Pipe internal defects of the weld penetration is not the workpiece and the weld seam between layers of metal or a partial lack of fusion defects. Incomplete penetration welds weakened section of the work, resulting in severe stress concentration and greatly reduce joint strength, it often becomes the root of the weld cracking. Weld slag in the folder with non-metallic slag, which called slag. The weld slag reduces the work section, causing stress concentration, will reduce the strength and toughness of the weld.
Hole weld metal at high temperatures, the absorption of excessive gas (such as H2) or because of metallurgical reaction solution within the cell gases (eg CO), cooling and solidifying the molten pool in the time to discharge, and the weld surface of the inside or cavities, is the porosity. The presence of pores reduces the effective working of the weld cross-section, reducing the mechanical strength of the joint. If penetrating holes or continuity exists, it will seriously affect the tightness of the weldment. Cracking during welding or after welding, the welded joint of the metal within the region that appears is called partial rupture cracks. Cracks may occur in the weld, the weld on both sides may also arise in the heat affected zone. Sometimes with the metal surface, sometimes with the metal inside.
Cracks usually follow different mechanisms can be divided into two types of thermal cracking and cold cracking. Hot cracking in the weld metal from a liquid to a solid produced during the crystallization, resulting, in the weld metal. Its causes are mainly present in the weld low melting substances (such as FeS, melting point 1193 ℃), it weakens the link between the grains, when subjected to high welding stresses, it is easy to cause the rupture between the grains. Weldments and welding rod containing S, Cu impurities such a long time, it is prone to hot cracking. Hot cracks along the grain boundary characteristics. When the crack through the surface and the outside world, then a clear tendency hydrogenation. Cold crack is generated in the cooling process after welding, most of the base metal or produce the matrix metal and the weld at the junction of the fusion line. Which produces mainly due to the weld heat affected zone or the formation of a hardened structure, high stress, caused by the breakdown of grains, welding high carbon or alloy elements more easily quenched steels, the most prone to cold cracking. Excessive weld melt into hydrogen, can cause cold cracking. Crack is the most dangerous kind of defect, which in addition to reducing the bearing section, the will have serious stress concentration in the use of crack will gradually expand, eventually could lead to component damage. Therefore, there is generally not allowed in welded structures such defects, once discovered to be shoveling heavy welding.
