Galvanized steel pipe has the dual advantages of corrosion resistance and long service life, and the price is relatively low, so its usage rate is getting higher and higher now. However, some users do not pay attention when welding galvanized pipes. It has caused some unnecessary trouble. So what issues should we pay attention to when welding galvanized pipes?
1. The prerequisite for welding galvanized steel pipes is polishing
The galvanized layer at the welding joint must be polished off, otherwise, bubbles, trachoma, false welding, etc. will occur. It will also make the weld brittle and reduce its rigidity.
2. Characteristics of galvanized steel pipes
Galvanized steel is generally coated with a layer of zinc on the outside of low-carbon steel, and the galvanized layer is generally 20um thick. The melting point of zinc is 419°C and the boiling point is around 908°C. During welding, zinc melts into a 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. The zinc liquid will etch the weld metal deeply along the grain boundaries, and the 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 cause cracks under the action of tensile stress. If fillet welds are welded, especially those of T-shaped joints, they are most likely to produce penetration cracks. When welding galvanized steel, the zinc layer on the groove surface and edges will oxidize, melt, evaporate, and even volatilize white smoke and steam under the action of arc heat, which can easily cause weld porosity. ZnO formed due to oxidation has a high melting point, about 1800°C or above. If the parameters are too small during the welding process, ZnO slag inclusion will be caused because Zn becomes a deoxidizer. FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag inclusions are produced. Secondly, due to the evaporation of zinc, a large amount of white smoke is emitted, which is irritating and harmful to the human body. Therefore, the galvanized layer at the welding joint must be polished away.
3. Welding process control of the galvanized steel pipe
The preparation before welding of galvanized steel pipe is the same as that of ordinary low-carbon steel. It should be noted that the groove size and nearby galvanized layer must be carefully processed. To achieve welding penetration, the groove size must be appropriate, usually 60~65° and a certain gap must be left, usually 1.5~2.5mm; to reduce the penetration of zinc into the weld, before welding, the galvanized surface in the groove can be Do not weld until the layer is removed. In actual work, centralized beveling, centralized control without leaving blunt edges, and a two-layer welding process were used to reduce the possibility of incomplete penetration. The welding rod should be selected according to the base material of the galvanized steel pipe. Generally, J422 is more commonly used for low-carbon steel due to ease of operation.
4. Welding techniques
When welding the first layer of multi-layer welding, try to melt the zinc layer and allow it to vaporize and evaporate to escape the weld, which can greatly reduce the amount of liquid zinc remaining in the weld. When welding fillet welds, 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. When the zinc layer After melting, return to the original position and continue welding forward. In horizontal and vertical welding, if short slag electrodes such as J427 are used, the undercut tendency will be very small; if the back-and-forth rod transport technology is used, defect-free welding quality can be obtained.
Post time: May-20-2024