1. Shielded Metal Arc Welding (SMAW, Stick Welding)
Core Advantages: Exceptional flexibility (works for on-site, outdoor, or complex joint positions like overhead welding), no need for external gas shielding (adapts to windy, low-temperature construction environments), and high portability-ideal for field repairs or large structural welding (e.g., bridge supports, steel frames in cold regions).
Why It's Suitable for Low Temperatures: Relies on low-hydrogen covered electrodes (e.g., E5515-G, E5516-G), which reduce hydrogen absorption (critical for preventing "hydrogen embrittlement," a major risk for high-strength steels in cold conditions). The electrode coating also stabilizes the arc, refines weld metal grains, and 补充 alloying elements (e.g., Mn, Ni) to enhance low-temperature toughness.
Key Operational Guidelines:
Dry electrodes at 350–400°C for 1–2 hours before use; store unused electrodes in a low-temperature electrode oven (80–100°C) to prevent moisture absorption (moisture increases hydrogen content).
Use a short arc to reduce heat loss in low temperatures and avoid porosity or incomplete fusion.
Preheat the base metal to 100–150°C (increase to 150–200°C for plates ≥25mm thick) to slow cooling and prevent the formation of brittle martensite in the heat-affected zone (HAZ).
2. Gas Metal Arc Welding (GMAW, MIG Welding)
Core Advantages: High welding efficiency (continuous wire feeding suits long weld seams), stable arc, and consistent weld quality-well-suited for batch production or large-area structural welding (e.g., cold-region curtain walls, industrial equipment frames).
Why It's Suitable for Low Temperatures: Uses argon-rich shielding gas (e.g., 80% Ar + 20% CO₂) to isolate air, reducing oxidation and nitrogen absorption (both impair toughness). Matching solid wires (e.g., ER55-G) or metal-cored wires (e.g., E71T-8-Ni1) are formulated with controlled C, Mn, and Ni content to lower the "ductile-brittle transition temperature (DBTT)" of the weld, ensuring ductility at sub-zero temperatures.
Key Operational Guidelines:
Equip the wire feeder with a heater (if ambient temperature <0°C) to prevent wire stiffening or moisture condensation on the wire surface.
Control heat input within 20–35kJ/cm (adjust current to 180–250A, voltage to 22–28V) to avoid coarse grain growth in the HAZ (coarse grains drastically reduce low-temperature toughness).
Maintain a shielding gas flow rate of 18–25 L/min; use a gas lens or windscreen in windy conditions to prevent gas disruption (critical for low-temperature outdoor welding).
3. Submerged Arc Welding (SAW)
Core Advantages: Ultra-high deposition rate and deep weld penetration-ideal for thick-section Q460NH welding (e.g., pressure vessel shells, heavy-duty bridge girders in cold climates), where efficiency and weld integrity are paramount.
Why It's Suitable for Low Temperatures: Relies on low-hydrogen, low-temperature-tough flux (e.g., SJ101G, SJ102) paired with solid wires (e.g., H08Mn2NiMoA). The granular flux isolates the arc from air, absorbs impurities, and slows weld cooling-reducing residual stress and HAZ brittleness. The wire's alloying elements (Ni, Mo) further enhance the weld's resistance to brittle fracture at low temperatures.
Key Operational Guidelines:
Preheat the flux to 250–300°C for 1–2 hours to remove moisture (moisture in flux increases hydrogen content, a critical risk in cold conditions).
For plates ≥30mm thick, use multi-pass welding and maintain an interpass temperature of 150–250°C (overheating weakens strength; underheating causes HAZ embrittlement).
Remove residual flux immediately after welding to prevent moisture trapping (moisture accelerates corrosion in low-temperature, high-humidity environments, compromising weather resistance).
4. Gas Tungsten Arc Welding (GTAW, TIG Welding)
Core Advantages: Precise heat control and ultra-clean welds (no slag inclusion, minimal HAZ width)-best for thin-section Q460NH (3–12mm thick) or critical root passes (e.g., pressure vessel nozzles, load-bearing joints in cold-region infrastructure), where defect-free welds are non-negotiable.
Why It's Suitable for Low Temperatures: Uses high-purity argon shielding gas (99.99% Ar) to eliminate contamination, and matching filler wires (e.g., ER55-G, ER55NiCrMo-1) with low C content to avoid brittle carbide formation. The low heat input of GTAW minimizes HAZ softening and residual stress, preserving low-temperature toughness.
Key Operational Guidelines:
Use a water-cooled torch (even in low temperatures) to prevent torch overheating and maintain arc stability.
Preheat thin plates (3–8mm) to 80–120°C; for thicker thin plates (10–12mm), preheat to 120–150°C to avoid cold cracking.
For butt joints, use backing argon for root passes to prevent backside oxidation (oxidation weakens weld toughness in low temperatures).
Processes to Avoid for Q460NH in Low-Temperature Applications
Oxy-acetylene welding: High heat input and uneven cooling cause excessive HAZ grain growth, drastically reducing low-temperature toughness.
Self-shielded Flux-Cored Arc Welding (FCAW-S): Self-shielded wires often have higher hydrogen content and unstable arc performance in low temperatures, increasing crack risk and brittle fracture potential.
