What Is ASME SA588 Grade B Weathering Steel?
ASME SA588 Grade B is the ASME Boiler and Pressure Vessel Code designation of ASTM A588 Grade B, a high-strength low-alloy (HSLA) structural steel with a guaranteed level of atmospheric corrosion resistance. It is one of the most widely used weathering steels, often sold under the trade description corten, and it is specified for structural applications such as bridges and buildings where the steel is expected to remain uncoated. The weathering effect comes from a balanced alloy design: copper, chromium, nickel and vanadium work together so that the rust layer that forms on the surface becomes dense and self-limiting instead of continuing to consume the base material.
Chemical Composition
The table below lists the composition limits for SA588 Grade B as published by the supplier, consistent with the limits of ASTM A588/A588M.
| Element | C | Mn | P | S | Si | Cu | Ni | Cr | V |
|---|---|---|---|---|---|---|---|---|---|
| Limit (%) | ≤0.20 | 0.75–1.25 | ≤0.04 | ≤0.05 | 0.15–0.50 | 0.20–0.40 | ≤0.50 | 0.40–0.70 | 0.01–0.10 |
The copper content is the main driver of corrosion resistance, while chromium and nickel contribute to the protective patina and vanadium adds strength through fine-grain refinement.
Mechanical Properties
Minimum mechanical values depend on plate thickness. For plates 100 mm (4 in) and under, the minimum yield strength is 345 MPa (50 ksi) and minimum tensile strength is 485 MPa (70 ksi). For thicker sections up to 125 mm (5 in), the corresponding minimum values are 315 MPa (46 ksi) yield and 460 MPa (67 ksi) tensile, with a minimum elongation of 21% on a 2 in (50 mm) gauge length. Designers should always apply the values for the actual plate thickness in the governing edition of the standard.
How the Weathering Mechanism Works
When exposed to the atmosphere, weathering steel forms an initial rust layer that quickly converts into a stable patina once the surface has passed through repeated wetting and drying cycles. The patina slows further corrosion to a very low rate, giving the structure a service life far longer than unprotected mild steel in the same environment. The mechanism works well in rural and urban atmospheres; it performs poorly where the steel is permanently wet, buried, or exposed to heavy salt spray, so coastal marine applications require either a coating system or a different material.
Typical Applications
SA588 Grade B plates are used for bridge girders, building frames, facade cladding, noise barriers, industrial plant structures and engineering machinery. Typical machinery parts include drilling rigs, electric shovel components, electric wheel dump trucks, mine cars, excavator buckets, loader arms, bulldozer blades, crane booms and coal mine hydraulic supports. In architectural work the steel is often left bare for its distinctive brown-orange patina, and it can be combined with cutting, pre-treatment, galvanizing or painting as the project requires.
Welding and Fabrication Notes
SA588 Grade B welds readily with standard processes such as SMAW, GMAW, FCAW and SAW. For thicker plates and highly restrained joints, low-hydrogen consumables and preheat are recommended to avoid hydrogen cracking. The weathering performance of welded joints depends on the filler metal: consumables with matching alloying (for example low-alloy weathering electrodes) keep the weld zone compatible with the base plate, whereas ordinary carbon-steel filler can leave the weld area more prone to corrosion. Galvanizing or painting is possible but removes the self-protecting patina behaviour; where coatings are used they should be selected for the expected atmospheric exposure.
Frequently Asked Questions
What is the difference between ASME SA588 and ASTM A588?
ASME SA588 is the ASME code adoption of ASTM A588. The composition limits and mechanical requirements are essentially the same; the SA designation is used when the material is ordered and documented under the ASME Boiler and Pressure Vessel Code.
Does SA588 Grade B need to be painted?
No, painting is optional. The grade is designed to be used bare so that the protective patina develops. In marine or permanently wet environments, a coating system should be applied instead of relying on natural patination.
Can SA588 Grade B be welded?
Yes. It is welded with the standard arc processes using low-hydrogen consumables, with preheat recommended for thicker plates and restrained joints. Filler metal should ideally match the weathering alloy content of the base plate.
What is the minimum yield strength of SA588 Grade B?
345 MPa (50 ksi) for plates 100 mm and under; 315 MPa (46 ksi) for thicker sections up to 125 mm, in line with the thickness tiers of ASTM A588/A588M.
Is SA588 Grade B the same as S355J2W?
Not identical, but S355J2W+N under EN 10025-5 is frequently cited as the closest European equivalent grade for SA588 Grade B. Exact equivalence should be checked against both specifications for the project.
What thickness ranges are typically supplied?
SA588 Grade B plates are commonly supplied from 6 mm up to 100 mm, with heavier sections available up to 125 mm subject to mill capability and the applicable standard edition.
