Non-destructive testing of welded flanges is the most widely used method in the inspection of pressure vessels, which can be applied to all aspects of container manufacturing. In the inspection of raw materials, ultrasonic testing is required for steel plates, high-pressure seamless steel pipe, and forgings of various steel sizes above grade IV, and surface testing is required for the groove surface of high-strength grade steel after hot working. The welds on the container should be inspected by X-ray or ultrasonic inspection. In addition, non-destructive testing is also required in welder operation skills assessment, welding process assessment, product test plate, pressure vessel inspection, and other aspects.

Nondestructive testing (NDE or NDT) is a non-destructive testing method, so it plays a very important role in product testing. It is a method to detect various surface or internal defects of materials, parts or equipment and judge their position, size, shape, and type by using the interaction of sound, light, electricity, heat, magnetism, and rays with substances without damaging the service performance of the inspected object.

X-ray detection (R T)

(1) Types of X-ray detection

① According to the type of X-ray detection

a. X-ray can penetrate 60~70mm steel plate, commonly used.

b. Gamma rays can penetrate steel plates over 150 mm.

c. High-energy X-rays can penetrate steel plates over 500mm.

② According to the defect display method

a. Ionization method can be used for continuous testing, but cannot determine the shape and nature of defects; It is not suitable for testing butt welding flanges with changes in thickness.

b. Fluorescent screen method can be continuous test, immediate results; The sensitivity is very poor and can only be tested for thin parts with thickness less than 20mm.

c. Photographic defects display effect is very good, the most widely used.

(2) Principle of radiography

X-ray detection photography test weld with X-ray sources for X-ray and gamma rays, the basic properties of the two rays are the same, because the gamma ray wavelength is shorter, so the penetration ability is stronger. General X-ray is generated by the X-ray tube in the flaw detection machine. In order to improve the transmission ability to adapt to the detection of large and thick welding flanges, high-energy X-ray provided by charged particle accelerator with an energy of more than 1MeV is also used. Gamma rays use radioactive isotopes of iridium (Ir) and cobalt (Co) as sources of rays.

Radiography is to place the ray source on one side of the butt welding flange to be inspected, and the film loaded into the cartridge is pressed against the other side of the butt welding flange. The X-ray tube can pass through the butt welding flange and the film when it emits the ray in a straight line to the butt welding flange.

And because there is always some absorption when a ray passes through a substance, that is, the intensity of the ray passing through a substance decreases continuously, the degree of attenuation is related to the thickness of the ray passing through and the properties of the substance itself, such as density. The greater the thickness or density of material passed through, the greater the attenuation of the rays.

When the ray passes through the defect, the defect density is always smaller than that of the metal material, so the ray attenuation is small, that is, when it passes through the butt welding flange to reach the other side of the film, the ray intensity received there is larger. The film used in radiography is coated with a material that can produce a photochemical reaction. The depth of the reaction depends on the intensity of the radiation received when other conditions are given.

When the ray passes through the defective part of the film, the sensitive film is strong, and the film after development and fixing processing is called the negative, the negative can be observed on the viewing lamp, and you can find the defective part of the weld inside (the defective part appears black on the negative), and you can judge the nature of the defect according to the characteristics of the image.

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