What Makes Weather-Resistant Plate Different
Weather-resistant carbon steel plate, known in the trade as weathering steel, is a low-alloy structural steel with deliberate additions of copper, chromium, nickel and phosphorus. Under alternating wet and dry exposure the surface develops a dense, tightly adherent oxide layer that slows further atmospheric attack. The result is a plate that can be used unpainted in many outdoor structures where ordinary carbon steel would need a protective coating system maintained for the life of the asset.
Corten B and Q355NH are the two grades most often requested for structural and architectural work: Corten B to an A588-type chemistry, and Q355NH to GB/T 4171 with a minimum yield strength of 355 MPa.
Grades, Chemistry and Mechanical Properties
| Element, % | Corten B type | Q355NH |
|---|---|---|
| C | 0.19 max | 0.16 max |
| Si | 0.15 - 0.50 | 0.50 max |
| Mn | 0.75 - 1.25 | 0.50 - 1.50 |
| P | 0.04 max | 0.030 max |
| S | 0.05 max | 0.030 max |
| Cu | 0.20 - 0.40 | 0.25 - 0.55 |
| Cr | 0.40 - 0.70 | 0.30 - 1.25 |
| Ni | 0.50 max | 0.12 - 0.65 |
Equivalent designations in other systems are S355J0WP and S355J2WP to EN 10025-5, and the SPA-H container grade widely used in transport equipment. Because these specifications share the same weathering mechanism, an equivalent grade is often accepted by designers provided the corrosion index and the mechanical properties of the ordered plate are respected.
| Property | Typical requirement |
|---|---|
| Yield strength ReH, thickness up to 16 mm | 355 MPa minimum |
| Tensile strength Rm | 470 - 630 MPa |
| Elongation A | 20% minimum |
| Charpy V-notch impact, J2 variants | 27 J at minus 20 C |
These values describe normal hot-rolled plate; the actual figures for each heat are stated on the mill test certificate issued with the delivery.
Size Range and Delivery Condition
Thickness: 3 mm to 150 mm, cut to the ordered size.
Width: 1000 mm to 3000 mm.
Length: 4000 mm to 12000 mm, or cut to length.
Technique: hot rolled, normalised or thermomechanically rolled according to grade and thickness.
Surface: mill finish, shot blasted, or blasted and primed where a paint system is specified.
Edge: as-rolled, sheared or flame cut, with cut edges normally left bare so that the patina can form.
Tolerance: thickness and profile tolerance in line with the order specification, with tighter limits possible on thin sheet.
Plate can be supplied as full sheets, cut-to-length rectangles or profiled blanks, and heavier sections are lifted by plate clamps rather than hooks to avoid surface marking.
Where the Plate Is Used
The most common applications are bridge girders and bridge deck plate, highway and rail structures, transmission towers, freight wagon and container bodies, vehicle chassis and frames, industrial chimneys and ducts, and architectural facades, screens, planters and street furniture. In each case the value comes either from eliminating a paint system or from extending its service life, because the alloying additions that form the patina also stabilise a paint film against undercutting from edges and scratches.
Fabrication, Welding and Quality Inspection
Cutting by plasma, laser or oxy-fuel is straightforward, and flame-cut edges normally need no further edge protection in exposed service.
Welding consumables should suit the base chemistry, particularly the copper level, so that the weld metal and the heat-affected zone weather at a rate similar to the parent plate.
Cold forming performance is good; hot forming should respect the temperature limits recommended for the grade.
For buried, immersed or permanently wet service a coating or cathodic protection is still required, because a stable patina needs wet and dry cycling to develop.
Fasteners and adjacent components should be of a compatible material to avoid galvanic effects in damp conditions.
Each heat is released against chemical analysis, tensile testing and impact testing where specified, together with dimensional and surface checks. Independent third-party inspection can be arranged on request, and every consignment travels with mill test certificates traceable to the heat number. Export packing uses waterproof paper and steel strapping on wooden or steel pallets, suitable for road, rail and sea transport.
FAQ
Q: Is Corten B the same as Q355NH?
The two grades serve the same purpose but their chemistry is not identical. Both are weathering structural steels with copper, chromium and nickel additions, and one is frequently accepted for the other where the specification permits.
Q: Does weathering steel plate have to be painted?
No. It can be left bare in exposed structures where wet and dry cycles let a stable patina form, and it can equally be supplied blasted and primed for a paint system.
Q: What thickness and size range is available?
Hot-rolled plate is produced from about 3 mm up to 150 mm thick, with width up to 3000 mm and length up to 12000 mm, or cut to order.
Q: Can the plate be welded without losing corrosion resistance?
Yes, using consumables matched to the base plate chemistry, so that the joint and the heat-affected zone weather consistently with the parent metal.
Q: How is the plate supplied and documented?
As hot-rolled plate in as-rolled, shot blasted or blasted and primed condition, with mill test certificates and packing suitable for sea transport.
