Capillary flaw detection is a method that is based on the penetration of certain liquid components into the surface defect of the product under the influence of capillary pressure. As a result of this, the light and color contrast of the defective areas are relatively undamaged.
Indications for this procedure
Capillary flaw detection (that is, conducting capillary inspection) is designed to detect and inspect through and surface defects that are invisible or weakly visible to the naked eye. We are talking about cracks, pores, lack of penetration, intergranular corrosion, shells, fistulas and so on.
The methods of this procedure
In most situations, technical requirements have to detect such small defects that it is almost impossible to notice them in the process of visual inspection with the naked eye. The use of an optical measuring device, for example, a magnifier or a microscope, makes it impossible to detect a surface defect due to insufficient contrast of the image against the background of metals and a small field of view at high magnification. In such cases, a capillary control technique is used.
As part of the study of a control sample for capillary flaw detection, indicator liquids can penetrate into the cavities of through and surface discontinuities in the material of control objects. And the indicator traces formed are recorded visually or by means of a converter. The capillary technique is controlled in accordance with GOST “Capillary techniques. General requirements".
The presence of a cavity is a prerequisite
A necessary condition for detecting defects in the discontinuity of material by a capillary technique is the presence of a cavity free of contaminants and other substances that have access to the depth of distribution and the surface of objects significantly exceeding the width of the general opening.
Methods of capillary defectoscopy are divided into the main, which use capillary phenomena, and combined, which are based on a combination of two or more different in physical essence methods of non-destructive testing. One of such methods is capillary control (or in another way - capillary flaw detection).
Appointment
Capillary flaw detection (i.e. capillary inspection) is intended for the manifestation of invisible or poorly visible to the naked eye through and surface defects in the objects of control. This technique allows you to determine their location, length and orientation on the surface.
Capillary methods of non-destructive testing are based on capillary penetrations of the indicator fluid into the cavity of the surface and through material of the object. As part of the application of this technique, registration of indicator traces formed is carried out visually or using a converter.
Control Rules
The capillary control technique is used to control objects of any size and shape, which are made of non-ferrous and ferrous metals, alloy steel, cast iron, metal coating, plastic and so on. Materials such as glass and ceramics can also be used in the fields of energy, rocket technology, aviation and shipbuilding. This method, among other things, is also applicable in the construction of nuclear reactors, in the chemical industry, in the field of metallurgy, automotive, electrical engineering, engineering, foundry, stamping, instrumentation and other industries. For some products and materials, this technique is the only one to determine the suitability of a part or installation for work.
Capillary flaw detection is also used for non-destructive testing of objects that are made of ferromagnetic material, when their magnetic property, shape, type and location of the defect do not allow reaching the sensitivity required by GOST by the magnetic particle method and magnetic particle inspection technology.
A necessary condition for detecting defects such as discontinuities in material by capillary techniques is the presence of cavities free of all kinds of contaminants and other substances that have access to the surface of objects, and in addition, the depth of propagation, which significantly exceeds the width of their disclosure. Capillary control is also used for leak detection in conjunction with other methods and for monitoring objects during operation.
What does capillary flaw detection of welds mean? This will be described later.
Welds
Such a flaw detection technique has been known to mankind for a very long time. We can say for sure that even in the Middle Ages, craftsmen identified with its help surface cracks invisible to the naked eye on various products. It is suitable for inspection of the weld.
In order to carry out color defectoscopy by the capillary method, the prepared part is immersed in a special colored solution, which is often called penetrant. In this solution, the part is aged from five to ten minutes, and then washed in cold water. When the flushing is completed, a thin layer of white paint is applied to the controlled surface, it can also be clay. Drying, the solution is absorbed, and the part is painted. A well-visible pattern appears in the area of the defect.
The advantages of this technique
A similar technique for detecting a defect in welds is quite widespread, as it has a number of different important advantages. There is no need for sophisticated equipment. All the required materials are usually quite cheap, and you can buy them in a standard hardware store.
An examination usually does not take too long. Thus, this means that such technology can be used even in mass production. It does not matter at all what the sample for capillary flaw detection is made of. This can, for example, be cast iron or steel along with non-ferrous and non-magnetic alloys, various plastics and even ceramics.
This technique is quite accurate, as it allows to identify cracks with a size of one micron. The process of performing color flaw detection is quite simple, and even an ordinary person who does not have special skills is able to master it. Of course, there are various disadvantages that impose a restriction on the use of such methods for controlling the quality of welds. It is worth noting that there may be an inability to detect a hidden defect and cracks that do not come to the surface. This is important in situations where the general requirements for product strength are particularly high.
Immediately before the start of the examination, the parts must be thoroughly cleaned of dirt and degreased. It is at this stage that the most serious problems may arise. However, they will have to be solved, because otherwise the accuracy of the result obtained may be in great doubt. Considering the fact that bulky elements of a welded structure, such as a gas pipe or a building frame, cannot be placed in a penetrant tank, color defectoscopy is used very limited during construction.
Unfortunately, for the guaranteed detection of defects, short-term immersion of a control sample for capillary flaw detection in a solution is not enough. According to the recommendations, the period of such swimming should be about thirty minutes, and therefore it is recommended to use a simple technique only for selective control.
Developer for capillary inspection
Control processes begin after the developer completes their work and lasts approximately thirty minutes. The presence of color saturation indicates the size of the defect. The lower the color concentration, the smaller the size of the defective area. A more saturated gamma can manifest itself mainly within deep cracks. After completion of the control, it is mandatory to remove the developer with water or using a special cleaner.
It is due to the effects of penetrant that complete penetration into the most inaccessible areas of defects is achieved. And the developer, which was deposited on the surface of the product, provides a thorough dissolution of the dye located in the internal cavity of the defect. This process allows you to fully understand where exactly the defective areas are located. Against this background, a color mark appears that has a linear appearance, purposefully indicating a crack, scratch or pore.
What materials for capillary flaw detection are used?
Materials: penetrant
Penetrant is used in the process of flaw detection. This is such a material having the properties of a flaw detection capillary effect, which has the ability to penetrate into discontinuous sections of the product, and in addition, has the property of finding the required defects. The penetrant in its composition has substances that have coloring preparations or additional luminescent additives. Such components are an excellent auxiliary link in the detection of all kinds of cracks and defective areas.
Capillary flaw detection kit
Products that are required to perform color flaw detection are primarily luminescent materials from the Ziglo brand. The kits required for capillary control are called Magnaflux, Sherwin and Helling. Spray guns are also used along with pneumatic guns, UV lighting devices, test panels along with control materials used in color flaw detection.
Useful Tips
Dye color is usually not very important. The main thing is that it is distinguished by contrast. The current GOST clearly standardizes the degree of illumination in the framework of defectoscopy. They are also allowed to use luminescent paints, and they recommend using a light source with a non-translucent reflector for highlighting. In some situations, the part is heated to obtain a clearer image.
Of course, not all people have vision, which allows you to see even bright colored lines with a thickness of only a few microns. And in this regard, when working with standards, the use of a magnifying glass and even a microscope is allowed. It should also be added that using the method of color flaw detection, it is possible not only to check the quality of the weld, but also to easily find the joints of the parts that are tightly fitted together. This is extremely important in situations where you have to try to disassemble devices that differ in an unfamiliar design.