Galvanized pipe is corrosion-resistant and hard, which is now commonly used gas transmission equipment. As for the welding operability of galvanized pipe, it's a little more difficult for you to master. But it doesn't matter. Since life is often used, it's good to know about its welding knowledge. Today, Jinan galvanized pipe will share with you the welding precautions of galvanized pipe.
1. Before welding, the welding seam must be polished and the galvanized layer must be polished off, otherwise bubbles, sand holes, false welding, etc. will occur. It will also make the weld brittle and reduce the rigidity.
2. Welding technique introduction: when welding the first layer of multi-layer welding, try to melt the zinc layer and make it vaporize and evaporate to escape from the weld, which can greatly reduce the liquid zinc left in the weld. When welding the fillet weld, the zinc layer shall be melted and vaporized and evaporated in the first layer as much as possible to escape from the weld. The method is to move the end of the electrode forward about 5 ~ 7mm, and then return to the original position to continue welding forward after the zinc layer is melted. If the short slag electrode, such as J427, is used in the cross welding and vertical welding, the undercut tendency will be very small; if the back and forth strip transportation technology is used, the flawless welding quality can be obtained.
3. Analysis of welding characteristics of galvanized steel.
Generally, galvanized steel is coated with a layer of zinc outside the low carbon steel, and the galvanized layer is generally 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 molten pool or at the root of the weld. Zinc has a large solid solubility in iron. The liquid zinc will etch the weld metal along the grain boundary, and the low melting point zinc will form "liquid metal embrittlement".
At the same time, zinc and iron can form brittle compounds between metals. These brittle phases can reduce the plasticity of weld metal and cause cracks under tensile stress. If the fillet weld is welded, especially the T-joint fillet weld, it is most likely to produce through cracks. During the welding of galvanized steel, the zinc layer on the groove surface and edge will oxidize, melt and evaporate under the action of arc heat, and even emit white smoke and steam, which will easily cause weld porosity.
4. How to control the welding process of galvanized steel?
The preparation of galvanized steel before welding is the same as that of ordinary low-carbon steel. It is necessary to carefully handle the groove size and nearby galvanized layer. In order to achieve penetration, the groove size shall be appropriate, generally 60 ~ 65 °, with a certain gap, 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 the actual supervision work, the centralized beveling and no blunt edge are used for centralized control. The two-layer welding process reduces the possibility of incomplete penetration. The electrode shall be selected according to the base material of galvanized pipe. Generally, J422 is widely used for low carbon steel due to its operability