Comparison of advantages and disadvantages of three conventional nondestructive testing methods: ultrasonic testing (UT), magnetic particle testing (M

Nondestructive testing is the general term of all technical means to use the characteristics of sound, light, magnetism and electricity to detect whether there are defects or non-uniformity in the tested object, give the information such as the size, location, nature and quantity of defects, and then determine the technical state of the tested object (such as qualified or not, remaining life, etc.) without damaging or affecting the service performance of the tested object. Common nondestructive testing methods: ultrasonic testing (UT), magnetic particle testing (MT) and liquid penetrant testing (PT). Ultrasonic testing (UT)

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1. Definition of ultrasonic testing:

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Through the interaction between ultrasonic and the specimen, the reflected, transmitted and scattered waves are studied, and the macroscopic defect detection, geometric characteristic measurement, microstructure and mechanical property change detection and characterization of the specimen are carried out, and then its specific application is evaluated. 2. Working principle of ultrasonic wave:

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It is mainly based on the propagation characteristics of ultrasonic wave in the specimen. The ultrasonic wave is generated by the sound source, and a certain way is adopted to make the ultrasonic wave enter the test piece; Ultrasonic wave propagates in the test piece and interacts with the test piece material and defects therein, so that its propagation direction or characteristics are changed; The changed ultrasonic wave is received through the detection equipment and can be processed and analyzed; According to the characteristics of the received ultrasonic wave, evaluate whether the test piece itself and its interior have defects and the characteristics of defects. 3. Advantages of ultrasonic testing:

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a. It is suitable for nondestructive testing of metal, non-metal and composite materials;

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b. It has strong penetration ability and can detect the internal defects of the specimen in a large thickness range. For example, for metal materials, thin-walled pipes and plates with a thickness of 1 ~ 2mm can be detected, and steel forgings with a length of several meters can also be detected;

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c. Defect location is accurate;

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d. The detection rate of area defects is high;

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e. It has high sensitivity and can detect defects with small internal size;

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f. The detection has the advantages of low cost, fast speed, light equipment, no harm to human body and environment, and convenient use.

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4. Limitations of ultrasonic testing

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a. The precise qualitative and quantitative analysis of the defects in the specimen still needs to be deeply studied;

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b. It is difficult to carry out ultrasonic testing on specimens with complex or irregular shapes;

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c. The location, orientation and shape of defects have a certain influence on the test results;

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d. Material and grain size have great influence on the detection;

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e. When the commonly used manual A-type pulse reflection method is used for detection, the result display is not intuitive, and there is no direct witness record for the detection result. 5. Application scope of ultrasonic testing

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a. From the material of the test object, it can be used for metal, non-metal and composite materials;

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b. From the manufacturing process of the test object, it can be used for forgings, castings, weldments, cemented parts, etc;

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c. In terms of the shape of the detection object, it can be used for plate, bar, pipe, etc;

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d. In terms of the size of the test object, the thickness can be as small as 1mm or as large as several meters;

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e. From the defect position, it can be either surface defect or internal defect. A forging is an object in which the metal is pressed to shape the required shape or appropriate compressive force through plastic deformation. This force is typically achieved by using a hammer or pressure. The casting process builds a refined particle structure and improves the physical properties of the metal. In the practical use of parts, a correct design can make the particle flow in the direction of main pressure. Magnetic particle testing (MT)

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1. Principle of magnetic particle testing:

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After the ferromagnetic material and workpiece are magnetized, due to the existence of discontinuity, the magnetic field lines on the surface and near the surface of the workpiece are locally distorted, resulting in magnetic leakage field, which adsorbs the magnetic powder applied on the surface of the workpiece and forms a visually visible magnetic mark under appropriate light, thus showing the position, shape and size of discontinuity 2 Applicability and limitations of magnetic particle testing:

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a. Magnetic particle flaw detection is applicable to detect the discontinuities on the surface and near surface of ferromagnetic materials with very small size and very narrow gap (for example, 0.1mm long and micron wide cracks can be detected), which are difficult to be seen by visual inspection.

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b. Magnetic particle testing can detect raw materials, semi-finished products, finished products and parts in service, as well as plates, profiles, pipes, bars, weldments, steel castings and forged steel parts.

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c. Cracks, inclusions, hair lines, white spots, folds, cold shuts, looseness and other defects can be found.

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d. Magnetic particle testing cannot detect austenitic stainless steel materials and welds welded with austenitic stainless steel electrodes, nor can it detect non-magnetic materials such as copper, aluminum, magnesium and titanium. It is difficult to find shallow scratches on the surface, deeply buried holes and delaminations and folds with an included angle of less than 20 ° with the workpiece surface. Penetrant testing (PT)

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1. Basic principle of liquid penetrant testing:

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After the part surface is coated with penetrant containing fluorescent dye or colored dye, the penetrant can penetrate into the surface opening defect after a period of time under the action of capillary; After the excess penetrant on the part surface is removed, the developer is applied on the part surface. Similarly, under the action of capillary, the developer will attract the penetrant retained in the defect, and the penetrant will seep back into the developer. Under a certain light source (ultraviolet light or white light), the trace of penetrant at the defect will be realized (yellow green fluorescence or bright red), so as to detect the morphology and distribution of the defect. 2. Advantages of penetrant testing:

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a. Can detect all kinds of materials; Metallic and non-metallic materials; Magnetic and non-magnetic materials; Welding, forging, rolling and other processing methods;

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b. High sensitivity (0.1 can be found) μ M wide defect)

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c. It has the advantages of intuitive display, convenient operation and low detection cost. 3. Disadvantages and limitations of penetrant testing:

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a. It can only detect the defects of surface openings;

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b. It is not suitable for checking workpieces made of porous and loose materials and workpieces with rough surface;

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c. Penetrant testing can only detect the surface distribution of defects, and it is difficult to determine the actual depth of defects, so it is difficult to make quantitative evaluation of defects. The detection results are also greatly affected by the operator.

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Since various testing methods have certain characteristics, in order to improve the reliability of testing results, the most appropriate nondestructive testing method should be selected according to the equipment material, manufacturing method, working medium, service conditions and failure mode, and the possible defect type, shape, position and orientation.

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Any nondestructive testing method is not omnipotent. Each method has its own advantages and disadvantages. Several detection methods should be used as much as possible to learn from each other to ensure the safe operation of pressure equipment.