Welding of Coated Steels

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Introduction to Coated Steel Welding

Coated steel welding represents a critical process in modern manufacturing, requiring specific techniques and precautions to ensure successful outcomes. The most common type, galvanized steel, features a protective zinc coating that presents unique challenges during welding operations. Understanding these challenges and implementing appropriate solutions is essential for achieving high-quality welds while maintaining corrosion protection.

Galvanizing Methods and Applications

Galvanized steel has become increasingly vital in manufacturing, particularly where corrosion resistance is paramount. Two primary galvanizing methods exist: continuous hot-dip coating for sheet metal and individual item dipping. The continuous process produces eight distinct coating thickness classes, offering versatility for different applications.

The applications of galvanized steel span numerous industries. In transportation, manufacturers utilize galvanized steel for truck bodies, buses, and automobiles, particularly in regions where road salt creates corrosive conditions. Industrial applications include air conditioning housings and processing tanks, while infrastructure applications extend to electrical transmission towers and highway signage. The material has also found widespread use in household appliances such as washing machines and dryers.

Welding Processes and Techniques

Arc Welding Fundamentals

During arc welding of galvanized steel, the zinc coating vaporizes at 1600°F (871°C), well below steel's melting point of 2800°F (1538°C). This temperature differential requires careful control of heat input and welding speed. The coating disturbance varies with the heat input and base metal heat loss characteristics.

Electrode Selection and Application

Electrode selection must consider both material thickness and welding position. For thin materials, E-XX12 or 13 electrodes prove most effective, while E-XX10 or 11 electrodes are better suited for galvanized pipe and thicker components. When working with heavy-gauge materials, low-hydrogen electrodes offer optimal results.

Gas Metal Arc Welding (GMAW)

GMAW has emerged as a preferred method for galvanized steel joining. When working with thin gauges, the fine-wire short-circuiting technique proves most effective. Shielding gas selection typically involves either 100% CO₂ or a 75% argon/25% CO₂ mixture, with the choice depending primarily on material thickness and welding position.

Resistance Welding Considerations

While resistance welding causes less zinc coating disruption, it presents unique technical challenges. The zinc displaced during spot welding solidifies adjacent to the weld, maintaining corrosion protection around the weld nugget. This process requires precise control of welding parameters to achieve optimal results.

Safety and Quality Considerations

Proper ventilation stands as a crucial safety requirement when welding coated steels. A comprehensive ventilation system should include positive ventilation with suction hoses at the weld area. When using GMAW or FCAW processes, suction-type gun nozzles provide additional protection. Under no circumstances should welding occur in confined spaces without adequate ventilation.

Quality control in coated steel welding requires attention to several key factors. Joint design must facilitate zinc vapor escape to prevent porosity. Complete joint penetration ensures structural integrity, while appropriate heat input levels help maintain coating integrity where possible. Regular monitoring for porosity and cracking remains essential throughout the welding process.

Coating Restoration Methods

Post-welding restoration of zinc coating proves essential for maintaining corrosion resistance. The first method involves zinc base paste application, where specially formulated paste sticks are applied to heated bare metal. This technique requires skill and practice to achieve proper blending with the original coating, though it typically results in a slightly thicker coating than the original.

The second method employs flame spray application, particularly effective for larger areas requiring restoration. This process involves spraying molten zinc onto the prepared surface, creating a coating approximately 2-2.5 times the thickness of the original. While more efficient for extensive repairs, proper surface preparation and application technique remain crucial for optimal adhesion and protection.

Specialized Applications

Tern Plate Welding

Tern plate, consisting of steel hot-dipped with lead-tin alloy, serves specific industrial purposes, most notably in automotive fuel tank construction. The material's coating thickness is specified by weight per square foot of sheet metal. While resistance welding remains the primary joining method for tern plate, arc or oxyacetylene welding may occasionally be necessary. In such cases, the destroyed tern plating adjacent to the weld requires restoration through soldering techniques.

Aluminized Steel Processing

The automotive industry extensively uses aluminized steel, particularly in exhaust system components. This material features a high silicon-aluminum alloy coating applied through hot dipping to both sides of the sheet steel. Two standard coating weights exist: regular at 0.40 ounces per square foot and lightweight at 0.25 ounces per square foot. When welding disrupts the aluminum coating, restoration proves particularly challenging. As a result, painting typically serves as the preferred method for protecting welded areas.

Advanced Welding Techniques

Carbon arc welding has demonstrated particular effectiveness with galvanized steel. Both single and twin carbon torch configurations prove viable, with the twin carbon torch functioning similarly to an oxyacetylene flame. The single carbon technique, when properly executed, allows for extremely high welding speeds. Operators typically employ Type RBCuZn-A filler rod, composed of 60% copper and 40% zinc, directing the arc onto the filler rod to achieve fusion while minimizing zinc coating destruction.

Torch brazing offers another viable approach for joining galvanized steel. This process mirrors the carbon arc technique in many ways, with the torch directed toward the filler rod to accomplish joint filling. Liberal application of brazing flux helps minimize zinc loss in areas adjacent to the weld, contributing to better overall corrosion resistance.

Conclusion

Successful welding of coated steels demands thorough understanding of material properties, careful process selection, and precise technique implementation. The choice of welding method and restoration technique significantly impacts both structural integrity and corrosion resistance. Practitioners must balance these considerations while maintaining adherence to safety protocols and quality control measures. Through proper attention to these factors, welders can achieve high-quality joints while preserving the protective properties of coated steels.