First, grinding is the prerequisite.
The galvanized layer at the welding point must be polished off, otherwise, bubbles, sand holes, false welding, etc. will be generated. It will also make the weld brittle and reduce rigidity.
Second, the welding characteristics of galvanized steel
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 about 908°C. 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. The zinc liquid will deeply erode the weld metal along the grain boundary, and the low-melting-point zinc forms “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 the action of tensile stress.
If the fillet weld is welded, especially the fillet weld of the T-joint, it is most likely to produce penetrating cracks.
When galvanized steel is welded, the zinc layer on the groove surface and edge will be oxidized, melted, evaporated, and even volatilized into white smoke and steam under the action of arc heat, which can easily cause weld porosity.
ZnO formed by 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. At the same time, since Zn becomes a deoxidizer, FeO-MnO or FeO-MnO-SiO2 low melting point oxide slag inclusion will be produced. Secondly, due to the evaporation of zinc, a large amount of white smoke will be volatilized, which will irritate and harm the human body. Therefore, the galvanized layer at the welding point must be polished off.
Third, welding process control
The preparation before welding of galvanized steel 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. To achieve full penetration, the groove size should be appropriate, generally 60~65°, and a certain gap should be left, generally 1.5~2.5mm; to reduce the penetration of zinc into the weld, the galvanized layer in the groove can be removed before welding.
In actual work, the centralized groove is adopted, the process of not leaving a blunt edge is used for centralized control, and 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 steel pipe. Generally, low-carbon steel is more commonly selected for J422 due to its easy operation.
Welding technique: When welding the layer weld of multi-layer welding, try to melt the zinc layer and make it vaporize and evaporate to escape the weld, which can greatly reduce the liquid zinc left in the weld.
When welding the fillet weld, also try to melt the zinc layer in the 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 to continue welding forward after the zinc layer is melted.
When performing horizontal and vertical welding, if short-slag electrodes such as J427 are used, the tendency to undercut will be very small; if the back-and-forth electrode transport technology is used, defect-free welding quality can be obtained.
Post time: Sep-11-2024