Hot rolled weathering steel plate is a low-alloy structural steel that forms a stable, tightly adherent oxide layer when exposed to the atmosphere. That patina slows further corrosion, so the material is widely specified for bridges, facades, transmission towers and freight equipment that must serve for decades without painting. Two families dominate the market: the Corten A and Corten B grades, and the ASTM A588 grade range, both supplied as hot rolled plate in thicknesses from 2 mm to 300 mm.
What Hot Rolled Weathering Steel Plate Is
Weathering steel differs from ordinary carbon steel because small, deliberately balanced additions of copper, chromium, nickel and phosphorus change the chemistry of the surface layer. Instead of shedding loose red scale, the plate builds a dense, dark oxide film that adheres to the base metal and restricts the access of oxygen and moisture. The patina develops fastest under alternating wet and dry cycles with free air movement, which is why unpainted weathering plate performs well on open structures and performs poorly where it stays permanently wet or buried.
Production starts with a reheated slab rolled above the recrystallisation temperature, followed by controlled cooling. Hot rolled plate covers 2 mm to 300 mm, although 2 mm to 100 mm accounts for the great majority of orders. Thinner cold rolled coil from 0.5 mm to 2 mm is produced for roofing, cladding and equipment panels, and the surface can be supplied clean, descaled, blast cleaned or primed according to the drawing.
Corten A and Corten B Chemistry
The two classic grades divide the work according to how they will be used. Corten A carries more phosphorus, which accelerates patina formation and makes it a natural choice for architectural and decorative surfaces. Corten B keeps phosphorus low and raises manganese, so thick sections and welded assemblies retain toughness.
| Element (wt%) | Corten A | Corten B |
|---|---|---|
| Carbon C | 0.12 max | 0.16 max |
| Silicon Si | 0.25-0.75 | 0.30-0.50 |
| Manganese Mn | 0.20-0.50 | 0.80-1.25 |
| Phosphorus P | 0.07-0.15 | 0.035 max |
| Sulfur S | 0.030 max | 0.030 max |
| Chromium Cr | 0.50-1.25 | 0.40-0.65 |
| Nickel Ni | 0.65 max | 0.40 max |
| Copper Cu | 0.25-0.55 | 0.25-0.40 |
Mechanical requirements follow the same split, and both grades share a tensile window of 470-630 MPa with a minimum elongation of 20 percent. Corten A is the stronger of the pair in tension because of its higher phosphorus level, while Corten B keeps a slightly lower yield floor in exchange for better behaviour in thick welded joints.
| Property | Corten A | Corten B |
|---|---|---|
| Minimum yield strength | 355 MPa | 345 MPa |
| Tensile strength | 470-630 MPa | 470-630 MPa |
| Minimum elongation | 20% | 20% |
ASTM A588 Grades and Equivalent Specifications
ASTM A588 is the structural plate specification most often written into bridge and building contracts. It covers grades A, B, C and K with a minimum yield strength of 345 MPa and a minimum tensile strength of 485 MPa, combined with atmospheric corrosion resistance roughly four times that of plain carbon structural steel. Grades A and B are the general-purpose choices, grade C is used where higher copper improves marine exposure, and grade K suits thick members that also require notch toughness.
| Grade | C max | Mn | Si | Cu | Cr | Ni | P max | S max |
|---|---|---|---|---|---|---|---|---|
| A588 Gr.A | 0.19 | 0.80-1.25 | 0.30-0.65 | 0.25-0.40 | 0.40-0.65 | 0.40 | 0.04 | 0.05 |
| A588 Gr.B | 0.20 | 0.75-1.35 | 0.15-0.50 | 0.20-0.40 | 0.40-0.70 | 0.50 | 0.04 | 0.05 |
| A588 Gr.C | 0.15 | 0.80-1.35 | 0.30-0.50 | 0.20-0.50 | 0.30-0.50 | 0.25-0.50 | 0.04 | 0.05 |
The same weathering concept appears in several specification systems, and the grade is normally chosen from whichever standard governs the project. Common equivalents are ASTM A242, A606 Type 4 and A709 Grade 50W, EN 10025-5 grades S235J0W, S355J0W, S355J2W and S355K2W, JIS G3114 grades SMA400AW, SMA490AW and SMA570W, JIS G3125 SPA-H, and GB/T 4171 grades Q235NH, Q355NH and Q415NH. Where a project mixes systems, the corrosion index and the impact test temperature must be compared rather than the yield strength alone.
Dimensions, Delivery Condition and Fabrication
Weathering plate is ordered by thickness, width and length rather than by coil weight, and the mechanical properties are certified on the plate as delivered. Controlled rolling, normalising, normalising plus tempering and quenching plus tempering are all available, and the delivery state must be fixed at the time of order because it changes both the microstructure and the impact behaviour.
| Parameter | Range |
|---|---|
| Thickness | Cold rolled 0.5-2 mm; hot rolled 2-300 mm, with 2-100 mm used most often |
| Width | 1000-3000 mm, with 1000-2200 mm used most often |
| Length | 1000-12000 mm, or cut to the requested size |
| Surface | Clean, descaled, blast cleaned or primed to the agreed specification |
| Delivery condition | Hot rolled, controlled rolled, normalised, annealed, tempered, quenched, N+T or Q+T |
| Documentation | Mill test certificate to EN 10204 3.1 with every consignment; independent third-party inspection can be arranged |
| Packing | Export seaworthy packing suitable for road, rail and sea transport |
Plates cut cleanly by plasma or waterjet, and oxy-fuel cutting is acceptable on thicker sections after preheating. Bending radii should be generous on the transverse direction, and welding should follow a qualified procedure with low-hydrogen consumables and a matching weathering electrode or filler so that the joint does not become a corrosion path. Any weld spatter or embedded iron left on the surface turns into a rust streak, so the finished surface is normally cleaned before the structure is handed over.
Applications and Service Life
The economics of weathering plate come from eliminating paint cycles rather than from a lower first cost. In an exposed, freely draining application the patina stabilises within a few years and the corrosion rate falls to a small fraction of the initial value, which is why the grade is standard in long-life infrastructure.
Bridge girders, deck plates and guard rails
Architectural facades, canopies and exposed structural frames
Transmission towers, telecommunication masts and reaction towers
Lifting and transport machinery frames
Industrial furnace shells, boiler supports and ducting
Container frames, warehouse shelving and material handling equipment
Performance depends on detailing as much as on chemistry. Water must drain, crevices and pockets that trap moisture should be avoided, and the steel should not be used in permanently immersed or heavily chloride-contaminated conditions without a protective coating. Where the structure is detailed correctly, an unpainted weathering steel member routinely outlasts a painted carbon steel member while requiring far less maintenance.
Frequently Asked Questions
Q: What is the difference between Corten A and Corten B?
Corten A contains more phosphorus and is intended for architectural and decorative panels, while Corten B keeps phosphorus low and raises manganese so that thick sections and welded structures keep their toughness.
Q: Which ASTM grade replaces Corten A on a bridge project?
ASTM A588 grades A, B and C are the usual structural equivalents, providing a minimum yield strength of 345 MPa and atmospheric corrosion resistance well above plain carbon steel.
Q: Can weathering steel be left unpainted?
Yes, provided the member drains freely, dries between wet cycles and is not permanently immersed. In trapped-moisture or heavy chloride service a coating system is still required.
Q: What thickness range is available in hot rolled plate?
Hot rolled weathering plate is produced from 2 mm to 300 mm, with 2 mm to 100 mm covering most orders, and cold rolled coil from 0.5 mm to 2 mm for thin panels.
Q: How long does the patina take to stabilise?
On an exposed, well-drained structure the protective oxide layer normally darkens and stabilises over roughly two to five years, after which the corrosion rate drops sharply.
Q: Does the plate lose strength if it is welded?
No, but the welded joint must use a matching weathering filler and a qualified procedure so that the joint retains both its strength and its corrosion resistance.
