What are the welding processes for galvanized rectangular pipes?
The welding process for galvanized rectangular pipes should be selected based on the surface zinc coating's characteristics (low melting point, easy evaporation), the pipe wall thickness, and the intended use (load-bearing/decorative, indoor/outdoor). The core objectives are to reduce zinc fume hazards, avoid weld defects (porosity, zinc embrittlement), and ensure subsequent corrosion protection.
II. Key Operational Points for Each Process (Specific to the Characteristics of the Galvanized Coating)
Regardless of the process chosen, "treating the zinc coating," "controlling heat input," and "post-weld corrosion protection" are core commonalities, but operational details vary between processes:
1. Arc Welding (Manual Welding): Suitable for thick-walled, load-bearing pipes.
Welding Rod Selection: Anti-porosity, low-hydrogen electrodes are preferred to avoid porosity defects caused by zinc vapor. Commonly used types:
Ordinary carbon steel galvanized pipe: E4303 (acidic electrode, easy to operate, suitable for non-load-bearing applications);
Load-bearing structures/low-temperature environments: E4315 (alkaline, low-hydrogen electrode, requires drying to reduce hydrogen embrittlement in the weld).
Zinc Cleaning: Use an angle grinder to grind the weld seam (10-15mm per side) to completely remove the zinc layer. (For thick hot-dip galvanized layers, grind until the silvery-white steel is exposed.) This prevents zinc evaporation and the formation of pores.
Parameter Control: Use a low current, short arc. For example, for welding a 3mm thick pipe, use a current of 90-120A and an arc length ≤ 1/2 the rod diameter. Avoid excessive zinc evaporation caused by prolonged high temperatures.
Welding Technique: Use "broken arc welding" (tick welding), pausing to cool after each 10-15mm weld to reduce zinc vapor accumulation. Maintain a 45°-60° rod angle to avoid direct contact with residual zinc. 2. CO₂ Gas Shielded Welding: Suitable for batch welding of medium-thin-wall pipes
Welding Wire and Gas:
Welding Wire: Choose ER50-6 (a low-carbon steel wire containing Si and Mn to deoxidize and reduce porosity). The diameter should be selected based on the wall thickness: 0.8mm for thin-wall pipes (1.5-2mm) and 1.0mm for medium-thick pipes (2-4mm).
Gas: Preferable is an Ar+CO₂ mixture (e.g., 80% Ar+20% CO₂). This reduces spatter by over 50% compared to pure CO₂ and provides a more stable shielding effect. (Pure CO₂ is only suitable for outdoor applications with low winds.)
Parameter Control: The key is "low current, fast welding speed." For example, for welding a 2mm thick pipe: use a current of 80-100A, a voltage of 18-20V, and a welding speed of 30-40cm/min. This avoids excessive heat input that could cause pipe deformation or zinc embrittlement. Operating Tips: Maintain a welding torch angle of 15-20° relative to the pipe, and limit the wire extension to 10-15mm (a wire too short can burn the torch, while a wire too long can provide insufficient protection). A 0.5-1mm gap should be left for butt joints to prevent incomplete penetration.
3. TIG Welding: Suitable for thin-walled, high-precision welding.
Tungsten Electrode and Gas:
Tungsten Electrode: Choose a cerium tungsten electrode (WC20) with a diameter of 2.0-2.4mm (suitable for thin-walled pipe). Ground the tip to a 30-45° angle to reduce arc drift.
Argon gas purity: ≥99.99% (high-purity argon ensures an oxidation-free weld pool and prevents blackening of the weld). Flow rate: 8-12 L/min.
Zinc Cleaning: Thoroughly clean the weld area (including oil stains and the zinc layer) with sandpaper or alcohol. Otherwise, even trace amounts of zinc residue can cause porosity under argon protection (TIG welding is extremely sensitive to impurities). Welding Technique: Use "butt welding without filler wire" (thin-walled pipes ≤ 1.5mm) or "manual filler wire" (wall thickness 1.5-2mm). When filling the filler wire, gently feed the wire from the front of the molten pool (avoiding contact with the tungsten electrode). Keep the heat-affected zone within 5mm to minimize pipe deformation.
4. Resistance Welding (Spot Welding/Seam Welding): Suitable for lightweight, thin-walled lap joints.
Electrode and Parameters:
Electrode: Use a copper alloy electrode (excellent thermal conductivity) with a smooth surface to avoid adhesion to the zinc coating.
Parameters: Spot welding pressure 0.3-0.5MPa, current on time 0.5-1.0s, current adjusted according to wall thickness (approximately 5-8kA for a 1mm thick pipe). Ensure the weld spot diameter is ≥ 3 times the wall thickness (e.g., for a 1mm thick pipe, the weld spot diameter is ≥ 3mm). Joint Design: Lap joints must be used, with an overlap length ≥ 5 times the wall thickness (e.g., ≥ 5mm for a 1mm pipe). Avoid short overlaps that could cause welds to break. Cold-dip galvanized pipes can be lightly sanded before welding (no need to thoroughly clean; resistance heat can partially melt the zinc layer).
Post-weld treatment: Residues of zinc residue will remain on the weld surface, which should be smoothed with sandpaper and then reapplied with cold-dip galvanizing paint to prevent corrosion. III. Core Principles of Process Selection
Based on wall thickness: TIG welding/CO₂ welding is preferred for thin walls (≤2mm); CO₂ welding is preferred for medium-thick walls (2-4mm); and arc welding is preferred for thick walls (≥4mm);
Based on application: Arc welding/E4315 electrodes are preferred for load-bearing structures (such as factory buildings); CO₂ welding is preferred for mass-produced decorative parts (such as guardrails); TIG welding is preferred for high-precision thin-walled parts (such as medical devices); and resistance welding is preferred for lightweight furniture;
Based on environmental protection requirements: Resistance welding (zinc fume-free) is preferred for confined spaces/indoor welding; arc welding/CO₂ welding (with fume exhaust equipment required) is preferred for outdoor/well-ventilated environments.
Summary
There is no "optimal" welding process for galvanized rectangular pipes. The selection must be based on a comprehensive consideration of pipe wall thickness, application requirements, and production efficiency. The key is to balance weld strength and corrosion resistance through three key steps: cleaning the zinc layer, controlling heat input, and applying post-weld corrosion protection. For example, arc welding with E4315 electrodes is used for thick, load-bearing outdoor guardrails, TIG welding with argon shielding is used for thin-walled indoor decorative frames, and CO₂ mixed gas welding is used for mass-produced equipment frames. All of these methods can achieve stable and reliable welding results.