Why the Rust Layer is a Problem for Welding
The stable patina on weathering steel is not the issue; it's the loose scale and contaminants underneath or within it that cause problems. If you weld over it, you risk:
Porosity (Gas Entrapment): The rust layer is hydrated iron oxide (it contains moisture). The intense heat of the welding arc causes this moisture to instantly turn to steam. This steam becomes trapped in the molten weld pool, creating bubbles that solidify as pores (cavities). Porosity severely weakens the weld, making it brittle and prone to failure.
Inclusions: The physical layer of scale can break off and be engulfed by the weld pool. These non-metallic inclusions act like little cracks within the weld, creating stress points and reducing the weld's structural integrity and toughness.
Poor Fusion: The rust and scale act as a barrier between the welding arc and the clean base metal. This can prevent proper penetration and fusion between the weld metal and the base metal, resulting in a weak, defective bond.
Unstable Arc: The inconsistent surface can cause the welding arc to flicker and become unstable, leading to an irregular, poor-quality weld bead.
Contaminated Weld Chemistry: The patina contains elements like sulfur and phosphorus. Excessive amounts of these elements entering the weld pool can create hot cracking (solidification cracking) as the weld cools, making the joint useless.
Best Practices for Welding Weathering Steel (Q235NH, Q355NH, etc.)
To ensure a high-quality, strong weld, you must prepare the material correctly. This process is identical to the best practices for other steels.
1. Surface Preparation (The Most Critical Step)
Grinding: You must completely remove all rust, scale, and contaminants from the welding zone.
Width: Clean a band typically 25-50 mm (1-2 inches) on both sides of the joint and on all faces to be welded.
Goal: Grind down to bright, shiny, bare metal. There should be no visible rust or discoloration in the area to be welded.
2. Proper Consumable Selection
Use weathering steel electrodes/wire: The filler metal should have comparable weathering resistance to the base metal. Using standard carbon steel electrodes (like E7018) will create a weld that corrodes at a different rate, potentially leading to preferential corrosion at the weld line.
Common Choices: Electrodes conforming to specifications like EN ISO 2560-A: E Fe 8 2NiCu or AWS rods specifically formulated for welding weathering steels (often with "W" in the designation, indicating weathering resistance).
3. Maintain Dry Conditions
Moisture is the enemy. Do not weld on wet surfaces or in rainy conditions. If the prepared metal has been sitting and gets damp, dry it thoroughly with a flame or heater before welding.
Low-Hydrogen Practice: Use strict low-hydrogen welding procedures (e.g., storing electrodes in a heated quiver) to further prevent hydrogen-induced cracking (HIC), which weathering steel can be susceptible to.
4. Post-Weld Treatment (For Corrosion Resistance & Aesthetics)
After welding and grinding the weld bead smooth, the Heat-Affected Zone (HAZ) and the weld itself will be bare, bright metal.
Natural Weathering: Over time (1-3 years), this area will naturally develop a patina that matches the surrounding steel. However, it will initially rust faster, creating a visible contrast.
Accelerated Patina (Recommended for aesthetics): To create a uniform appearance immediately, you can apply a weathering steel patina solution (a chemical accelerator) to the welded area. This encourages the protective rust layer to form quickly on the new metal, blending it with the parent material.
