SMA490CP is a hot-rolled atmospheric corrosion resisting steel plate specified in JIS G3114, the Japanese standard for weldable structural steels that are intended to be used unpainted in the open air. It belongs to the SMA490 family, a 490 MPa class weathering steel in which copper, chromium and nickel are balanced so that a dense, tightly adherent rust layer forms and then slows further attack. The trailing letter designates the flue-gas and exposed-face variant of the family, produced with tightly controlled residual elements and used where a stable brown patina is wanted instead of a painted finish.
Chemical Composition to JIS G3114
The alloying additions are deliberate rather than incidental. Copper and chromium promote a compact, low-porosity oxide, while nickel raises toughness in the heat affected zone and limits the copper-rich surface embrittlement that can occur during hot forming. Values below are heat analysis in mass percent for the SMA490CP grade.
| Element | C | Si | Mn | P | S | Cu | Cr | Ni |
|---|---|---|---|---|---|---|---|---|
| SMA490CP | max 0.18 | 0.15-0.65 | max 1.40 | max 0.035 | max 0.035 | 0.30-0.50 | 0.45-0.75 | 0.05-0.30 |
For welded construction a carbon equivalent limit is normally agreed at the ordering stage, because the permitted value changes with the thickness class being rolled and with the delivery condition. Buyers who need a guaranteed CE should state the requirement on the enquiry together with the thickness range so that the heat is trimmed accordingly.
Mechanical Properties and Impact Toughness
Yield strength falls as plate thickness rises, because the through-thickness cooling rate slows during rolling; the tensile band and the minimum elongation stay constant. Charpy V-notch impact testing is specified at 0 degrees C with a 47 J minimum absorbed energy for the SMA490P grades.
| Thickness (mm) | Yield strength, min (MPa) | Tensile strength (MPa) | Elongation, min (%) | Charpy V at 0 C, min (J) |
|---|---|---|---|---|
| t <= 16 | 365 | 490-610 | 21 | 47 |
| 16 < t <= 40 | 355 | 490-610 | 21 | 47 |
| 75 < t <= 100 | 325 | 490-610 | 21 | 47 |
Thickness, Width and Delivery Condition
Plate is routinely rolled from 6 mm to 300 mm thick, in widths of 1500 mm to 4050 mm and lengths of 3000 mm to 15000 mm. Delivery condition is quoted to the plate thickness and to the fabrication route: hot rolled or as-rolled for thinner sections, normalized or thermo-mechanically controlled rolled where toughness has to be guaranteed, and quenched and tempered only when a specific project calls for it. Plate is supplied with mill test certificates, and third-party inspection can be arranged when the end user requires witness testing.
Fabrication Rules for Unpainted Service
The patina only develops properly where the surface stays alternately wet and dry and where water cannot collect. Detailing therefore matters more than grade selection: provide drainage and air access, avoid horizontal ledges, seal or weld crevices that would hold standing water, and keep the steel clear of chloride-rich run-off. Welding is carried out with consumables that match the base chemistry in copper, chromium and nickel so that the joint weathers at the same rate as the parent plate. Cutting edges and weld spatter should be dressed back, since an undisturbed surface oxidises more evenly.
Typical Applications
SMA490CP is selected for air preheaters and economisers in power and process plants, where flue gas condensate attacks ordinary carbon steel; for rail wagons, containers and frames that are exposed for decades; for bridges and gantries where inspection access is limited; and for building facades, screens and landscape structures where the patina is part of the architectural intent. In all of these cases the design life depends as much on drainage geometry as on the alloy content of the plate.
Frequently Asked Questions
Q: What is the minimum yield strength of SMA490CP?
A: 365 MPa for plate up to 16 mm thick and 355 MPa for plate from 16 mm to 40 mm, with lower values permitted as thickness increases further.
Q: What impact test does SMA490CP have to pass?
A: Charpy V-notch testing at 0 degrees C with a minimum absorbed energy of 47 J, tested to the plate thickness class specified in JIS G3114.
Q: How does SMA490CP differ from the structural weathering grades of the same family?
A: The chemical basis is the same copper-chromium-nickel system, but the grade is intended for flue gas and exposed decorative faces, so residual elements and surface quality are held tighter and the plate is often ordered to a brighter, more uniform patina.
Q: Can SMA490CP be left unpainted in a coastal plant?
A: It can, but only where chloride deposition is low and surfaces drain freely. In splash zones or chloride-rich atmospheres the protective layer breaks down repeatedly and a coating system is normally specified instead.
Q: What thickness range is available?
A: Rolling covers 6 mm to 300 mm in thickness, 1500 mm to 4050 mm in width and 3000 mm to 15000 mm in length, subject to mill rolling schedules.
Q: Which welding consumables suit SMA490CP?
A: Consumables that match the copper, chromium and nickel content of the parent plate, welded with controlled heat input so that the heat affected zone toughness required at 0 degrees C is preserved.
