News

The existence of zinc layer has brought some difficulties to the welding of galvanized steel. The main problems are: the increased sensitivity of welding cracks and pores, zinc evaporation and smoke, oxide slag inclusion, and the melting and damage of zinc coating. Among them, welding crack, air hole and slag inclusion are the main problems,
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 far lower than that of iron, the iron in the molten pool crystallizes first, and the wavy zinc will infiltrate into it along the grain boundary of steel, leading to the weakening of intergranular bonding. Moreover, it is easy to form intermetallic brittle compounds Fe3Zn10 and FeZn10 between zinc and iron, which further reduces the plasticity of the weld metal, so it is easy to crack along the grain boundary and form cracks under the effect of welding residual stress.
Factors affecting crack sensitivity: ① Thickness of zinc layer: the zinc layer of galvanized steel is thin and the crack sensitivity is small, while the zinc layer of hot-dip galvanized steel is thick and the crack sensitivity is large. ② Workpiece thickness: the greater the thickness, the greater the welding restraint stress and the greater the crack sensitivity. ③ Groove gap: gap
Larger, greater crack sensitivity. ④ Welding method: the crack sensitivity is small when manual arc welding is used, but greater when CO2 gas shielded welding is used.
Methods to prevent cracks: ① Before welding, open V-shaped, Y-shaped or X-shaped groove at the welding position of the galvanized sheet, remove the zinc coating near the groove by oxyacetylene or sand blasting, and control the gap not to be too large, generally about 1.5mm. ② Select welding materials with low Si content. Welding wire with low Si content shall be used for gas shielded welding, and titanium type and titanium-calcium type welding rod shall be used for manual welding.
(2) Stomata
The zinc layer near the groove will oxidize (form ZnO) and evaporate under the action of arc heat, and emit white smoke and steam, so it is very easy to cause pores in the weld. The greater the welding current is, the more serious the zinc evaporation is and the greater the porosity sensitivity is. It is not easy to produce pores in the medium current range when using titanium type and titanium-calcium type bright strips for welding. However, when cellulose type and low hydrogen type electrodes are used for welding, pores are easy to occur under low current and high current. In addition, the electrode angle should be controlled within 30 °~70 ° as far as possible.
(3) Zinc evaporation and smoke
When the galvanized steel plate is welded by electric 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. The main component of this kind of smoke is ZnO, which has a great stimulating effect on workers’ respiratory organs. Therefore, good ventilation measures must be taken during welding. Under the same welding specification, the amount of smoke produced by welding with titanium oxide type electrode is low, while the amount of smoke produced by welding with low hydrogen type electrode is large. (4) Oxide inclusion
When the welding current is small, ZnO formed in the heating process is not easy to escape, which is easy to cause ZnO slag inclusion. ZnO is relatively stable and its melting point is 1800 ℃. Large ZnO inclusions have a very adverse effect on weld plasticity. When titanium oxide electrode is used, ZnO is fine and evenly distributed, which has little effect on plasticity and tensile strength. When cellulose type or hydrogen type electrode is used, ZnO in the weld is larger and more, and the weld performance is poor.