A709 Gr.50 Preheat for GMAW — 1-1/2" to 2-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 Gr.50 welded with GMAW at 1-1/2" to 2-1/2" is 150°F (65°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.
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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 150°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 1-1/2" to 2-1/2"
Heavy plate with significant restraint and thermal mass — preheat is critical to maintain slow cooling for hydrogen escape.
Other Steels with GMAW at 1-1/2" to 2-1/2"
| Steel | Category | Preheat |
|---|---|---|
| A709 HPS50W | B | 150°F (65°C) |
| A709 HPS70W | C | 225°F (110°C) |
| A710 Gr.A | C | 225°F (110°C) |
| A913 Gr.70 | C | 225°F (110°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 in the 1-1/2 to 2-1/2 inch range with GMAW is the controlled-shop heavy-section bridge combination — primary girder splice plates, gusset plates, and connection plates on AASHTO LRFD-designed bridges where the section is heavy enough that the 150°F preheat floor is binding and the gas-shielded process delivers the cleaner deposit profile that bridge-grade rejection criteria demand at heavy section.
Pre-weld notes
Crossing the 1-1/2 inch threshold changes the preheat-extent rule per Clause 7.6 — for base metal 1-1/2 inch and greater, the heated zone shall extend at least equal to the base-metal thickness, but not less than 3 inches. Real preheat infrastructure is required: induction blankets, electric resistance pads, or oxy-fuel rosebud burners with surface-vs-through-thickness gradient handling. Bridge-grade GMAW work at this thickness adds a fourth concern — Charpy V-notch (CVN) testing is commonly specified by the bridge owner for heavy primary members, which moves the WPS to qualified rather than prequalified.
What a CWI verifies
A CWI on A709-50 GMAW heavy-section bridge work verifies (1) preheat through-thickness, not just surface — sampling 3-6 inches from the arc on the back side catches the through-thickness lag during heat-up, (2) the heated-zone extent against Clause 7.6 (at least equal to base-metal thickness, not less than 3 inches), (3) interpass temperature held above 150°F with a contact pyrometer between pass groups, (4) shielding gas conforms to Table 5.10, and (5) where CVN is specified, the inspection covers Clause 6 essential-variable controls (Table 6.6 + Table 6.8) on top of the prequalified gate.