The existence of the zinc layer brings certain difficulties to the welding of galvanized steel sheets. The main problems are: increased sensitivity of welding cracks and pores, evaporation and smoke of zinc, oxide slag inclusion, and melting and destruction of the galvanized layer. Among them, welding cracks, pores and slag inclusions are the most important problems.
Weldability
⑴Cracks.
During the welding process, molten zinc floats on the surface of the molten pool or is located 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 steel along the grain boundaries, causing the intergranular bonding to weaken. Moreover, intermetallic brittle compounds Fe3Zn10 and FeZn10 are easily formed between zinc and iron, further reducing the plasticity of the weld metal. Therefore, under the action of welding residual stress, it is easy to split along the grain boundaries and form cracks.
1) Factors affecting crack sensitivity.
① The thickness of the zinc layer. The zinc layer of galvanized steel is thin and has low crack sensitivity, while the zinc layer of hot-dip galvanized steel is thicker and has greater crack sensitivity.
② Thickness of the workpiece. The greater the thickness, the greater the welding restraint stress and the greater the crack sensitivity.
③ Groove gap, the larger the gap, the greater the crack sensitivity.
④ Welding method. The crack sensitivity is small when welding with manual arc welding, but the crack sensitivity is greater when welding with CO2 gas shielded welding.
2) Methods to prevent cracks.
① Before welding, make a V, Y-shaped or .
② Select welding materials with low Si content. Welding wire with low Si content should be used for gas shielded welding, and titanium type and titanium calcium type electrodes should be used for manual welding.
⑵Stomata.
The zinc layer near the groove is oxidized (to form ZnO) and evaporated under the action of arc heat, and evaporates white smoke and vapor, so it is easy to cause pores in the weld. The greater the welding current, the more serious the evaporation of zinc and the greater the pore sensitivity. When welding with titanium type or titanium calcium type electrodes, pores are less likely to occur in the medium current range. When welding with cellulose and low-hydrogen electrodes, pores are prone to occur under both low and high currents. In addition, the angle of the welding rod should be controlled within the range of 30° to 70°.
⑶Evaporation and smoke of zinc.
When arc welding is used to weld galvanized steel sheets, the zinc layer near the molten pool is oxidized into ZnO under the action of arc heat and evaporates, forming a large amount of smoke. The main component of this smoke is ZnO, which has a great irritating effect on workers' respiratory organs. Therefore, good ventilation measures must be taken during welding. Under the same welding specifications, the amount of smoke generated when welding with titanium oxide electrodes is lower, while the amount of smoke generated when welding with low hydrogen electrodes is larger.
⑷Oxide slag inclusion.
When the welding current is small, the ZnO formed during the heating process is not easy to escape, which can easily cause ZnO slag inclusion. ZnO is relatively stable, with a melting point of 1800°C. Large pieces of ZnO slag have a very negative impact on the plasticity of the weld. When titanium oxide type welding rods are used, ZnO is distributed finely and evenly, and has little effect on plasticity and tensile strength. When using cellulose-type or hydrogen-type electrodes, the ZnO in the weld is larger and more abundant, and the weld performance is poor.
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