Are there any special precautions for welding Q355NH weathering steel plate?

Use Cu/Cr/Ni alloyed weathering steel welding wires/electrodes that are compatible with the GB/T 4171 weathering steel system. Matching the weld metal chemistry with the Q355NH base material helps the welded area develop a protective patina with corrosion behavior closer to the surrounding weathering steel.

Recommended options include MMA electrode E5015-G, MIG/TIG wire ER50-G, and submerged arc welding wire H08Mn2SiCuA + SJ101.

Do not use ordinary carbon steel welding materials where weathering performance is required. A weld made with non-matching carbon steel consumables can develop a different corrosion rate from the Q355NH base metal. The weld zone may rust earlier and lose the intended weathering consistency, creating a potential corrosion initiation area.

Q355NH is a low-alloy weathering steel. Excessive heat input can lead to weld-zone softening, grain coarsening, and changes in mechanical and corrosion performance.

Thin plates (3–12 mm): Adopt low-heat-input welding methods such as TIG, MIG, or laser welding. Use an appropriate travel speed and avoid unnecessary multi-pass heat accumulation.

Medium/heavy plates (≥14 mm): Preheat the base material to 80–150°C, with the preheat temperature not exceeding 200°C, to reduce the risk of cold cracking. The interpass temperature should be controlled at ≤250°C. For multi-pass welding, keep the heat input of each pass controlled rather than allowing excessive heat accumulation.

For medium and heavy plates, a typical welding heat input may be around 15–25 kJ/cm, depending on plate thickness, joint design, welding process, and the applicable welding procedure specification (WPS). Thin plates generally require a lower heat input.

Preheat temperature should be checked at an appropriate location near the joint, typically about 75 mm from the weld line, rather than directly on the weld bead.

On-site MMA welding: Use small-diameter electrodes such as 2.5–3.2 mm and an appropriately controlled welding current to reduce heat accumulation.

Cutting and welding edges are fresh steel surfaces without the stable oxide layer present on naturally weathered areas, making them more susceptible to initial localized corrosion. Post-weld treatment helps maintain more consistent weathering across the structure.

Grind weld seams to remove spatter, burrs, and visible undercutting, producing a smooth transition between the weld and base metal. This reduces surface irregularities that can act as local corrosion initiation points.

Apply a thin transparent weathering sealer or suitable temporary passivation treatment to weld seams, cutting edges, and heat-affected zones (HAZ) when temporary protection during the initial oxidation stage is required. Full coating is generally avoided where natural weathering is intended.

Avoid leaving welding slag or other residues on the surface, as they can interfere with uniform patina formation and promote localized corrosion.

Q355NH's weathering and mechanical performance depend partly on its designed alloy composition and original microstructure. Therefore, unnecessary high-temperature post-weld heat treatment should be avoided.

Do not perform high-temperature annealing or normalizing above 600°C unless specifically required and qualified by the applicable welding procedure or design code. Such temperatures can cause recrystallization and significant microstructural changes, potentially affecting the original mechanical properties and the consistency of the weathering response.

For heavy plates (≥50 mm): When residual-stress reduction is required, a low-temperature stress-relief treatment of approximately 150–200°C for 1–2 hours may be considered as a recommended approach, subject to the applicable WPS and project requirements. This is intended to reduce welding residual stress without applying a high-temperature annealing or normalizing cycle.

Remove oil, grease, rust, scale, paint, and other contaminants from the welding area before welding. Cleaning approximately 10–20 mm on both sides of the weld joint helps reduce the risk of porosity, inclusions, and other weld defects.

Use suitable mechanical cleaning methods such as sandblasting or wire brushing, together with an appropriate degreasing agent where necessary.

Ensure that the welding surface is dry before welding. Moisture can increase the risk of hydrogen-related cracking, particularly in thicker plates.

On-site welding should preferably be carried out under dry, controlled conditions.

Avoid welding during rain, snow, or high-humidity conditions such as RH >80%. If welding must be performed under difficult site conditions, suitable weather protection and moisture-control measures should be provided.

For welding in coastal or salt-spray environments, the weld area should be cleaned after welding and protected as necessary to prevent salt deposits from accelerating corrosion of the fresh weld surface.