Non-destructive testing and radiographic testing of butt welding flanges

Non-destructive testing of butt welding flanges is the most widely used method in pressure vessel inspection. It can be applied to all aspects of vessel manufacturing. In the inspection of raw materials, steel plates for pressure vessels, high-pressure seamless steel pipes, IV Forgings of various steel grades above grade are required to be subjected to ultrasonic testing, the groove surface of high-strength grade steel after thermal processing must be surface tested, and the welds on the container must be subjected to radiographic or ultrasonic testing. In addition, non-destructive testing is also required for welders’ operating skills assessment, welding process qualification, product test panels, and in-use pressure vessel inspection.

　　 Since non-destructive testing (NDE or NDT) is a non-destructive testing method, it occupies a very important position in the inspection of products. It uses the interaction of sound, light, electricity, heat, magnetism and radiation with the substance to detect various surface or internal defects of materials, parts or equipment, and determine their location without damaging the performance of the inspected object. , Size, shape and type.

　　Radiation Testing (RT)

　　(1) Type of radiographic inspection

　　① According to the type of radiographic inspection

　　A. X-rays can penetrate 60~70mm steel plates and are 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 display method of the defect

　　A. The ionization method can be used for continuous inspection, but the shape and nature of defects cannot be judged; it is not suitable for testing butt welded flanges with varying thickness.

　　B. The fluorescent screen method can be tested continuously, and the results can be obtained immediately; the sensitivity is very poor, and only thin parts with a thickness of less than 20mm can be tested.

　　C. Photographic method The defect display effect is very good, and the most widely used.

　　(2) Principles of radiography

　　 X-ray and γ-ray are used as the radiation source for welding seam detection by 　　 radiographic inspection. The basic properties of these two kinds of rays are the same. Because the wavelength of γ-ray is shorter, the penetrating ability is stronger. General X-rays are generated by the X-ray tube in the flaw detector. In order to improve the transillumination ability to meet the detection of large and thick butt welded flanges, there are also high-energy X-rays provided by charged particle accelerators with an energy of more than 1MeV. Gamma rays use radioisotopes of iridium (Ir) and cobalt (Co) as the radiation source.

The 　　 radiographic method is to place the radiation source on one side of the butt-welded flange to be inspected, and put the film in the cassette to the other side of the butt-welded flange. When the rays generated by the X-ray tube are directed to the butt welding flange with the film in a straight line, it can pass through the butt welding flange and the film.

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

　　 When the rays pass through the defect, because the defect density is always less than the metal material, the ray attenuation is small, that is, when passing through the butt welding flange to the other side of the film, the intensity of the ray received there is greater. The film used for radiographic inspection and photography is coated with a substance that can produce a photochemical reaction on the base, which will produce a certain chemical reaction after radiation, and the depth of the reaction depends on the intensity of the received radiation under certain other conditions.

When the radiation passes through the defective part, the film has strong sensitivity. The photosensitive film is developed and fixed and called the negative film. The negative film can be observed on the film viewer to find the defective part inside the weld (defect The parts appear darker on the film), and the nature of the defect can be judged according to the characteristics of the image.