A709 Gr.50 Preheat for SAW — up to 3/4"

SAW (Submerged Arc Welding)

SAW submerges the arc beneath granular flux for highest deposition rates, flat/horizontal only. Category B in Table 5.11.

SAW with F7A2-EM12K wire/flux delivers the highest deposition rates for flat-position fillet welds on building steel. Typical parameters: 500-700 amps, 28-32 volts, 18-30 IPM travel speed. Flux consumption runs approximately equal to wire consumption by weight. Unfused flux recovery and recycling systems are standard in production shops to control consumable costs.

A709 Gr.50

ASTM A709 Grade 50 is the standard bridge plate and shape grade with 50 ksi minimum yield and 65 ksi minimum tensile, commonly specified for highway bridge plate girder flanges, webs, floor beams, and cross-frames. It falls under Category B only in Table 5.11, requiring low-hydrogen welding processes. Chemistry mirrors A572 Gr.50 (0.23% max carbon, Nb/V microalloying) with CVN testing per AASHTO temperature zone requirements. A709 Gr.50 accounts for the majority of bridge steel tonnage in North America. Flange plate thicknesses routinely reach 2-3" on large plate girders, making preheat compliance at the upper Table 5.11 tiers a significant production consideration for bridge fabrication shops during cold-weather operations.

Why This Preheat for A709 Gr.50 with SAW

Standard 50 ksi bridge plate for girders and cross-frames. This steel is prequalified only with low-hydrogen processes under Table 5.11, which is why it appears in Category B but not Category A. The 32°F minimum preheat with SAW balances the steel's strength level and carbon equivalent against the controlled hydrogen input from the consumable. Non-low-hydrogen SMAW is not an option for this grade under D1.1 prequalified WPS.

Typical Applications for A709 Gr.50

Standard for highway bridge plate girder flanges, box girder webs, cross-frame angles, bearing sole plates, splice plates in bolted-welded connections, and composite deck studs. A709 Gr.50 is the baseline strength grade for most modern highway bridge design per AASHTO LRFD. Flange butt splices, web-to-flange continuous fillet welds, and bearing stiffener clips are the dominant weld types in girder fabrication. Girder flanges typically range from 3/4" to 3" thick with widths from 12" to 30", requiring extended preheat soak times on thicker flange splices. Bridge fabrication shops certified to AISC Major Steel Bridge category maintain dedicated preheat tracking logs for each flange splice throughout the production sequence. Web-to-flange fillet welds on plate girders often exceed 100 feet of continuous weld per girder, making SAW the standard process for these joints. Flange splice CJP groove welds undergo 100% UT examination per D1.5.

Why Preheat Matters at up to 3/4"

Thin material sheds heat quickly, allowing hydrogen to escape the HAZ readily — lowest preheat tier in Table 5.11.

Other Steels with SAW at up to 3/4"

A709 Gr.50 with SAW

Try Different Combinations

Use the interactive preheat calculator to look up any steel, process, and thickness combination from D1.1:2025 Table 5.11.

A709 Gr.50 Welding Guides