A709 Gr.36 Preheat for SMAW (low-hydrogen) — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 Gr.36 welded with SMAW (low-hydrogen) 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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SMAW (Low-Hydrogen)
Low-hydrogen SMAW (E7018/E7016) uses basic-coated electrodes requiring rod oven storage, assigned to Category B in Table 5.11.
E7018 is the default electrode for structural fillet and groove welds on common building steels. Rod ovens should hold at a minimum of 250°F per D1.1 Clause 7.3.2.1; exposure time out of the oven is limited to 4 hours maximum per Table 7.1. For overhead position, use 3/32" diameter rods to control puddle size. Vertical-up stringer beads provide the best fusion on thicker members.
SMAW-LH Tips for Common Structural Steels
For A709 Grade 36 bridge steel (36 ksi yield, CVN per AASHTO zone), E7018 handles secondary member welds (stiffeners, floor beams, bracing) in bridge fabrication shop and field. The mandatory CVN documentation requirement distinguishes bridge fabrication from structural building — electrode batch certifications must be retained as quality records with the bridge fabricator. Category A and B both apply; SMAW-LH.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Filler Metal for SMAW-LH
Electrode: E7018 (AWS A5.1). Table 5.7 lists it for Group I and Group II base metals only; Groups III, IV and V take AWS A5.5 low-alloy electrodes from Table 5.7. Diameter: 1/8" (general/out-of-position), 5/32" (production), 3/16" (heavy plate flat only). Storage: 250°F rod oven minimum per D1.1 §7.3.2.1. Exposure limit: 4 hours out of oven per Table 7.1, then re-bake at 500-800°F for minimum 2 hours per §7.3.2.4 (A5.1 classification).
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 SMAW-LH
Bridge-specific 36 ksi grade with CVN testing for highway construction. With low-hydrogen SMAW-LH, this combination falls under Category B rather than Category A — E7018 low-hydrogen electrodes produce typically 4-8 mL/100g diffusible hydrogen under proper rod oven conditions. The 50°F minimum preheat is lower than what non-low-hydrogen SMAW would require at the same thickness because SMAW-LH 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 SMAW (low-hydrogen) 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 SMAW (low-hydrogen)
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 SMAW low-hydrogen sits at the heavier end of 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 thick enough that the 50°F preheat floor is binding.
Pre-weld notes
Three discipline layers stack 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, the LH electrode storage discipline of Clause 7.3.2.1 holds — 250°F holding oven, no rebake more than once, no use after wetting — with the atmospheric-exposure clock per Table 7.1 column A running on every working can. Third, bridge-grade traceability — the electrode lot must trace back to the manufacturer's certification per the WPS, and bridge-owner project documents may specify CVN testing under Clause 6 qualified-WPS rather than the prequalified path.
What a CWI verifies
A CWI on A709-36 SMAW-LH mid-thickness work verifies (1) preheat at perimeter and samples mid-joint with a contact pyrometer after the first pass group, (2) electrode-lot traceability to the WPS-cited heat lot, (3) holding-oven temperature and the contractor's rebake log, and (4) atmospheric-exposure tag on the working can against Table 7.1 column A. The 50°F floor is closer to binding at this thickness than at thin section — winter shop or field conditions can put base metal below the floor without obvious signal.