one Galvanizing steel arc welding

The existence of zinc layer brings some difficulties to the welding of galvanized steel. The main problems are:increasing sensitivity of welding cracks and pores, zinc evaporation and dust, oxide slag inclusion and melting and destruction of galvanized steel. Among them, welding cracks, porosity and slag inclusion are the most important problems.

1.1 Weldability

(1) Crack

During the welding process, the molten zinc floats on the surface of the weld 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 the liquid zinc seeps into it along the grain boundaries of the steel, resulting in weak intergranular bonding. Moreover, the intermetallic brittle compounds Fe3Zn10 and FeZn10 are easily formed between zinc and iron, which further reduces the plasticity of weld metal. Therefore, under the action of welding residual stress, it is easy to crack along the grain boundary and form cracks.

1) Factors affecting crack sensitivity

The zinc layer of galvanized steel is thinner and the crack sensitivity is small, while the zinc layer of hot-dip galvanized steel is thicker and the crack sensitivity is larger.

The greater the thickness of the workpiece, the greater the welding constraint stress and the greater the crack sensitivity.

(3) The greater the gap of groove clearance, the greater the crack sensitivity.

The crack sensitivity is small when welding with manual arc welding, and the crack sensitivity is larger when welding with CO2 gas shielded welding.

2) Methods to prevent cracks

① Before welding, open a V, Y or X groove at the welding place of the galvanized plate, remove the galvanized layer near the groove with oxygen acetylene or sand blasting, and control the gap should not be too large, generally about 1.5mm.

② Select welding materials with low Si content. Welding wire with low Si content should be used for gas shielded welding, and titanium and titanium-calcium electrode should be used for manual welding.

(2) Stomata

The zinc layer near the groove produces oxidation (forming ZnO) and evaporation under the action of arc heat, and volatilizes white dust and vapor, so it is easy to cause porosity in the weld. The greater the welding current, the more serious the zinc evaporation and the greater the stomatal sensitivity. When welding with titanium and titanium calcium electrode, it is not easy to produce porosity in the medium current range. However, when welding with cellulose type and low hydrogen type electrode, small current and large current are easy to produce porosity. In addition, the Angle of the electrode should be controlled within the range of 30° ~ 70° as far as possible.

(3) Zinc evaporation and smoke

When the galvanized steel plate is welded by arc welding, the zinc layer near the molten pool is oxidized to ZnO under the action of arc heat and evaporates, forming a large soot. The main component of this smoke is ZnO, which has a great stimulating effect on the workers' respiratory organs, so good ventilation measures must be taken when welding. Under the same welding specification, the amount of smoke produced by titanium oxide welding rod is lower, and the amount of smoke produced by low hydrogen welding rod is larger.

(4) oxide slag 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 bulk ZnO slag inclusion has a very adverse effect on weld plasticity. When titanium oxide electrode is used, ZnO is fine and evenly distributed, which has little influence on plasticity and tensile strength. When cellulose or hydrogen electrode is used, the ZnO in the weld is larger and more, and the weld performance is poor.

1.2 Welding process of galvanized steel

Galvanized steel can be welded by manual arc welding, MIG welding, argon arc welding, resistance welding and other methods.

(1) Manual arc welding

1) Preparation before welding

In order to reduce welding smoke and prevent welding cracks and pores, in addition to opening the appropriate groove before welding, the zinc layer near the groove should be removed. The removal method can be flame baking or sand blasting. The groove clearance should be controlled within 1.5 ~ 2mm as far as possible, and when the workpiece thickness is large, it can be relaxed to 2.5 ~ 3mm.

2) Selection of electrode

The selection principle of the welding rod is that the mechanical properties of the weld metal are as close as possible to the base metal, and the silicon content of the welding rod deposited metal should be controlled below 0.2%.

The joint strength obtained by using ilmenite type electrode, titanium oxide type electrode, cellulose type electrode, titanium-calcium type electrode and low hydrogen type electrode can reach satisfactory indicators, as shown in Table 1. However, slag inclusion and porosity are easy to occur in the welds of low-hydrogen and cellulose welding rods, so they are generally not used.

For low carbon steel galvanized steel, J421/J422 or J423 electrode should be preferred. For galvanized steel plates with strength levels above 500MPa, E5001, E5003 and other types of welding rods can be selected. For galvanized steel plates with a strength above 600MPa, E6013, E5503 and E5513 welding rods should be selected.

When welding, try to use short arc, do not make the arc swing, in order to prevent the expansion of the melting area of the galvanized layer, to ensure the corrosion resistance of the workpiece and reduce the amount of smoke.

(2) gas arc welding

CO2 gas shielded welding or Ar+CO2, Ar+O2 and other mixed gas shielded welding can be used for welding. The protective gas has a significant effect on the content of Zn in the weld. When pure CO2 or CO2+O2 is used, the content of Zn in the weld is higher, while when Ar+CO2 or Ar+O2 is used, the content of Zn in the weld is lower. The current has little effect on the Zn content in the weld, and the Zn content in the weld decreases slightly with the increase of the welding current.

When welding galvanized steel with gas shielded welding, welding dust is much larger than manual arc welding, so special attention should be paid to exhaust air. The main factors affecting the amount and composition of soot are current and protective gas. The greater the current, or the greater the CO2 or O2 content in the protective gas, the larger the welding smoke, and the ZnO content in the smoke also increases, and the maximum da ZnO content can reach about 70%.

Under the same welding specification, the penetration depth of galvanized steel is larger than that of non-galvanized steel. The porosity sensitivity of T-shaped joints, lap joints and downward vertical welding is higher, and the higher the welding speed, the easier the porosity is. For galvanized alloy steel, the effect of welding speed is particularly obvious. In multi-pass welding, the porosity sensitivity of the subsequent pass is greater than that of the previous pass.