ASTM A709/A709M Grade 50W
High-strength low-alloy weathering structural steel for bridges · ASTM A709/A709M-26 — Structural Steel for Bridges · ASTM A6/A6M — General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling · AASHTO M 270M/M 270 — Structural Steel for Bridges · ASTM A673/A673M — Sampling Procedure for Impact Testing of Structural Steel · ASTM G101 — Estimating Atmospheric Corrosion Resistance of Low-Alloy Steels · AASHTO/AWS D1.5M/D1.5 — Bridge Welding Code
ASTM A709 Grade 50W is the conventional 50 ksi yield-strength weathering steel in the ASTM bridge-steel specification. It combines Grade 50 structural strength with chemistry controlled for enhanced atmospheric corrosion resistance and may be used unpainted only where exposure, drainage, detailing, inspection, and maintenance permit a stable protective patina to form. The complete designation must address tension or fracture-critical status and AASHTO temperature zone whenever Charpy V-notch toughness is required.
- 50 ksi [345 MPa]Minimum yield strength · Applies to covered plates through 4 in [100 mm] and structural shapes within the specification's product limits.
- 70 ksi [485 MPa]Minimum tensile strength · No general maximum is imposed unless the applicable supplementary requirement is ordered.
- ASTM G101 corrosion index ≥ 6.0Weathering requirement · Calculated from heat analysis; enhanced atmospheric resistance does not make the steel immune to corrosion.
- Type A or Type BChemistry routes · Correspond chemically to ASTM A588/A588M Grades A and B, respectively; the composition type is included in material marking.
- Up to 4 in [100 mm], inclusivePlate thickness covered by tensile table · Actual mill availability and project-specific restrictions may be narrower.
- 50W, 50WT1/2/3, or 50WF1/2/3Critical ordering distinction · Plain Grade 50W does not by itself invoke Charpy impact testing.
Overview
- Designation system
- ASTM bridge structural-steel grade designation
- Product forms
- Structural plate, Rolled structural shapes, Structural bars
- Condition
- Killed steel made to fine-grain practice, No single heat-treatment condition defines conventional Grade 50W, Impact-tested T or F designation with temperature zone when required
- Density
- 7850 kg/m³ (Representative engineering density for structural steel; not a grade acceptance requirement.)
What the designation means
A709/A709M is a product specification for bridge structural steel, not merely a chemistry designation. “50” identifies the 50 ksi specified minimum yield-strength level, while “W” identifies enhanced atmospheric corrosion resistance. The SI designation is Grade 345W. Grade 50W is the conventional weathering grade and must not be confused with HPS 50W, which has different chemistry, processing, toughness, and welding characteristics.
ASTM A6/A6M supplies the general dimensional tolerances, testing, workmanship, repair, marking, certification, and delivery rules. Project specifications may add owner requirements, AASHTO provisions, fracture-control requirements, welding rules, domestic-origin provisions, nondestructive examination, or supplementary testing. Compliance therefore depends on the complete purchase specification, not the grade name alone.
Specified strength and ductility
| Product/test basis | Applicable size | Yield strength, minimum | Tensile strength, minimum | Elongation, minimum |
|---|---|---|---|---|
| Plates and bars — 8 in [200 mm] gage length | Plate thickness through 4 in [100 mm] | 50 ksi [345 MPa] | 70 ksi [485 MPa] | 18% |
| Plates and bars — 2 in [50 mm] gage length | Plate thickness through 4 in [100 mm] | 50 ksi [345 MPa] | 70 ksi [485 MPa] | 21% |
| Structural shapes — 8 in [200 mm] gage length | All covered shape thicknesses | 50 ksi [345 MPa] | 70 ksi [485 MPa] | 18% |
| Structural shapes — 2 in [50 mm] gage length | Flange or leg thickness through 3 in [75 mm] | 50 ksi [345 MPa] | 70 ksi [485 MPa] | 21% |
| Wide-flange shapes — 2 in [50 mm] gage length | Flange thickness over 3 in [75 mm] | 50 ksi [345 MPa] | 70 ksi [485 MPa] | 18% |
For plates wider than 24 in [600 mm], ASTM A6/A6M adjustments reduce the specified elongation by two percentage points. Floor plate is exempt from elongation determination. Values are specification minima, not typical mill properties.
A high reported yield strength is not automatically advantageous. It can affect Charpy test-temperature adjustments and weld-metal matching. Review actual certified properties where toughness, forming, fracture control, or welding procedure qualification is sensitive to strength overrun.
Chemical composition and weathering classification
| Element | Type A, mass % | Type B, mass % |
|---|---|---|
| Carbon | 0.19 max | 0.20 max |
| Manganese | 0.80–1.25 | 0.75–1.35 |
| Phosphorus | 0.030 max | 0.030 max |
| Sulfur | 0.030 max | 0.030 max |
| Silicon | 0.30–0.65 | 0.15–0.50 |
| Nickel | 0.40 max | 0.50 max |
| Chromium | 0.40–0.65 | 0.40–0.70 |
| Copper | 0.25–0.40 | 0.20–0.40 |
| Vanadium | 0.02–0.10 | 0.01–0.10 |
For each 0.01 percentage-point reduction below the carbon maximum, manganese may be increased by 0.06 percentage point, to 1.50% maximum. Phosphorus to 0.04% and sulfur to 0.05% are permitted for structural shapes, bars, and plates not over 15 in [380 mm] wide.
Type A and Type B are alternative alloy systems, not quality levels. Both use combinations of copper, chromium, nickel, silicon, and vanadium to produce weathering behavior. ASTM requires an atmospheric corrosion-resistance index of at least 6.0, calculated from the heat analysis using the predictive method in ASTM G101. This index is a compositional qualification; it is not a guaranteed corrosion rate or service-life prediction for a bridge site.
The mill test report should identify the heat chemistry, and the product marking must include the Grade 50W composition type. Confirm that the reported chemistry satisfies both the selected Type A or B limits and the required G101 index.
Charpy toughness and complete grade designation
Bridge components requiring notch toughness must be ordered with more than the designation “Grade 50W.” The suffix T identifies a non-fracture-critical tension component; F identifies a fracture-critical tension component. A following number identifies AASHTO temperature Zone 1, 2, or 3. Examples are 50WT2 and 50WF2. Material ordered simply as 50W has no ASTM A709 Charpy requirement and is limited by the specification to non-tension components or secondary members.
| Zone | Minimum service-temperature range | CVN test temperature for conventional Grade 50W |
|---|---|---|
| 1 | 0°F [−18°C] and above | 70°F [21°C] |
| 2 | Below 0 to −30°F [−18 to −34°C] | 40°F [4°C] |
| 3 | Below −30 to −60°F [−34 to −51°C] | 10°F [−12°C] |
The governing bridge design specification and owner establish the applicable zone.
| Designation/application | Thickness | Minimum average energy | Minimum individual test value |
|---|---|---|---|
| 50WT1, 50WT2, or 50WT3 — non-fracture-critical tension component | Through 2 in [50 mm] | 15 ft·lbf [20 J] | Controlled through ASTM A673/A673M acceptance rules |
| 50WT1, 50WT2, or 50WT3 — non-fracture-critical tension component | Over 2 through 4 in [50–100 mm] | 20 ft·lbf [27 J] | Controlled through ASTM A673/A673M acceptance rules |
| 50WF1, 50WF2, or 50WF3 — fracture-critical tension component | Through 2 in [50 mm] | 25 ft·lbf [34 J] | 20 ft·lbf [27 J] |
| 50WF1, 50WF2, or 50WF3 — fracture-critical tension component | Over 2 through 4 in [50–100 mm] | 30 ft·lbf [41 J] | 24 ft·lbf [33 J] |
Tests are performed at the zone temperature shown above. ASTM temperature reductions apply when the reported yield strength substantially exceeds the 50 ksi minimum. Fracture-critical base metal is also subject to restrictions on mill or supplier weld repair.
Plans and purchase documents should designate the member category and zone rather than replacing the standard designation with an isolated impact-energy statement. This preserves the associated sampling, frequency, orientation, retest, and marking provisions.
Using Grade 50W as uncoated weathering steel
Grade 50W can form an adherent protective oxide layer when exposed to alternating wet and dry periods. It is not stainless steel and it does not stop corroding. Successful unpainted service depends on the atmospheric environment and on details that drain, dry, and remain accessible for inspection. The initial patina may be nonuniform and runoff can stain concrete or other adjacent surfaces.
| Exposure or detail | Technical concern | Practical consequence |
|---|---|---|
| Marine or chloride-laden atmosphere | Chlorides can prevent formation of a stable protective patina | Site-specific corrosion assessment or a protective system may be necessary |
| Road-deicing salt spray, especially tunnel-like grade separations | Repeated salt deposition and sheltered surfaces can cause continuing corrosion | Do not assume the W suffix makes bare steel suitable |
| Leaking deck joints and uncontrolled drainage | Contaminated water produces localized wetting and section loss | Eliminate joints where practical; intercept drainage and protect vulnerable steel |
| Water, debris, pack-rust, or crevice traps | Persistent moisture prevents drying | Provide positive drainage, ventilation, sealing, and cleaning access |
| Persistent fog, condensation, high humidity, or sheltered low crossings | Long time of wetness undermines weathering performance | Evaluate the site microclimate rather than relying on regional climate alone |
| Direct industrial chemical exposure | Deposits may destabilize the oxide layer | Specialist corrosion assessment may be required |
A weathering-steel bridge still requires inspection and maintenance. Drainage systems, joints, deposits, vegetation, screens, covers, and localized coatings must be maintained.
Painting Grade 50W is permissible. When only part of a member is coated—commonly near joints—the coating system, termination details, surface preparation, and future maintenance strategy should be explicitly designed.
Fabrication and welding
Grade 50W is weldable using qualified bridge-fabrication procedures, but ASTM A709 does not supply a universal welding procedure. Bridge work is normally governed by AASHTO/AWS D1.5 together with owner specifications. Procedure selection must address product thickness, joint type, process, restraint, hydrogen control, heat input, preheat and interpass temperature, toughness requirements, and whether the weld will remain exposed.
For exposed unpainted work, filler-metal selection must satisfy mechanical and toughness requirements while providing suitable atmospheric-corrosion behavior and an acceptable appearance. Weathering filler metals commonly produce weld deposits stronger than the 70 ksi minimum tensile strength of the base steel; this established slight overmatch must still be considered in weld design and procedure control. Do not select consumables solely by tensile classification or color.
| Operation | What to control |
|---|---|
| Thermal cutting and edge preparation | Removal of scale, slag, gouges, cracks, and unacceptable cut-edge discontinuities under the fabrication specification |
| Welding | Approved WPS, low-hydrogen practice, consumable storage, preheat/interpass control, heat input, and required weld-metal toughness |
| Cold bending or forming | Inside radius, orientation, edge condition, thickness, and project-specific qualification; minimum tensile properties alone do not establish bendability |
| Heat curving or straightening | Owner-approved procedure and temperature limits; uncontrolled heating can alter properties and geometry |
| Blast cleaning | Surface preparation appropriate to either patina development or the specified coating system |
| Mixed weathering and non-weathering steel | Positive material identification to prevent visually similar grades from being installed in the wrong location |
Preheat values from an example WPS or another project are not universal requirements.
Specifying, purchasing, and accepting the material
| Item | What should be stated or verified |
|---|---|
| Specification edition | Applicable ASTM A709/A709M or AASHTO M 270M/M 270 edition and any owner amendments |
| Complete grade designation | 50W for eligible non-tension/secondary use, or 50WT1/2/3 or 50WF1/2/3 where CVN testing is required |
| Product and dimensions | Plate, shape, or bar designation; thickness, width, length, section, quantity, and dimensional tolerances |
| Chemistry type | Record Type A or B; specify a required type only when the project has a technical reason |
| Surface condition | Bare weathering, shop-coated, metallized, or locally coated, with the appropriate preparation requirements |
| Fabrication status | Identify tension components, fracture-critical components, weld requirements, and prohibited repairs |
| Supplementary requirements | Ultrasonic examination, additional tension-test frequency, single-heat bundles, maximum tensile strength, or other project options when needed |
| Certification | Heat identity, chemical analysis, tensile results, G101 corrosion index, required CVN results, composition type, and traceability through fabrication |
Supplementary requirements do not apply automatically; they must be invoked by the contract or purchase order.
Receiving inspection should reconcile the material markings, mill test report, purchase designation, dimensions, and intended member category. Particular attention is warranted when plates from the same project include plain 50W, T-designated material, and F-designated material, because their required certification and permitted applications differ even though the surfaces may look identical.
Substitution boundaries
ASTM A588/A588M Grades A and B use the same chemical types represented by Grade 50W Types A and B, but an A588 certificate is not automatically an A709 bridge-steel certificate. A709 can add bridge-specific impact testing, designation, marking, and supplementary requirements. Conversely, Grade 50W is not a permitted unilateral substitute for A709 Grade 50, 50S, or 36; ASTM requires purchaser–supplier agreement.
HPS 50W is a separate high-performance bridge steel even though it has the same nominal yield-strength level and a W suffix. It has a substantially different low-carbon chemistry, processing options, toughness profile, and fabrication considerations. Commercial descriptions such as “Corten,” “weathering steel,” or “A588-type” are not adequate substitutes for the complete ASTM A709 designation.
Sources
- ASTM A709/A709M-26 — Standard Specification for Structural Steel for BridgesASTM International
- ASTM A709/A709M-21 — Standard Specification for Structural Steel for BridgesASTM International
- AASHTO M 270M/M 270 — Standard Specification for Structural Steel for BridgesAmerican Association of State Highway and Transportation Officials
- Bridge Welding Reference Manual, FHWA-HIF-19-088Federal Highway Administration
- Load and Resistance Factor Design for Highway Bridge Superstructures Reference Manual, FHWA-NHI-15-047Federal Highway Administration
- Uncoated Weathering Steel in Structures, Technical Advisory T 5140.22Federal Highway Administration
- FHWA Memorandum: Inspection and Maintenance of Bridges with Uncoated Weathering Steel ComponentsFederal Highway Administration
- AASHTO/NSBA Steel Bridge Fabrication Guide Specification — Errata and commentaryAASHTO/NSBA Steel Bridge Collaboration
Ask MetalMate about ASTM A709/A709M Grade 50W
Discussing: ASTM A709/A709M Grade 50W
AI assistant · Responses are generated and can contain errors; verify specifications before acting.