Galvanized pipe, also known as galvanized steel pipe, is divided into hot-dip galvanizing and electro-galvanizing. The hot-dip galvanizing layer is thick, with uniform plating, strong adhesion and long service life. The cost of electroplating is low, the surface is not very smooth, and its corrosion resistance is much worse than that of hot-dip galvanized pipes.
The molten metal is reacted with an iron matrix to produce an alloy layer, thereby combining the substrate and the plating layer. Hot-dip galvanizing is to pickle the steel pipe first, in order to remove the iron oxide on the surface of the steel pipe, after pickling, it is washed by ammonium chloride or zinc chloride aqueous solution or a mixed aqueous solution of ammonium chloride and zinc chloride, and then fed. Hot dip plating tank. Hot-dip galvanizing has the advantages of uniform plating, strong adhesion and long service life. Most of the northern processes use galvanized strips to directly coil the zinc.
Hot dip galvanizing process
Workpiece degreasing → washing → pickling → washing → drying solvent dip fluxing preheated hot dip galvanized → cooling → finishing → rinsing → drying → passivation test
Characteristics of galvanized steel welding
Galvanized steel is generally low carbon steel plated with zinc, the zinc coating is generally 20um thick. At 419 ° C, the boiling point of about 908 ° C melting point of zinc. In welding, the melted liquid zinc bath floating on the surface or in the root of the weld location. Zinc has a greater solubility in iron, zinc etching liquid will go into the weld metal along the grain boundaries, low-melting zinc to form a “liquid metal embrittlement.” Meanwhile, zinc and iron brittle intermetallic compounds can be formed, these brittle phases so that the weld metal plastic lower under tensile stress and cracks. If the fillet weld, especially the fillet weld T-joints most prone to penetrate the cracks. When welding galvanized steel, zinc layer and the surface of the groove at the edge of the arc under the action of heat, oxidation, melting, evaporation as well as volatile white smoke and steam, can easily cause weld porosity. ZnO due to oxidation and the formation of its high melting point of about 1800 ° C or more, if small parameter in the welding process, will cause the ZnO slag, at the same time. Since Zn become deoxidizer. Produces FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag. Second, due to the evaporation of zinc, volatile lot of white smoke, to stimulate the human body, harmful effects, therefore, must be welded zinc coating at the polished off.
Welding process control
Preparation before welding galvanized steel and low carbon steel is generally the same, people need to pay attention is to seriously handle the groove dimensions and the nearby zinc coating. For penetration, groove size should be appropriate, generally 60 ~ 65 °, to leave some space, usually 1.5 ~ 2.5mm; zinc in order to reduce the penetration of the weld, before welding can be galvanized inside groove Clear after layer and then welded. In practice, supervision, using a centralized playing groove, leaving blunt edge technology for centralized control, two welding process, reducing the possibility of incomplete penetration.Galvanized pipe welding rod should be based on the choice of substrate material, usually low-carbon steel on account ease, use J422 is more common.Welding techniques: When welding the first layer of the multilayer welding seam, try to make it melt and vaporize the zinc layer, evaporation and escape the weld, can greatly reduce the liquid zinc to remain in the weld. When fillet welds, as far as possible in the first layer to melt and vaporize the zinc coating, evaporation and escape the weld, the method is first removed from the ends of the electrode forward about 5 ~ 7mm, when the zinc layer melted down and continue to move back to the original position welding. Again when horizontal welding and vertical welding, such as welding slag, such as the choice of short-J427, undercut tends to be minimal; roundtrip transportation around if strip technology, but can be defect-free weld quality.
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