The Knowledge of Welding Process of Galvanized Steel Pipe

With the development of the economy, the demand of galvanized steel pipes increasing day by day. But most of people may not know the precautions for welding galvanized pipes.

The seam must be polished before welding, otherwise will have bubbles, trachoma, false welding, etc. It will also drop the welds brittle and the rigidity. 

Characteristics of welding galvanized steel
Usually galvanized steel coated with zinc on the low carbon steel, and the galvanized layer is about 20um thick. The melting point of zinc is around 419 ° C and the boiling point is around 908 ° C. In welding, zinc melts into a liquid and floats on the surface of the molten pool or at the root of the weld. Zinc has greater solid solubility in iron, zinc liquid will deeply etch the weld metal along the grain boundary, and low melting point zinc forms "liquid metal embrittlement". At the same time, zinc and iron can form brittle compounds between metals, these brittle phases reduce the plasticity of the weld metal, and produce cracks under the action of tensile stress.

How to control the process of welding galvanized steel?
The preparation of galvanized steel before welding is the same as the low carbon steel. In order to weld well, the groove size should be around  60 ~ 65 °, gap should be 1.5 ~ 2.5mm. In the work operation, do centralized beveling with out blunt edge which is used for centralized control. The two-layer welding process reduces the possibility of under-welding. The electrode should be selected according to the base material of the galvanized pipe. Generally, low-carbon steel is generally used because of its ease of operation.

Welding method: When welding the first layer of multi-layer welding, try to melt the zinc layer and vaporize and evaporate it to escape the weld, which can reduce the liquid zinc remaining in the weld.

In the case of fillet welds, the zinc layer is also melted as much as possible in the first layer and vaporized and evaporated to escape the weld. The method is to move the end of the electrode forward about 5 ~ 7mm. When the zinc layer is made After melting, return to the original position to continue welding. In the case of horizontal and vertical welding, if the short slag electrode such as J427 is used, the tendency of undercut will be very small; if the front and back reciprocating bar technology is used, the defect-free welding quality can be obtained.

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