A709 Gr.50 Preheat for FCAW — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 Gr.50 welded with FCAW at 3/4" to 1-1/2" is 50°F (10°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
FCAW (Flux Cored Arc Welding)
FCAW uses tubular flux-cored wire, available gas-shielded (E71T-1) or self-shielded (E71T-8) for field work. Category B in Table 5.11.
E71T-1 gas-shielded wire is the workhorse for structural steel erection fillet welds. Self-shielded E71T-8 is preferred for field welding where wind makes gas shielding unreliable. Deposition rates run 8-12 lb/hr depending on wire diameter and position. The flux core provides a protective slag that supports the puddle in vertical-up and overhead positions.
Why FCAW for A709 Gr.50 at 3/4" to 1-1/2"
Why FCAW for A709 Gr.50 at 3/4" to 1-1/2"? FCAW delivers 8-12 lb/hr deposition — compared to <a href="/welding/preheat-calculator/a709-50/saw/3-4-to-1-1-2-inch/">SAW</a> at 15-40 lb/hr. Position capability: all positions. Suitability: field and shop.
Filler Metal for FCAW
Gas-shielded: E71T-1C (AWS A5.20, classified with 100% CO2) or E71T-1M (classified with 75 to 80 percent argon, balance CO2). The C and M designators identify the shielding gas used for classification; the actual gas must follow the electrode manufacturer and the qualified WPS. Self-shielded: E71T-8 (no external gas, field-ready). Diameter: 0.045" standard, 1/16" for high-deposition. Stick-out: 3/4" to 1-1/4" (longer than GMAW due to resistive heating of flux core).
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 FCAW
Standard 50 ksi bridge plate for girders and cross-frames. This steel is prequalified only with low-hydrogen processes under Table 5.11. With FCAW, flux-cored wire in FCAW provides a combination of deoxidizers and low-moisture flux formulations that control hydrogen. The 50°F minimum preheat balances the steel’s strength level and carbon equivalent against the hydrogen control provided by FCAW. 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 3/4" to 1-1/2"
Preheat climbs at this range as thicker material slows heat dissipation, trapping hydrogen at crack-susceptible grain boundaries.
Other Steels with FCAW at 3/4" to 1-1/2"
| Steel | Category | Preheat |
|---|---|---|
| A709 HPS50W | B | 50°F (10°C) |
| A709 HPS70W | C | 150°F (65°C) |
| A710 Gr.A | C | 150°F (65°C) |
| A913 Gr.70 | C | 150°F (65°C) |
A709 Gr.50 with FCAW
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 3/4 to 1-1/2 inch range with FCAW is the field-erection bridge combination for primary girder splice fillet runs, gusset-plate welds, and connection-plate field welds — sections heavy enough that the 50°F preheat floor is binding and the higher deposition rate of cored wire moves more steel per shift than SMAW or GMAW under outdoor conditions.
Pre-weld notes
Three constraints layer at this combination. First, the 50°F preheat floor needs active verification per Clause 7.6 — the heated zone must extend at least twice the base-metal thickness from the welding point (1.5 to 3 inches at this band). Second, FCAW classification on the spool must match the WPS — self-shielded vs gas-shielded is an essential variable per Table 6.6, and the variants are not interchangeable mid-job because arc characteristics, electrode classifications, and supplementary atmospheric-exposure rules differ. Third, inter-pass slag removal across the increased pass count of mid-thickness work — bridge-grade rejection criteria for slag inclusions are stricter than general structural.
What a CWI verifies
A CWI on A709-50 FCAW mid-thickness field work verifies (1) preheat at perimeter and samples mid-joint with a contact pyrometer or thermocouple after the first pass group, (2) the FCAW classification on the spool against the WPS-cited variant, (3) inter-pass slag removal across the joint sequence, and (4) amperage/voltage against Table 5.4 prequalified limits. The 50°F floor is closer to binding at this thickness than at thin section — winter field conditions or large-section heat-sink effects can put base metal below the floor.