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Repair of steel pipe weld internal defects

Pipe weld cracks often follow different mechanisms can be divided into two types of thermal cracking and cold cracking.

It is produced by the thermal cracking of a liquid to a solid state crystallization process in the weld metal, mostly generated in the weld metal. The main causes of its low-melting substance is present in the weld, which weakens the contact between the grains when subjected to high welding stress, it is easy to cause breakdown between the grains. Weldments and welding rod containing S, Cu impurities such a long time, it is prone to hot cracking.

Thermal cracking characteristic distribution along the grain boundaries. When the crack through the surface and the outside world, it is a clear tendency hydrogenation. Cold cracks are generated in the cooling process after welding, mostly produced in the base metal or base metal and the weld fusion line junction. The main reason is due to its generation or the weld heat affected zone formed within the quenched structure, under high stress, causing rupture of grains, the higher the carbon content of the welding elements, or more easily hardened alloy steel, the most prone to cold cracking. Excessive weld melt into hydrogen, can also cause cold cracking. A defect of the most dangerous cracks, which in addition to reducing the cross-section than the bearing, but also produce severe stress concentration at the crack will gradually expand in use, may eventually lead to the destruction member. So welded structures are generally not permitted the existence of such defect has been found to be a shovel to re-welding.

Steel pipe internal defects of weld lack of penetration is refers to the workpiece and the weld metal or weld layer between local incomplete fusion of a defect. Incomplete penetration welds weakened section of the work, causing serious stress concentration, greatly reduce joint strength, it is often a source of weld cracking. Weld slag sandwiched nonmetallic slag, that is called slag. Weld slag reduces work-section, resulting in stress concentration will reduce weld strength and toughness.

Porosity of the weld metal at high temperatures, the absorption due to too much gas or gas dissolved within the cell is generated by the reaction of metallurgical, too late when the bath is discharged is cooled and solidified, and the weld hole is formed inside or the surface, i.e. pores. The presence of pores reduces the effective work of the weld cross-section, to reduce the mechanical strength of the joint. If penetrating pores or continuity exists, it will seriously affect the tightness of the weldment. After cracking during welding or welding metal in the welded joints appearing partial rupture area called crack. Cracks may occur in the weld may also be produced in the weld heat affected zone on both sides. Sometimes occurs on the metal surface, the metal may be generated internally.





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