The quality requirements for welded structures, welded products, and welded joints are multifaceted. It includes internal requirements such as joint performance and organization. At the same time, there must be no defects in appearance, shape, dimension accuracy, weld shape, surface, and interior. In order to find out as soon as possible When dealing with problems, it is often a macroscopic analysis followed by a detailed microscopic analysis if necessary.
The most important content of macro analysis is to analyze the defects of welded joints. Mainly adopt the low-magnification analysis method of the metallographic microscope, through low-magnification metallographic inspection on the internal defects of welded joints, cooperate with high-magnification microstructure analysis to determine the cause of defects, find out the methods to avoid and eliminate, and improve the quality of welded joints quality.
Through sampling, grinding, etching and low-magnification photography, we can clearly and intuitively check the macroscopic defects of welded joints, and combine the corresponding welding standards to judge whether the welding process, welding workers and welding structure can meet the relevant requirements. Requirements.
According to the cause of formation and defect form, the macroscopic defects of welds can be mainly divided into the following categories:
1. Stomata
During the crystallization process of the welding pool, some gases may remain in the welding mirror to form pores because some gases have no time to escape. Porosity is a common defect in welded joints. Pores not only appear on the surface of the weld but also often appear inside the weld. It is difficult to detect it with simple methods in welding production, and it will cause serious harm. The welding porosity generated inside the weld is called internal porosity, and the porosity that opens outside is mostly called surface porosity.
2. Slag inclusions
Slag inclusions are slag or other non-metallic inclusions contained in welds, and are a common defect in welds. In welding using filled flux wire, such as submerged arc welding, slag is formed due to poor deposition, or in CO2 welding without flux, the product of deoxidation produces slag, which remains inside the multilayer welding metal. Slag inclusions are formed.
3. Lack of penetration and lack of fusion
Incomplete penetration refers to the part left by the incomplete penetration of the root of the joint during welding. Incomplete fusion is a common defect, which refers to the local residual gap between the molten metal and the base metal or between the adjacent weld bead and the weld layer, and the incomplete melting and bonding between the base metal and the base metal during spot welding. Some are called unfused.
4. Cracks
Welding cracks are divided into hot cracks (crystallization cracks, high-temperature liquefaction cracks, polygonal cracks), cold cracks (delayed cracks, hardened embrittlement cracks, low plasticity cracks), reheat cracks, and lamellar tear cracks according to their shape and cause. Crack and so on.
5. Undercut
Undercutting is sometimes called undercutting. It is a groove lower than the surface of the base metal produced at the welding toe because the deposited metal does not completely cover the melted part of the base metal during welding. After the welding arc melts the edge of the weldment, Not getting the gap left by the replenishment of molten metal from the electrode. An undercut that is too deep will weaken the joint and may also cause structural failure at the undercut.
6. Other Defects
In addition to the above-mentioned defects, the common defects of welds include porosity, cold shut, burn-through, weld bumps, shrinkage cavities, pits, sags, uneven weld leg size, excessive concavity/convexity, and wrong weld toe angles Wait.
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