way as to fix with one probe the signal radiated from another probe and twice reflected from the existing defect.
The main advantage of the method: it can be used to evaluate 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 a probe with the same sensitivity. This version of ultrasound is actively used to check thick-walled products and their welds.
Delta method
The specified ultrasonic inspection of the welds uses ultrasonic energy reradiated by the defect. A transverse wave, which falls on a defect, is partially reflected, partially converted into a longitudinal wave, and also re-radiates the diffracted wave. As a result, the required probes are captured. The disadvantage of this method is the cleaning of the seam, the rather high complexity of decoding the received signals during the control of welded joints up to 15 millimeters thick.
The advantages of ultrasound and the subtleties of its application
The study of welded joints using high-frequency sound is, in fact, non-destructive testing, because this method is not able to cause any damage to the investigated area of the product, but it quite accurately determines the presence of defects. Also of particular interest is the low cost of the work and their high speed. It is also important that the method is absolutely safe for human health. All studies of metals and welds based on ultrasound are carried out in the range from 0.5 MHz to 10 MHz. In some cases, it is possible to carry out work using ultrasonic waves having a frequency of 20 MHz.
An analysis of a welded joint by means of ultrasound must necessarily be accompanied by a whole range of preparatory measures, such as cleaning the test joint or surface, applying specific contact fluids (special purpose gels, glycerin, machine oil) to the controlled area. All this is done to ensure proper stable acoustic contact, which ultimately provides the necessary picture on the device.
Inability to use and disadvantages
Ultrasonic testing is absolutely irrational to use for examining welding joints of metals having a coarse-grained structure (for example, cast iron or an austenitic weld with a thickness of more than 60 millimeters). And all because in such cases there is a sufficiently large dispersion and strong attenuation of ultrasound.
It is also not possible to unambiguously fully characterize the detected defect (tungsten inclusion, slag inclusion, etc.).