Product Forms Available for Weathering Steel
Weathering steel reaches the market in four practical forms, and the size range depends on which one is ordered. Hot rolled coil is the thinnest product, cut-to-length sheet is produced from the same coil, plate is rolled individually on a plate mill, and wide heavy plate comes from a heavy plate line for bridge and infrastructure work. The alloy content follows the same recipe across all of them, but the rolling route, the achievable thickness and the tolerance class change with the product form.
Weathering steel is a low-alloy steel made by adding small amounts of copper, phosphorus, chromium and nickel so that a protective layer forms on the metal surface instead of loose rust. Structural weathering steel is divided into two service families: the high weathering grades with the strongest atmospheric corrosion resistance, and the weldable weathering grades with better welding behaviour. Because the material is specified for exposed service, the thickness and surface condition together decide the life of the structure, so the size range is not a catalogue detail but part of the engineering choice.
Typical Size Ranges by Product Form
| Product form | Thickness | Width | Length | Usual standard |
|---|---|---|---|---|
| Hot rolled coil and sheet | 1.5 mm to 16 mm | 1000 mm to 1600 mm | Coil, or cut to length | JIS G3125, GB/T 4171-2008 |
| Cut-to-length sheet | 1.5 mm to 12 mm | 1000 mm to 1500 mm | 2000 mm to 6000 mm | JIS G3125, GB/T 4172-2008 |
| Plate | 3 mm to 100 mm | 1500 mm to 3000 mm | 3000 mm to 12000 mm | GB/T 709-2019, EN 10025-5 |
| Wide heavy plate | 50 mm to 200 mm | 2000 mm to 4000 mm | Up to 12000 mm | ASTM A588/A588M, GB/T 4171-2008 |
The practical limit on the thin end comes from the rolling mill rather than the standard: below about 1.5 mm the low-alloy chemistry makes scale control and flatness difficult. The limit on the thick end comes from both the standard and the through thickness properties, because toughness and homogeneity fall away as section increases and the toughness requirement must still be met at the specified impact test temperature.
How the Grades Are Commissioned Across the Size Range
SPA-H to JIS G3125 is normally supplied in sheet and plate thicknesses up to around 16 mm, which covers containers, rolling stock, guard rails and architectural panels. The GNH grades of GB/T 4171-2008 and the ASTM A588 grades continue into much heavier sections for bridge girders and tower legs, where plate of 50 mm to 100 mm and above is routine. The weldable NH grades of GB/T 4172-2008 are used where the design calls for extensive welding and the phosphorus level must stay at or below 0.035 %.
When a project needs a mix of thin sheet and heavy plate, the alloys are not interchangeable across the whole range. A thin sheet grade ordered to JIS G3125 cannot simply be replaced by a heavy plate grade to ASTM A588, because the phosphorus, copper and chromium contents differ and the weldability assumptions change with them. The grade number, the standard and the thickness range should be fixed together on the purchase order.
Choosing Thickness for Corrosion Allowance and Stiffness
Two separate decisions drive the thickness. The first is structural: the section must carry the load within the allowable stress and deflection. The second is the corrosion allowance, which is the extra thickness set aside to cover metal lost before the oxide film stabilises and to cover any local attack. Weathering steel in an exposed, well drained urban or rural location typically needs a much smaller allowance than painted carbon steel, but the allowance is not zero, particularly where the surface stays wet for long periods.
Weathering steel can be used in a thinning design, exposed without coating, or with a simplified coating system, which extends the corrosion resistant life of the component, saves steel weight and cuts labour and energy compared with a fully protected carbon steel structure. The equivalent effect in cost terms is often a saving of the order of a fifth to a third of the steel mass against carbon steel in the same application, before finishing costs are considered.
Tolerances, Edges and Ordering Details to Fix in Writing
Thickness tolerance class and, where applicable, the restricted tolerance classes defined in GB/T 709-2019, which matter for container and bridge work.
Edge condition: as-rolled, sheared or flame cut, with the amount of edge dressing agreed.
Flatness and waviness limits, especially for thin sheet used as visible cladding.
Surface finish: mill finish for natural weathering, pickled and oiled where a coating follows, or pre-rusted for immediate installation.
Length and width cut to drawing, with the quantity of offcuts and the stacking method specified to reduce transit damage.
Certification: mill test certificate against the grade standard, with chemical analysis, tensile and impact results, plus ultrasonic testing to GB/T 2970 when the application requires it.
Ordering by product form and thickness alone is not sufficient. Two sheets of identical size can behave differently if one is coiled product with a different finishing temperature or if the tolerance class differs, so the purchase specification should name the standard, the grade, the delivery condition and the inspection level together.
Frequently Asked Questions
Q: What is the thinnest weathering steel normally available?
A: Hot rolled coil is produced from about 1.5 mm, and 2 mm sheet is a common stock thickness for panels, screens and containers. Thinner gauges are limited by scale control and flatness on the rolling mill.
Q: What is the maximum thickness of weathering steel plate?
A: Plate is commonly rolled to about 100 mm, and grades such as ASTM A588 Grade A and the GNH grades of GB/T 4171-2008 can be ordered heavier, up to around 200 mm, where the toughness requirement at the specified test temperature can still be met.
Q: What widths are available?
A: Coil is generally 1000 mm to 1600 mm wide, cut-to-length sheet up to about 1500 mm, and plate 1500 mm to 3000 mm. Wide heavy plate lines extend this to about 4000 mm for large girder and tower work.
Q: Can weathering steel plate be cut to size before shipment?
A: Yes. Plate and sheet are normally offered cut to length and cut to width, with edge condition specified, and cutting to drawing before dispatch reduces site waste and avoids leaving unprotected flame cut edges in exposed positions.
Q: Does plate thickness change the corrosion behaviour?
A: The corrosion rate per unit area is essentially a surface property, so thick and thin sections weather at similar rates. Thickness changes how long the section can tolerate the loss, which is why the corrosion allowance is set as a proportion of the section.
