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Welding Methods and Precautions for Galvanized Steel Pipes

Release time:

2025-12-12

Galvanized steel pipes are increasingly widely used in water supply and drainage, fire protection, heating, ventilation, and industrial transportation fields due to their excellent corrosion resistance, long service life, and cost-effective advantages. However, the welding process of galvanized steel pipes is special. The galvanized layer on the surface is prone to melting, evaporation and other reactions under the high-temperature welding environment. If the operation is not standardized, it is likely to cause defects such as porosity, cracks, and slag inclusion in the weld. This will not only reduce the joint strength and tightness, but also may shorten the overall service life of the pipe and even cause potential safety hazards. To help relevant practitioners master scientific welding technology, avoid construction risks, and ensure project quality, this article will systematically sort out the core points and precautions of galvanized steel pipe welding from key links such as pre-welding preparation, characteristics of galvanized steel pipes, process control, and operating techniques.

I. Core Preparation Before Welding: Grinding of Galvanized Layer

It is necessary to thoroughly grind the galvanized layer in the weld area before welding, which is a prerequisite for ensuring welding quality. If the unground galvanized layer is directly welded, the vaporization of zinc at high temperature will cause defects such as bubbles, sand holes, and cold welding in the weld. At the same time, the residual zinc element will also make the weld metal brittle, significantly reducing the rigidity and bearing capacity of the joint. The grinding range should cover the groove and 20-30mm areas on both sides until the metal color is exposed, ensuring that the welding surface is clean and free of impurities.

II. Welding Characteristics of Galvanized Steel Pipes

Galvanized steel pipes usually take low-carbon steel as the base material, with a galvanized layer about 20μm thick plated on the surface. The melting point of zinc is only 419℃, and the boiling point is about 908℃, which is much lower than the temperature required for steel welding. During the welding process, the galvanized layer will quickly melt into a liquid state, floating on the surface of the molten pool or accumulating at the root of the weld. Due to the high solid solubility of zinc in iron, liquid zinc will penetrate and erode the weld metal along the grain boundaries, forming the phenomenon of "liquid metal embrittlement"; at the same time, zinc and iron will also form intermetallic brittle compounds such as Fe₃Zn₁₀. These brittle phases will greatly reduce

the weld plasticity and easily produce cracks under tensile stress, among which the fillet weld of T-joint is most prone to through cracks.

In addition, the galvanized layer on the groove surface will oxidize and evaporate under the action of arc heat, producing a large amount of white zinc fume and steam. If these substances are involved in the molten pool, they are likely to form porosity; the melting point of ZnO generated by oxidation is as high as 1800℃ or more. If the welding parameters are too small, ZnO will remain to form slag inclusion; at the same time, zinc as a deoxidizer may also induce low-melting oxide slag inclusion such as FeO-MnO. It should be specially noted that the zinc fume generated by evaporation has a strong irritating and harmful effect on the human body. Therefore, in addition to grinding the galvanized layer, it is also necessary to take good construction ventilation and personal protection measures.

III. Key Points of Welding Process Control

The selection of welding materials should match the base material of the galvanized steel pipe. For common low-carbon steel galvanized pipes, J422 electrodes are preferred. They have excellent operability, strong adaptability to slight residual impurities, and can better ensure welding stability. If other welding methods are adopted, it is necessary to match the welding materials and protective media targetedly. For example, CO₂ gas shielded welding requires the selection of special welding wire and appropriate protective gas ratio.

IV. Key Welding Techniques

During multi-layer welding, the core goal of the first layer of weld is to promote the full melting, vaporization and escape of the residual galvanized layer from the weld, reducing the residual liquid zinc. When welding fillet welds, the end of the electrode can be moved forward by 5~7mm first, and then returned to the original position to continue welding after the galvanized layer is fully melted.

For horizontal welding and vertical welding operations, it is recommended to use short-slag electrodes such as J427, which can effectively reduce the undercut tendency; if the back-and-forth electrode movement technology is adopted, the weld formation quality can be further optimized and defects can be avoided. During the welding process, it is necessary to maintain short-arc operation and appropriately control the welding speed, so as to ensure good weld formation and reduce the residence time of zinc vapor in the molten pool.

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