First, the premise will be polished
However, the galvanized coating must be polished off, otherwise it will have bubbles, trachoma, false welding and so on. Will make the weld brittle, rigid decline.
Second, the welding characteristics of galvanized steel
Galvanized steel is generally coated with a layer of low carbon steel zinc, zinc coating is generally 20um thick. Zinc melting point at 419 ° C, the boiling point of about 908 ° C. In welding, zinc melts into a liquid that floats on the bath surface or at the root of the weld. Zinc has a large solid solubility in iron. Zinc liquid will in-depth etch the weld metal along grain boundaries and low-melting point zinc forms "liquid metal embrittlement."
At the same time, zinc and iron can form brittle intermetallic compounds. These brittle phases reduce the plasticity of the weld metal and cause cracks under tensile stress.
If the fillet weld, especially T-fillet weld the most prone to crack through. When galvanized steel is welded, the zinc layer on the surface and edge of the groove will oxidize, melt and evaporate under the action of electric arc to volatilize white fume and steam, which can easily lead to the fouling of the weld seam.
Due to the oxidation of ZnO formed, its higher melting point, about 1800 ° C or more, if the parameters in the welding process is too small, will cause ZnO slag, at the same time. Since Zn becomes a deoxidizer. Resulting in FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag. Secondly, due to the evaporation of zinc, a large amount of white smoke is volatilized, causing irritation and injury to the human body. Therefore, the galvanized layer at the welding place must be polished and polished.
Third, the welding process control
Galvanized steel welding preparation and general mild steel is the same, it should be noted that be careful to deal with the bevel size and the vicinity of the zinc coating. In order to penetrate, bevel size to be appropriate, generally 60 ~ 65 °, to leave a certain gap, usually 1.5 ~ 2.5mm; In order to reduce the infiltration of zinc on the weld before welding, the groove can be galvanized Remove the layer after the welding.
In the actual supervision work, using a centralized beating, without leaving blunt edge process for centralized control, two-layer welding process, reducing the possibility of incomplete penetration.
Welding rod should be based on galvanized pipe substrate selection, the general low carbon steel due to ease of operation, the selection of J422 is more common.
Welding practices: welding the first layer of multi-layer welding seam, try to make the molten layer of zinc and make it vaporize, evaporate and escape the weld, which can greatly reduce the liquid zinc to stay in the weld.