The galvanized steel pipe is characterized by its excellent benefits such as corrosion resistance, long life, low initial cost and maintenance costs, has been wieldy used in outdoor and indoor, marine and underground applications
Galvanized steel pipe is a 20um thick zinc coating on the outside of the low carbon steel pipe. The melting point of zinc is at 419°C and the boiling point is around 908° ℃. In welding, zinc is melted on the surface of the molten pool or at the root of the weld. Since zinc has a relatively high solid solubility in iron, the zinc liquid will deeply erode the weld metal along the grain boundary, forming "liquid metal embrittlement". At the same time, zinc and iron can form an intermetallic brittle compound, which reduces the plasticity of the weld metal and leads to cracks under the action of tensile stress.
When welding galvanized steel, the surface of groove and the edge of the zinc layer under the action of arc heat, oxidation, melting and evaporation of white smoke and steam easily lead to weld porosity. ZnO formed by oxidation has a high melting point, above 1800°C. If the parameters are too small in the welding process, ZnO slag inclusion will be caused. Meanwhile, as Zn becomes a deoxidizer, FeO-MnO or Feo-Mno-Sio2 oxide slag inclusion with low melting point will be generated. It must be noted that the white dust of zinc volatilization is harmful to human body and the galvanized layer at welding must be polished off.
Step 1: Polishing
The galvanized layer must be polished off at the welding part, otherwise will occur bubbles, trachoma, false welding and other defects, which leads to brittle welding seam and rigidity decreases.
Step 2: Weld groove
The preparation of galvanized steel before welding is similar to the general low carbon steel, it is necessary to pay attention to the groove size and galvanized layer. The groove size is generally 60~65° and there is a certain gap, generally 1.5~2.5mm; In order to reduce the penetration of zinc into the weld, the galvanized layer in the groove can be removed before welding and then welded. In practical welding, the groove is generally concentrated and no blunt edge is left. Two-layer welding reduces the possibility of incomplete welding. The welding rod should be selected according to the base material of galvanized steel tube. Generally speaking, J422 is an ideal choice.
Step 3: Welding
In the first layer of multilayer welding weld, try to melt the zinc layer and make it vaporized, evaporation, this can greatly reduce the zinc liquid remained in the weld. The same method is used for fillet welds. First move the electrode tail forward about 5~7mm, and then return to the original position to continue welding after melting the zinc layer. If the choice of short slag electrode such as J427, there will be a very small bite edge tendency; Back and forth input electrode can get the welding quality without defects