Filler Metal Chemistry Drives Long-Term Weld Performance
Weathering steel protects itself with a dense, tightly adherent oxide layer that forms only when the alloy contains enough copper, chromium and nickel to stabilise the patina. A weld that is chemically leaner than the plate becomes the weak point of the structure: it oxidises first, sheds a loose rust layer and stains the surrounding surface. On an exposed bridge, facade or retaining structure, consumable selection is therefore a corrosion decision rather than only a strength decision, and it has a direct effect on inspection intervals over the service life of the asset.
Base Metal Chemistry and Which Standard Applies
Under EN 10025-5 the two common structural grades are S235J2W and S355J2W. The standard limits them to approximately 0.25-0.55% Cu, 0.40-0.80% Cr and a maximum of 0.65% Ni, with phosphorus capped near 0.035%; the J2 suffix certifies 27 J impact energy at -20 °C. The North American counterpart, ASTM A588/A588M Grade A, works in the same window with 0.25-0.40% Cu, 0.40-0.65% Cr, 0.40-0.65% Ni, a minimum yield strength of 345 MPa and a minimum tensile strength of 485 MPa. Filler metals for both systems are selected to land near or just above that analysis, so the deposit weathers at a similar rate to the parent metal instead of forming a preferential corrosion cell at the joint line.
Consumables by Welding Process
The filler classification systems already contain weathering options. The correct choice depends on the process, the joint configuration and the mechanical requirement.
GMAW (MIG): copper-bearing ER80S-G wire to AWS A5.28 is the matched choice, with a minimum tensile strength of 550 MPa and the small Cu, Cr and Ni additions needed for patina compatibility. ER70S-6 to AWS A5.18, at 480 MPa minimum tensile, is sound for non-exposed or intermittently wet joints but is not a weathering deposit.
SMAW (stick): E8018-C3 to AWS A5.5 is a basic low-hydrogen electrode carrying 0.80-1.10% Ni with a minimum yield of 460 MPa and a minimum tensile of 550 MPa. E7018 to AWS A5.1 is a lower-strength alternative for non-critical fillets but has no weathering alloy package.
FCAW: W-classifications such as E81T1-W2 to AWS A5.29 are formulated with Cu, Cr and Ni for weathering service and are widely used for site welding where deposition rate and out-of-position work matter.
SAW: a copper-bearing wire combined with a neutral agglomerated flux keeps silicon and oxygen in the deposit within the limits of the A5.17 and A5.23 classifications, which is important for thick plate.
Heat Input, Preheat and Interpass Control
Weathering steel is not a quenched and tempered product, but its heat-affected zone still needs management. Excessive heat input coarsens the HAZ and can leave a band that never develops the protective layer; insufficient heat produces lack of fusion and cold lap at the root. A working band of roughly 1.0-2.5 kJ/mm suits 6-20 mm sections welded with basic consumables, with stringer beads preferred over wide weaves so each pass cools predictably. AWS D1.1 sets minimum preheat by thickness and carbon equivalent; for A588 material the table starts at 10 °C for sections up to 20 mm and increases for heavier thicknesses, and EN 1090-2 follows the same principle for execution class EXC2 and above. Electrodes are taken from a correctly redried low-hydrogen condition, holding ovens are used on site, and interpass temperature is normally capped near 250 °C to limit softening in the heat-affected zone.
Quality Control, Certificates and Delivery Formats
For B2B supply the consumable package is verified in the same way as the plate. Weathering plate is released against an EN 10204 Type 3.1 mill test certificate to EN 10025-5 or ASTM A588/A588M, and each consumable batch carries its own certificate to AWS A5.28, A5.5 or A5.29 with lot numbers traceable to the weld map. Diffusible hydrogen is specified through the H5 or H10 classification of ISO 3690, which matters for restrained joints and for thicknesses above about 25 mm. Procedure qualification follows EN ISO 15614-1 or AWS D1.1 Clause 4, welders are qualified to EN ISO 9606-1, and production inspection covers visual examination, dye penetrant or ultrasonic testing on defined joint categories. Plate and consumable chemistry can be specified as one package: material is offered as cut-to-length plate from 3 to 150 mm, sheared and plasma-cut blanks, and profiled components, with deposit chemistry matched to the certified plate analysis before fabrication starts.
Frequently Asked Questions
Q: Can ordinary ER70S-6 wire be used on S355J2W or A588 plate?
A: It produces a structurally sound weld, but the deposit lacks the Cu, Cr and Ni that drive patina formation, so the joint weathers differently from the plate. Exposed architectural and bridge work normally specifies a weathering-classified filler.
Q: Is preheat required for weathering steel?
A: Preheat is governed by thickness and carbon equivalent. AWS D1.1 and EN 1090-2 start at low values for thin sections and increase with thickness; thin plate in mild ambient conditions generally needs little or none, while heavy restrained joints need controlled preheat and interpass limits.
Q: Does the weld have to be painted or coated?
A: No. A matched weathering deposit develops its own protective layer at the same rate as the plate. Painting is used only where the aesthetic requirement calls for a uniform colour during the initial run-off period.
Q: Which certificate should accompany the filler metal?
A: A batch certificate to the relevant AWS filler classification, with lot number, heat analysis and mechanical results. The plate itself is supplied with an EN 10204 Type 3.1 certificate to EN 10025-5 or ASTM A588/A588M.
Q: How is diffusible hydrogen controlled?
A: By selecting H5 or H10 classified consumables tested to ISO 3690, by storing electrodes in holding ovens, by redrying according to the manufacturer schedule, and by cleaning the joint of moisture, scale and rust before welding.
Q: Can S355J2W be welded to carbon steel?
A: Yes. The transition joint is welded with a filler that suits the stronger or more highly restrained side, and the weathering grade is protected by detailing that avoids trapping water at the bimetallic interface.
