How to Specify Corten Weathering Steel

What Weathering Steel Actually Is

Weathering steel is ordinary carbon steel with a small addition of corrosion resistant elements such as copper and nickel. That puts it in a low alloy series between carbon steel and stainless steel, keeping the strength ductility forming welding abrasion and high temperature behavior of a structural steel while adding atmospheric corrosion resistance.

The commercial effect is thinner sections and longer life for the same duty. Because the steel resists rust rather than being coated against it, the component carries corrosion resistance through its whole section rather than in a film that can be scratched off.

• Low alloy steel positioned between carbon steel and stainless steel
• Copper and nickel added to ordinary carbon steel in small amounts
• Strength ductility forming welding and abrasion behavior retained
• Weather resistance stated at 2 to 8 times ordinary carbon steel
• Paintability stated at 1.5 to 10 times ordinary carbon steel

How the Protective Rust Layer Forms

Weathering steel uses rust rather than fighting it. Left bare in the atmosphere it rusts at first exactly as ordinary steel does, but the alloying elements cause a dense protective rust to build on the surface over time, and that layer then inhibits further rust development.

This is why the material can be used with no coating at all. The buyer gets the economic effect of removing recoating cost from the maintenance budget and the visual effect of the settled color of the protective rust.

• Initial rusting looks the same as on ordinary steel
• Alloy elements build a dense protective rust over time
• The protective layer inhibits later rust development
• Bare specification removes recoating from the maintenance cycle
• The settled rust color is the finish rather than a defect

Grade Designations You Can Order Against

Weathering steel is written differently on every drawing that crosses a desk. The grades held here are Q235NH Q355NH SPA-H 09CuPCrNi-A Grade K S235W and S355W, and the A588 designation is carried as its own plate and sheet item. Standards quoted across the range are BS EN GB AISI ASTM DIN and JIS.

Order under the designation printed on your drawing rather than translating it yourself. The mill test certificate is then issued under that name and your inspector has nothing to reconcile.

• Q235NH and Q355NH for Chinese designations
• SPA-H for Japanese designations
• 09CuPCrNi-A where the chemistry is named on the drawing
• S235W and S355W for European structural equivalents
• Grade K and A588 for American practice

Thickness Width and Delivery Condition

Corten steel plate is listed in thickness from 0.1mm to 10mm or customized as required for the sheet and coil range, and the plate specification block carries a thickness of 0 to 100mm with size customized. Width is supplied to requirement rather than to a fixed list.

Delivery condition is quoted as Q plus T, hot rolled or normalized. That matters. A normalized plate and a quenched and tempered plate of the same grade behave differently under forming and under welding heat, so the condition belongs on the order line next to the grade.

• Sheet and coil thickness listed from 0.1mm to 10mm or customized
• Plate thickness block listed as 0 to 100mm with customized size
• Delivery condition quenched and tempered hot rolled or normalized
• Surface supplied natural or painted
• Samples available before a production order is placed

Welding Cutting and Forming

Welding is straightforward. Manual welding and submerged arc welding are both usable, and spot welding is possible on thin material. Workability is the same story, with cold working hot working and gas cutting all carried out as on an ordinary steel of the same strength level.

The point that catches fabricators out is the weld metal rather than the parent plate. A weld made with an ordinary consumable will not weather like the plate around it, so match the consumable to the weathering grade if the joint is going to be left bare.

• Manual and submerged arc welding both suitable
• Spot welding possible on thin material
• Cold working hot working and gas cutting as for ordinary steel
• Match the welding consumable when the joint stays uncoated
• Bending welding decoiling cutting and punching available before shipment

Where Bare Weathering Steel Works

The applications listed for this material are bridges, guard rails, sign poles and highway structures, and those cases share a pattern. The steel is freely exposed to the atmosphere, it goes through wetting and drying cycles, and it would otherwise need repainting on a cycle that costs more than the steel.

Detailing matters as much as grade selection. Water that sits in a pocket instead of draining, or a face that never dries, will not form the stable protective layer the material depends on.

• Bridges and highway structures exposed on all faces
• Guard rails and sign poles where repainting access is expensive
• Architectural facades using the settled rust color as the finish
• Avoid details that trap standing water against the plate
• Allow for initial run off staining onto adjacent surfaces

What to Confirm Before the Order Is Placed

A complete weathering steel order names the grade designation, the delivery condition, the thickness, the cut sizes and whether the surface is natural or painted. Processing services covering bending welding decoiling cutting and punching mean the plate can arrive as finished parts rather than as stock to be cut again.

Ask for the mill test certificate so the weathering grade is documented against the structure record. The ISO9001, BV, SGS and RoHS certificates can be issued alongside it.

• Grade designation exactly as printed on the drawing
• Delivery condition quenched and tempered hot rolled or normalized
• Thickness and the cutting list for finished parts
• Natural or painted surface
• Mill test certificate requested with the shipment
• Sample requested where a finish trial is needed first