A709 Gr.36 Preheat for FCAW — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 Gr.36 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.
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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.
FCAW Tips for Common Structural Steels
For A709 Grade 36 bridge steel (36 ksi yield, mandatory CVN per AASHTO zone), FCAW E71T-1M at 220–260 A handles floor beam stiffener fillets, lateral bracing welds, and secondary member connections. Bridge fabrication quality plans require FCAW wire lot numbers and test data to be logged and retained as part of the quality record for each bridge girder.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
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.36
ASTM A709 Grade 36 is the bridge-specific equivalent of A36, used exclusively for highway bridge construction under AASHTO LRFD Bridge Design Specifications. With 36 ksi minimum yield, 58-80 ksi tensile, and mandatory Charpy V-notch testing requirements based on AASHTO temperature zone (Zone 1: 70°F, Zone 2: 40°F, Zone 3: 10°F test temperatures), it qualifies for both Category A and Category B preheat in Table 5.11. Chemistry matches A36 (0.26% max carbon) but the CVN requirements add a fracture resistance guarantee absent from plain A36. A709 Gr.36 is primarily used for secondary bridge members — lateral bracing, floor beam stiffeners, diaphragm plates, and bearing components — where Gr.50 strength is not needed but bridge-code CVN compliance is required.
Why This Preheat for A709 Gr.36 with FCAW
Bridge-specific 36 ksi grade with CVN testing for highway construction. With low-hydrogen FCAW, this combination falls under Category B rather than Category A — flux-cored wire in FCAW provides a combination of deoxidizers and low-moisture flux formulations that control hydrogen. The 50°F minimum preheat is lower than what non-low-hydrogen SMAW would require at the same thickness because FCAW significantly reduces the driving force for hydrogen-induced cracking in the heat-affected zone.
Typical Applications for A709 Gr.36
Specified exclusively for highway bridge deck plate, floor beams, lateral bracing, secondary framing, bridge railing posts, abutment bearing stiffeners, and diaphragm plates. A709 Gr.36 requires CVN testing per AASHTO zone classification, distinguishing it from plain A36 by guaranteeing fracture resistance at the bridge’s design service temperature. Transverse stiffener fillet welds and floor beam web-to-flange joints are high-frequency fabrication details in bridge shops. The fracture-critical designation on certain bridge members imposes additional NDT requirements (UT in lieu of RT for CJP welds) and welder qualification testing beyond standard D1.5 provisions. Temperature zones (1 through 3) determine the CVN test temperature for the Charpy specimens: Zone 1 at 70°F for moderate climates, Zone 2 at 40°F for cold climates, and Zone 3 at 10°F for severe cold. Mill certifications must show CVN results meeting the specified zone before plate release to the bridge fabricator.
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 Gr.50 | 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.36 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.36 Welding Guides
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A709 Grade 36 in the 3/4 to 1-1/2 inch range with FCAW is the field-erection light-bridge combination for primary girder splices on pedestrian bridges, walkway main support brackets, and connection plates on highway-bridge secondary members — 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. Second, FCAW classification on the spool against the WPS-cited variant — self-shielded (FCAW-S) and gas-shielded (FCAW-G) are not interchangeable, and the choice is an essential variable per Table 6.6. 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 A709-36 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 can put base metal below the floor without obvious signal.