Ultrasonic inspection of welded joints, methods and technology of control

There is virtually no industry where welding is not performed. The overwhelming majority of metal structures are mounted and connected together by means of welding seams. Of course, the quality of this kind of work in the future depends not only on the reliability of the erected building, structure, machine or any unit, but also on the safety of people who will somehow interact with these structures. Therefore, to ensure an appropriate level of performance of such operations, ultrasonic inspection of welding seams is used, due 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.

History of occurrence

Ultrasonic flaw detection as such was developed in the 1930s. 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 received worldwide recognition, and the number of manufacturers of such equipment has increased dramatically.

The ultrasonic flaw detector, the price of which starts today from 100,000 to 130000 thousand rubles, originally consisted of vacuum tubes. Such devices were cumbersome and heavy. They worked exclusively from AC power supplies. But already in the 1960s, with the advent of semiconductor circuits, the flaw detectors were significantly reduced in size and were able to work from batteries, which eventually enabled the devices to be used even in the field.

Step into the digital reality

In the early stages, the devices described used analog signal processing, which, like many other similar devices, was prone to drift at the time of calibration. But already in 1984 Panametrics company gave a start to the first portable digital flaw detector called EPOCH 2002. Since that time, digital units have become highly reliable equipment, ideally providing the necessary stability of calibration and measurements. The ultrasonic flaw detector, whose price directly depends on its technical characteristics and brand of the manufacturer, also received the function of data recording and the possibility of transmitting the readings to a personal computer.

In modern conditions, more and more interest is attracted to systems with phased arrays, which use a complex technology based on multi-element piezoelectric elements that generate directed beams and create transverse images similar to medical ultrasound imaging.

Scope of application

The ultrasonic method of control 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 welded base metal thickness of more than 4 millimeters. In addition, the method is actively used to test the connection of joints of gas and oil pipelines, various hydraulic and water supply systems. And in such cases, as control of seams of large thickness, obtained as a result of electroslag welding, ultrasonic flaw detection is the only acceptable method of control.

The final decision on whether the part or welding seam is suitable for operation is taken on the basis of three basic indicators (criteria) - amplitude, coordinates, conditional dimensions.

In general, ultrasonic testing is exactly the method that is most fruitful from the point of view of image formation in the process of studying the seam (details).

Causes of relevance

The described method of ultrasound control is good because it has a much higher sensitivity and reliability of indications in the process of detection of defects in the form of cracks, lower cost and high safety during use in comparison with the classical methods of radiographic inspection. To date, ultrasonic inspection of welded joints is used in 70-80% of cases of inspections.

Ultrasonic transducers

Without the use of these devices, non-destructive ultrasonic testing is simply unthinkable. Devices serve for the formation of excitation, as well as for the reception of oscillations of ultrasound.

The units are different and are classified according to:

• The method of making contact with the product under investigation.
• The method of connecting piezoelements to the electrical circuit of the flaw detector itself and the dislocation of the electrode relative to the piezoelectric element.
• Orientations are acoustic with respect to the surface.
• The number of piezoelements (one-, two-, multi-element).
• The width of the band of operating frequencies (narrowband - a band less than one octave, broadband - bandwidth exceeds one octave).

Measured characteristics of defects

In the world of technology and industry, GOST manages everything. Ultrasonic inspection (GOST 14782-86) in this matter is also no exception. The standard regulates that defects are measured by the following parameters:

• Equivalent defect area.
• The amplitude of the echo signal, which is determined taking into account the distance to the defect.
• The coordinates of the defect at the welding point.
• To the conditional sizes.
• The conditional distance between defects.
• The number of defects on the selected length of the weld or joint.

Operation of the flaw detector

Non-destructive testing, which is ultrasonic, has its own technique of use, which states that the main measured parameter is the echo amplitude obtained directly from the defect. To differentiate echoes from 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 beginning of operation of the flaw detector is accompanied by its adjustment. For this, the rejection sensitivity is set. After this, during the conducted ultrasonic tests, the received echo is compared from the detected defect with the fixed rejection level. In the event that the measured amplitude exceeds the reject level, the 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: non-penetration, incomplete penetration, cracking, porosity, slag inclusions. It is these abnormalities that effectively detect flaw detection using ultrasound.

Ultrasound research options

Over time, the validation process has received several valid methods for studying welding joints. Ultrasound control provides a fairly large number of options for acoustic study of the metal structures in question, but the most popular are:

• Echo method.
• Shadow.
• Mirror-Shadow Method.
• Echo-mirror.
• The delta method.

Method number one

Most often in industry and rail transport, the echo-pulse method is used. It is thanks to him that more than 90% of all defects are diagnosed, which is possible due to the registration and analysis of almost all signals reflected from the surface of the defect.

In itself, this method is based on the sound of a metal product by pulses of ultrasonic vibrations followed by their recording.

Advantages of the method are:

- the possibility of unilateral access to the product;

- quite high sensitivity to internal defects;

- the highest accuracy in determining the coordinates of the detected defect.

However, there are disadvantages, including:

- low resistance to interference from surface reflectors;

- a strong dependence of the signal amplitude on the location of the defect.

The described defectoscopy implies the sending of an ultrasonic pulser to the product. Reception of the response signal occurs either by him or by a second seeker. In this case, the signal can be reflected both directly from the defects, and from the opposite surface of the part, the product (seam).

The Shadow Method

It is based on a detailed analysis of the amplitude of ultrasonic vibrations transmitted from the emitter to the receiver. In the event that this indicator decreases, it indicates a defect. In this case, 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 must 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 difficulty of orienting the PES (piezoelectric transducers) relative to the central beams of the directional pattern. However, there are advantages, which include high resistance 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 bars. The main sign that the defect is detected, is the weakening of the signal amplitude, which is reflected from the opposite surface (most often it is called the 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.

Echo mirror method

The most effective way to detect vertically located defects. The check is carried out with the help of two PETs, which move along the surface near the seam on one side of it. At the same time, their motion is made in such a way as to fix a signal from one PET emitted from another PEP by one PEP and reflected twice from the existing defect.

The main advantage of the method is that it can be used to evaluate the shape of defects that exceed 3 mm and which deviate more than 10 degrees in the vertical plane. The most important thing is to use a PEP with the same sensitivity. This version of ultrasound is actively used to test thick-walled products and their welding seams.

The delta method

This ultrasonic inspection of welds uses ultrasonic energy re-emitted by the defect. The transverse wave, which falls on the defect, is reflected partially in a mirror image, partially transformed into a longitudinal wave, and also re-emits a diffracted wave. As a result, the required PEP waves are trapped. Disadvantage of the method can be considered a sweep of the seam, rather high complexity of deciphering the received signals during the control of welded joints of thickness up to 15 millimeters.

Advantages of ultrasound and fineness of its application

Investigations of welded joints with the help of high-frequency sound are, in fact, non-destructive testing, because such a method is not capable of causing any damage to the investigated section of the product, but at the same time rather accurately determines the presence of defects. Also, special attention should be paid to the low cost of the works 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 carry out work using ultrasonic waves having a frequency of 20 MHz.

The analysis of the welded joint by means of ultrasound must be accompanied by a whole complex of preparatory measures, such as cleaning the test weld or surface, applying specific contact liquids (special purpose gels, glycerin, engine oil) to the controlled area. All this is done to ensure proper stable acoustic contact, which ultimately provides the necessary image on the device.

Inability to use and disadvantages

Ultrasound control is absolutely irrational to apply for examination of welded metal compounds having a coarse-grained structure (for example, cast iron or an austenitic suture 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.

Also it is not possible to unambiguously fully characterize the detected defect (tungsten inclusion, slag inclusion, etc.).