Radiation control methods based on the detection and analysis of ionizing radiation interacting with a controlled product. The most commonly used methods to control the transmitted radiation, based on the different absorption of ionizing radiation in passing through the defect and defect-free section of the welded joint (Fig. 178). The intensity of the transmitted radiation is greater in the areas of lesser thickness or of lesser density, particularly in the field of defects - discontinuities or inclusions.
Methods of radiation monitoring are classified primarily by type (and source) of ionizing radiation and by the form of ionizing detector study.
Ionizing called the study, whose interaction with the environment leads to the formation of electric charges. As the ionizing radiation, consisting of charged particles has a small penetration capability, then for radiation monitoring welds typically used radiation photons or neutrons. The most commonly used x-rays (X-rays). This photon radiation with a wavelength of 6 ... 1 * 10-13 * 10-9 m. With the same nature as that of visible light but shorter wavelength (visible light 4 ... 7 * 10-7 m) , X-rays have high penetrating power and can pass through a sufficiently large thickness of structural materials. The interaction with the material of controlled items X-ray intensity decreases, which is used in the control. X-rays provides the greatest sensitivity control.
Get X-ray radiation in the X-ray tubes. Emitted from a heated cathode, electrons are accelerated to high voltage in an airtight cylinder, from which the air is pumped out, and pass through the anode. When the deceleration of electrons at the anode of the energy is released as photons of different wavelengths, including X-ray. The higher the accelerating voltage, the more energy is generated photons and their penetrating power.
The scheme of the radiation monitoring of the transmitted radiation:
1 - source of radiation;
2 - the product;
3 - defect;
4 - the detector (film);
5 - radiation density.
Another common type of ionizing radiation used in the testing of welded joints is γ-radiation. This photon radiation with a wavelength of 1 * 4 * 10-13 ... 10-12m arising from the decay of radioactive isotopes, the source of γ-radiation in the radiation control usually is a radioactive isotope of thulium, iridium, cesium, cobalt, 170Tu, 192Ir, 137Cs , 60Co, and others. γ-radiation sources are compact and do not require large amounts of electricity (only for lighting and maybe move the radioactive isotope in the working position and vice versa). However, γ-radiation is more harmful to humans and, unlike X-rays can not be turned off. The penetrating ability of γ-radiation is higher than X-ray, so may shine product of greater thickness, but the sensitivity control with the lower difference between the defective and defect-free portions is less noticeable. Therefore the application area γ-inspection - inspection of products of large thickness (small defects in this case are less dangerous), control in the field of assembly and, in particular - pipelines and large reservoirs, translucence of complex shapes, if you can not place the x-ray machine.
Depending on the method of detection (detection and registration) ionizing distinguish radiography study in which the internal structure of the image fixing products occurs on film or paper, radioscopy (image seen on the screen) and radiometry (recorded electrical signals). Radiography is most popular with regard to simplicity, clarity and documented inspection results. When radiographic inspection for the registration of the intensity of radiation transmitted through the metal used radiographic film or photo paper (the method of direct exposure), activated metal screens or charged wafers (the method of image transfer).
More common method of direct exposure. If it can be used all the above types of ionizing radiation. The optical density of blackening of radiographic film or photographic paper depends on the dose of ionizing radiation, it is more on the ground, covered with less dense regions of the controlled object. Therefore, defects such as pores, cracks, lack of fusion and slag inclusion, will appear on the radiographic film in the form of dark spots corresponding shape. Inclusion of more dense than the base metal (eg, tungsten welding aluminum consumable electrode) will be on view radiograms have bright spots. To better identify the direction of the radiation defect should if possible coincide with the direction of its maximum size.
Translucence butt joint (Fig. 2) is usually carried out perpendicular to the surface or in the direction of cutting edges as possible the formation of defects in the weld fusion line. When monitoring the fillet welds of radiographic direction chosen by the bisector of the angle or the direction of cutting edges. When testing pipe welds and box designs best option is to place the radiation source within the product, as in this case, firstly, there is a panoramic radiographic a single exposure, and secondly weaken the walls of the product stream of ionizing radiation in the environment. If it is impossible within the premises of the radiation source translucence is carried out, including through two walls at an angle to the axis of the joint to avoid superimposing images of joints at each other (Fig. 2c). Only about 1% of the photons of ionizing radiation passing through the film, interact with it. Therefore, to increase the sensitivity of the control and acceleration of radiographic intensifying use fluorescent or metal foil screens of heavy metals (usually lead), glued on a flexible plastic.
Fluorescent Screens are plastic or cardboard substrate on which a phosphor layer - substances glowing under influence of ionizing radiation. They are used with a special film that is apparently sensitive to ultraviolet and infrared radiation. The lowest exposure obtained using fluorescent screens, and the best sensitivity - using metal screens.
Schemes of radiographic testing:
and - the butt;
b - fillet welds;
in - pipe;
1 - source of radiation;
2 - Film
Radiographic films are sensitive to radiation and contrast. The greater the sensitivity of the film, the better the performance control. The higher the contrast of the film, the higher the sensitivity control. Therefore, high-contrast film used in transmission responsible products, as well as light metal parts and small thickness.
Detection of defects in radiographic inspection also depends on the sharpness of the image. The causes of confusion may be the formation of the emulsion layer of the film photoelectrons (internal blur), the scattering of the radiation in the material goods (especially in transmission products large thickness), displacement or oscillation relative location of the source and detector products (eliminated rigidly fixed) and the difference between the real shape of the radiation source from the point (geometric unsharpness). To reduce the geometric blur used radiation sources with the smallest possible size of the focal spot, best approximate the film to a controlled product and increases the focal length (from the radiation source to the film).
Preparing for the radiography radiography is at the preliminary examination of the welded joint and clean it from the slag, oil and other contaminants. External defects are removed. Lots marked with the seam of lead characters, or mark film or fluorescent screen. On the surface of the article near the controllable seal established standards sensitivity often - trench: the plate with the grooves of varying depth and width.
The time is determined by the radiographic exposure nomograms which are usually built for each material, depending on its thickness, the radiation energy (in particular, the voltage at the anode X-ray tube), the focal length; type applied film and intensifying screens.
Method of image transfer is relatively rare: in the control of radioactive products and kseroradiografy.
When radiography of radioactive products are used as neutron radiation, as well as a detector - metal activate the screen, which are activated in the neutron flux and are not sensitive to γ-radiation. Then, the latent image is transferred onto a radiographic film, applying it to the metallic screen.
Kseroradiografy avoids the use of radiographic film. This delivers improved performance monitoring by eliminating time-consuming photo processing, as well as a reduction of expenses in connection with the exception of the consumption of silver, which is part of the film. The detector when used kseroradiografy special kseroradiografic plate consisting of a conductive substrate (aluminum, brass, glass, or paper with a conductive coating) coated with a semiconductor coating (often selenium). The radiation source is mainly used X-ray machines, at least - radioisotope sources braking or γ-radiation. When kseroradiografy charge kseroradiografic plate by corona discharge and placed in a light-tight cassette. In the process of radiographic selenium becomes a conductor, the charge leaks. The greater the intensity of the transmitted radiation, the smaller the residual charge. Then the plate is applied colorant. The dye is transferred onto the paper contact, in a manner fixed thereon with acetone or other solvent. Time control compared with conventional radiography is reduced tenfold. One kseroradiografic plate may be used up to 1000 times.
Radioscopy based on candling controlled objects by ionizing radiation, converting the transmitted radiation in black and white or electronic image, followed by amplification, transmission and analysis of images on the screen or the optical device of the television receiver. The use of television systems provide radiation safety personnel, allows you to boost the brightness and contrast, and zoom.
As a source of ionizing radiation during radioscopy increasingly used X-ray machines, rarely linear and cyclic accelerators and radioisotope sources of high power. Prospective application of neutron radiation produced in nuclear reactors or neutron generator.
Radioscopy lets examine the internal structure of the object at the moment of radiographic and saves the dignity of radiography: the ability to determine the type, nature and form of the defect. The low inertia of the radiation image conversion allows for a short time to explore the object at different angles, which increases the probability of detection of latent defects. Sensitivity radioscopy lower sensitivity radiography performance - higher. In installations for radioscopy may provide a mark and subsequent radiography revealed defective sites.
Radiometry is based on transmission products to ionizing radiation, and converting the flux density or spectral content of the transmitted radiation into an electrical signal. The radiation source used mainly radioisotopes (γ-rays), accelerators, at least - X-ray machines and neutron sources. The detectors use the ionization chambers, gas-discharge counters (proportional and Geiger counters), fixing the ionization or gas discharge under the action of ionizing radiation, and scintillation counters based on measurements using electron multipliers intensity light flashes in phosphors.
Unlike radiographic and radioscopic methods in object-rayed radiometry narrow beam of radiation. If the item is defective rayed, the registration unit will celebrate the change in intensity of the radiation.
Benefits radiometry: high sensitivity (higher than the radiographic method), high performance, non-contact monitoring of quality moving products, which is particularly useful when the maquila sector (possibly exercise feedback process). The main drawback of radiometry: integrating properties - Simultaneous recording of the signal from the defect and changes in the thickness of the product. This makes it difficult to determine the shape, size and depth of the defect - sometimes necessary to remove or smooth out the weld reinforcement.
With radiation control methods revealed cracks, lack of fusion, inclusions, pores, undercuts and other defects. The inspection results are evident (besides the usual radiometry), so compared to other NDT methods during radiation control is easier to determine the type of defect. As a rule, does not require high purity surface welds and products can be controlled by a comparatively great thickness.