Galvanized steel is widely used in various industries due to the protective layer of zinc oxide that forms in the air, preserving the internal structure of the steel.
Despite its resistance to corrosion, the presence of a galvanized layer can cause cracking, porosity and slag inclusion during welding, often resulting in inferior welding quality.
Typically, a layer of zinc, about 20 µm thick, is applied to low carbon steel. Zinc has a melting point of 419°C and a boiling point of around 908°C. During welding, zinc melts into a liquid and floats on the surface of the welding puddle or deposits at the root of the weld seam.
Zinc, having substantial solubility in iron, can infiltrate the weld metal along grain boundaries, causing “liquid metal embrittlement”. At the same time, zinc and iron can form brittle intermetallic compounds such as Fe3Zn10 and FeZn10, which reduce the plasticity of the weld metal and lead to cracking under tensile stress.
Fillet welds, especially T-joints, are particularly prone to cracking.
During welding of galvanized steel, the zinc layer on the chamfer and edge of the weld, under the action of the arc heat, oxidizes, melts, evaporates and volatilizes, releasing white smoke and vapor, which can cause porosity in the weld.
The high melting point of ZnO, formed due to oxidation, at around 1800°C or above, can lead to ZnO slag inclusions if the welding parameters are too small.
Additionally, the creation of low-melting oxide slag compounds such as FeO-MnO or FeO-MnO-SiO2 can occur due to zinc's role as a deoxidant. Improper welding standards and techniques can lead to expansion of the fusion area, potentially damaging the galvanized layer.
This is especially likely during long arc operations and large swings. The evaporation of zinc produces a large amount of white smoke, which can be irritating and harmful to humans.
Therefore, using welding methods and materials that produce less smoke is a critical consideration.
Various welding methods are used for galvanized steel, including gas welding, manual metal arc welding, CO2 gas protected welding, automatic submerged arc welding, and tungsten inert gas welding.
Gas welding, once commonly used to weld galvanized pipes, is now largely obsolete due to its unconcentrated heat input, which can lead to defects and poor mechanical performance of the weld. Gas welding causes significant damage to the galvanized layer.
CO2 gas shielded welding is favorable for welding galvanized steel. With the correct welding specifications and corresponding shielding gases and welding materials, high-quality weld joints can be obtained.
However, this method is rarely used in engineering practice.
Tungsten inert gas welding, with its concentrated arc energy, causes less damage to the galvanized layer and can easily form good double-sided forming joints for one-sided welding.
However, its slower speed and higher cost make it a less attractive option.
Manual metal arc welding is the most commonly used method for pipe installation. With the correct choice of electrodes, such as J421, J422, J423, titanium oxide type and titanium-calcium type electrodes are used.
These electrodes have a high melting rate, which increases the melting speed. If operated without oscillation, only the galvanized layer at the edge of the weld pool will be damaged, generally not expanding the melting area, thus reducing the penetration of liquid zinc into the weld metal.
With the correct operation method and welding material, good mechanical performance of the joint can be obtained, free from defects.
Due to its lower cost and faster speed compared to tungsten inert gas welding, manual metal arc welding is adopted when qualified welders are available.
The pre-welding preparation of galvanized steel is the same as that of low-carbon steel in general. Close attention should be paid to the size of the chamfer and the nearby galvanized layer.
To ensure full penetration, the chamfer size must be appropriate, generally 60 ~ 65°, with a certain gap, generally 1.5 ~ 2.5mm. To reduce zinc penetration into the weld, the galvanized layer inside the bevel can be cleaned before welding.
In actual work, a centralized chamfering process without leaving blunt edges is adopted for centralized control, and a two-layer welding process is used to reduce the possibility of incomplete fusion.
The electrode should be chosen based on the base material of the galvanized pipe. For general low carbon steel, J422 is commonly chosen because of its easy operability.
Welding Technique:
When welding the first layer of multilayer welds, try to melt the zinc layer and allow it to vaporize and evaporate from the weld, which can greatly reduce the amount of liquid zinc remaining in the weld.
When welding fillet welds, try to melt the zinc layer and allow it to vaporize and evaporate from the weld in the first layer. This is done by first moving the electrode tip forward about 5 to 7 mm, melting the zinc layer, and then returning to the original position to continue welding.
After welding, it is necessary to immediately clean the weld seam, brush with zinc-rich primer and implement corrosion protection measures.
Quality assurance measures for welding
Control is implemented in five aspects: personnel, materials, machines, methods and environment.
1. The personal factor is the focus of welding control:
Therefore, before welding, welders who are technically proficient and hold welding certificates must be selected for the necessary technical training and instructions. No random replacement is allowed to ensure the stability of pipeline welding personnel.
2. Welding materials control:
Ensure that purchased materials come from regular channels, with quality assurance, certificates of conformity and meet process requirements; control of recycled welding electrode heads is strict to ensure flow and usage; welding materials must be cooked strictly according to the process, and the quantity issued at one time must not exceed half a day of use.
3. Welding machines:
Welding machines must be reliable and meet process requirements; welding machines must have calibrated current and voltage meters to ensure the correct execution of the welding process. Welding cables cannot be too long, and welding parameters must be adjusted when they are long.
4. Welding process methods:
Ensure strict implementation of special operating methods for galvanized pipes, pre-welding inspection of the welding process groove, control of welding process parameters and operating techniques, post-welding appearance quality inspection, and additional non-destructive testing after welding , if necessary. Control welding levels and the amount of welding material for each joint.
5. Welding environment control:
Ensure that the temperature, humidity and wind speed during welding meet the process requirements.
Conclusion
Hot-dip galvanized pipe welding in construction adopts the correct welding process, strictly checks and accepts according to specifications, and promptly carries out anti-corrosion treatment (zinc-rich paint) on the weld after welding.
It has certain feasibility in the construction process of open and closed air conditioning systems, which can increase the construction speed and improve the firmness of pipe connections.
Therefore, under permitted construction conditions, and on the premise of implementing relevant protective and anti-corrosion measures, hot-dip galvanized pipes can be welded together.
