ASTM A709 and Its Weathering Grades
ASTM A709 is the specification for structural steel for bridges and covers a range of grades in one document. The conventional grades 36 and 50 are carbon steels, while the W suffix identifies weathering grades such as 50W and 70W, and the HPS series covers high-performance steels with enhanced toughness and weldability. For grade 50W the minimum yield strength is 345 MPa and the minimum tensile strength is 485 MPa, with atmospheric corrosion resistance at least twice that of carbon steel as demonstrated by the corrosion index procedure of ASTM G101.
Bridge specifications also set Charpy V-notch requirements through the toughness zones of the AASHTO and AWS D1.5 welding provisions. The zones are usually illustrated with test temperatures of about 21 degrees Celsius, 4 degrees Celsius and minus 12 degrees Celsius, with a minimum average energy of 27 J for non-fracture-critical members and a higher requirement for fracture-critical members. That toughness demand is the reason welding procedures for A709 are qualified rather than assumed.
Why Weathering Steel Welds Behave Differently
The corrosion resistance of grade 50W and similar grades comes from a balanced addition of copper, chromium and nickel. In a welded joint the filler metal dilutes the parent chemistry, and the heat-affected zone loses the fine microstructure produced by controlled rolling. If the dilution is too great or the heat input too high, the joint can weather at a different rate from the plate, showing as a darker or lighter band along the weld after several years of exposure.
The practical consequence is simple: consumables, heat input and post-weld cleaning all matter more than they do on ordinary structural carbon steel, because the appearance and the long-term corrosion performance of the joint are judged together.
Preheat, Interpass Temperature and Heat Input
Preheat is the first control. Thick sections and low ambient temperatures increase the cooling rate and the risk of hydrogen-assisted cracking in the heat-affected zone, so a preheat of at least about 100 degrees Celsius is common for sections above roughly 25 mm, with the actual value calculated from carbon equivalent, thickness and joint restraint following the bridge welding provisions of AWS D1.5. Interpass temperature is capped in the qualified procedure so that toughness is not lost in the coarse-grained zone.
Measure preheat on the plate surface at a distance from the joint so the reading is representative.
Keep interpass temperature within the qualified range, and allow thick assemblies to cool in a controlled way.
Control heat input, since an excessive value coarsens the heat-affected zone while an excessively low value encourages cracking.
Use dry electrodes and clean, dry joint faces to keep hydrogen out of the weld.
Consumables and Joint Preparation
Low-hydrogen consumables are standard for A709 work. For exposed weathering joints the filler metal should be of a matching weathering type so that the weld surface develops a patina close to that of the plate. Joint faces should be free of rust, scale, oil and moisture, and edges cut by thermal processes should be dressed to remove the hardened layer before welding. Where a joint is accessible, a slight excess of weld reinforcement dressed back flush gives better weathering appearance than a rough, undressed cap.
Post-Weld Cleaning, Cutting and Inspection
Post-weld cleaning is not cosmetic. Slag, spatter, weld fume and anti-spatter compounds all act as local corrosion sites, and steel that has been left contaminated shows streaking within a few months of service. Brushing and, where required, light grinding restore a uniform surface so that the patina forms evenly. Cutting for fabrication is carried out by plasma, oxy-fuel or laser processes, with the cut edges dressed where fatigue or coating performance demands it. Forming should be done progressively and, on thick plate, with local heating to reach tight radii without damaging the microstructure. Inspection covers visual examination of every weld, dimensional checks and, for complete joint penetration welds in fracture-critical members, volumetric testing such as ultrasonic examination.
Frequently Asked Questions
Q: What is the difference between ASTM A709 grade 50 and grade 50W?
A: Both have the same minimum yield and tensile strength, but grade 50W is a weathering steel with copper, chromium and nickel additions that give significantly better atmospheric corrosion resistance.
Q: What preheat should be used when welding A709 plate?
A: A minimum of about 100 degrees Celsius is common for thick sections, with the exact value determined from carbon equivalent, thickness and restraint under AWS D1.5, and capped interpass temperature stated in the qualified procedure.
Q: Which filler metal suits weathering bridge steel?
A: A low-hydrogen, weathering-grade consumable matched to the parent metal. Matching the corrosion behaviour of the plate matters as much as matching tensile strength.
Q: Why is post-weld cleaning so important on weathering steel?
A: Slag and spatter prevent the patina from forming uniformly, so the weld zone stains and corrodes preferentially. Cleaning lets the whole surface weather at the same rate.
Q: Does A709 steel need post-weld heat treatment?
A: Normally not. Stress relief is applied only where the design or a thick, highly restrained joint requires it, and it must be evaluated for its effect on toughness.
Q: Can A709 plate be cold formed?
A: Yes, within defined bend radii, but thick plate should be heated locally for tight bends, and forming should be carried out in progressive passes to avoid cracking.
