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In actual welding quality inspection, two methods, non-destructive inspection and destructive testing, are generally used.

Welding quality inspection~non-destructive testing

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.

Welding quality inspection~ Visual inspection of welds

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.

Welding quality inspection~Samples and inspection of welds
Welding quality inspection~Application of universal energy gauge in inspecting welds

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.

Welding quality inspection~Weld tightness test

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.

Weld tightness test-water test

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.

Weld tightness test-hydrostatictest

Water pressure test of boiler gas bag

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.

Weld tightness test-Air pressure test

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.

Weld tightness test-Kerosene test

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 testing

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:

Welding quality inspection
  • preprocessing. Before spraying the solution, remove the surrounding oil stains, rust spots, etc., then dry-wipe the inspected surface with acetone, then use a cleaning agent to clean the inspected surface, and then dry or air-dry.
  • penetration. Spray or brush the penetrant onto the surface to be inspected. When spraying, the distance between the nozzle and the surface to be inspected is preferably 20 to 30 mm, and the penetration time is 15 to 30 minutes. In order to detect small defects, the workpiece can be preheated to 40 ~ 50%C and then penetrated.
  • Clean. After the specified penetration time is reached, wipe off the excess penetrant on the surface with cotton cloth, and then clean it with cleaning agent. Be careful not to wash away the penetrant in the defects. When using an emulsified penetrant, it is required to apply the emulsifier on the surface to be inspected by dipping, brushing or spraying before cleaning. It is particularly effective in inspecting crack-shaped surface defects such as wires. The emulsified penetrant generally stays on the surface under inspection for 1 to 5 minutes, and then rinses with clean water.
  • imaging. Brush or spray a thin and even layer of developer on the surface with a thickness of 0.05 ~ 0.07mm, keep it for 15 ~ 30 minutes and then observe.
  • observe. Observe with the naked eye or a magnifying glass. When the surface being inspected is defective, a red pattern will appear on the white developer. The fluorescence method uses a black light or ultraviolet flaw detection lamp to illuminate in a dark place, and the defective area will display a bright fluorescent image.

Magnetic particle testing

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.

Magnetic particle testing-Schematic diagram of magnetic particle flaw detection test principle

The basic operation of magnetic particle inspection is shown in Figure 6-13. The basic procedure is:

Basic operations of magnetic particle inspection
  • Clean up. Before magnetic particle inspection, the surface of the weld to be inspected and the 30 mm area near it should be dried and cleaned. When the surface under inspection hinders the display, it should be polished or sandblasted.
  • Magnetization. According to the shape of the inspected surface and the direction in which defects are prone to occur, the magnetization method and magnetization current are selected, and the power-on time is 0.5~1S. Magnetization methods include the electrode electric shock method (using two electrodes to contact two points on the surface of the weld to be inspected, and energizing to generate a magnetic field) and the yoke method (electromagnet method or permanent magnet method). The electrode shock method is used. The magnetizing current range is shown in Table-Magnetizing current range. The electrode shock distance is Generally 80~200mm. Using the yoke method requires that the magnet used has a certain magnetomotive force, the lifting force of the AC electromagnetic yoke is not less than 50 N; the lifting force of the permanent magnet yoke is not less than 200 N, and the distance between the two magnetic poles should be between 80 and 160mm.
Magnetizing current range
  • Examine. The inspection operation should be carried out continuously. Magnetic powder should be applied when the magnetizing current passes. Dry magnetic powder should be sprayed or spread. The particle size of the magnetic powder should be uniform. Generally, it should be screened with a sieve of no less than 200 mesh. The magnetic suspension should be poured slowly, paying attention to the appropriate amount. When applying fluorescent magnetic powder, it needs to be carried out in the dark. Turn on the ultraviolet inspection lamp (or black light) 5 minutes before the inspection to make the fluorescent magnetic powder emit obvious fluorescence.The ratios of common fluorescent magnetic powder suspensions are shown in Table-Fluorescent magnetic powder suspension ratio. In order to prevent missed inspections, each weld generally needs to be inspected twice, and the direction of the magnetic field lines during the two inspections should be generally vertical.
Fluorescent magnetic powder suspension ratio
  • Ultrasonic flaw detection test
    Ultrasonic flaw detection is a non-destructive testing method that uses ultrasonic waves to detect internal defects in materials. Since ultrasonic waves propagate very far in metal, they can be used to detect defects in welds of large weldments (thickness greater than 40 mm), and can more sensitively find the location of defects, but it is difficult to determine the nature, shape and size of defects. Figure-Schematic diagram of ultrasonic flaw detection test is a schematic diagram of the ultrasonic flaw detection test, and Figure-Application scope of ultrasonic flaw detection is the applicable range of ultrasonic flaw detection. When the probe is at the M position, the ultrasonic wave does not encounter defects in the weld, reflects at K, and continues to propagate forward, and the probe cannot receive it. To the reflected wave, there is only one “final pulse” a on the fluorescent screen indicating that the ultrasonic wave is emitted to the weldment; when the probe moves to the N position, the ultrasonic wave encounters the defect c in the weld and is reflected back from the original path. After the probe receives it, it will “Defect pulse” c appears on the fluorescent screen. The depth h of the defect can be calculated from the distance from pulse a to c. The defect area can be determined from the pulse height of c. The characteristics of the defect wave are shown in Figure-Characteristics of defect waves
Schematic diagram of ultrasonic flaw detection test
Application scope of ultrasonic flaw detection
Characteristics of defect waves
  • Radiographic testing
    Radiographic inspection is a non-destructive inspection method that uses X-rays or gamma rays to irradiate welded joints to detect internal defects. Figure-Schematic diagram of the principle of radiographic flaw detection is a schematic diagram of the principle of radiographic flaw detection. Ultrasonic flaw detection is usually used to determine whether there are defects. After the defects are found, radiographic flaw detection is used to determine their nature, shape and size.
Schematic diagram of the principle of radiographic flaw detection

Welding quality inspection~Destructive testing

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 test

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.

Stretching test
Bending test
Hardness test of butt joints
Impact test

Metal physical and chemical tests

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 test

Welding performance tests are mainly fracture toughness tests, cold and hot crack tests, etc.