Galvanized steel pipe because it has the dual advantages of corrosion resistance and long service life, and the price of galvanized pipe is also relatively low, so now its utilization rate is higher and higher, but the users do not pay attention to the welding of galvanized pipe, resulting in some unnecessary trouble, so what problems should we pay attention to when welding galvanized pipe? Let Jinan Galvanized pipe manufacturers to take you to understand
Jinan galvanized steel pipe
1、 The premise must be polished
It is necessary to grind off the zinc coating at the welding position, otherwise bubbles, sand holes and false welding will be produced. It will also make the weld brittle and rigid.
2、 Welding characteristics of galvanized steel
Galvanized steel is generally coated with a layer of zinc on low carbon steel, and the zinc coating is generally in the thickness of 20um. 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 molten pool or at the root of weld. Zinc has a large solid solubility in iron, and liquid zinc can deeply etch the weld metal along the grain boundary, resulting in "liquid metal embrittlement" of low melting point zinc.
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 tensile stress. If the fillet weld is welded, especially the fillet weld of T-joint, it is easy to produce through crack. During the welding of galvanized steel, the zinc layer on the surface and edge of the groove will oxidize, melt, evaporate and even volatilize white smoke and steam under the action of arc heat, which will easily cause weld porosity. The melting point of ZnO formed by oxidation is higher than 1800 ° C. if the parameters are too small during welding, it will cause slag inclusion in ZnO. Because Zn becomes the deoxidizer. FeO MnO or feo-mno-sio2 low melting point oxide inclusions are produced. Secondly, due to the evaporation of zinc, a large amount of white smoke and dust will be emitted, which will stimulate and damage the human body. Therefore, the zinc coating on the welding joint must be polished off.
3、 Welding process control
The preparation before welding of galvanized steel is the same as that of ordinary low carbon steel. It is necessary to carefully handle the groove size and the nearby zinc coating. In order to weld through, the groove size should be appropriate, generally 60 ~ 65 ° and a certain gap should be reserved, generally 1.5 ~ 2.5mm; in order to reduce the penetration of zinc to the weld, the zinc coating in the groove can be removed before welding.
In the actual work, the centralized beveling process without blunt edge is adopted for centralized control, and the two-layer welding process reduces the possibility of incomplete penetration. Welding rod shall be selected according to the base material of galvanized steel pipe. J422 is generally used for low carbon steel due to easy operation. Welding method: when welding the first layer weld of multi-layer welding, try to make the zinc layer melt and make it vaporize and evaporate out of the weld, which can greatly reduce the liquid zinc remaining in the weld.
In the welding of fillet weld, the zinc layer shall be melted in the first layer as much as possible and vaporized and evaporated out of the weld. The method is to move the end of the welding rod forward about 5 ~ 7mm, and then return to the original position after melting the zinc layer and continue to weld forward. If short slag electrode such as J427 is selected for horizontal and vertical welding, undercut tendency will be very small; if back and forth rod transportation technology is adopted, the welding quality without defects can be obtained.