A709 Gr.36 Preheat for GMAW — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 Gr.36 welded with GMAW 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
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 36 bridge steel (36 ksi yield, CVN per AASHTO temperature zone), GMAW spray transfer at 210–280 A with 0.035" or 0.045" ER70S-6 handles floor beam, stiffener, and bracing fillet welds. The mandatory CVN testing distinguishes A709 Gr.36 from plain A36 — verify that ER70S-6 lot certifications show CVN test results meeting the zone requirement.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Why GMAW for A709 Gr.36 at 3/4" to 1-1/2"
Why GMAW for A709 Gr.36 at 3/4" to 1-1/2"? GMAW delivers 8-12 lb/hr deposition — compared to <a href="/welding/preheat-calculator/a709-36/saw/3-4-to-1-1-2-inch/">SAW</a> at 15-40 lb/hr. Position capability: all positions (not GMAW-S). Suitability: primarily shop.
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 GMAW
Bridge-specific 36 ksi grade with CVN testing for highway construction. With low-hydrogen GMAW, this combination falls under Category B rather than Category A — the continuous solid wire and gas shielding in GMAW produce inherently low hydrogen levels, typically 2-4 mL/100g. The 50°F minimum preheat is lower than what non-low-hydrogen SMAW would require at the same thickness because GMAW 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 GMAW 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 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.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 GMAW is the controlled-shop combination for heavier light-bridge components — pedestrian-bridge primary girder splices, walkway main support brackets, and modest secondary stiffeners on highway bridges where the bridge-spec traceability is required and the section is heavy enough that the 50°F preheat floor is binding.
Pre-weld notes
Crossing into the mid-thickness band changes the binding constraint. 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. Shielding-gas integrity per Table 5.10 holds firm; bridge-grade rejection criteria for porosity at start-of-arc are tighter than general structural. Travel-speed control matters more at this section because over-rapid GMAW deposition into a 1-inch A709-36 section reduces heat input enough to alter the HAZ width.
What a CWI verifies
A CWI on A709-36 GMAW mid-thickness work verifies preheat at perimeter and samples mid-joint with a contact pyrometer after the first pass group, then checks gas-flow rate at the torch (not the regulator), then confirms travel speed and amperage/voltage against Table 5.3 prequalified GMAW limits. Where the bridge owner specifies CVN testing per the project documents, the WPS may be qualified rather than prequalified.