The existence of the zinc layer brings some difficulties to the welding of hot-dip galvanized steel sheets. The main problems are: weld cracks and increased porosity, zinc evaporation and soot, oxide inclusions, and galvanized layer melting and destruction . Among them, welding cracks, porosity and slag inclusion are the main problems.
During the welding process, the molten zinc floats on the surface of the molten pool or at the root of the weld. Since the melting point of zinc is much lower than that of iron, the iron in the molten pool crystallizes first, and liquid zinc will penetrate into the grain boundary of the steel, resulting in weaker intercrystalline bonding. Moreover, the intermetallic brittle compounds Fe32n10 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.
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) Bevel gap: The greater the gap, the greater the crack sensitivity.
(4) Welding method: crack sensitivity is small when welding by manual arc welding, while crack sensitivity is higher when welding by CO2 gas shielded welding.
Ways to prevent cracks:
(1) Before welding, open the groove V, Y or X-shaped groove at the welding place of galvanized sheet, use oxyacetylene or sandblasting to remove the galvanized layer near the groove, and the control gap should not be too large, generally 1.5 mm or so.
(2) Select welding materials with low Si content. Welding wire with low S content should be used for gas shielded welding, and titanium type and titanium calcium type electrode should be used for manual welding.
The zinc layer near the groove is oxidized (formed as Zno) and evaporated under the action of arc heat, and evaporates white smoke and steam, so it is easy to cause porosity in the weld. The greater the welding current, the more serious the evaporation of zinc, and the greater the stomata sensitivity. When welding with titanium-type and titanium-calcium-type electrodes, it is not easy to produce pores in the medium current range. When welding with cellulose-type and low-hydrogen electrode, stomata are likely to occur under both small current and large current. In addition, the electrode angle should be controlled as much as possible within the range of 300 ~ 70.
Zinc evaporation and soot
When welding galvanized steel plates by arc welding, the zinc layer near the molten pool is oxidized to Zn0 and evaporated by the action of arc heat, forming a large amount of smoke. The main component of this smoke is Zno, which has a great stimulating effect on workers’ respiratory organs. Therefore, good ventilation measures must be taken when welding. Under the same welding specification, the amount of fumes generated when welding with titanium oxide electrodes is lower, while the amount of fumes generated when welding with low hydrogen electrodes is larger.
When the welding current is small, the ZnO formed during heating is not easy to escape, and it is easy to cause Zn0 slag inclusion. ZnO is relatively stable, its melting point is 1800 ℃, large Zn0 slag inclusions have a very adverse effect on the weld plasticity. When using titanium oxide electrode, ZnO is finely and uniformly distributed, and has little effect on plasticity and tensile strength. When using cellulose or hydrogen electrode, the Zno in the weld is larger and more, and the weld performance is poor.
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