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Hot dip galvanized sheet welding process

1. Hot galvanized sheet arc welding
The existence of zinc layer brings some difficulties to the welding of hot-dip galvanized sheet. The main problems are: the increase of the welding crack and porosity, the evaporation of zinc and the melting and destruction of dust, oxide inclusions and zinc coating. Welding cracks, porosity and slag are the main problems.
1.1 Weldability
(1) Crack
During the welding process, molten zinc floats on the surface of the bath or at the root of the weld. Since the melting point of zinc is much lower than that of iron, the iron in the bath first crystallizes, and the liquid zinc infiltrates into the grain boundaries of the steel, resulting in weaker intercrystalline bonding. And easy to form between the zinc and iron intermetallic compounds brittle Fe3Zn10 and FeZn10, further reducing the plasticity of the weld metal. Therefore, under the action of welding residual stress, it is easy to crack along the grain boundary to form cracks.
1) Factors that affect crack sensitivity
① The thickness of zinc layer The galvanized sheet has a thin zinc layer and a small crack sensitivity. However, the zinc layer of the hot-dip galvanized sheet is thick and the crack sensitivity is high.
② The greater the thickness of the workpiece thickness, the greater the welding restraint stress, the greater the sensitivity of the crack.
③ groove gap gap, the greater the crack sensitivity.
④ welding method with manual arc welding when the crack sensitivity is small, and with CO2 gas shielded welding welding susceptibility larger.
2) Crack prevention methods
① before welding in the galvanized sheet weld groove V, Y-shaped or X groove, with acetylene or sandblasting and other methods to remove the groove near the galvanized layer, while controlling the gap should not be too large, usually about 1.5mm .
② choose a low amount of Si-containing welding materials. Gas-shielded welding should be used with a low amount of Si wire, hand welding using titanium type, titanium calcium type electrode.
(2) Stomata
The zinc layer near the bevel generates oxidation (ZnO) and evaporates under the action of the arc heat and volatilizes white fumes and vapors, thus easily causing porosity in the weld. The greater the welding current, the more serious the evaporation of zinc, the greater the stomatal sensitivity. With titanium, titanium calcium type electrode welding, in the medium current range is not easy to produce pores. The use of cellulose and low hydrogen electrode welding, small current and high current are prone to stomatal. In addition the electrode angle should be controlled in the range of 30 ° ~ 70 °.
(3) zinc evaporation and smoke
When arc welding is used to heat hot-dip galvanized sheet, the zinc layer in the vicinity of the molten pool is oxidized to ZnO under the action of arc heat and evaporates to form a large soot. The main component of this soot is ZnO, which has a great stimulating effect on the respiratory organs of workers. Therefore, good ventilation must be taken during welding. Under the same welding specifications, the amount of soot generated when welding with a titanium oxide type electrode is low, whereas the amount of soot generated when welding a low-hydrogen type electrode is large.
(4) oxide slag
When the welding current is small, the formation of ZnO during heating is not easy to escape, easily lead to ZnO slag. ZnO is relatively stable, the melting point of 1800 ℃. Bulk ZnO inclusions have a very detrimental effect on weld plasticity. The use of titanium oxide electrode, ZnO was small and uniform distribution, the plasticity and tensile strength are not affected. The use of cellulose-based or hydrogen-type electrode, the weld ZnO larger, more, poor weld performance.

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