Before the galvanized steel pipe is welded, the galvanized layer of the welding place must be worn away, otherwise the bubble, the trachoma, the false welding and so on will be produced. It will also make the weld brittle and the rigidity decrease. The galvanized steel tube is generally plated with a layer of zinc outside the low carbon steel, and the galvanized layer is generally 20um thick. Zinc has a larger solid solubility in iron, and the zinc solution has deeply eroded the weld metal along the grain boundary, and the low melting point zinc forms liquid metal embrittlement.
Zinc and iron can form brittle intermetallic compounds. These brittle phases reduce the plastic properties of the weld metal and produce cracks under the action of tensile stress. When galvanized steel pipe is welded, the zinc layer on the surface and edge of the groove will generate oxidation, melting and evaporation under the action of arc heat, which will cause white smoke and steam to volatilize, which will easily cause the weld porosity.
The pre - welding preparation of galvanized steel pipe is the same as that of low carbon steel. It is necessary to pay attention to the proper handling of the groove size and the galvanized layer in the vicinity. For welding penetration, the groove size should be appropriate, generally 60~65 degrees, leaving a certain gap, usually 1.5~2.5mm. In order to reduce the penetration of zinc to the weld seam, before welding, the zinc coating in the groove will be removed before welding.
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