1. Galvanized steel arc welding
The existence of zinc layer has brought certain difficulties to the welding of galvanized steel. The main problems are: increased sensitivity of welding cracks and pores, evaporation of zinc and soot, oxide slag inclusion, and melting and destruction of galvanized layer. Among them, welding cracks, pores and slag inclusion are the main problems.
1.1 Weldability
(1) Crack
During welding, molten zinc floats on the surface of the molten pool or at the root of the weld. Because the melting point of zinc is much lower than that of iron, the iron in the molten pool crystallizes first, and the liquid zinc will infiltrate along the grain boundaries of the steel, resulting in weaker intercrystalline bonding. Moreover, the intermetallic brittle compounds Fe3Zn10 and FeZn10 are easily formed between zinc and iron, which further reduces the plasticity of the weld metal. Therefore, under the action of welding residual stress, it is easy to crack along the grain boundary and form cracks.
Welding process of galvanized steel sheet
1) Factors affecting crack sensitivity
① Thickness of zinc layer The zinc layer of galvanized steel is thinner and has less crack sensitivity, while the zinc layer of hot-dip galvanized steel is thicker and has greater crack sensitivity.
② Workpiece thickness The larger the thickness, the greater the welding restraint stress and the greater the crack sensitivity.
③ Gap clearance The larger the clearance, the greater the crack sensitivity.
④ Welding method When using manual arc welding, the crack sensitivity is small, but when using CO2 gas shielded welding, the crack sensitivity is higher.
2) Methods to prevent cracks
① Before welding, open the groove V, Y or X groove at the welding place of galvanized sheet, use oxyacetylene or sandblasting to remove the galvanized layer near the groove, and control the gap should not be too large, generally about 1.5mm .
② Use welding materials with low Si content. Gas shielded welding should use a low Si content wire, manual welding using titanium, titanium calcium electrode.
(2) Stomata
The zinc layer near the bevel is oxidized (forms ZnO) and evaporated under the action of arc heat, and volatilizes white smoke and vapor, so it is easy to cause porosity in the weld. The greater the welding current, the more severe the evaporation of zinc, and the greater the susceptibility to stomata. When welding with titanium-type, titanium-calcium-type electrodes, it is not easy to generate pores in the medium current range. When welding with cellulose and low-hydrogen electrodes, pores are easily generated at low and large currents. In addition, the electrode angle should be controlled within the range of 30 ° to 70 °.
(3) Zinc evaporation and soot
焊接 When welding galvanized steel sheet by arc welding, the zinc layer near the molten pool is oxidized to ZnO and evaporated under the action of arc heat, forming a large amount of smoke and dust. The main component of this soot is ZnO, which has a great stimulating effect on the respiratory organs of workers. Therefore, good ventilation measures must be taken during welding. Under the same welding specifications, the amount of soot generated when welding with titanium oxide electrodes is low, while the amount of soot generated when welding with low hydrogen electrodes is large.