How to perform nondestructive testing of pressure vessels?

1. Radiographic testing (RT)
Testing principles and equipment:
Radiographic testing uses the different attenuation of X-rays or γ-rays at defective and non-defective parts when they penetrate an object, making the film sensitive to different levels to detect defects. For example, when the ray passes through a weld with pores, the attenuation of the ray at the pores is less than that of the surrounding metal, forming a darker image on the film. The equipment mainly includes a ray source (X-ray machine or γ-ray source), film, dark box, etc. For the inspection of thick-walled containers or large containers, the γ-ray source is more suitable due to its strong penetration; for thinner parts, the X-ray machine can provide clearer images.
Testing steps and key points:
First, clean the surface to be inspected, remove impurities such as oil and rust, and ensure that the surface is flat. Then select a suitable ray source and film according to the shape, thickness and inspection requirements of the container. Place the film on the back of the container to be inspected, and the ray source at a certain distance from the front, and expose it according to the specified exposure parameters (such as voltage, current, exposure time, etc.). After exposure, the film is developed and fixed in a darkroom. When observing the film, pay attention to identifying different types of defect image features, such as round or oval black images for pores and linear black images for cracks. At the same time, the defects should be evaluated according to the quality grading method specified in the standard.
2. Ultrasonic testing (UT)
Detection principle and equipment:
Ultrasonic testing is to emit ultrasonic waves to the object to be tested. When the ultrasonic waves encounter defects or interfaces, reflection, refraction and waveform conversion will occur, and the defects will be judged according to the changes in the received signals. The equipment mainly includes ultrasonic flaw detectors and probes. The flaw detector generates high-frequency electrical pulse signals, which stimulate the probe to generate ultrasonic waves and receive reflected echo signals. Different types of probes (such as straight probes and oblique probes) are suitable for different detection objects and defect types. For example, straight probes are used to detect defects parallel to the detection surface, and oblique probes are used to detect defects such as oblique cracks in welds.
Detection steps and key points:
Before testing, the surface to be tested should be polished so that the probe can have good contact with the surface. Then set appropriate detection parameters on the flaw detector, such as probe frequency, gain, scanning range, etc. During the inspection process, move the probe along the inspection surface and observe the echo signal on the flaw detector screen. The location, size and nature of the defect are judged according to the amplitude, position and shape of the signal. For example, when the echo amplitude is high and the position does not match the normal bottom surface echo position, there may be a defect. Unlike X-ray inspection, the ultrasonic inspection results are displayed by electronic signals, and the inspectors need to have rich experience and professional knowledge to accurately judge the defect situation.
3. Magnetic Particle Testing (MT)
Detection principle and equipment:
Magnetic particle testing is mainly used to detect defects on the surface and near the surface of ferromagnetic materials. When the inspected part is magnetized, a leakage magnetic field will be generated at the defect. The magnetic powder (divided into fluorescent magnetic powder and non-fluorescent magnetic powder) is sprinkled on the surface. The magnetic powder will be adsorbed by the leakage magnetic field to form magnetic traces, thereby showing the location and shape of the defect. The equipment mainly includes magnetizing devices such as magnetizing power supply, magnetizing coil or magnetic yoke. According to the shape and size of the inspected part, different magnetizing methods can be selected, such as circumferential magnetization, longitudinal magnetization and composite magnetization.
Inspection steps and key points:
Before inspection, clean the inspected surface and remove impurities such as oil and rust, as these will affect the adsorption of magnetic powder. Then select the appropriate magnetization method and magnetization current to magnetize the inspected part. While magnetizing, sprinkle the magnetic powder evenly on the surface or spray the magnetic suspension (a mixture of magnetic powder and liquid) on the surface with a spray can. Observe the location, shape and concentration of the magnetic traces to determine the type and severity of the defect. For example, the magnetic traces of surface cracks are generally linear and clear; while the magnetic traces of near-surface defects may be lighter. After inspection, the parts should be demagnetized to avoid residual magnetism affecting subsequent use.
4. Penetrant testing (PT)
Inspection principle and equipment:
Pentnetrant testing is suitable for detecting defects in non-ferromagnetic materials and surface openings. The penetrant containing color dyes or fluorescent agents is applied to the inspected surface, and the penetrant will penetrate into the defects of the surface opening. After waiting for a certain period of time, remove the excess penetrant, and then apply the developer. The penetrant in the defect will be adsorbed on the developer, forming an obvious display mark. The equipment mainly includes penetrant, developer, spray can, etc. Penetrant is divided into water-washable type, post-emulsification type and solvent removal type. The appropriate type should be selected according to different testing requirements and materials to be tested.
Testing steps and key points:
First, pre-treat the surface to be tested to ensure that the surface is clean, dry and free of oil. Then apply the penetrant evenly on the surface, and wait for the appropriate penetration time (generally 10-30 minutes) according to the type of penetrant and the material to be tested. Remove excess penetrant with appropriate methods (such as rinsing with water, wiping with solvent, etc.). Then apply the developer, wait for the development time (generally 7-15 minutes), and observe whether there are any display marks on the surface. If so, judge the defect according to the shape, size and color of the mark. For example, a circular mark may be a pore, and a linear mark may be a crack. After the test is completed, remove the penetrant and developer residue on the surface.