1 Introduction
With the development of the times and the advancement of science and technology, higher and higher requirements are put forward for the safety status of products, product quality and performance in use. Due to the superiority of non-destructive testing technology, it is used in the manufacture and use of pressure vessel products. It is widely used.
2. Commonly used non-destructive testing techniques for pressure vessels
2.1 Radiographic testing
At present, radiographic testing can be divided into four categories according to the definition of the American Society for Testing and Materials: photographic testing, real-time imaging testing, tomographic testing and other radiographic testing technologies. Among them, radiography is the most widely used radiographic testing method in pressure vessels. .
Radiography refers to the use of X-rays or γ-rays to penetrate a test piece. The difference in intensity of the test piece due to defects that affect the absorption of rays is detected by measuring this difference, and the film is used as a non-destructive equipment for recording information. Detection method. Radiographic equipment can be divided into: X-ray inspection machine, high-energy ray inspection equipment, gamma-ray inspection machine.
Features of radiography:
⑴ Intuitive images of defects can be obtained, and the positioning can be accurate.
⑵ The test results can be recorded directly and can be stored for a long time.
⑶ The detection rate of volume defects (such as pores, slag inclusions, etc.) is very high. For area defects (such as cracks, unfused, etc.), if the camera angle is inappropriate, it is easy to miss the detection.
⑷ It is suitable for inspecting butt welds, but the effect of inspecting fillet welds is poor, and it is not suitable for inspecting plates, bars, forgings, etc.
2.2 Ultrasonic testing
Ultrasonic testing is mainly used to detect buried defects in butt welds and internal surface cracks of pressure vessel welds. It is also used to detect possible cracks in pressure vessel forgings and high-pressure bolts. Ultrasonic testing can be divided into ultrasonic testing, ultrasonic thickness measurement, ultrasonic grain size measurement, stress measurement, etc. There is a pulse reflection method that judges based on the echo of the defect and the echo of the bottom surface;
There is a penetration method to judge the defect condition based on the shadow of the defect, and a resonance method to judge the defect condition or judge the thickness of the board by the standing wave generated by the test object. At present, the pulse reflection method is most used.
Features of ultrasonic testing:
⑴ The detection rate of area defects is higher, while the detection rate of volume defects is lower.
⑵ Suitable for inspecting thicker workpieces.
⑶ Suitable for various test pieces, including butt welds, fillet welds, T-shaped welds, plates, pipes, bars, forgings, composite materials, etc.
⑷ The inspection cost is low, the speed is fast, the inspection instrument is small in size, light in weight, and convenient to use on site.
⑸ Unable to obtain visual images of defects, difficult to locate, and low quantitative accuracy.
⑹ There is no direct witness record of the inspection results.
2.3 Magnetic particle testing
Magnetic particle testing is a non-destructive testing method that displays surface and near-surface defects of ferromagnetic materials based on the interaction between the leakage magnetic field and the magnetic powder at the defect. The principle is that the ferromagnetic material produces a strong magnetic induction after magnetization, and the magnetic field density increases hundreds to thousands of times.
If there are discontinuities in the material (mainly including discontinuities caused by defects and discontinuities caused by structure, shape, material, etc.), the magnetic field lines will be distorted, and some of the magnetic field lines may overflow the surface of the material and pass through the space to form Leakage magnetic field, the local poles of the leakage magnetic field can attract ferromagnetic materials.
Features of magnetic particle testing:
⑴ It is suitable for the detection of ferromagnetic materials, but cannot be used for the detection of non-ferromagnetic materials.
⑵ It can detect surface and near-surface defects, but cannot be used to inspect internal defects.
⑶ The detection sensitivity is very high, and very small cracks and other defects can be found.
⑷ The detection cost is low and the speed is fast.
⑸ The shape and size of the workpiece sometimes affect the detection, so it is difficult to magnetize and cannot be detected.
2.4 Penetration detection
After the penetrant containing fluorescent dye or coloring dye is applied to the surface of the element, the penetrating liquid can penetrate into the defects of the surface opening after a certain period of time under the action of capillary; Applying developer, similarly, under capillary action, the developer will attract the penetrant remaining in the defect, and the penetrant will seep back into the developer. Under a certain light source, the traces of the penetrant at the defect will be displayed to detect The morphology and distribution of defects.
Features of penetration testing:
⑴ It can be used for any kind of materials except loose porous materials.
⑵ There are defects in several directions at the same time, and a comprehensive inspection can be roughly achieved with one operation.
⑶ The surface finish of the components has a large impact, and the test results are easily affected by the level of the operator.
⑷ It can detect the defects of the surface opening, but it cannot detect the buried defects or closed surface defects.
(5) The detection sensitivity is lower than that of magnetic particle detection.
2.5 Ultrasonic Testing by Time-of-Flight Diffraction (TOFD)
TOFD detection technology is to generate waveform conversion at the tip of discontinuous defects. When it is converted, a diffracted wave is generated. This diffracted wave covers a larger angle range. Then the diffracted wave will detect the existing defect and record the flight time of the signal. The height of the defect can be measured, and then the defect can be quantified. The defect size is usually defined as the flight time difference of the diffraction signal, and the signal amplitude has nothing to do with the defect quantification.
TOFD detection technology uses two pulse probes, one sending and one receiving, and the probes are arranged symmetrically with respect to the center line of the weld. The transmitting probe generates an unfocused longitudinal wave beam incident at a certain angle into the inspected workpiece. When the sound wave pulse is transmitted to one end of the receiving probe, the first signal reaches the end of the receiving probe. This is the side wave, and the side wave is in the inspection. Spread under the surface of the workpiece.
If there is no defect, then the second signal that reaches the receiving probe will be the bottom echo. The receiving probe determines the position and height of the defect by receiving the diffraction signal from the tip of the defect and its time difference.
Features of TOFD testing:
⑴ One scan can almost cover the entire weld area, and a very high inspection speed can be achieved.
⑵ The detection rate is very high, and it is easy to detect defects with poor directionality.
⑶ Various types of defects can be found, and they are not sensitive to the direction of defects.
⑷ Can identify defects extending to the surface.
⑸ The quantification and positioning of defects in the vertical direction are very accurate, and the accuracy error is less than 1mm.
⑹ The detection effect is better when combined with the pulse reflection method, and the coverage rate is 100%.
3 Application characteristics of non-destructive testing on pressure vessels
3.1 Non-destructive testing should be combined with destructive testing
Non-destructive testing technology has many advantages, but it has certain limitations and cannot replace destructive testing. During the evaluation of pressure vessel equipment, the non-destructive test results should be compared and verified with the destructive test results in order to make a correct judgment.
For example, in addition to non-destructive testing, blasting tests are required for LPG cylinders. To evaluate the quality of welded joints, in addition to non-destructive testing, specimens must be cut for mechanical performance analysis, and sometimes metallographic and fracture inspections are also required.
3.2 Correctly choose the timing of non-destructive testing
In the non-destructive testing of pressure vessels, the timing of the non-destructive testing should be correctly selected according to the testing purpose, combined with the characteristics of equipment working conditions, materials and manufacturing processes. For example, splicing heads should be subjected to non-destructive testing after forming; materials with a tendency to delay cracks should be non-destructive testing at least 24 hours after welding is completed;
Materials with a tendency to reheat cracks should be subjected to a non-destructive test after heat treatment; for low-alloy steel pressure vessels with a standard tensile strength lower limit greater than or equal to 540MPa, after the pressure test, the welded joints should also be subjected to surface non-destructive testing [1].
3.3 Choose an appropriate non-destructive testing method
When performing non-destructive testing of pressure vessel equipment, various testing methods have certain characteristics and cannot be applied to all workpieces and all defects. In order to improve the reliability of testing results, it should be based on the material, manufacturing method, working medium, and failure mode of the equipment. And the type, shape, location and orientation of the expected defects, select the most suitable non-destructive testing method.
3.4 Comprehensive application of various non-destructive testing methods
In non-destructive testing, it must be recognized that any non-destructive testing method is not omnipotent, and each method has its own advantages and disadvantages. Therefore, in non-destructive testing, if possible, do not use only one non-destructive testing method. Several testing methods should be used as much as possible to learn from each other’s strong points and obtain more defect information, so as to have a clearer understanding of the actual situation. Ensure the safe long-term operation of pressure vessel equipment.
4. Conclusion
The above briefly introduces several commonly used non-destructive testing methods and characteristics of pressure vessels. In actual application, we should combine the characteristics of various non-destructive testing methods and select suitable non-destructive testing methods to make non-destructive testing technology better in pressure vessel products. Good application.
