SMAW is a manual welding process that uses a coated electrode as both the filler metal and arc stabilizer. The electrode coating decomposes during welding to generate shielding gas and slag, which protect the molten pool from atmospheric contamination. This method features simple equipment, strong adaptability to on-site construction, and suitability for welding carbon steel, low-alloy high-strength steel, and weathering steel components. It is widely used in field welding of steel structure nodes, such as beam-column connections and truss joints. Limitations include low welding efficiency, high labor intensity, and significant dependence on operator skills.

 

    SAW operates by burying the arc under a layer of granular flux, which isolates the arc and molten pool from air, suppresses arc light radiation, and reduces spatter. The process uses continuous bare wire as filler metal, enabling high-current, high-efficiency welding with deposition rates 5–10 times higher than SMAW. It is ideal for welding thick plates (≥8 mm) of carbon steel and low-alloy steel, such as steel structure base plates, box-section columns, and pressure vessel shells. SAW excels in flat and fillet welding positions but is not suitable for vertical or overhead welding due to flux flow constraints.

 

    GMAW uses a continuous filler wire fed through a welding gun, with shielding gas delivered coaxially to protect the weld zone. MIG welding (using inert gas like argon) is suitable for welding stainless steel and non-ferrous metals, while MAG welding (using mixed gas like Ar+CO₂) is preferred for carbon steel and low-alloy steel. This method offers high welding speed, stable arc, and excellent weld formation, with minimal post-weld cleaning required. It is widely applied in automated production lines for steel structures, such as prefabricated steel beam welding and steel pipe truss fabrication. The pulsed GMAW variant further reduces heat input, minimizing thermal deformation of thin-walled steel components.

 

    GTAW uses a non-consumable tungsten electrode to generate an arc, with shielding gas (argon or helium) protecting the electrode and molten pool. Filler metal is added manually or automatically as needed. The process produces high-quality, precise welds with minimal spatter, making it suitable for root pass welding of thick steel plates, welding of stainless steel structural components, and joining dissimilar steel materials. GTAW is characterized by low heat input and narrow heat-affected zone (HAZ) but has lower efficiency, limiting its use to critical weld seams requiring strict quality control.

 

    ESW is a high-efficiency welding method for thick steel plates (≥50 mm), utilizing the resistance heat of molten slag to melt the base metal and filler wire. The process is typically used in vertical welding positions for heavy steel structures, such as hydraulic press frames, ship hull plates, and large-scale steel bridge nodes. ESW achieves single-pass full penetration welding of thick plates, significantly reducing welding layers and time. However, it produces a large HAZ with coarse grains, necessitating post-weld heat treatment to improve weld toughness.

 

    - Select welding consumables (electrodes, wires, flux) that match the base steel grade. For Q355 low-alloy high-strength steel, use E50 series electrodes or ER50 series wires to ensure weld strength and toughness match the base metal.

    - For weathering steel structures, choose weather-resistant welding consumables to maintain corrosion resistance of the weld zone.

    - Avoid mixing consumables of different grades; store electrodes and flux in a dry environment to prevent moisture absorption, which can cause weld porosity and hydrogen-induced cracking (HIC).

 

    - Design weld joints according to structural load-bearing requirements: use full-penetration butt welds for primary load-bearing components, and fillet welds for secondary components, with weld size determined by force calculation.

    - Perform thorough surface cleaning: remove rust, oil, paint, and mill scale from the weld zone (at least 20 mm on both sides of the joint) using grinding, sandblasting, or chemical cleaning to ensure good fusion between filler metal and base metal.

    - Preheat the workpiece for high-carbon steel, high-strength steel, or thick plate welding. The preheating temperature is determined by steel carbon equivalent (CEV) and plate thickness; typically 80–150℃ for Q460 steel plates ≥25 mm thick. Preheating reduces temperature gradients, slows cooling rates, and prevents cold cracking.

 

    - Control heat input strictly: excessive heat input leads to HAZ coarsening, reduced weld toughness, and increased thermal deformation; insufficient heat input causes incomplete fusion and lack of penetration. For Q355 steel butt welding (10 mm thickness), use a welding current of 220–280 A, voltage of 24–28 V, and speed of 150–200 mm/min.

    - Maintain proper arc length and welding angle: for SMAW, arc length should be 1–2 times the electrode diameter; for GMAW, keep the contact tip-to-work distance at 12–20 mm to ensure stable shielding gas coverage.

    - Adopt multi-layer multi-pass welding for thick plates, with each pass thickness controlled at 3–5 mm. Clean slag between layers thoroughly to avoid slag inclusion defects.

 

    - Use rational welding sequences: for box-section steel components, adopt symmetric welding to offset unilateral thermal deformation; for long weld seams, use segmented welding (backstep welding or skip welding) to reduce cumulative deformation.

    - Apply rigid clamping fixtures to fix workpieces during welding, limiting free deformation. Fixtures should be removed gradually after welding to avoid residual stress concentration.

    - Perform pre-deformation design: preset reverse deformation of 0.5–1.5 mm for components prone to angular deformation, counteracting welding-induced deformation.

 

    - Conduct stress relief heat treatment for critical steel structures (e.g., pressure vessels, crane beams): heat the component to 550–650℃, hold for 1–2 hours per 25 mm thickness, then cool slowly to room temperature to reduce residual stress.

    - Remove weld spatter, slag, and excess weld reinforcement; grind weld toes to smooth transitions, reducing stress concentration factors.

    - Implement tiered quality inspection:

        - Visual inspection (VT) to check for surface defects such as cracks, porosity, and incomplete fusion.

        - Non-destructive testing (NDT) including ultrasonic testing (UT) for internal defects in thick plates, radiographic testing (RT) for critical welds, and magnetic particle testing (MT) for surface cracks in ferromagnetic steels.

        - Perform mechanical property testing (tensile, impact, bend tests) on weld specimens to verify compliance with design standards.