The welding technology of galvanized steel pipe

Galvanized steel welding characteristics:

Galvanized steel is generally in the low-carbon steel outer layer of zinc-plated, galvanized layer is generally in the 20um thick. Melting point of zinc in 419 °C, bp 908 °C or so. In welding, the zinc melted into the liquid floating on the surface or in the root of the weld pool position. Zinc has a greater degree of solid solution of iron, zinc, liquid etching along the grain boundary deep weld metal, low melting point of zinc to form a “liquid metal embrittlement.” Meanwhile, zinc and iron can form brittle intermetallic compound phase of the weld metal of these brittle plastic reduction in the tensile stress and cracks. If the welding fillet welds, especially T-joint fillet most prone to penetrate the cracks. When welding galvanized steel, bevel edge surface and a zinc layer under the action of the arc heat, oxidation, melting, evaporation and even volatile white smoke and steam, can easily cause weld porosity. Due to oxidation and the formation of ZnO, the high melting point of about 1800 °C or more, if the welding process parameter is too small, will cause slag ZnO, simultaneously. Since Zn as a deoxidizer. Produce FeO-MnO or FeO-MnO-SiO2 slag melting oxides. Second, because the evaporation of zinc, a large amount of white smoke evaporation, stimulate the human body, harmful effects, therefore, necessary to weld galvanized layer of polishing off. 



Galvanized welding process control 

Galvanized steel welding preparation and the general low-carbon steel is the same, you need to pay attention is to seriously handle groove size and the nearby zinc coating. To weld penetration, beveling size should be appropriate, generally 60 ~ 65 °, to leave a certain gap, is generally 1.5 ~ 2.5mm; zinc in order to reduce the penetration of the weld, the weld before, the groove may be galvanized layer clear later welding. In actual supervision, using a centralized hit groove, leaving a blunt edge technology for centralized control, two welding process, reducing the possibility of lack of penetration. Welding galvanized steel pipe should be based on a matrix material selection, general ease of low carbon steel due consideration, the choice of J422 is more common. 



Welding techniques: the first layer of multi-layer welding weld seam, try to make it melt the zinc layer vaporization, evaporation and escape the weld, which can greatly reduce the liquid zinc remain in the weld. In fillet welding, the same as in the first layer and make the molten zinc vaporization, evaporation and escape the weld, which is shifted forward end portion of the first electrode of about about 5 ~ 7mm, when the zinc layer melted and then continue to move back to its original position welding. Horizontal and vertical welding again, if use short-slag welding, such as J427, tend to be small undercut; roundtrip transportation if the front and rear of technology, but also can be non-defective weld quality.

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