Nondestructive testing and radiographic testing of butt weld flanges

Non-destructive testing of butt welding flanges is the most widely used method in pressure vessel inspection, and 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, and forgings of various steel grades above grade IV with a thickness exceeding a certain value shall be subjected to ultrasonic testing, and the groove surface after hot processing of high-strength grade steel shall be subjected to surface inspection. For inspection, the welds on the container shall be subjected to radiographic inspection or ultrasonic inspection. In addition, non-destructive testing is also required in welder operation skill assessment, welding procedure evaluation, product test plate, and in-use pressure vessel inspection...

 

 

Since non-destructive testing (NDE or NDT) is a non-destructive testing method, it plays an important role in product testing. It uses the interaction of sound, light, electricity, heat, magnetism and ray with matter to detect various surface or internal defects of materials, parts or equipment without damaging the performance of the object to be inspected, and to determine its position , size, shape and variety of methods.

 

Radiographic Inspection (RT)

 

(1) Types of radiographic testing;

 

① According to the type of radiographic 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 display method of defects

 

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

 

b. The fluorescent screen method can be continuously tested 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 defect display effect is very good, the most widely used.

 

(2) Principle of radiography

 

X-rays and γ-rays are the radiation sources used to detect welds by radiographic inspection. The basic properties of these two kinds of rays are the same. Because the wavelength of γ-rays 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 adapt to 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 are sourced from radioactive isotopes of iridium (Ir) and cobalt (Co).

 

The radiographic inspection method is to place the radiation source on one side of the butt welding flange to be inspected, and attach the film loaded into the cassette to the other side of the butt welding flange. When the radiation generated by the X-ray tube hits 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 will always be some absorption when the ray passes through the material, that is, the intensity of the ray passing through the material is continuously attenuated, and the degree of attenuation is related to the thickness of the ray passing through and the properties of the material itself, such as density. The greater the thickness passed through or the greater the density of material passed through, the more the ray is attenuated.

 

When the ray passes through the defect, since the defect density is always smaller than that of the metal material, the attenuation of the ray is small, that is, when the ray passes 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 is coated with a substance that can produce photochemical reaction on the film base. A certain chemical reaction will occur after irradiation with radiation, and the depth of the reaction depends on the received ray intensity under certain other conditions.

 

When the ray passes through the defective part, the film has a strong light sensitivity. The photosensitive film is called negative film after developing and fixing treatment. When the negative film is observed on the film viewing lamp, the defective parts (defects) inside the weld can be found. The parts appear darker on the negative), and the nature of the defect can be judged from the characteristics of the image.