A588 Preheat for GMAW — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A588 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 A588 weathering steel (50 ksi yield, CE-IIW 0.45–0.52), GMAW with standard ER70S-6 is acceptable on unexposed interior joints but exposed weld faces require a weathering-type wire such as ER80S-G or ER70S-G1 with Cu/Cr/Ni additions to match the corrosion-protective alloy chemistry. Category B applies — minimum preheat 32°F up to 3/4", stepping up to 225°F above 2-1/2".
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Filler Metal for GMAW
Common wire: ER70S-6 (AWS A5.18). Diameter: 0.035" for thin sections and out-of-position, 0.045" for production flat/horizontal. Shielding gas: 75/25 Ar/CO2 (standard), 90/10 Ar/CO2 (less spatter, better profile), or 100% CO2 (deeper penetration, more spatter). Contact-tip-to-work distance: 1/2" to 3/4".
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
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 GMAW
Weathering steel forming protective patina for unpainted bridge applications. This steel is prequalified only with low-hydrogen processes under Table 5.11. With GMAW, the continuous solid wire and gas shielding in GMAW produce inherently low hydrogen levels, typically 2-4 mL/100g. The 50°F minimum preheat balances the steel’s strength level and carbon equivalent against the hydrogen control provided by GMAW. 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 GMAW 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 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.
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 GMAW is the controlled-shop bare-finish weathering combination for bridge fascia primaries, transmission-tower main legs, and exposed-architectural primary members — sections heavy enough that the 50°F preheat floor is binding and the gas-shielded process delivers the cleaner deposit profile that exposed-finish work demands.
Pre-weld notes
Three constraints 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, shielding-gas integrity per Table 5.10 — production gas must conform to one of the Ar/CO₂, Ar/O₂, or 100% CO₂ family per A5.18/A5.18M and any OE designator on the electrode classification. Third, for exposed unpainted applications the electrode must conform to Table 5.9 per Clause 5.6.3 (B2L, G, Ni1, Ni2, or Ni3 analysis under A5.28/A5.28M for GMAW; for G, footnote a requires deposited weld metal with a chemical composition the same as that for any one of the weld metals in the table).
What a CWI verifies
A CWI on bare A588 GMAW mid-thickness work verifies (1) preheat at perimeter and samples mid-joint with a contact pyrometer after the first pass group, (2) electrode classification matches Table 5.9 for exposed weathering applications, (3) shielding gas conforms to Table 5.10 with flow-rate sampled at the torch, and (4) travel speed against the WPS bounds — over-rapid GMAW deposition into a 1-inch A588 section can leave a brittle HAZ that the preheat floor exists to prevent. The single-pass exemptions of Clause 5.6.3.1 and 5.6.3.2 do not apply to multi-pass joints in this thickness range.