Precautions for welding galvanized steel pipes

Galvanized steel pipes are generally coated with a layer of zinc on the outside of low-carbon steel, and the galvanized layer is generally 20μm thick. The melting point of zinc is 419°C and the boiling point is about 908°C.

1. The weld must be polished before welding

The galvanized layer at the weld must be polished off, otherwise, bubbles, sand eyes, false welding, etc. will be generated. It will also make the weld brittle and reduce rigidity.

2. Analysis of the characteristics of galvanized steel pipe welding

During welding, zinc melts into liquid and floats on the surface of the molten pool or at the root of the weld. Zinc has a large solid solubility in iron. Liquid zinc will deeply erode the weld metal along the grain boundary, and low-melting-point zinc will form "liquid metal embrittlement". At the same time, zinc and iron can form intermetallic brittle compounds. These brittle phases reduce the plasticity of the weld metal and produce cracks under tensile stress. Welding fillet welds, especially fillet welds of T-joints, are most likely to be produced through cracks. When galvanized steel pipes are welded, the zinc layer on the surface and edge of the groove is oxidized, melted, and evaporated under the action of arc heat, emitting white smoke and steam, which can easily cause weld porosity. The ZnO formed by oxidation has a high melting point, above 1800°C. If the parameters are too small during welding, ZnO slag inclusion will occur. At the same time, since Zn becomes a deoxidizer, FeO-MnO or FeO-MnO-SiO2 low-melting oxide slag inclusion will be generated. Secondly, due to the evaporation of zinc, a large amount of white smoke is volatilized, which is irritating and harmful to the human body. Therefore, the galvanized layer at the weld must be polished off.

3. How to control the welding process of galvanized steel pipes?

The pre-welding preparation of galvanized steel pipes is the same as that of general low-carbon steel. It should be noted that the groove size and the nearby galvanized layer should be carefully handled. In order to weld through, the groove size should be appropriate, generally 60°~65°. A certain gap should be left, generally 1.5~2.5mm. In order to reduce the penetration of zinc into the weld, the galvanized layer in the groove can be removed before welding. In actual supervision work, the centralized groove is used without leaving a blunt edge process for centralized control. The two-layer welding process reduces the possibility of incomplete penetration. The welding rod should be selected according to the base material of the galvanized pipe. Generally, J422 is more commonly used for low-carbon steel due to its easy operation. Welding technique: When welding the first layer of multi-layer welds, try to melt the zinc layer and make it vaporize and evaporate to escape the weld, which can greatly reduce the situation of liquid zinc remaining in the weld. When welding fillet welds, also try to melt the zinc layer in the first layer and make it vaporize and evaporate to escape the weld. The method is to first move the end of the welding rod forward about 5~7mm, and then return to the original position after the zinc layer melts and continue to weld forward. In horizontal welding and vertical welding, if a short slag welding rod such as J427 is selected, the edge-biting tendency will be very small. If the back-and-forth rod moving technology is used, it is more likely to obtain a defect-free welding effect.