Technical Requirements for Welding of Galvanized Pipes

On-site welding construction of hot-dip galvanized steel pipes, It focuses on controlling zero weld defects, qualified structural strength and complete post-welding anti-corrosion performance, so as to eliminate quality problems such as pores and cracks caused by zinc layer vaporization.

Pre-welding Preparation Requirements

Pipe Inspection: Check the specifications, wall thickness and material of galvanized pipes before construction to ensure compliance with design requirements. The pipes shall be free from deformation, damage and severe corrosion with uniform and intact galvanized layers. Unqualified pipes are prohibited from welding.

Groove Machining: For pipes with wall thickness ≥ 3mm, V-shaped grooves are required with a groove angle of 55°-65° and a root face of 1-1.5mm. The cutting surface shall be smooth without burrs and curling. Thin-walled pipes (wall thickness < 3mm) can be directly butt-welded without grooving.

Complete Zinc Removal (Core Procedure): Grind the inner and outer surfaces 20-50mm on both sides of the groove thoroughly with angle grinders and steel wire brushes to completely remove galvanized layers, oxide scales, oil stains and zinc tumors until the metallic luster is fully exposed. Residual zinc layers are strictly forbidden to avoid dense pores and slag inclusions in welds.

Pipe Alignment Requirements: The misalignment of pipe butt joints shall not exceed 10% of the pipe wall thickness and shall not exceed 2mm at maximum. The butt gap is controlled within 2-3mm with straight and offset-free alignment.

Welding Material Preparation: Adopt E43 series low-carbon steel welding rods or matching welding wires. Welding rods shall be dried and insulated in accordance with specifications before use. Moist welding rods are prohibited to ensure qualified weld fusion quality.

Welding Process Requirements

Welding Parameter Control: Adopt shielded metal arc welding (SMAW) or CO₂ gas metal arc welding (GMAW). Follow the principles of low current, short arc and fast welding speed. The welding current shall be reduced by 10%-15% compared with that for ordinary carbon steel pipes to reduce high-temperature vaporization of zinc layers and shorten the residence time of the molten pool.

Welding Operation Specifications: Adopt straight-line slight weaving welding. Prolonged fixed-point arcing and large-amplitude swinging are forbidden to prevent pipe wall burn-through and weld overheating. For multi-layer and multi-pass welding, completely remove welding slag and spatter after each pass and confirm no defects before welding the next layer.

Construction Environment Requirements: Wind shielding facilities shall be erected when the on-site wind speed exceeds 2m/s. Welding is prohibited in rainy, snowy and humid conditions. Preheat the welding area when the ambient temperature is lower than 0℃ to prevent weld cold cracks.

Safety Protection: Keep full ventilation during welding. Operators shall wear protective equipment to prevent inhalation of harmful zinc oxide welding fume.

Post-welding Treatment and Anti-corrosion Requirements

Weld Cleaning: After welding, thoroughly remove welding slag, spatter and burrs on the weld surface, and polish the weld and surrounding areas to keep the surface flat and smooth.

Mandatory Anti-corrosion Repair: The galvanized layer around the welding area will be burned out by high welding temperature, so special anti-corrosion treatment is mandatory. Polish off welding oxide layers first, then apply special galvanized repair coatings such as cold zinc spray and zinc-rich primer. The thickness of the repaired coating shall not be less than that of the original galvanized layer. Ordinary paint only is prohibited to ensure consistent overall anti-corrosion performance of the pipeline.

Pressure Test: Conduct hydrostatic test or air tightness test on pressure-bearing galvanized pipes in accordance with specifications to verify weld tightness. Qualified standards include no leakage and no pressure drop.

Weld Quality Acceptance Standards

Visual Quality: Welds shall be uniform in forming and width without cracks, pores, slag inclusions, burn-through, incomplete fusion and other defects. The undercut depth shall be ≤ 0.5mm without continuous excessive undercut.

Non-destructive Testing: Conduct radiographic testing (RT) or ultrasonic testing (UT) in specified proportion for key parts such as process and pressure pipelines, and the results shall meet the qualified specification standards.

Overall Requirements: The welds shall reach the standard structural strength without hidden defects. The post-welding anti-corrosion repair shall be complete to ensure the pipeline meets the service working conditions.