A588 Preheat for FCAW — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A588 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.
FCAW Tips for Common Structural Steels
For A588 weathering steel, FCAW fill metal must match the weathering composition for exposed joints — standard E71T-1 does NOT provide atmospheric corrosion resistance matching. Use weathering-type FCAW wire (E81T1-W or equivalent) on exposed faces. On unexposed joints, standard E71T-1M is acceptable. The 50 ksi yield and CE-IIW (0.45-0.52) place it in Category B.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Why FCAW for A588 at 3/4" to 1-1/2"
Why FCAW for A588 at 3/4" to 1-1/2"? FCAW delivers 8-12 lb/hr deposition — compared to <a href="/welding/preheat-calculator/a588/saw/3-4-to-1-1-2-inch/">SAW</a> at 15-40 lb/hr. Position capability: all positions. Suitability: field and shop.
A588
ASTM A588 is a weathering steel specification (50 ksi minimum yield, 70 ksi minimum tensile) that forms a protective iron oxide patina when exposed to atmospheric wet-dry cycling. The copper (0.25-0.40%), chromium (0.40-0.65%), and nickel (0.25-0.40%) alloying creates a dense, adherent rust layer that stabilizes after 2-5 years of exposure. Used in unpainted bridges and exposed structural members, it requires low-hydrogen processes (Category B) per Table 5.11 due to its higher alloy content pushing CE-IIW to 0.45-0.52. Weld filler metal must be a matching weathering composition (e.g., E8018-W2) to achieve corrosion-matching at exposed joints. A588 is produced as plate up to 8" thick and is available in structural shapes, though A709 Gr.50W is more commonly specified for bridge applications.
Why This Preheat for A588 with FCAW
Weathering steel forming protective patina for unpainted bridge applications. 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 A588
Used in unpainted highway bridge girders, exposed pedestrian bridges, architectural weathering facades, transmission tower legs, rail bridge stringers, and marine navigation light structures. A588 welds must use compatible weathering-type filler metals (e.g., E8018-W2 or ER80S-G) to achieve matching corrosion resistance in the exposed weld face. Girder flange splices and stiffener fillet welds are primary fabrication joints. The distinctive brown-orange patina develops over 2-5 years of atmospheric exposure to form a stable, adherent oxide layer that does not require repainting. In coastal locations with salt spray or in areas where the surface stays wet for extended periods, the protective oxide layer may not form properly, limiting A588 to inland applications with reliable wet-dry cycling. Bolt holes and copes must be deburred to prevent corrosion concentration. Drainage details in the steel design prevent water traps that would undermine the patina formation process.
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 |
|---|---|---|
| A633 Gr.A/C/D | B | 50°F (10°C) |
| A633 Gr.E | C | 150°F (65°C) |
| A709 HPS70W | C | 150°F (65°C) |
| A710 Gr.A | C | 150°F (65°C) |
A588 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.
A588 Welding Guides
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A588 plate in the 3/4 to 1-1/2 inch range with FCAW is the heavy-deposition mid-thickness combination for bare weathering steel — bridge primary girder splices, transmission-tower main-leg connections, and exposed-architectural primary members where the higher deposition rate of cored wire moves steel faster than SMAW or GMAW and the patina-finish requirement drives the consumable selection.
Pre-weld notes
Stepping into the mid-thickness band, the 50°F preheat floor needs active verification rather than the ambient-satisfies-it pattern of thin work. Three constraints layer in. First, the preheat distance per the WPS — for sub-1.5-inch base metal, the heated zone must extend at least twice the base-metal thickness in all directions from the point of welding (1.5 to 3 inches). Second, the FCAW electrode must conform to Table 5.9 per Clause 5.6.3 for exposed bare applications (B2L, K2, Ni1, Ni2, Ni3, Ni4, or WX analysis under A5.29/A5.29M). Third, inter-pass slag removal across the increased pass count of mid-thickness work — incomplete removal traps inclusions across multiple pass groups.
What a CWI verifies
A CWI on A588 FCAW mid-thickness work verifies preheat at perimeter and samples mid-joint after the first pass group with a contact pyrometer or thermocouple, then confirms electrode classification against Table 5.9, then walks inter-pass slag removal through the joint sequence. The 50°F floor is closer to binding at this thickness than at thin section — winter shop conditions or large-section heat-sink effects can put base metal below the floor without obvious signal, and the through-thickness reading lags surface by minutes during heat-up.