Color method of non-destructive testing. Capillary testing, color flaw detection, capillary non-destructive testing

Capillary inspection (capillary / luminescent / color flaw detection, penetrant inspection)

Capillary control, capillary flaw detection, fluorescent / color flaw detection- these are the most common names of the method of non-destructive testing by penetrating substances among specialists, - penetrants.

Capillary control method - best way detection of defects emerging on the surface of products. Practice shows high economic efficiency of capillary flaw detection, the possibility of its use in a wide variety of shapes and controlled objects, ranging from metals to plastics.

At a relatively low cost Supplies, equipment for fluorescent and color flaw detection is simpler and less expensive than for most other non-destructive testing methods.

Sets for capillary control

Color flaw detection kits based on red penetrants and white developers

Standard kit for operation in the temperature range -10°C ... +100°C

High temperature kit for operation in the range 0°C ... +200°C

Kits for capillary flaw detection based on luminescent penetrants

Standard kit for operation in the temperature range -10°C ... +100°C in visible and UV light

High temperature kit for operation in the range 0°C ... +150°C using a UV lamp λ=365 nm.

Set for testing critical products in the range of 0°C ... +100°C using a UV lamp λ=365 nm.

Capillary flaw detection - an overview

History reference

Method for studying the surface of an object penetrating penetrants, which is also known as capillary flaw detection(capillary control), appeared in our country in the 40s of the last century. Capillary control was first used in the aircraft industry. Its simple and clear principles have remained unchanged to this day.

Abroad, around the same time, a red-white method for detecting surface defects was proposed, and soon patented. Subsequently, it received the name - the method of control penetrating liquids (Liquid penetrant testing). In the second half of the 1950s, materials for capillary flaw detection were described in the US military specification (MIL-1-25135).

Quality control with penetrants

The ability to control the quality of products, parts and assemblies with penetrating substances - penetrants exists due to such a physical phenomenon as wetting. The flaw detection liquid (penetrant) wets the surface, fills the mouth of the capillary, thereby creating conditions for the appearance of the capillary effect.

Penetrating power is a complex property of liquids. This phenomenon is the basis of capillary control. Penetration depends on the following factors:

• properties of the investigated surface and the degree of its purification from contamination;
• physical and chemical properties of the material of the control object;
• properties penetrant(wettability, viscosity, surface tension);
• temperature of the object of study (affects the viscosity of the penetrant and wettability)

Among other types of non-destructive testing (NDT), the capillary method plays a special role. First, in terms of the combination of qualities, it is perfect way surface control for the presence of microscopic discontinuities invisible to the eye. It is favorably distinguished from other types of NDT by its portability and mobility, the cost of controlling a unit area of ​​a product, and the relative ease of implementation without the use of sophisticated equipment. Secondly, capillary control is more versatile. If, for example, it is used only for testing ferromagnetic materials with a relative magnetic permeability of more than 40, then capillary flaw detection is applicable to products of almost any shape and material, where the geometry of the object and the direction of defects do not play a special role.

Development of capillary testing as a method of non-destructive testing

The development of methods for flaw detection of surfaces, as one of the areas of non-destructive testing, is directly related to scientific and technological progress. Manufacturers of industrial equipment have always been concerned about saving materials and manpower. At the same time, the operation of equipment is often associated with increased mechanical loads on some of its elements. As an example, consider the turbine blades of aircraft engines. In the mode of intense loads, it is cracks on the surface of the blades that are a known danger.

In this particular case, as in many others, capillary control proved to be very useful. Manufacturers quickly appreciated it, it was adopted and received a sustainable development vector. The capillary method has turned out to be one of the most sensitive and popular non-destructive testing methods in many industries. Mainly in mechanical engineering, serial and small-scale production.

Currently, the improvement of capillary control methods is carried out in four directions:

• improving the quality of flaw detection materials aimed at expanding the sensitivity range;
• decline harmful effects materials on the environment and humans;
• the use of systems for electrostatic spraying of penetrants and developers for their more uniform and economical application to controlled parts;
• introduction of automation schemes into the multi-operational process of surface diagnostics in production.

Organization of a section for color (luminescent) flaw detection

The organization of a site for color (luminescent) flaw detection is carried out in accordance with industry recommendations and standards of enterprises: RD-13-06-2006. The site is assigned to the non-destructive testing laboratory of the enterprise, which is certified in accordance with the Certification Rules and the basic requirements for non-destructive testing laboratories PB 03-372-00.

Both in our country and abroad, the use of color flaw detection methods in large enterprises is described in internal standards, which are completely based on national ones. Color flaw detection is described in the standards of Pratt & Whitney, Rolls-Royce, General Electric, Aerospatiale and others.

Capillary control - pros and cons

Advantages of the capillary method

1. Low cost of consumables.
2. High objectivity of control results.
3. Can be applied to almost everyone hard materials(metals, ceramics, plastics, etc.) except for porous ones.
4. In most cases, capillary control does not require the use of technologically sophisticated equipment.
5. Implementation of control in any place under any conditions, including stationary, using the appropriate equipment.
6. Due to the high inspection performance, it is possible to quickly check large objects with a large area of ​​the surface to be examined. Using this method in enterprises with continuous production cycle in-line control of products is possible.
7. The capillary method is ideal for detecting all types of surface cracks, providing a clear visualization of defects (when properly monitored).
8. Ideal for inspecting complex geometries, light metal parts such as turbine blades in the aerospace and power industries, and engine parts in the automotive industry.
9. Under certain circumstances, the method can be used for leak tests. To do this, the penetrant is applied to one side of the surface, and the developer to the other. At the leak site, the penetrant is pulled to the surface by the developer. Leak testing for detecting and locating leaks is extremely important for products such as tanks, tanks, radiators, hydraulic systems etc.
10. Unlike X-ray inspection, capillary flaw detection does not require special safety measures, such as the use of radiation protection equipment. During the research, it is enough for the operator to exercise elementary caution when working with consumables and use a respirator.
11. No special requirements regarding the knowledge and qualifications of the operator.

Limitations for color flaw detection

1. The main limitation of the capillary testing method is the ability to detect only those defects that are open to the surface.
2. The factor that reduces the efficiency of capillary testing is the roughness of the object of study - the porous structure of the surface leads to false readings.
3. Special cases, although quite rare, include the low wettability of the surface of some materials by penetrants, as in water based and based on organic solvents.
4. In some cases, the disadvantages of the method include the complexity of performing preparatory operations associated with the removal coatings, oxide films and drying parts.

Capillary control - terms and definitions

Capillary non-destructive testing

Capillary non-destructive testing is based on the penetration of penetrants into cavities that form defects on the surface of products. penetrant is a dye. Its trace, after appropriate surface treatment, is recorded visually or with the help of instruments.

In capillary control various testing methods are used based on the use of penetrants, surface preparation materials, developers and for capillary studies. There are now a sufficient number of capillary inspection consumables on the market to enable the selection and development of methods that meet essentially any requirement for sensitivity, compatibility, and ecology.

Physical basis of capillary flaw detection

The basis of capillary flaw detection- this is a capillary effect, as a physical phenomenon and a penetrant, as a substance with certain properties. The capillary effect is influenced by such phenomena as surface tension, wetting, diffusion, dissolution, emulsification. But in order for these phenomena to work for the result, the surface of the test object must be well cleaned and degreased.

If the surface is properly prepared, a drop of penetrant that falls on it quickly spreads, forming a stain. This indicates good wetting. Wetting (adhesion to the surface) is understood as the ability of a liquid body to form a stable interface at the boundary with a solid body. If the interaction forces between liquid molecules and solid body exceed the forces of interaction between the molecules inside the liquid, then the surface of the solid body is wetted.

pigment particles penetrant, many times smaller than the width of the opening of microcracks and other damage to the surface of the object of study. In addition, the most important physical property of penetrants is low surface tension. Due to this parameter, penetrants have sufficient penetrating power and well wet various types of surfaces - from metals to plastics.

Penetrant penetration into discontinuities (cavities) of defects and the subsequent extraction of the penetrant during the developing process occurs under the action of capillary forces. And the decoding of the defect becomes possible due to the difference in color (color flaw detection) or glow (luminescent flaw detection) between the background and the surface area above the defect.

Thus, at normal conditions, very small defects on the surface of the test object are not visible to the human eye. In the process of step-by-step surface treatment with special compositions, on which capillary flaw detection is based, an easily readable, contrasting indicator pattern is formed above the defects.

In color flaw detection, due to the action of the penetrant developer, which "pulls" the penetrant to the surface by diffusion forces, the size of the indication is usually significantly larger than the size of the defect itself. The size of the indicator pattern as a whole, subject to the control technology, depends on the volume of the penetrant absorbed by the discontinuity. When evaluating the results of control, one can draw some analogy with the physics of the "amplification effect" of signals. In our case, the "output signal" is a contrast indicator pattern, which can be several times larger in size than the "input signal" - an image of a discontinuity (defect) that is unreadable by the eye.

Defectoscopy materials

Defectoscopy materials for capillary control, these are means that are used in the control of liquid (penetration control) penetrating into the surface discontinuities of the tested products.

Penetrant

A penetrant is an indicator liquid, a penetrating substance (from English penetrate - to penetrate) .

Penetrants are called capillary flaw detection material, which is able to penetrate into the surface discontinuities of the controlled object. The penetration of the penetrant into the damage cavity occurs under the action of capillary forces. As a result of small surface tension and the action of wetting forces, the penetrant fills the void of the defect through the mouth, open to the surface, thus forming a concave meniscus.

Penetrant is the main consumable for capillary flaw detection. Penetrants are distinguished by the method of visualization into contrast (color) and luminescent (fluorescent), by the method of removal from the surface into water-washable and removed by a cleaner (post-emulsifiable), by sensitivity into classes (in descending order - I, II, III and IV classes according to GOST 18442-80)

Foreign standards MIL-I-25135E and AMS-2644, in contrast to GOST 18442-80, divide the sensitivity levels of penetrants into classes in ascending order: 1/2 - ultra-low sensitivity, 1 - low, 2 - medium, 3 - high, 4 - ultra-high .

A number of requirements are imposed on penetrants, the main of which is good wettability. The next important parameter for penetrants is viscosity. The lower it is, the less time is required for complete impregnation of the surface of the test object. In capillary control, such properties of penetrants are taken into account as:

• wettability;
• viscosity;
• surface tension;
• volatility;
• flash point (flash point);
• specific gravity;
• solubility;
• sensitivity to pollution;
• toxicity;
• smell;
• inertia.

The composition of the penetrant usually includes high-boiling solvents, dyes (phosphors) based on pigment or soluble, surface-active substances (surfactants), corrosion inhibitors, binders. Penetrants are available in cans for aerosol application (the most suitable form of release for field work), plastic cans and barrels.

Developer

The developer is a material for capillary non-destructive testing, which, due to its properties, brings to the surface the penetrant located in the defect cavity.

The penetrant developer is typically white and acts as a contrasting background for the indicator image.

The developer is applied to the surface of the test object in a thin, uniform layer after it has been cleaned (intermediate cleaning) from the penetrant. After the intermediate cleaning procedure, a certain amount of penetrant remains in the defect zone. The developer, under the action of forces of adsorption, absorption or diffusion (depending on the type of action), "pulls out" the penetrant remaining in the capillaries of defects to the surface.

Thus, the penetrant under the action of the developer "tints" the surface areas above the defect, forming a clear defectogram - an indicator pattern that repeats the location of defects on the surface.

According to the type of action, developers are divided into sorption (powders and suspensions) and diffusion (paints, varnishes and films). Most often, developers are chemically neutral sorbents from silicon compounds, white color. Such developers, covering the surface, create a layer having a microporous structure, into which, under the action of capillary forces, the coloring penetrant easily penetrates. In this case, the developer layer above the defect is colored in the color of the dye (color method), or wetted with a liquid with the addition of a phosphor, which begins to fluoresce in ultraviolet light (luminescent method). In the latter case, the use of a developer is not necessary - it only increases the sensitivity of the control.

The right developer should provide uniform coverage of the surface. The higher the sorption properties of the developer, the better it "pulls" the penetrant from the capillaries during development. These are the most important properties of the developer, which determine its quality.

Capillary control involves the use of dry and wet developers. In the first case, we are talking about powder developers, in the second, water-based developers (water-based, water-washable), or based on organic solvents (non-aqueous).

The developer as part of the flaw detection system, as well as other materials of this system, is selected based on the requirements for sensitivity. For example, to detect a defect with an opening width of up to 1 micron, in accordance with the American standard AMS-2644 for the diagnosis of moving parts of a gas turbine installation, a powder developer and a luminescent penetrant should be used.

Powder developers have good dispersion and are applied to the surface by an electrostatic or vortex method, with the formation of a thin and uniform layer, which is necessary to guarantee the extraction of a small volume of penetrant from the cavities of microcracks.

Water-based developers do not always provide a thin and even layer. In this case, if there are small defects on the surface, the penetrant does not always come to the surface. Too thick a layer of developer may mask the defect.

Developers can chemically interact with indicator penetrants. According to the nature of this interaction, the developers are divided into chemically active and chemically passive. The latter are the most widely used. Reactive developers react with the penetrant. Detection of defects, in this case, is carried out by the presence of reaction products. Chemically passive developers act only as a sorbent.

Penetrant developers are available in aerosol cans (the most suitable form for field application), plastic canisters and drums.

Penetrant emulsifier

Emulsifier (penetrant quencher according to GOST 18442-80) is a flaw detection material for capillary control, used for intermediate surface cleaning when using a post-emulsifiable penetrant.

During emulsification, the penetrant remaining on the surface interacts with the emulsifier. Subsequently, the resulting mixture is removed with water. The purpose of the procedure is to clean the surface from excess penetrant.

The emulsification process can have a significant impact on the quality of visualization of defects, especially when testing objects with a rough surface. This is expressed in obtaining a contrasting background of the required purity. To obtain a well-read indicator pattern, the background brightness should not exceed the brightness of the indication.

In capillary control, lipophilic and hydrophilic emulsifiers are used. Lipophilic emulsifier - is made on an oil basis, hydrophilic - on a water basis. They differ in the mechanism of action.

The lipophilic emulsifier, covering the surface of the product, passes into the remaining penetrant under the action of diffusion forces. The resulting mixture is easily removed from the surface with water.

The hydrophilic emulsifier acts on the penetrant in a different way. When exposed to it, the penetrant is divided into many smaller particles. As a result, an emulsion is formed, and the penetrant loses its properties for wetting the surface of the test object. The resulting emulsion is removed mechanically (washed off with water). The basis of hydrophilic emulsifiers is a solvent and surface-active substances (surfactants).

Penetrant cleaner(surfaces)

Penetrant Control Cleaner is an organic solvent for removing excess penetrant (intermediate cleaning), cleaning and degreasing the surface (pre-cleaning).

A significant influence on the wetting of the surface is exerted by its microrelief and the degree of purification from oils, fats and other contaminants. In order for the penetrant to penetrate even the smallest pores, in most cases, mechanical cleaning is not enough. Therefore, before carrying out the control, the surface of the part is treated with special cleaners made on the basis of high-boiling solvents.

The most important properties of modern surface cleaners for capillary control are:

• ability to degrease;
• absence of non-volatile impurities (ability to evaporate from the surface without leaving traces);
• minimum content harmful substances that have an impact on humans and the environment;
• Operating temperature range.

Compatibility of consumables for capillary control

Flaw detection materials for capillary testing in terms of physical and chemical properties must be compatible both with each other and with the material of the test object. Components of penetrants, cleaning agents and developers should not lead to loss of operational properties of controlled products and damage to equipment.

Compatibility table for Elitest consumables for capillary control:

⚫ - recommended to use; ∨ - can be used; × - can not use
Download the table of compatibility of consumables for capillary and magnetic particle testing:

Equipment for capillary control

Equipment used in capillary testing:

• reference (control) samples for capillary flaw detection;
• sources of ultraviolet lighting (UV lamps and lamps);
• test panels (test panel);
• pneumohydroguns;
• pulverizers;
• chambers for capillary control;
• systems for electrostatic application of flaw detection materials;
• water purification systems;
• drying cabinets;
• tanks for immersion application of penetrants.

Detectable defects

Capillary flaw detection methods make it possible to detect defects emerging on the product surface: cracks, pores, shells, lack of penetration, intergranular corrosion and other discontinuities with an opening width of less than 0.5 mm.

Control samples for capillary flaw detection

Control (standard, reference, test) samples for capillary control are metal plates with artificial cracks (defects) of a certain size applied to them. The surface of the control samples may have a roughness.

Control samples are manufactured according to foreign standards, in accordance with European and American standards EN ISO 3452-3, AMS 2644C, Pratt & Whitney Aircraft TAM 1460 40 (the standard of the enterprise - the largest American manufacturer of aircraft engines).

Control samples are used:

• to determine the sensitivity of test systems based on various flaw detection materials (penetrant, developer, cleaner);
• to compare penetrants, one of which can be taken as a model;
• to assess the quality of washability of luminescent (fluorescent) and contrast (color) penetrants in accordance with AMS 2644C;
• for a general assessment of the quality of capillary control.

The use of control samples for capillary control in the Russian GOST 18442-80 is not regulated. Nevertheless, in our country, control samples are actively used in accordance with GOST R ISO 3452-2-2009 and enterprise standards (for example, PNAEG-7-018-89) to assess the suitability of flaw detection materials.

Capillary control techniques

To date, quite a lot of experience has been accumulated in the use of capillary methods for the purposes of operational control of products, assemblies and mechanisms. However, the development of a working procedure for capillary testing often has to be done on a case-by-case basis. This takes into account factors such as:

1. sensitivity requirements;
2. the state of the object;
3. the nature of the interaction of flaw detection materials with the controlled surface;
4. compatibility of consumables;
5. technical capabilities and conditions for the performance of work;
6. the nature of the expected defects;
7. other factors affecting the effectiveness of capillary control.

GOST 18442-80 defines the classification of the main capillary control methods depending on the type of penetrating substance - penetrant (solution or suspension of pigment particles) and depending on the method of obtaining primary information:

1. brightness (achromatic);
2. color (chromatic);
3. luminescent (fluorescent);
4. luminescent color.

Standards GOST R ISO 3452-2-2009 and AMS 2644 describe six main methods of capillary control by type and group:

Type 1. Fluorescent (luminescent) methods:

• method A: water-washable (Group 4);
• method B: post-emulsification (Groups 5 and 6);
• method C: solvent soluble (Group 7).

Type 2. Color Methods:

• method A: water-washable (Group 3);
• method B: post-emulsification (Group 2);
• method C: solvent soluble (Group 1).

capillary control. Color flaw detection. Capillary method of non-destructive testing.

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Capillary flaw detection- a flaw detection method based on the penetration of certain contrast agents into the surface defective layers of the controlled product under the action of capillary (atmospheric) pressure, as a result of subsequent processing by the developer, the light and color contrast of the defective area increases relative to the undamaged one, with the identification of the quantitative and qualitative composition of damage (up to thousandths of millimeter).

There are luminescent (fluorescent) and color methods of capillary flaw detection.

Mainly according to technical requirements or conditions, it is necessary to detect very small defects (up to hundredths of a millimeter) and it is simply impossible to identify them with a normal visual inspection with the naked eye. The use of portable optical instruments, such as a magnifying loupe or microscope, does not allow revealing surface damage due to insufficient visibility of the defect against the background of metal and the lack of a field of view at multiple magnifications.

In such cases, the capillary control method is used.

During capillary testing, indicator substances penetrate into the cavities of surface and through defects in the material of the test objects; subsequently, the resulting indicator lines or points are recorded visually or using a transducer.

Control by capillary method is carried out in accordance with GOST 18442-80 “Non-destructive control. capillary methods. General requirements."

The main condition for the detection of defects such as discontinuity of the material by the capillary method is the presence of cavities free from contaminants and other technical substances that have Free access to the surface of the object and the depth of occurrence, several times greater than the width of their opening at the exit. A cleaner is used to clean the surface before applying the penetrant.

Purpose of capillary inspection (capillary flaw detection)

Capillary flaw detection (capillary control) is designed to detect and inspect surface and through defects invisible or poorly visible to the naked eye (cracks, pores, lack of penetration, intergranular corrosion, shells, fistulas, etc.) in controlled products, determining their consolidation, depth and orientation on the surface.

Application of the capillary method of non-destructive testing

The capillary method of control is used in the control of objects of any size and shape, made of cast iron, ferrous and non-ferrous metals, plastics, alloyed steels, metal coatings, glass and ceramics in power engineering, rocket technology, aviation, metallurgy, shipbuilding, chemical industry, in the construction of nuclear reactors, in mechanical engineering, automotive, electrical engineering, foundry, medicine, stamping, instrumentation, medicine and other industries. In some cases, this method is the only one for determining the technical serviceability of parts or installations and their admission to work.

Capillary flaw detection is used as a method of non-destructive testing also for objects made of ferromagnetic materials, if they magnetic properties, shape, type and location of damages do not allow achieving the required sensitivity according to GOST 21105-87 by the magnetic particle method or the magnetic particle inspection method is not allowed to be used according to specifications object operation.

Capillary systems are also widely used for tightness control, in combination with other methods, when monitoring critical objects and objects in operation. The main advantages of capillary flaw detection methods are: simplicity of operations during testing, ease of handling devices, a wide range of tested materials, including non-magnetic metals.

The advantage of capillary flaw detection is that, using a simple control method, one can not only detect and identify surface and through defects, but also obtain, by their location, shape, extent and orientation over the surface, complete information about the nature of the damage and even some of the causes of its occurrence (concentration power voltages, non-observance of technical regulations during manufacture, etc.).

As developing liquids, organic phosphors are used - substances that have bright intrinsic radiation under the action of ultraviolet rays, as well as various dyes and pigments. Surface defects are detected by means that allow the penetrant to be removed from the cavity of defects and detected on the surface of the controlled product.

Devices and equipment used in capillary control:

Sets for capillary flaw detection Sherwin, Magnaflux, Helling (cleaners, developers, penetrants)
. Spray guns
. Pneumohydroguns
. Sources of ultraviolet illumination (ultraviolet lamps, illuminators).
. Test panels (test panel)
. Control samples for color flaw detection.

Parameter "sensitivity" in the capillary method of flaw detection

The sensitivity of capillary control is the ability to detect discontinuities of a given size with a given probability when using a specific method, control technology and penetrant system. According to GOST 18442-80, the control sensitivity class is determined depending on the minimum size of the detected defects with a transverse size of 0.1 - 500 μm.

The detection of surface defects with an opening size of more than 500 µm is not guaranteed by capillary inspection methods.

Sensitivity class Defect opening width, µm

II From 1 to 10

III From 10 to 100

IV From 100 to 500

technological Not standardized

Physical bases and technique of the capillary control method

The capillary method of non-destructive testing (GOST 18442-80) is based on the penetration of an indicator substance into a surface defect and is designed to detect damage that has a free exit to the surface of the test item. The color flaw detection method is suitable for detecting discontinuities with a transverse size of 0.1 - 500 microns, including through defects, on the surface of ceramics, ferrous and non-ferrous metals, alloys, glass and other synthetic materials. It has found wide application in the control of the integrity of adhesions and welds.

A colored or coloring penetrant is applied with a brush or sprayer to the surface of the test object. Due to the special qualities that are provided at the production level, the choice of the physical properties of the substance: density, surface tension, viscosity, penetrant under the action of capillary pressure, penetrates into the smallest discontinuities that have an open exit to the surface of the controlled object.

The developer, applied to the surface of the test object in a relatively short time after careful removal of the unassimilated penetrant from the surface, dissolves the dye located inside the defect and, due to mutual penetration into each other, “pushes” the penetrant remaining in the defect onto the surface of the test object.

Existing defects are visible quite clearly and contrast. Indicator traces in the form of lines indicate cracks or scratches, individual color dots indicate single pores or exits.

The process of detecting defects by the capillary method is divided into 5 stages (carrying out capillary control):

1. Preliminary cleaning of the surface (use a cleaner)
2. Application of the penetrant
3. Removal of excess penetrant
4. Applying the developer
5. Control

capillary control. Color flaw detection. Capillary method of non-destructive testing.

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Capillary control welded joints used to identify external (surface and through) and. This method of verification allows you to identify defects such as hot and, lack of penetration, pores, shells and some others.

With the help of capillary flaw detection, it is possible to determine the location and size of the defect, as well as its orientation along the metal surface. This method applies to both and . It is also used in welding plastics, glass, ceramics and other materials.

The essence of the capillary control method is the ability of special indicator liquids to penetrate into the cavities of weld defects. Filling defects, indicator liquids form indicator traces, which are recorded during visual inspection, or with the help of a transducer. The order of capillary control is determined by standards such as GOST 18442 and EN 1289.

Classification of capillary flaw detection methods

Methods of capillary testing are divided into basic and combined. The main ones imply only capillary control with penetrating substances. Combined are based on the combined use of two or more, one of which is capillary control.

Basic control methods

The main control methods are divided into:

1. Depending on the type of penetrating agent:

• testing with penetrating solutions
• testing with filter suspensions

1. Depending on the method of reading information:

• luminance (achromatic)
• color (chromatic)
• luminescent
• luminescent color.

Combined methods of capillary control

Combined methods are subdivided depending on the nature and method of exposure to the surface being checked. And they are:

1. Capillary-electrostatic
2. Capillary-electroinduction
3. Capillary magnetic
4. Capillary radiation absorption method
5. Capillary-radiation method of radiation.

Technology of capillary flaw detection

Before capillary testing, the surface to be tested must be cleaned and dried. After that, an indicator liquid - panetrant is applied to the surface. This liquid penetrates the surface defects of the seams and, after some time, an intermediate cleaning is carried out, during which excess indicator liquid is removed. Next, a developer is applied to the surface, which begins to draw out the indicator liquid from the welded defects. Thus, defect patterns appear on the controlled surface, visible to the naked eye, or with the help of special developers.

Stages of capillary control

The process of capillary control can be divided into the following stages:

1. Preparation and pre-cleaning
2. Intermediate cleaning
3. Manifestation process
4. Detection of welding defects
5. Drawing up a protocol in accordance with the results of the check
6. Final surface cleaning

Materials for capillary control

The list of necessary materials for capillary flaw detection is given in the table:

Preparation and preliminary cleaning of the surface to be checked

If necessary, contaminants such as scale, rust, oil stains, paint, etc. are removed from the controlled surface of the weld. These contaminants are removed using mechanical or chemical cleaning, or a combination of these methods.

Mechanical cleaning is recommended only in exceptional cases, if there is a loose film of oxides on the controlled surface or there are sharp drops between the weld beads, deep undercuts. Limited use mechanical cleaning obtained due to the fact that when it is carried out, often surface defects are closed as a result of mashing, and they are not detected during inspection.

Chemical cleaning is carried out using various chemical cleaners that remove contaminants such as paint, oil stains, etc. from the surface being checked. Chemical residues can react with indicator liquids and affect the accuracy of the control. Therefore, chemicals after preliminary cleaning should be washed off the surface with water or other means.

After preliminary cleaning of the surface, it must be dried. Drying is necessary so that neither water, nor solvent, nor any other substances remain on the outer surface of the joint being checked.

Application of indicator liquid

The application of indicator liquids to the controlled surface can be carried out in the following ways:

1. capillary way. In this case, the filling of welded defects occurs spontaneously. The liquid is applied by wetting, dipping, streaming or spraying. compressed air or inert gas.
2. Vacuum way. With this method, a rarefied atmosphere is created in the defect cavities and the pressure in them becomes less than atmospheric, i.e. a kind of vacuum is obtained in the cavities, which sucks the indicator liquid into itself.
3. compression method. This method is the opposite of the vacuum method. Filling of defects occurs under the influence of pressure on the indicator liquid, exceeding Atmosphere pressure. Under high pressure, the liquid fills the defects, displacing air from them.
4. ultrasonic method. Defect cavities are filled in an ultrasonic field using ultrasonic capillary effect.
5. deformation method. Defect cavities are filled under the influence of elastic oscillations of a sound wave on the indicator liquid or under static loading, which increases minimum size defects.

For better penetration indicator liquid in the cavity of defects, the surface temperature should be in the range of 10-50°C.

Intermediate surface cleaning

Intermediate surface cleaning agents should be applied in such a way that the indicator liquid is not removed from surface defects.

Water cleaning

Excess indicator liquid can be removed by spraying or wiping with a damp cloth. At the same time, mechanical impact on the controlled surface should be avoided. The water temperature should not exceed 50°C.

Solvent cleaning

First, excess fluid is removed with a clean, lint-free cloth. After that, the surface is cleaned with a cloth dampened with solvent.

Purification with emulsifiers

Water-sensitive emulsifiers or oil-based emulsifiers are used to remove indicator liquids. Before applying the emulsifier, wash off excess indicator liquid with water and immediately apply the emulsifier. After emulsification, it is necessary to wash the metal surface with water.

Combined cleaning with water and solvent

With this method of cleaning, first, excess indicator liquid is washed off the controlled surface with water, and then the surface is cleaned with a lint-free cloth moistened with a solvent.

Drying after intermediate cleaning

To dry the surface after intermediate cleaning, several methods can be used:

• wiping with a clean, dry, lint-free cloth
• evaporation at ambient temperature
• drying at elevated temperature
• air drying
• a combination of the above drying methods.

The drying process must be carried out in such a way that the indicator liquid does not dry out in the defect cavities. To do this, drying is carried out at a temperature not exceeding 50°C.

The process of manifestation of surface defects in the weld

The developer is applied to the controlled surface in an even thin layer. The development process should be started as soon as possible after the intermediate cleaning.

dry developer

Dry developer can only be used with fluorescent indicator liquids. Dry developer is applied by spraying or electrostatic spraying. Controlled areas should be covered uniformly, evenly. Local accumulations of developer are not allowed.

Liquid developer based on aqueous suspension

The developer is applied uniformly by dipping the controlled compound into it or by spraying with the help of an apparatus. When using the immersion method, for best results, the duration of the immersion should be as short as possible. After that, the controlled compound must be dried by evaporation or blowing in an oven.

Solvent based liquid developer

The developer is sprayed onto the surface to be inspected in such a way that the surface is uniformly wetted and a thin and uniform film is formed on it.

Liquid developer in the form of an aqueous solution

Uniform application of such a developer is achieved by immersing controlled surfaces into it, or by spraying with special devices. The immersion should be short, in which case the best test result is achieved. After that, the controlled surfaces are dried by evaporation or blowing in an oven.

The duration of the development process

The duration of the development process continues, as a rule, for 10-30 minutes. In some cases, an increase in the duration of manifestation is allowed. The countdown of the development time begins: for dry developer immediately after its application, and for liquid developer - immediately after the surface has dried.

Identification of welding defects as a result of capillary flaw detection

If possible, inspection of the surface to be inspected begins immediately after the developer has been applied or after it has dried. But the final control occurs after the completion of the process of manifestation. As auxiliary devices, with optical control, magnifying glasses are used, or glasses with magnifying lenses.

When using fluorescent indicator liquids

Photochromic glasses are not allowed. It is necessary that the inspector's eyes adjust to the darkness in the test booth for at least 5 minutes.

Ultraviolet radiation must not enter the eyes of the inspector. All controlled surfaces must not fluoresce (reflect light). Also, objects that reflect light under the influence of ultraviolet rays should not fall into the field of view of the controller. General UV lighting may be used to allow the inspector to move freely around the test chamber.

When using colored indicator liquids

All controlled surfaces are inspected in daylight or artificial lighting. Illumination on the tested surface must be at least 500 lx. At the same time, there should be no glare on the surface due to the reflection of light.

Repeated capillary control

If there is a need for re-inspection, then the entire process of capillary flaw detection is repeated, starting with the pre-cleaning process. To do this, it is necessary, if possible, to provide more favorable conditions control.

For repeated control, it is allowed to use only the same indicator liquids, of the same manufacturer, as during the first control. Use of other liquids, or the same liquids, but different manufacturers, not allowed. In this case, it is necessary to perform a thorough cleaning of the surface so that no traces of the previous check remain on it.

According to EN571-1, the main stages of capillary control are presented in the diagram:

Video on the topic: "Capillary flaw detection of welds"

Non-destructive testing becomes important when the development of the coating has already been completed and it is possible to proceed to its industrial application. Before a coated product enters service, it is checked for strength, cracks, discontinuities, pores or other defects that could cause failure. The more complex the coated object is, the more likely it is to have defects. Table 1 presents and describes below the existing non-destructive methods for determining the quality of coatings.

Table 1. Non-destructive methods quality control of coatings before their operation.

VISUAL INSPECTION

The simplest quality assessment is an external inspection of a coated product. Such control is relatively simple, it becomes especially effective when good lighting, when using a magnifying glass. As a rule, an external examination should be carried out qualified personnel and in combination with other methods.

SPRAYING WITH PAINT

Cracks and depressions on the surface of the coating are detected by the absorption of paint. The surface to be tested is sprayed with paint. Then it is carefully wiped and an indicator is sprayed on it. After a minute, the paint emerges from cracks and other small defects and colors the indicator, thus revealing the contour of the crack.

FLUORESCENT CONTROL

This method is similar to the paint soak method. The test specimen is immersed in a solution containing fluorescent paint, which is applied to all cracks. After cleaning the surface, the sample is covered with a new solution. If the coating has any defects, the fluorescent paint in that area will be visible under UV light.

Both methods based on absorption are used only to detect surface defects. Internal defects are not detected. Defects lying on the surface itself are difficult to detect, since when wiping the surface before applying the indicator, the paint is removed from them.

RADIOGRAPHIC CONTROL

Inspection by penetrating radiation is used to detect pores, cracks and voids within the coating. X-rays and gamma rays pass through the material being tested and onto photographic film. The intensity of x-ray and gamma radiation changes as they pass through the material. Any pores, cracks, or changes in thickness will be registered on the film, and with the appropriate interpretation of the film, the position of all internal defects can be established.

Radiographic control is relatively expensive and slow. The operator must be protected from exposure. Difficult to analyze products complex shape. Defects are defined when their dimensions are more than 2% of the total thickness of the coating. Therefore, radiographic technique is not suitable for detecting small defects in large structures of complex shape, it gives good results on less complex products.

EDGE CURRENT CONTROL

Surface and internal defects can be determined using eddy currents induced in the product by introducing it into the electromagnetic field of the inductor. When moving the part in the inductor, or the inductor relative to the part, the induced eddy currents interact with the inductor and change its impedance. The induced current in the sample depends on the presence of conduction defects in the sample, as well as its hardness and size.

By applying appropriate inductances and frequencies, or a combination of both, defects can be detected. Eddy current control is impractical if the configuration of the product is complex. This type of inspection is unsuitable for detecting defects on edges and corners; in some cases, the same signals can come from an uneven surface as from a defect.

ULTRASONIC CONTROL

In ultrasonic testing, ultrasound is passed through a material and changes in the sound field caused by defects in the material are measured. The energy reflected from defects in the sample is perceived by the transducer, which converts it into an electrical signal and feeds it to the oscilloscope.

Depending on the size and shape of the sample, longitudinal, transverse or surface waves are used for ultrasonic testing. Longitudinal waves propagate in the material under test in a straight line until they meet a boundary or discontinuity. The first boundary the incoming wave encounters is the boundary between the transducer and the product. Part of the energy is reflected from the boundary, and the primary pulse appears on the oscilloscope screen. The rest of the energy passes through the material until it encounters a defect or opposite surface, the position of the defect is determined by measuring the distance between the signal from the defect and from the front and back surfaces.

The discontinuities can be arranged so that they can be identified by directing the radiation perpendicular to the surface. In this case, the sound beam is introduced at an angle to the surface of the material to create shear waves. If the entry angle is sufficiently increased, then surface waves are formed. These waves travel along the contour of the sample and can detect defects near its surface.

There are two main types of installations for ultrasonic testing. The resonant test uses radiation with a variable frequency. When the natural frequency corresponding to the thickness of the material is reached, the oscillation amplitude increases sharply, which is reflected on the oscilloscope screen. The resonance method is mainly used to measure thickness.

In the pulse echo method, pulses of constant frequency with a duration of fractions of a second are introduced into the material. The wave passes through the material and the energy reflected from the defect or back surface is incident on the transducer. The transducer then sends another pulse and receives the reflected one.

The transmission method is also used to detect defects in the coating and to determine the adhesion strength between the coating and the substrate. In some coating systems, the measurement of reflected energy does not adequately identify the defect. This is due to the fact that the interface between the coating and the substrate is characterized by such a high reflection coefficient that the presence of defects only slightly changes the total reflection coefficient.

The use of ultrasonic testing is limited. This can be seen from the following examples. If the material has a rough surface, sound waves dissipate so strongly that the test is meaningless. To test objects of complex shape, transducers are needed that follow the contour of the object; surface irregularities cause spikes to appear on the oscilloscope screen, making it difficult to identify defects. Grain boundaries in metal act similarly to defects and scatter sound waves. Defects located at an angle to the beam are difficult to detect, since reflection occurs mainly not towards the transducer, but at an angle to it. It is often difficult to distinguish between discontinuities located close to one another. In addition, only those defects are detected, the dimensions of which are comparable with the sound wavelength.

Conclusion

Screening tests are undertaken during the initial stage of coating development. Because during the search optimal mode the number of different samples is very large, a combination of test methods is used to weed out unsatisfactory samples. This selection program usually consists of several types of oxidation tests, metallographic examination, flame tests and tensile tests. Coatings that have successfully passed the selection tests are tested under conditions similar to operational ones.

Once a particular coating system has been found to have withstood field testing, it can be applied to protect the actual product. It is necessary to develop a technique for non-destructive testing of the final product before putting it into operation. The non-destructive technique can be used to detect surface and internal holes, cracks and discontinuities, as well as poor adhesion of the coating and substrate.