Welding characteristics of galvanized steel pipe
- Aug 07, 2018 -

Galvanized steel is widely used in all walks of life. The selection of the length of galvanized steel is to protect the internal steel structure by using the metal zinc that can form a fine oxide protective layer in the air. In the case of welding and scratching, the relatively vivid galvanized part can be used as a dedicated anode because of the existence of the Zn-Fe battery. The corrosion of steel is delayed and the corrosion resistance is excellent. However, due to the existence of galvanized layer, cracks, porosity, slag inclusion occur in welding, it is difficult to obtain excellent welding quality.

Galvanized steel is usually plated with zinc on the low carbon steel, and the zinc coating is usually 20um thick. The melting point of zinc is 419 C, and the boiling point is about 908 C. In welding, zinc melts into liquid and floats on the surface of the weld pool or at the root of the weld. Zinc has a high solid solubility in iron. Zinc liquid will further etch weld metal along grain boundary. Low melting point zinc forms "liquid metal embrittlement".  Together, zinc and iron can form intermetallic brittle compounds, such as Fe3Zn10 and FeZn10. These brittle phases reduce the plasticity of weld metal and crack under tensile stress. It is assumed that the fillet weld, especially the T joint, is most likely to penetrate cracks. When the galvanized steel is welded, the zinc layer on the surface of the bevel and the edge of the groove is oxidized, melted and evaporated, resulting in the volatiles of white smoke and steam, which are very easy to cause the weld hole. Due to oxidation, the melting point of ZnO is about 1800 degree C. If the parameter is too small during welding, it will cause ZnO.

Slag, together with Zn as a deoxidizer, attack FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag. It is assumed that the selection of welding specifications is not suitable and the operation method is improper. It is very simple to melt the galvanized layer at the edge of the weld to expand the melting zone. It is possible to damage the galvanized layer, especially in the condition of elongated arc and large shaking operation, which widen the melting zone and damage the galvanized layer more seriously. Together, because of the evaporation of zinc, a lot of white smoke is announced, which has influence and damage to the human body. Therefore, the selection of welding methods and materials with low smoke and dust is also the essential factor to be considered.

There are many methods for welding of galvanized steel, usually in galvanized steel structure, such as gas welding, manual arc welding, CO2 gas shielded welding, submerged arc welding, tungsten arc welding and so on.

In the past, the gas welding is commonly used in the welding of galvanized pipe. Because of the incompatibility of the heat input of gas welding, the brief attack defect and the poor mechanical function of the welding seam, it is now screened in the work of the equipment. Gas welding has a great damage to the galvanized layer.

CO2 gas shielded welding has excellent welding function for galvanized steel. When selecting suitable welding specifications and matching protective gas and welding materials, high quality welding joint can be obtained. This method is less selected in engineering practice.

Tungsten arc welding arc energy convergence, less damage to the galvanized layer, and easier to form a good single side welding double-sided forming joint, is a kind of welding method worthy of selection, but the welding speed, slow, cost more expensive.

Manual arc welding is the most widely used welding method in pipeline equipment now. In the correct selection of welding rod, such as J421, J422, J423 and titanium oxide and titanium dioxide welding electrode, the melting rate of the electrode is larger because the skin of these electrodes contains a lot of rutile and ilmenite, and the melting rate is increased relatively. It is assumed that under the condition of not shaking, it is only possible to damage the galvanized layer, such as the front of the molten pool, not to expand the melting zone, and to reduce the penetration of the zinc liquid to the weld metal. In the case of the correct operation method and welding material, the mechanical function of the joint can be obtained, and the welding quality with no defect can be obtained. Because manual arc welding is cheaper and quicker than tungsten argon arc welding, manual arc welding technology is selected in the case of skilled welders.

The preparation of galvanized steel before welding is the same as that of ordinary low carbon steel. It should be noted that the groove standard and nearby galvanized layer should be carefully handled. In order to weld through, the groove standard should be appropriate, general 60~65 degrees, to leave a certain space, generally 1.5~2.5mm; in order to reduce the penetration of zinc to the weld, before welding, the galvanized layer in the groove can be removed from the future rewelding. In practice, the jointing groove is selected, and the two-layer welding process is adopted to reduce the possibility of non-penetration.

The electrode should be selected according to the matrix material of galvanized pipe. Generally speaking, low carbon steel is widely used because of its ease of operation. J422 is widely used. Welding method: when welding the first layer weld of multi-layer welding, the zinc layer is melted and vaporized and evaporated to escape the weld, and the liquid zinc can be greatly reduced in the weld seam. In the welding angle weld, it is also on the first layer to melt the zinc layer and make it vaporize and evaporate and escape the weld. The method first moves the end of the electrode forward about 5~7mm, when the zinc layer is melted back to the normal position and continuously forward welding. After welding, clean up the weld seam and brush the zinc rich primer in good time.