Steel pipe weld cracks are often pided into two types of hot and cold cracks according to different mechanisms. It is produced by thermal cracking of liquids, the solid-state crystallization process in the weld metal, which is mainly produced in the weld metal. The main reason for its low melting point material exists in the weld, when subjected to high welding stress, which weakened the contact between the grains, easy to cause breakdown between the grains. Welding and welding rod containing S, Cu impurities such a long time, prone to thermal cracking.
Distribution of Thermal Crack Characteristics along Grain Boundary. When the crack through the surface and the outside world, it is a clear tendency to hydrogenation. In the cooling process after welding cold cracks, mainly in the base metal or base metal and weld fusion line produced. The main reason is that, due to the weld heat affected zone formed or formed in the quenched structure, the alloy steel with the carbon content of the welding element is higher or harder is more likely to be cracked. Over-welding into hydrogen, may also lead to cold cracks. The most dangerous crack defects, in addition to reducing the cross-section than the bearings, but also in the crack at the serious stress concentration, in use will gradually expand, eventually leading to damage to components. Therefore, the presence of such defects, which normally do not allow for welded structures, is a shovel for re-welding.
The lack of penetration of the internal defects of steel pipe is the lack of penetration of the workpiece and the weld metal or weld layer between the local incomplete fusion of defects. Incomplete penetration weld thinning of the workpiece section, resulting in severe stress concentration, greatly reduce the joint strength, often a source of weld cracks. Welding slag into the non-metallic slag, that is slag. Welding slag will reduce the working cross-section, resulting in stress concentration will reduce the welding strength and toughness.
The porosity of the weld metal at high temperatures, the absorption due to the excessive gas or gas dissolving in the cell, is caused by the metallurgical reaction, the cooling and solidification occurs too late when the bath is discharged, and the welding hole forms a surface inside, hole. The presence of the holes reduces the effective work of the welding cross-section to reduce the mechanical strength of the joint. If there are through holes or continuity, it will seriously affect the tightness of the welding parts. In the welding cracking or welding, the metal in the weld in the local cracked area known as the crack. Cracks may occur in the weld, or both sides of the welding heat-affected zone. Sometimes occurs on the metal surface, the metal may be generated internally.
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