Non-destructive testing is the most widely used method in the inspection of pressure vessels. It can be applied to all aspects of vessel manufacturing. In the inspection of raw materials, steel plates for pressure vessels with a thickness exceeding a certain value, high-pressure seamless steel pipes, and various types above grade IV Forgings of steel grades need to be subjected to ultrasonic testing, the groove surface of high-strength grade steel after thermal processing needs to be surface tested, and the welds on the container need to be subjected to radiographic or ultrasonic testing. In addition, non-destructive testing is also required in aspects such as welder operation skill assessment, welding process qualification, product test board, 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.
2. 14. 1 Radiographic testing (RT)
(1) Types of radiographic testing
① According to the type of radiographic inspection
a. X-rays can penetrate 60~70mm steel plates and are commonly used.
b. γ-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 inspecting workpieces 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 the radiation sources used by radiographic inspection to detect welds. The basic properties of these two 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 workpieces, there are also high-energy X-rays provided by a charged particle accelerator 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 workpiece, and put the film in the cassette tightly on the other side of the workpiece. When the rays generated by the X-ray tube are directed toward the workpiece with the film in a straight line, they can pass through the workpiece 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 ray passes through the defect, because the defect density is always less than that of the metal material, the ray attenuation is small, that is, when the ray passes through the workpiece 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 a strong sensitivity. The photosensitive film is developed and fixed and called the negative film. The negative film is 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.
Ultrasonic Testing (UT)
(1) Types of ultrasonic testing
①According to the coupling method
a. Contact method A method of direct detection with a layer of coupling agent such as glycerin or engine oil on the surface of the probe and the workpiece.
b. Water immersion method There is a layer of water on the surface of the probe and the workpiece, and the thickness of the water layer is adjusted so that the propagation time of the sound wave in the water is an integer multiple of the metal. Divided into full immersion (workpiece and probe are all immersed in water) and partial immersion type (workpiece and probe are partially immersed in water).
②According to the signal receiving mode
a. Reflective method Use a probe to reflect and receive ultrasonic waves, and what is received is the ultrasonic waves reflected by the defect or the bottom surface of the workpiece. This method is commonly used.
b. Penetration method One probe reflects ultrasonic waves, and the other probe receives ultrasonic waves. The two probes are on both sides of the workpiece, and the received ultrasonic waves are the parts blocked by the reflected ultrasonic waves to remove defects.
③According to the continuity of ultrasound
a. Continuous wave detection The emitted ultrasound is continuous, and it is often used for ultrasound image display.
b. Pulse wave detection The emitted ultrasonic wave is pulsed, and it is commonly used in field detection.
④According to wave type
a. Longitudinal wave detection The wave pattern transmitted and received by a straight probe is mainly used for steel plate detection (Figure 2-2).
b. Transverse wave detection The wave pattern transmitted and received by the inclined probe is mainly used for the detection of welds (Figure 2-3).
As a special case, the wave pattern emitted and received by an oblique probe whose probe angle is equal to the second critical angle (incidence angle α = 55°) is specifically used to find defects on the surface or very close to the surface. This is (α =55°) a method of oblique probe detection.
c. Rayleigh wave detection When the thickness of the workpiece is greater than the wavelength used, it is a Rayleigh wave detection. It is used to find defects near or at the surface of the workpiece and perpendicular to the surface of the workpiece (Figure 2-4).
d. Lamb wave detection When the thickness of the workpiece is less than the wavelength used, it is a Lamb wave detection. It is used to inspect shallow flaws that are close to the surface and parallel to the surface of the workpiece (Figure 2-5).
(2) Principle of ultrasonic testing
At present, the ultrasonic detection method widely used in the industry is the pulse reflection method according to its working principle. According to the reflected wave display mode, there are A type, B type, C type, 3D type and so on. Type A is the wave amplitude display, that is, the reflected wave shows whether the defect exists and its relative position, and the size of the defect is determined from the height of the wave amplitude. The other types are the image display of defects. The current ultrasonic testing standard JB 4730-1994 for pressure vessels refers to the use of A-type display to detect defects.
The pulse detection method is to add the high-frequency electrical pulse signal generated by the transmitter circuit in the ultrasonic detector to the piezoelectric wafer of the probe. The wafer receives the high-frequency electrical pulse, and due to the inverse piezoelectric effect, the high frequency with the same frequency as the electrical pulse will be generated. Mechanical vibration, touch the probe to the workpiece, and coat the contact surface between the probe and the workpiece with a good sound-permeable coupling agent such as oil, glycerin or water. Its function is to eliminate the air gap between the contact surfaces and make the sound beam energy. It is better to enter the workpiece through the interface, this method is called the contact method.
The workpiece and the probe head can also be immersed in the coupling liquid. The commonly used coupling liquid is water, and the probe does not touch the workpiece. This method is called liquid immersion or water immersion. Whether it is the contact method or the water immersion method, the vibration of the wafer on the probe can enter the workpiece at a certain angle. According to the linearity and directivity of the ultrasonic wave, the ultrasonic wave propagates forward in a certain direction and within a certain range.
If it encounters a heterogeneous interface, such as a defect surface or the outer surface of a workpiece, the ultrasonic wave will be reflected back in a certain direction according to the reflection law and be received by the probe, causing the probe wafer to vibrate. Due to the positive piezoelectric effect, this mechanical vibration is again It is converted into an electric pulse signal and received by the instrument. The pulse signal processed by circuits such as amplification and detection is displayed on the fluorescent screen of the instrument. This is the reflected wave. According to the characteristics of the reflected wave on the phosphor screen, the relative position, and the height of the wave amplitude, determine whether there is a defect, the location of the defect, the size and the nature.
During detection, the transmitter circuit outputs pulse signals intermittently at fixed time intervals, so the wafer is also in intermittent working state. When it receives electrical pulses and generates vibration, it acts as an ultrasonic generator; during the intermittent time when the vibration is stopped , It acts as an ultrasonic receiver and waits for the reflected ultrasonic signal to vibrate the wafer and convert it into an electrical signal.
The probe used as the receiver may be the probe originally used as the transmitter. This is the single-probe detection method; for example, the two probes are used for transmitting and receiving, which is the double-probe method. No matter which method, the output is an electrical pulse signal, and the reflected wave is used to find and determine the defect, so it is called the pulse reflection method.
Ultrasonic Testing by Time-of-Flight Diffraction (TOFD)
TOFD inspection is an ultrasonic inspection method that mainly uses the diffraction wave signal at the end of the defect to detect and determine the size of the defect. Its basic feature is the use of one-shot-one-receive probe pair working mode.
TOFD usually uses a longitudinal wave oblique probe. After the ultrasonic pulse is transmitted at the flawless part of the workpiece, the first through wave to the receiving probe is followed by the bottom surface reflected wave. When there is a defect, between the through wave and the bottom reflected wave, the receiving probe will also receive the diffracted wave or reflected wave generated by the defect. In addition to the above-mentioned waves, there are also transverse waves generated by the wave form conversion at the defect site and the bottom surface, which generally arrive at the receiving probe later than the reflected waves from the bottom surface. The ultrasonic propagation path in the workpiece is shown in Figure 2-6, and the A scan signal at the defect is shown in Figure 2-7.
Magnetic Particle Testing (MT)
(1) Magnetic particle detection method
Magnetic powder detection includes dry magnetic powder, wet magnetic powder, fluorescent and non-fluorescent magnetic powder detection methods.
(2) Principle of magnetic particle detection
Magnetic particle testing is to use ferromagnetic materials to magnetize and sprinkle magnetic powder on the surface of the workpiece. Magnetic powder is used to show the magnetic leakage caused by defects during magnetization. The largest amount of magnetic leakage caused by defects on the surface and near the surface, cracks and incomplete penetration Defects such as not fusion, etc., when the extension direction is perpendicular to the direction of the magnetic force line, large magnetic leakage will also occur.
Penetration Testing (PT)
(1) Penetration detection method
Penetration detection includes non-fluorescent and stained penetration detection methods.
According to the different types of imaging agents and penetrants, see Table 2-17 and Table 2-18 for the classification of penetration detection methods, and Table 2-19 for the detection steps used in combination of various methods.
(2) Selection of penetration testing methods
The selection of the penetrant detection method can be determined according to the surface roughness of the inspected workpiece, the detection sensitivity, the size of the inspection batch, and the water source and power supply at the inspection site.
Workpieces with smooth surface and high detection sensitivity should adopt emulsification coloring method or post-emulsification fluorescence method, and solvent removal fluorescence method can also be used.
Workpieces with rough surfaces and low detection sensitivity requirements should be washed with water-washed coloring method or water-washed fluorescent method.
The detection of on-site no water source and power source should adopt solvent removal fluorescence method.
For the detection of large batches of workpieces, it is advisable to use the water-washed coloring method or the water-washed fluorescent method.
For local detection of large workpieces, solvent removal type coloring method and solvent removal type fluorescence method should be adopted.
The fluorescence method has higher detection sensitivity than the color method.
