Welding is usually used in installation activities, if you need to provide a high degree of reliability of the joints. In many cases, there is no alternative to thermal alloying, but there are many different techniques for its implementation, not to mention the conditions of work. Accordingly, methods for checking the quality of the obtained seam also differ. Specialists use non-destructive testing of welded joints, which allows preserving the structure of the material in the joint zone and the operational qualities of the structure as a whole.
Regulatory Standards (GOST)
Implementation of non-destructive testing methods is carried out in accordance with established technical standards. Especially for welding, the GOST section is provided under number 3242-79. Guided by the rules of this section, the wizard can apply this or that control method. The standards describe not only the inspection technique, but also the equipment. In some cases, deviation from the requirements stipulated by this GOST is also allowed. Non-destructive testing of welded joints in this case is already focused on quality assessment methods that are recommended for defect detection operations in relation to specific metals and alloys. However, in such situations, one should rely on the requirements of GOST, but in another section - 19521-74.
What defects are detected?
There are several groups of defects that help detect non-destructive testing technologies. At the basic level, surface flaws of the seam are revealed. Such deviations from the norm can be recorded already at the external examination, even without the use of special equipment. For example, external non-destructive testing of welded joints helps to fix discontinuity zones that come to the surface. Internal defects cannot be detected without the appropriate technical means. It determines the shape of the seam, its characteristics and degree of reliability.
Moreover, the presence of a defect as such does not always indicate the unsuitability of the design or product for further use as intended. Again, according to the regulations, the welding seam can have critical and insignificant deviations. The control task consists precisely in the detection of critical defects, which are defined as inadequate to the requirements for the operation of the material.
Equipment for acoustic control method
This method of checking structures for defects in welds is one of the most technologically advanced, accurate and efficient. Compared to other modern control techniques, it is also versatile. It can be used both indoors and in the field without power supply. An ultrasonic flaw detector, consisting of several functional modules , is involved in the verification. In particular, this non-destructive testing of welded joints involves the use of piezoelectric transducers, which contain hardware components for receiving and scattering ultrasonic waves. The device generates pulses of ultrasonic vibrations, and also receives reflected signals, which are presented to the operator in a form convenient for analysis. Studying the amplitude of the signals, the user of the equipment determines the parameters of the defects.
Radiation Monitoring Equipment
This technique is called radiation flaw detection of welded joints. The principle of research is based on the supply of ionizing radiation. During the passage of the rays through the seam, their intensity decreases depending on the thickness and density of the material. The changes in the radiation indices allow the operator to determine the presence of discontinuities in the thickness of the compound. In the implementation of this operation using various sources of x-ray radiation. The most common equipment for non-destructive testing of welded joints of this type in the form of electron accelerators and gamma detectors. These devices are combined with the ability to work with radioisotope radiation. Russian manufacturers of x-ray machines for testing welded joints produce equipment that provides the ability to control the average energy range of photon radiation from 15 keV to 30 MeV.
Equipment for thermal control
Evaluation of the quality of the weld through thermal scanning allows you to work with a wide range of alloys used in both industry and the construction industry. As for the detected defects, the thermal analysis reveals hidden cavities, cracks, areas of lack of fusion, foreign inclusions, etc. Directly heating and recording of suspicious zones is carried out by a radiometer. This is a device that implements non-destructive testing of welded joints of metal structures over the entire area. In the process of analysis, the operator checks both the basic undeformed structure and the junction. By comparing pristine areas and seams, the reliability of the structure is determined. Today, there are different directions of this method. In particular, the thermal thermal imaging method involves the analysis of vibrations during energy transfer to an object.
Electrical Control Equipment
The formation of an electric field around the object under study also allows you to determine the characteristics of the internal structure of the structure at the connection points. For the application of this method, electrical capacitors of various types are used. For example, overhead systems are highly heterogeneous in the electrostatic field they form. This property is useful in that the operator captures the oscillations against the background of high sensitivity in the supply of reverse pulses from the material. Electrical non-destructive testing of the quality of welded joints in linear-broaching structures involves straight-through transducers. Such equipment, in particular, is used in assessing the quality of welds made on wire, metal tapes, rods, etc. Depending on the electrodes, different current supply schemes can be used.
Capillary Control Devices
This is an extensive complex of methods that are aimed at detecting and determining the parameters of internal defects. As working equipment, capillary flaw detectors are used . They record the characteristics of the same cavities, their structure, direction, depth and spatial location. However, their function is impossible without the use of penetrants. These are liquid or loose substances, which, if possible, are introduced into the seam and spread along its internal cavities. Capillary methods of non-destructive testing of welded joints involve the use of penetrants of different characteristics. These are some kind of developers giving information about the structure of the compound to capillary flaw detectors. There are substances that are activated by ultrasonic, magnetic, color and other pulses. Some compounds have a pronounced chemical activity, so immediately after the control is performed, it is required to process the seams with the so-called quenchers. They exclude the negative impact of capillary penetrants on the material of the object, which allows us to attribute this method to non-destructive.
Seam leak detection as a control method
This technique is largely related to the principles of the previous control technology, but has several significant differences. If the capillary method is focused on the exact determination of the parameters of the internal cavities, then leak detection aims at the fundamental finding of areas in which the tightness is broken. In this case, the weld can be checked not only with the help of liquid substances, but also with the help of air and gas mixtures. Often this method is used before capillary technology, because leak detection alone only records the fact of a leak in the connection node, but does not provide information about the characteristics of the defects.
How is the optimal control method selected?
Specialists build on tasks that need to be done through monitoring. For example, if we are talking about surface testing, then we can do with the mentioned leak detection technology or an experienced visual inspection. For a deeper and more accurate analysis, ultrasonic, electrical and x-ray machines are used. Further, it is taken into account how one or another non-destructive testing of welded joints can be effective when implemented in specific conditions. So, the ultrasound technique can be used in almost any environment, but it is more expensive. The electric method for scanning defects is more affordable, but it can only be used if there is a stable current source.
Conclusion
Seam inspection in metal structure joints is the most important reliability verification operation. Subject to positive verification results, the product or design can be used for its intended purpose. In addition, non-destructive testing of welded joints can provide information on old facilities in operation. Over time, even high-quality seams undergo wear and tear, so inspection should be performed regularly. After it and according to the results of the analysis, a conclusion is given on the technical condition of the structure. On the basis of this document, the responsible engineer makes a decision either on the elimination of defects, or on the admission of the object to further use.