Welding Weathering Steel: Filler Metals, Procedures and Patina Match
Jan 22, 2024
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Yes, Weathering Steel Is Welded Every Day
Weathering steel is a weldable material and it is joined in bridges, buildings, sculptures and garden products. The alloy combines higher strength with better atmospheric corrosion resistance than ordinary structural carbon steel, and low-phosphorus grades offer good impact toughness together with sound welding behaviour. The decisive point is not whether the material can be welded, but that the joint must weather at approximately the same rate as the surrounding plate. That requirement drives the choice of filler metal, the joint preparation and the heat input.
Because most weathering steel structures are left unpainted, welded joints are also part of the visible surface. A mismatched consumable does not only affect corrosion; it leaves a permanent streak on an otherwise uniform patina.
Choosing the Filler Metal
The most common mistake is using standard mild steel filler. The resulting weld deposit rusts faster than the base material and bleeds brown streaks onto the plate around the joint. For weathering steel, use filler metals with a matching alloy chemistry, typically copper-bearing electrodes and wires developed for weathering grades. These consumables produce weld metal that forms a patina at a rate close to that of the base plate, which keeps the structure visually uniform and corrosion resistant.
Match the filler to the base chemistry, not just to the tensile class
Use low-hydrogen consumables where the procedure requires them
Store electrodes according to the manufacturer's instructions to avoid moisture pickup
Record the consumable batch in the welding procedure specification
Welding Processes and Joint Preparation
Shielded metal arc welding, gas metal arc welding and flux-cored arc welding are all used on weathering steel. Ordinary structural steel equipment and procedures apply, with adjustments to the consumables and to the heat input. The joint area must be clean, dry and free of oil, paint and loose rust before welding. Mill scale and contaminants are removed from the joint faces, and heat input is controlled to avoid excessive grain growth in the heat-affected zone.
| Process | Consumable type | Typical use |
|---|---|---|
| Shielded metal arc welding | Copper-bearing covered electrode | Site work, repairs, short runs |
| Gas metal arc welding | Copper-bearing solid wire | Shop fabrication, production welding |
| Flux-cored arc welding | Copper-bearing flux-cored wire | Thicker sections, higher deposition |
For thick plates, preheating may be necessary depending on the grade, the thickness and the ambient temperature, following the approved welding procedure specification. Fit-up gaps and restraint should be planned so that shrinkage stresses stay within acceptable limits.
Strength Grades and Structural Use
Weathering steels are supplied in a range of strength levels, with yield strength ratings from about 345 MPa up to 960 MPa in the high-strength grades. Structural grades such as ASTM A588 plate and the EN 10025-5 grades, for example S355J2W, are used for load-bearing frames, bridges and heavy equipment. Low-phosphorus grades are preferred for welded load-bearing structures because their combination of toughness and weldability is more predictable. The grade must be matched to the design loads and to the qualified welding procedure.
Post-Weld Treatment and Patina Matching
After welding, remove slag and spatter and grind the weld smooth where appearance matters. Weld zones develop patina slightly differently from the base material during the first year, and that is normal. Do not paint only the welds: a painted weld creates a patchy appearance and can trap moisture against the steel. With matching filler metal, the joint weathers to a similar colour over time. For critical structures, follow the project specification for inspection, including non-destructive testing where it is required.
Frequently Asked Questions
Q: Can I weld weathering steel with an ordinary arc welder?
Yes. Standard arc welding equipment works well. The important detail is the filler metal, which should match the weathering chemistry of the base steel.
Q: What happens if mild steel filler is used by mistake?
The weld deposit corrodes faster than the surrounding plate and can show rust streaks and premature section loss. Matching filler metal is strongly recommended.
Q: Does welding weaken weathering steel?
Welding is a normal joining method for this material. With a correct procedure, low-hydrogen consumables and proper heat control, joints can meet the strength requirements of the structure.
Q: Should the welds be painted afterwards?
No. Painting only the welds creates an uneven appearance and traps moisture. With matching filler metal the weld forms its own patina and blends with the plate.
Q: Which grades are best suited to welding?
Low-phosphorus weathering grades offer excellent toughness and weldability and suit load-bearing structures. Typical examples are the EN 10025-5 grades and ASTM A588 structural plate.
Q: How is the heat input controlled?
The procedure specification sets the current, travel speed and interpass temperature so that excessive grain growth in the heat-affected zone is avoided.