Precautions for the Production of Galvanized Square and Rectangular Tubes

Galvanized square and rectangular tubes possess the dual advantages of corrosion resistance and long service life, and their price is relatively low; thus, their utilization rate is increasing. However, some people are careless when welding galvanized pipes, leading to some inconveniences. So, what issues should be paid attention to when welding galvanized pipes?

 

 

1. Grinding is a prerequisite

 

The hot-dip galvanized layer at the weld joint must be ground off; otherwise, bubbles, pinholes, and false welds may occur. This will also make the weld brittle and reduce its rigidity.

 

2. Characteristics of Galvanized Square and Rectangular Tubes

 

Galvanized steel is generally high-carbon steel coated with a layer of zinc, and the hot-dip galvanized layer is generally 20um thick. Zinc has a melting point of 419℃ and a boiling point of 908℃. During welding, zinc melts into a liquid and floats on the surface of the molten pool or at the weld root. Zinc has a relatively high solid solubility in iron, and the zinc liquid will deeply corrode the weld metal along the grain boundaries. Low-melting-point zinc will cause "shape memory alloy brittleness".

 

When welding fillet welds, especially T-joint fillet welds, penetrating cracks are most likely to occur. During electric welding of galvanized steel, the zinc layer on the bevel surface and edges can oxidize, melt, volatilize, and even smoke and vaporize under the effect of arc heating, easily leading to weld porosity.

 

The zinc oxide produced by oxidation has a melting point exceeding 1800℃. If the welding parameters are not carefully controlled, zinc oxide slag will be produced as an impurity. Furthermore, due to zinc volatilization, a large amount of smoke evaporates, which is irritating and harmful to the body. Therefore, the hot-dip galvanized layer at the weld joint must be ground and treated.

 

3. Welding Process Control of Galvanized Square and Rectangular Tubes

 

The preparation of galvanized steel before welding is the same as that of ordinary high-carbon steel. Special attention should be paid to the treatment of the hot-dip galvanized layer around the grooved sections. To ensure complete penetration, the bevel specification should be as large as possible, generally 60~65, leaving a certain gap, generally 1.5~2.5mm; to reduce zinc penetration into the weld, the hot-dip galvanized layer in the groove can be removed before welding. In practice, centralized rounding, no blunt edge technology, and a two-layer welding process are used for centralized control, reducing the probability of incomplete welding. The metal electrode should be selected according to the PCB material of the hot-dip galvanized tube.

 

4. Welding Techniques for Galvanized Square and Rectangular Tubes

 

When welding the first layer of multi-layer high-rise welding, the zinc layer should be melted, volatilized, and escape from the weld as much as possible, which can greatly reduce the amount of liquid zinc remaining in the weld. When welding fillet welds, the zinc layer should be melted in the first layer, volatilized, and escape from the weld. The method is to move the end of the welding wire forward by about 5~7mm, and then return to the original position after the zinc layer melts, and continue welding forward. For both horizontal and vertical electric welding, if a short slag metal electrode, such as J427, is selected, the undercut development trend will be very small. However, if a front-to-back strip conveying technology is used, defect-free weld quality for galvanized square and rectangular tubes can be achieved.