Every welder knows that welded structures have operational properties that are determined by the quality of the welds.
For a reliable and durable construction, it is necessary to make a high-quality seam.
Inadequate quality of the weld seam causes rapid destruction of the welds, and if density is insufficient, their tightness will be impaired. The process of destruction is accelerated when using the structure in high pressure conditions.
The purpose of the weld inspection
Before sending the welded finished product to a subsequent inspection, it is necessary to control the quality level of its welds, and this helps to identify not only external defects, but also hidden ones, in order to eliminate them. Welded construction is applied only after careful control.
After welding, you must carefully inspect the seam for cracks and lack of penetration.
At the first stage, the product is subjected to external inspection and identify various defects of the welds, including the presence of cracks and lack of penetration. Detection of these defects is also available with the naked eye, and the remaining types of deficiencies are determined by the use of special equipment.
There are control welds, which are classified as destructive and non-destructive. The latter types include not only one method of external examination, but also the following types of seam control:
The use of non-destructive methods of control is not associated with a change in the appearance of the product itself. Destructive methods are used when checking the release of welded parts in sufficient quantities using a constant type of welding and the same conditions.Back to table of contents
Inspection and testing of welds: inspection methods
Factors affecting the quality of welds.
Welds can be controlled in various ways, for which their classification is provided, which includes the following methods used in the inspection:
- Visual inspection.
If we consider each of them in detail, then one of the most common and accessible is an external examination, which is not associated with any material costs. The use of any welded joints is subjected to this method of control, regardless of the process of further operations, followed by other methods of control of the seams.
You can conduct an external examination with the identification of almost all types of defects in welds. This method of control is associated with the identification of poor penetration, undercuts, sagging or other defects that are available for viewing. For the implementation of the external examination, you can use a magnifying glass, which increases 10 times.
Under the conditions of external examination, observation can be not only visual, but also related to the measurement of the weld, conducting measurements of edges that are prepared in advance. The process of mass production of welded products is associated with the use of special templates that allow you to make measurements of each parameter of the weld as accurately as possible.Back to table of contents
Physical control methods
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The application of the magnetic method of welding seams
Diagram of the magnetic method of quality control of the weld.
The detection of defects in the process of magnetic control is based on such a magnetic field ability as its dispersion. For this purpose, it is necessary to connect the electromagnetic core and then place it in the solenoid. The magnetized compound should have a surface, pre-coated with iron filings, scale, etc. Such elements that react to the appearance of magnetic fields should be used.
If the weld has defects, the formation of powder clusters occurs, which, when interacting with a magnetic field, begins to move with the formation of a magnetic directional spectrum. For easier movement of the powder under the influence of magnetic fields, the welded products knock, which gives the greatest mobility to small grains and grains.
Fixation of the magnetic field of dispersion is carried out using a special device, that is, a magnetographic flaw detector. To determine the quality of the connection, you can use the method of comparison with the standard. This method is simple and reliable as well as cheap. Magnetic control method of welding seams is indispensable in the process of laying pipelines, which play a responsible role. It is often used during construction.Back to table of contents
Radiation monitoring method and ultrasonic method
Radiographic control scheme: 1 — radiation source; 2 — straight bundle; 3 — weld; 4 — cassette with detector and screens; 5 — plot of radiation intensity.
The radiation method of control is associated with the determination of defects in any cavity of the welds that are invisible. Translucent seams using x-rays or using gamma radiation, which is able to penetrate the metal structure of the product.
Radiation is created by special sources, x-ray or gamma-based installation. The seam and radiation source must be in front of each other. The arrangement of the x-ray film is connected with its installation in a special cassette that is opaque. When the film is irradiated with X-rays passing through the metal structure, spots remain that are darker. This is due to the smallest absorption of rays by defective places.
Using the x-ray method is safe for human health, but the source of radiation, that is, the x-ray machine, is too cumbersome. Use it exclusively in the hospital. The intensity of the gamma radiation source is significant, which allows you to control the welds, which are characterized by the greatest thickness. Portable equipment and the availability of cheap method makes this type of control the most common in construction.
Gamma radiation is more dangerous than X-rays, due to the passage of special training. The use of the radiographic control method has the disadvantage of not being able to detect the presence of cracks that are not located in accordance with the direction of the flow of rays.
Ultrasonic inspection of welds.
The use of the ultrasonic method is associated with the use of acoustic control method. This allows you to identify various shortcomings, that is, gas pores and cracks, including such defects, for the sake of which the method of radiation flaw detection is not used.
Ultrasonic method conduct tests related to the reflection of a sound wave from the boundary of the separation of media. The use of the piezoelectric method in creating a sound wave has become widespread as a method for controlling a weld. The basis of this method is the principle of creating a mechanical oscillation as a result of the imposition of an alternating electric field.Back to table of contents
Destructive inspection check
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Conditions for the mechanical control method
Pattern of weld defects.
Mechanical destruction of products in the process of testing by the metallographic method is necessary to identify the characteristics of the welds.
The object of the tests are the samples with welded seams, which are cut from the welds of specially welded control samples of the joints. They are samples made on the basis of the relevant requirements for the technological level of welding products in special conditions. It is required to take the samples having the corresponding form and a certain size.
The main purpose of the control checks is the analysis and evaluation:
- strength of the weld;
- quality of the metal;
- right choice of technology;
- qualification level of worker-welder.
You can compare the characteristics of the welds with the properties of the metal. In case of unsatisfactory results, the correspondence of the totals for the given level is not noted. Mechanical testing is carried out on the basis of GOST 6996-66, where the relevant types of control tests of welded metal joints for:
Parameters for quality control of the weld.
- Various individual sections and in general.
- The subject of static stretching and bending.
- The nature of the shock of bending.
- The level of resistance in aging conditions.
- The result of measuring the characteristics of hardness.
Conducting control tests to identify statistical stretching associated with determining the level of strength of the weld. The definition of statistical bending involves the study of the plasticity properties of the weld. Tests related to the detection of defects are carried out before the beginning of the formation of the first defects in the area of the stretched zone, taking into account the angle of bend.
Tests on the identification of statistical bending produced using technological samples taken from nodes with transverse or longitudinal seam. When testing for impact bending and rupture, the impact strength of the weld is determined. According to his revealed hardness, each of the structural changes of the metal, the values of its degree of piercing after welding and during subsequent cooling are investigated.Back to table of contents
Application of macro and microstructural control methods
Microstructure diagram of the weld.
The study of metals, which is carried out on the basis of the macro- and microstructural method, includes not only the process of detecting the presence of defects of a certain nature, but also determining the quality of the welded joint.
The macrostructural method involves the study of macro sections that are present in the metal fractures, which are determined using a magnifying glass or visual inspection. Macro research determines the nature of each defect, its location, if they are visible in different zones of the welds.
Microstructural analysis is associated with the analysis of the structure of metals. During the research process, a visual magnification of 50–2000 times is possible if an optical microscope is used. Microscanning is associated with determining the amount of metal, detecting its burn-through, the presence of oxides in its structure, the clogging of welds, the presence of non-metallic inclusions. This makes it possible to study the metal grains, determining their size, to investigate changes in the composition of the metal. Detection of microscopic pores, cracks, and other defects that fill the welds is possible.
The use of appropriate methods to produce thin sections in metallographic studies is associated with the preparation of samples for inspection by cutting them out of the main product. Conducting metallographic analysis is associated with the additional use of chemical methods.
Metallographic studies of the weld.
Testing allows to obtain the corresponding characteristics of welds, which take into account the appropriate conditions for the use of welded metal structures. The control process is associated with the identification of the corrosion resistance of products operating in aggressive environments.
All relevant control methods are applied in the process of destruction of structures. The course of these tests is based on the ability of the structure to withstand a given design load and the determination of destructive loads with a certain safety margin.
Conducting tests of destructible structures, monitor the status of the destruction scheme, which should be compared with the conditions of operation of the structure. The number of products to be tested must meet the established specifications, which is determined by the existing organization of the production process.Back to table of contents
Method of nondestructive testing of welds
The control, which is non-destructive, is associated not only with conducting an external examination, including the study of the tightness of the seams. It allows you to detect defects on the surface, which can be internal and hidden.
Measurement associated with conducting an external inspection of the weld can be considered common because of its simplicity. Acceptance of any welded finished units is carried out after the first control operations.
Inspection of weld tightness implies compliance with product standards in terms of tightness, i.e. impermeability, taking into account the gases and liquids used.
Circuit control leak with a vacuum chamber.
The control of products and assemblies is made when they are already fully prepared.
If an external examination shows that there are any defects, then they must be repaired before testing begins. Weld seals are monitored using methods such as:
- Capillary (kerosene).
- Chemical (ammonia).
- Bubble (hydraulic or air pressure).
Let us dwell on these methods of control.Back to table of contents
The application of the capillary method is based on a certain property of liquids associated with the possibility of penetration through the capillaries, which are microscopic cracks, into the structure of metals. The capillary method is associated with the activity of the fluid in the capillaries. The liquid is able to retract by penetrating the capillaries. The use of certain of them (kerosene) makes it possible to detect the presence of through defects in welds.
Table for capillary inspection of welds.
The use of capillary control is associated with the verification of any welded products without taking into account their shapes, sizes and materials from which they are made. This method is more suitable for detecting various flaws on the surface of the seams. The flaws are invisible, so they cannot be detected with a simple naked eye, as well as with a magnifying glass.
To test density welds based on the capillary method allow special substances, which are called penetrants. Getting into the structure of the investigated materials with defects on the surface of the welded joint, they paint them in a bright color, allowing you to quickly see the crack.
With a high level of wettability of the liquid and the smallest radius of the capillary there is an increase in the depth and speed of penetration of the liquid. As a result of applying the capillary method, it is possible to detect not only invisible or weakly visible defects with an open cavity. However, this method allows you to identify and end-to-end types of defects, if in the process of their detection to use kerosene.Back to table of contents
Chemical control and its application
To control the quality of welds using a chemical method, a solution of chalk in water is used.
To cover the welds, a chalk solution is used in water that is used to treat the accessible side of the weld, which allows for a thorough inspection and identification of all defects. After drying the painted surface, the seam should be abundantly moistened with kerosene. If the seams are loose, this is revealed when there is a coating of chalk and traces of kerosene on it.
Ammonia control is carried out in the presence of indicators that can change their color in the presence of an alkaline medium that affects it. The reagent in this process is ammonia gas, which is used for control.
Conducting tests, on the one side of the seam stack tape of paper, which is moistened in a 5% solution of a special indicator. On the opposite side of the seam, the compound is treated with a mixture of gaseous ammonia and air. When ammonia penetrates through loose welded joints, it is stained in the places of presence of defects.Back to table of contents
Control methods for hydraulic and air pressure
Air pressure control is carried out when checking on the basis of analysis of the tightness of welded joints. To do this, they are immersed completely in a bath filled with water. Next, it is supplied with compressed air, taking into account the level of pressure, which exceeds the working by 10-20%.
If the structures are large, they are covered with foam indicators after the internal pressure has been applied along the weld. A soap solution can act as a foam indicator. If there are various leaks in the welds, air bubbles begin to appear. Testing by compressed air or gas must be accompanied by compliance with the rules of the required safety.
The strength and density of the products is checked by monitoring in the presence of hydraulic pressure. Pre-seams are completely sealed using waterproof plugs. Drying of air seams is carried out by air blowing. After that, the product should be filled under excessive pressure with water, which is 1.5–2 times higher than the working pressure level. Further it is necessary to sustain the required period. Where a leak occurs, defects can be detected.
Vacuum control involves testing a weld that cannot be tested using kerosene, water or air. In the process of testing, the seam is recorded for air infiltration through weld gaps. Registration is carried out on the same seam area on the same side of the product where a vacuum is created.