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Inspection of welding quality

Before and in the welding process, it must carefully examine the factors affecting the welding quality, in order to prevent and reduce generation of welding defects; and after welding, it must detect welding part quality and capability according to the technical requirement of product, to ensure safe to use. 

  Inspection methods of weld products can be divided into two types: destructive inspection and nondestructive inspection. Destructive inspection includes analysis weld joint chemical components analysis, microstructure analysis and mechanical capability analysis, and it is mainly used for research and new products. There are several methods in non-destructive inspection, because these methods do not harm products, so they play an important role. 

  There are usually non-destructive inspection methods: 

1. Exterior inspection. Inspect with the naked eyes or by use of sample, low-multiple magnifying glass (5~20 times), to inspect weld joint figuration, and inspect weld dimensions are to meet the requirements or not, is there any defects on the surface of weld joint. All weld joints must to be passed by exterior inspection after welding. 

2. Seal tightness test. It is necessary for containers of gas storage, liquid storage, and liquefied gas, reactors and pipeline system. All weld joints and sealing surfaces must to be done. Here are usual methods as following: 

(1) Hydraulic test. It inspects containers and pipes withstand high pressure enough. This kind inspection is not only to test defect penetration for materials, but to test weld joint strength. In the testing process, filled with water to the container at first, then increased water pressure to 1.2~1.5 times of working pressure, and maintained more than 5 minutes, and then step-down to working pressure, next to knock along the weld joint with round-hammer percussion. Inspect weld joints if any leakages will be happen. 

  (2) Air pressure test (also called “air tightness test”). Inspect leak tightness of low pressure containers, pipeline, and ship cabins. Fill compressed air into the containers or into the pipelines when test, daub coated subs on the surface of weld joints, then inspect where will leak. It can also put the containers or pipelines into tank full of water, and fill compressed air into the containers or pipelines, see if there are bubbles emerge. 

  (3) Kerosene test. It is used for inspect leak of non-pressure weld joints or containers. Method is daub chalk powder liquid on the weld joint surface, when the surface is dry, daub kerosene on the opposite surface of weld joints. If there is defect of penetrability, there will show oil spot on the side of coated chalk powder, and it can determine the defected location. If does not appear oil spot within 15~20 minutes, you can consider it is qualified. 

3. Magnetic particle. It is used for inspect the surface or near surface defect (such cracks, porosity, slag, etc.) of  

weldment. Put the weldment in the place of magnetic field to magnetization, let magnetic field through magnetic line of force evenly distributed in weldment interior. When sprinkle fine magnet particle on the weld surface, magnet particle distributed evenly if there is no defect on weld surface. Otherwise, magnetic particle centralized in the defective place because magnetic of force left the defective place, exposed in the air, and formed left out magnetic field. According to the location, shape and size of magnetic particle, we can judge defective situation. 

4. Penetrant inspection. This method is only to inspect defect on the workpiece surface when it is difficult to discover with naked eyes. It could not inspect defect that under the surface. There are two usual methods: fluorescence inspection and color inspection. 

  Fluorescence inspection is the fluorescent liquid (mineral oil contained MgO) daub on weld surface, for this liquid has a strong infiltration ability to infiltrate the defect surface. When clean up the weld surface, in the ultraviolet irradiation, and remnant fluorescent liquid in the defect will show yellow green reflect. According to the reflective situation, it can judge weld surface defective situation. Fluorescence inspection is generally used in non-iron alloy surface detect. 

  Color inspection is the coloring agent (containing Sudan Red dye, kerosene, turpentine, etc.) daub on weld surface, the coloring agent will infiltrate into the crack. After a certain time, clean up the weld surface, spray white imaging agent, to maintain 15~30 minutes, if there is red stripe on the white underside, it means where exists defect. 

5. Ultrasonic inspection. This method is used to detect internal defects in materials. Ultrasound passes through the welding piece from the surface into the interior by probe, when it reaches the defective place and the bottom, there are two reflected waves early or late back to the instrument. Reflected waves signals are received by instrument and will appeared pulses at the screen. According to the level pulse signal high or low, interval, and location, it can determine whether there is (are) defect (defects) or not, location(s) and size(s), but it is uncertain of character and shape of defect. Ultrasonic inspection is mainly used for checking the heavy weldment with smooth surface and simple shape, and always cooperates with ray inspection. Ultrasonic inspection determines whether there is (are) defect (defects) or not, and when it discovered defect(s), then use ray inspection to confirm its character, shape, and size. 

6. Ray inspection. Irradiate weld joints by use of X ray or γ-ray, detect welding defect according to sensitization of negative film. Because the density of weld defect is lower than the metal, so sensitization is higher than other places. After developing the film, it will appear black stripes or spots, according to the black spot location, shape, and size, and it can judge defect location, size, and type. X-ray inspection is suitable for weldment thickness less than 50mm, and γ-ray inspection is suitable for weldment thickness within 50~150mm.