A709 Gr.36 Preheat for SMAW (low-hydrogen) — 1-1/2" to 2-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 Gr.36 welded with SMAW (low-hydrogen) 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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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 150°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 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 SMAW (low-hydrogen) at 1-1/2" to 2-1/2"
| Steel | Category | Preheat |
|---|---|---|
| A709 Gr.50 | 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.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 1-1/2 to 2-1/2 inch range with SMAW low-hydrogen is the heavy-section light-bridge combination — pedestrian-bridge primary girder splices on heavy plate, walkway main support brackets at heavy section, and modest secondary-stiffener field repair on highway bridges where the bridge-spec traceability is required and the section is heavy enough that the 150°F preheat floor is binding.
Pre-weld notes
Three discipline layers stack at this combination, with the 150°F preheat floor as the first binding one. First, the preheat-extent rule shifts at 1-1/2 inch 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. Second, real preheat infrastructure is required — induction blankets, electric resistance pads, or oxy-fuel rosebud burners. Third, the LH electrode storage and atmospheric-exposure discipline of Clause 7.3.2.1 and Clause 7.3.2.2 holds firmly — at heavy section the slow deposition pace lengthens the on-the-can exposure time per Table 7.1 column A, especially in field-rework conditions on existing bridges.
What a CWI verifies
A CWI on A709-36 SMAW-LH heavy-section 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, and (4) the LH electrode atmospheric exposure on the working can. Bridge owners may specify supplementary CVN testing on heavy primary members that elevate the WPS to qualified rather than prequalified.