In actual welding quality inspection, two methods, non-destructive inspection and destructive testing, are generally used.
Non-destructive testing, also known as non-destructive testing, is a method of inspecting defects of materials or semi-finished products without damaging their performance.
The appearance inspection of welds is a commonly used simple inspection method, which is to use the naked eye, a sample (Figure-Welding quality inspection~Samples and inspection of welds), a measuring tool (Figure-Application of universal energy gauge in inspecting welds) or a low-power magnifying glass (generally no more than 10 times) to inspect the welds The appearance dimensions and welding seam forming conditions are inspected. The appearance inspection of welds is conducive to the analysis and discovery of internal defects to a certain extent.
The method is to use your eyes to get close enough to the weldment being inspected to directly observe and distinguish the defect morphology. Under normal circumstances, the visual distance is 400 ~ 600mm. . During the inspection process, use appropriate lighting, adjust the angle of illumination and observation, or use a low-power magnifying glass to improve the ability to detect and distinguish welding defects. If you cannot directly approach the weldment being inspected with your eyes, you must use an industrial endoscope for observation. The resolution of these devices should be equivalent to the direct resolution capability.
The main contents of appearance inspection include cleaning quality, weld appearance size, welding defects and surface condition. Among them, the cleaning quality mainly refers to the slag, spatter and attachments that hinder the appearance inspection of the weld and its base metal; the appearance size of the weld mainly refers to the reinforcement and height difference of the weld, the weld ratio and the groove ratio. Side widening and width difference, the weld straightness and width difference of the I-shaped groove, the welding legs at the tube plate shape and the convexity and concavity of the weld; welding defects mainly refer to cracks, incomplete welding, slag inclusions, pores, and welding defects. Nodules, undercuts, lack of fusion and concavity, etc.; surface condition mainly refers to primary welding, repair welding and rework welding.
For various containers, reactors and piping systems that store gases, liquids and liquefied gases, tightness tests are required on welds and sealing surfaces. Commonly used tightness tests include sealing inspection and air tightness inspection. The tightness test is a test method to check whether there is water leakage, air leakage, oil leakage, etc.; the air tightness test is to press compressed air (or ammonia, Freon, helium, halogen gas, etc.) into the welding container, and use the gas inside and outside the container to Test method for checking whether there is leakage by pressure difference.
Water-filled inspection is the simplest sealing inspection method and is often used in situations where there is no pressure or only static pressure generated by the weight of the liquid in the container.
Used to test the tightness and pressure resistance of the weld. The test method is to fill the container with water, plug all the holes and eyes on the container, and use a water pump to increase the water pressure to 1.25~1.5 times the working pressure of the container to conduct a strength test. During the strength test, attention should be paid to placing the container in a safe place. Test personnel and other personnel are not allowed to approach the pressure test container to prevent losses caused by abnormal explosions. After this pressure continues for a period of time (generally specified as 5 minutes), it is then reduced to the working pressure and a tightness test is carried out. At this time, the staff can approach the container and observe that if there are water droplets or fine water lines on the weld, it means that the weld is not dense and should be marked for repair. This method is often used for closed containers, as shown in Figure-Water pressure test of boiler gas bag Shows the water pressure test of the boiler gas bag. For vessels that require perspective inspection and post-weld heat treatment, the overpressure hydrostatic test should be conducted after heat treatment and perspective.
The pressure of compressed air used for testing is generally lower than the working pressure of the welded parts. Apply soapy water to the weld to check for leaks. This method is suitable for unpressurized or low-pressure containers, pipeline storage tanks, etc. This method should not be used for strength testing.
First, brush a layer of lime water on the outer weld of the container. After it is dry and white, apply kerosene on the inner side of the weld. Due to the small surface tension of kerosene, it has the ability to penetrate very small pores. When the weld has penetrating defects, kerosene can penetrate and produce obvious oil spots or strips on the side with the lime powder layer. In order to accurately determine the size and location of the defect, it should be observed immediately after applying kerosene. The general observation time is 15 to 30 minutes. This method is suitable for general containers and circulating water pipes that are not under pressure.
Penetrant inspection is a non-destructive inspection method that uses penetrants with fluorescent dyes (fluorescence method) or red dyes (coloring method) to show traces of defects. The fluorescence method is generally used for surface damage of non-ferrous metals; the coloring method is not limited by the type of material, nor is it affected by the shape and size of defects, but it is only applicable to open defects on the surface of weldments.
The basic operation process of penetrant testing is shown in Figure-The basic operation process of penetrant testing. The main procedures are:
Magnetic particle inspection is a non-destructive inspection method that utilizes the phenomenon of magnetic particle adsorption by the leakage magnetic field generated by defects on the surface of ferromagnetic materials in a strong magnetic field. Figure-Schematic diagram of magnetic particle flaw detection test principle is a schematic diagram of the principle of magnetic particle flaw detection test. Sprinkle magnetic iron oxide powder on the surface of the weld. According to the iron powder is the location and size of the defect can be judged by the traces of adsorption. After magnetic particle inspection, the weldment should be demagnetized.
The basic operation of magnetic particle inspection is shown in Figure 6-13. The basic procedure is:
Destructive testing is an indispensable method for performance testing of welds and joints. Destructive testing is mainly used for welding process qualification, weldability testing, welder skill evaluation and other inspection methods to evaluate welds and welded joints. Commonly used destructive tests include mechanical property tests, metal physical and chemical tests and welding performance tests.
Mechanical property tests include tensile (Figure-Stretching test), bending (Figure-Bending test) hardness (Figure-Hardness test of butt joints), impact (Figure-Impact test), fatigue, creep and other items to measure tensile strength , Yield strength, elongation, area reduction, bending angle, hardness, impact toughness, fatigue limit, creep strength and other indicators.
There are many methods of physical and chemical testing, which are generally carried out by professional inspectors. Welders can conduct macroscopic inspections by themselves for reference when analyzing problems.
Macroscopic inspection is to observe the test piece with the naked eye or with the help of a 5 to 10 times magnifying glass. The boundaries of each area of the weld, lack of penetration, cracks, serious structural unevenness, etc. can be clearly seen. During macroscopic inspection, cut the cross-section of the weld and welded joint, grind it with a grinding wheel and then polish it with sandpaper, and then etch it. All etched samples should be cleaned in clean water and dried with hot air from a hair dryer.
Welding performance tests are mainly fracture toughness tests, cold and hot crack tests, etc.
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