A709 Gr.50 Preheat for GMAW — up to 3/4"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 Gr.50 welded with GMAW at up to 3/4" is 32°F (0°C), Category B. Preheat below this raises hydrogen-cracking risk in the heat-affected zone; the same temperature is the minimum interpass limit maintained through the weld.
Built on AWS D1.1:2025 Table 5.11 — every value traced to the clause.
Have a preheat question? Ask Flux
GMAW (Gas Metal Arc Welding)
GMAW (MIG) feeds continuous solid wire with shielding gas — an inherently low-hydrogen process assigned to Category B in Table 5.11.
ER70S-6 wire at 0.035" or 0.045" diameter handles most structural work on common grades. Spray transfer at 250-350 amps provides high deposition for shop fillet welds. For thinner material under 1/4", short-circuit transfer at lower parameters reduces heat input. Gas flow rates of 35-45 CFH through a standard nozzle provide adequate shielding in typical shop environments without excessive turbulence.
GMAW Tips for Common Structural Steels
For A709 Grade 50 bridge steel (50 ksi yield, Category B only), GMAW spray transfer at 260–300 A with 0.045" ER70S-6 is standard for shop fabrication of girder flange attachment plates, cross-frame clips, and bearing stiffeners. Flange plate thicknesses on large plate girders commonly reach 2–3", requiring preheat to 225°F at the upper thickness tiers.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Filler Metal for GMAW
Common wire: ER70S-6 (AWS A5.18). Diameter: 0.035" for thin sections and out-of-position, 0.045" for production flat/horizontal. Shielding gas: 75/25 Ar/CO2 (standard), 90/10 Ar/CO2 (less spatter, better profile), or 100% CO2 (deeper penetration, more spatter). Contact-tip-to-work distance: 1/2" to 3/4".
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
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 GMAW
Standard 50 ksi bridge plate for girders and cross-frames. This steel is prequalified only with low-hydrogen processes under Table 5.11. With GMAW, the continuous solid wire and gas shielding in GMAW produce inherently low hydrogen levels, typically 2-4 mL/100g. The 32°F minimum preheat balances the steel’s strength level and carbon equivalent against the hydrogen control provided by GMAW. 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 GMAW at up to 3/4"
| Steel | Category | Preheat |
|---|---|---|
| A709 HPS50W | B | 32°F (0°C) |
| A709 HPS70W | C | 50°F (10°C) |
| A710 Gr.A | C | 50°F (10°C) |
| A913 Gr.70 | C | 50°F (10°C) |
A709 Gr.50 with GMAW
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
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A709 Grade 50 at or below 3/4 inch with GMAW is the controlled-shop combination for bridge components where deposition rate matters and the section is thin enough that ambient preheat is adequate — connection plates, gusset plates, splice-plate fillet runs, and secondary girder elements in shop fabrication.
Pre-weld notes
Two factors dominate at this combination, neither of them the 32°F preheat floor. First, shielding-gas integrity — gas-flow rate and cup cleanliness drive porosity at start-of-arc, and bridge-grade work has tighter rejection criteria for porosity than general structural. Second, travel-speed control — bridge fabrication WPSs frequently bound travel speed tightly to control HAZ width, and a faster-than-WPS travel reduces heat input enough to alter the HAZ even at this thin section. Confirm the WPS amperage, voltage, and travel-speed window match the production parameters before each shift.
What a CWI verifies
A CWI on A709-50 GMAW thin-section work verifies amperage and voltage against the WPS limits, samples gas-flow rate at the torch with a flow meter (not the regulator), and checks travel speed against the WPS-pinned range. The 32°F preheat floor is rarely the binding constraint at this thickness; the binding constraints are gas integrity and parameter conformance to the prequalified WPS limits in Tables 5.3 and 5.10.