ASTM A588 Grade A
High-strength low-alloy atmospheric-corrosion-resistant structural steel · ASTM A588/A588M-24 · ASTM A6/A6M-26a
ASTM A588 Grade A is a weldable, fine-grain HSLA structural steel whose copper-chromium-silicon-vanadium chemistry provides enhanced resistance to atmospheric corrosion. It is commonly selected for exposed structural work, but successful unpainted service depends on repeated wetting and drying, good drainage and limited chloride contamination. Its nominal 50 ksi [345 MPa] minimum yield level applies only through 4 in. [100 mm] plate or bar thickness and to structural shapes; thicker plate and bar have lower specified minima.
- ASTM A588/A588M-24Current governing edition · Used together with the current edition of ASTM A6/A6M for general delivery, testing, tolerances and certification requirements.
- Shapes, plates and bars through 8 in. [200 mm]Covered products · Sheet, strip and structural tubing are covered by other weathering-steel specifications.
- 50 ksi [345 MPa] through 4 in. [100 mm]Minimum yield point · Falls to 46 ksi [315 MPa] over 4–5 in. and 42 ksi [290 MPa] over 5–8 in. for plates and bars.
- 70 ksi [485 MPa] through 4 in. [100 mm] and for shapesMinimum tensile strength · ASTM A588 specifies minima rather than a bounded tensile-strength range.
- 6.0 minimumCorrosion-resistance index · Calculated from heat analysis using the predictive method referenced in ASTM G101.
- Not mandatory in the base gradeNotch toughness · Charpy V-notch testing must be separately specified, such as through ASTM A6/A6M Supplementary Requirement S5 or an applicable bridge specification.
Overview
- Designation system
- ASTM product specification and grade designation
- Product forms
- Structural shapes, Structural plates, Structural bars, Finished structural products processed from coil
- Condition
- As furnished under ASTM A588/A588M and ASTM A6/A6M; ASTM A588 does not define a single mandatory heat-treatment delivery, Bare or coated in service, as determined by the structure and exposure
- Density
- 7850 kg/m³ (Representative engineering value for structural steel, not a specified ASTM A588 acceptance requirement.)
What the designation defines
ASTM A588/A588M covers high-strength low-alloy structural steel intended principally for welded bridges and buildings where reduced structural weight, enhanced durability or both are important. Grade A identifies one of the permitted alloy chemistries. The designation is a product specification: it controls chemistry, tensile properties and atmospheric-corrosion index for specified rolled structural products, rather than defining a universal alloy with condition-independent properties.
The scope is limited to structural shapes, plates and bars not exceeding 8 in. [200 mm] in thickness. Coil is not itself qualified to ASTM A588; qualification occurs after a processor converts the coil into a finished structural product and performs the additional testing, inspection and certification required by ASTM A6/A6M. Thin sheet or strip should normally be specified to ASTM A606/A606M, and weathering structural tubing to ASTM A847/A847M.
“Weathering steel” is a material category, not a complete purchase designation. Likewise, “Corten” or “COR-TEN” is often used colloquially but is a proprietary trade name and should not be treated as proof of ASTM A588 Grade A compliance.
Grade A specification requirements
| Element | Grade A composition, mass % | Comment |
|---|---|---|
| Carbon | 0.19 max | Carbon below the maximum permits a corresponding increase in manganese under the specification footnote. |
| Manganese | 0.80–1.25 | May exceed 1.25 by 0.06 percentage point for each 0.01 percentage point reduction in carbon below 0.19, to 1.50 maximum. |
| Phosphorus | 0.030 max | Up to 0.040 is permitted for shapes, bars and plates not exceeding 15 in. [380 mm] in width. |
| Sulfur | 0.030 max | Up to 0.050 is permitted for shapes, bars and plates not exceeding 15 in. [380 mm] in width. |
| Silicon | 0.30–0.65 | Part of the Grade A deoxidation and weathering-alloy system. |
| Nickel | 0.40 max | No specified minimum. |
| Chromium | 0.40–0.65 | Contributes to atmospheric-corrosion resistance. |
| Copper | 0.25–0.40 | Contributes to formation of the protective weathering patina. |
| Vanadium | 0.02–0.10 | Microalloying and strengthening element. |
| Iron | Remainder | Subject to normal residual elements and steelmaking practice. |
These are heat-analysis requirements. Product analysis is subject to the applicable ASTM A6/A6M tolerances.
In addition to meeting the element limits, the steel must be produced to fine-grain practice. The atmospheric-corrosion-resistance index calculated from the heat analysis by the ASTM G101 predictive method must be at least 6.0. If required by the purchase documents, the manufacturer must provide corrosion-resistance evidence satisfactory to the purchaser.
| Product and governing thickness | Yield point, min | Tensile strength, min | Elongation, min |
|---|---|---|---|
| Plate and bar, 4 in. [100 mm] and under | 50 ksi [345 MPa] | 70 ksi [485 MPa] | 18% in 8 in. [200 mm] or 21% in 2 in. [50 mm] |
| Plate and bar, over 4 through 5 in. [over 100 through 125 mm] | 46 ksi [315 MPa] | 67 ksi [460 MPa] | 21% in 2 in. [50 mm] |
| Plate and bar, over 5 through 8 in. [over 125 through 200 mm] | 42 ksi [290 MPa] | 63 ksi [435 MPa] | 21% in 2 in. [50 mm] |
| Structural shapes, all covered sizes | 50 ksi [345 MPa] | 70 ksi [485 MPa] | 18% in 8 in. [200 mm] or 21% in 2 in. [50 mm] |
Specimen orientation and preparation are governed by ASTM A6/A6M. For plates wider than 24 in. [600 mm], the applicable elongation requirement is reduced by two percentage points. Elongation need not be determined for floor plate. Wide-flange shapes with flange thickness over 3 in. [75 mm] have an 18% minimum elongation in 2 in. [50 mm].
The designation does not impose a maximum tensile or yield strength, a yield-to-tensile ratio, mandatory hardness, mandatory Charpy energy, fatigue properties or through-thickness properties. Add any application-critical requirements explicitly rather than assuming they are inherent in Grade A.
Atmospheric-corrosion performance
The alloy additions promote formation of a comparatively dense, adherent oxide layer when the steel experiences alternating wet and dry periods. This patina slows subsequent atmospheric corrosion; it does not stop corrosion and does not make the steel stainless, maintenance-free in every environment or suitable for unrestricted immersion and burial.
| Condition | Implication for bare A588 Grade A |
|---|---|
| Open atmospheric exposure with regular drying | Generally favorable for protective patina development, provided details shed water and remain inspectable. |
| Persistent dampness, condensation or trapped water | Patina may remain porous and nonprotective; localized or continuing section loss can result. |
| Marine salt, salt fog or direct seawater spray | Chlorides can prevent development of a stable protective layer. Site-specific corrosivity assessment or a coating system is normally required. |
| Road-deicing salt and traffic spray | Particularly severe at leaking joints, depressed roadways and tunnel-like bridge locations. Bare use should not be assumed acceptable. |
| Buried, submerged or enclosed unventilated service | Outside the favorable atmospheric wet-dry mechanism; use a service-specific corrosion-control strategy. |
| Industrial chemical contamination | Deposited chemicals may destabilize the patina; assess the actual atmosphere rather than relying only on the grade designation. |
Detailing is as important as alloy selection. Provide positive drainage, avoid water traps and debris shelves, protect vulnerable crevices or overlapping surfaces, prevent roadway runoff from flowing over steel and allow access for inspection and cleaning. Rust-colored runoff can stain concrete, masonry, glazing and adjacent finishes, particularly during the early weathering period.
The ASTM G101 index ranks the relative atmospheric-corrosion potential of a heat from its composition. It is not a prediction of actual service life independent of geometry, chloride deposition, time of wetness, maintenance and corrosion allowance.
Fabrication and joining
ASTM A588 Grade A is intended for welded, bolted or riveted structural construction, but the material specification does not supply a complete welding procedure. Welding must follow the applicable structural welding code and a qualified or prequalified procedure appropriate to material thickness, joint restraint, heat input, hydrogen control, service temperature and required toughness. Preheat and interpass controls should be established from the governing code and the actual heat chemistry rather than from the grade name alone.
For exposed multipass welds, weathering-compatible low-alloy filler metal is normally preferred to maintain atmospheric-corrosion behavior and obtain a closer color match. Certain carbon-steel fillers may be acceptable for particular single-pass or painted applications because of dilution from the base metal, but this must be justified by the welding specification and project requirements. Weld-metal strength matching alone does not address appearance, toughness or corrosion behavior.
Use compatible weathering fastener assemblies where connections will remain unpainted. Connection design must also address faying-surface condition, slip resistance where relevant, sealing or drainage of crevices and access for inspection. The structural connection specification, not ASTM A588 alone, governs bolts, nuts, washers and joint installation.
Cutting, drilling and cold forming are generally performed with conventional structural-steel equipment. Relative to mild carbon steel, the higher yield strength increases forming force and springback. Bend radius, edge quality, rolling direction, thickness and temperature should be evaluated for the actual part. ASTM A6/A6M provides general information on cold bending but does not replace a qualified fabrication procedure.
Mill scale, fabrication markings, oil, weld spatter and uneven surface preparation can produce nonuniform early weathering. Blast cleaning may be specified where a more uniform architectural appearance is required; this is an appearance and project requirement, not a default ASTM A588 condition.
Specification, purchasing and verification
| Item | Why it matters |
|---|---|
| Complete designation and edition | State ASTM A588/A588M, Grade A and the required edition. Do not order merely as “weathering steel.” |
| Unit system | Specify ASTM A588 for inch-pound ordering or A588M for SI ordering; ASTM warns against mixing the two systems for conformance. |
| Product form and dimensions | Mechanical requirements and ASTM A6/A6M tolerances depend on whether the item is plate, bar or shape and on thickness, width and section. |
| Structural product made from coil | Identify this route where applicable and require the additional coil-product testing and reporting prescribed by ASTM A6/A6M. |
| Notch toughness | Specify Charpy V-notch energy, temperature, orientation and test frequency when fracture behavior or low-temperature service is important. |
| Additional examination | State any required ultrasonic examination, product analysis, bend testing, through-thickness requirements or other supplementary provisions. |
| Unpainted-service requirements | Identify exposed surfaces, required surface preparation, weathering-compatible weld metal and fasteners, and any requirement to report the corrosion index or supporting heat chemistry. |
| Certification | Require material test reports traceable to heat and product, including heat analysis and tensile results. |
| Bridge work | Confirm whether ASTM A709/A709M Grade 50W and project/AASHTO requirements govern instead of a basic ASTM A588 order. |
| Availability | Confirm the selected shape, plate thickness and quantity with qualified producers or service centers; not every nominally covered form is routinely stocked. |
Incoming verification should compare the certificate and product markings against the purchase order, heat number, grade, product form, dimensions and specified supplementary requirements. Confirm that the reported tensile values correspond to the applicable thickness range. For bare service, review the complete heat analysis and any required corrosion-index documentation rather than accepting a generic “weathering steel” statement.
Substitution based only on a 50 ksi yield rating is unsafe. A substitute must satisfy the governing product form, chemistry or corrosion index, dimensional tolerances, weldability, toughness, testing, certification and project-specific environmental requirements.
Sources
- ASTM A588/A588M-24 — Standard Specification for High-Strength Low-Alloy Structural Steel, up to 50 ksi [345 MPa] Minimum Yield Point, with Atmospheric Corrosion ResistanceASTM International
- ASTM A6/A6M-26a — Standard Specification for General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet PilingASTM International
- ASTM G101-04(2025) — Standard Guide for Estimating the Atmospheric Corrosion Resistance of Low-Alloy SteelsASTM International
- ASTM A709/A709M-26 — Standard Specification for Structural Steel for BridgesASTM International
- Uncoated Weathering Steel in Structures — Technical Advisory 5140.22Federal Highway Administration
- A Primer on Weathering SteelNational Steel Bridge Alliance / American Institute of Steel Construction
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