A709 HPS50W Preheat for GMAW — up to 3/4"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A709 HPS50W welded with GMAW at up to 3/4" is 32°F (0°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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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 HPS50W high-performance weathering bridge steel (50 ksi yield, 0.11% max carbon, CE-IIW ~0.38–0.42), GMAW with weathering-compatible wire (ER80S-G or matching chemistry) handles both shop and field splice welds. The reduced CE-IIW compared to conventional A588 means GMAW preheat sensitivity is lower, reducing rejections on thick flange CJP splices during cold-weather fabrication. Calcium-treated sulfide shape control virtually eliminates lamellar tearing.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Why GMAW for A709 HPS50W at up to 3/4"
Why GMAW for A709 HPS50W at up to 3/4"? GMAW delivers 8-12 lb/hr deposition — compared to <a href="/welding/preheat-calculator/a709-hps50w/saw/up-to-3-4-inch/">SAW</a> at 15-40 lb/hr. Position capability: all positions (not GMAW-S). Suitability: primarily shop.
A709 HPS50W
ASTM A709 HPS50W is a high-performance weathering steel (50 ksi yield, 70 ksi minimum tensile) developed specifically for unpainted bridge construction. The HPS designation indicates enhanced weldability through lower carbon (0.11% max), controlled sulfur (0.006% max with calcium treatment), and copper-nickel-chromium alloying for atmospheric corrosion resistance. These chemistry controls reduce the CE-IIW to approximately 0.38-0.42, well below conventional weathering steels. It falls under Category B in Table 5.11. The FHWA-funded development program that created HPS grades (starting in the 1990s) aimed to eliminate the weldability problems, lamellar tearing, and inconsistent toughness that plagued earlier weathering steel bridge designs. HPS50W has largely replaced conventional A709 Gr.50W in new unpainted bridge designs.
Why This Preheat for A709 HPS50W with GMAW
High-performance weathering bridge steel with enhanced weldability at 50 ksi. 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 32°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 A709 HPS50W
Deployed in unpainted bridge plate girders across humid or coastal environments, curved girders on interchange ramps, signature pedestrian bridges in architectural applications, and rural highway bridges where lifecycle paint costs exceed the HPS premium. The enhanced weldability of HPS50W reduces fabrication risk at flange splices and allows wider groove angles with reduced risk of lamellar tearing in thick flange plates through controlled sulfur and inclusion shape control via calcium treatment. The chemistry control (0.11% max carbon, controlled sulfur at 0.006% max) differentiates HPS from conventional weathering grades. Material cost premium over standard A709 Gr.50W is typically 15-25% per ton but eliminates lifetime repainting cycles estimated at $15-25 per square foot every 20-25 years. The lower CE-IIW of HPS50W compared to conventional weathering grades means it welds with less preheat sensitivity, reducing reject rates on thick flange CJP splices during cold-weather bridge fabrication.
Why Preheat Matters at up to 3/4"
Thin material sheds heat quickly, allowing hydrogen to escape the HAZ readily — lowest preheat tier in Table 5.11.
Other Steels with GMAW at up to 3/4"
| Steel | Category | Preheat |
|---|---|---|
| A913 Gr.50/60/65 | B | 32°F (0°C) |
| A709 HPS70W | C | 50°F (10°C) |
| A710 Gr.A | C | 50°F (10°C) |
| A913 Gr.70 | C | 50°F (10°C) |
A709 HPS50W 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 HPS50W Welding Guides
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A709 HPS50W at or below 3/4 inch with GMAW is the controlled-shop high-performance-steel combination — modern bridge components designed under AASHTO LRFD where the high-performance grade delivers controlled toughness for fracture-critical members at modest section thickness, and the gas-shielded process delivers the cleaner deposit profile that bridge-grade rejection criteria demand.
Pre-weld notes
Three constraints sit on top of the 32°F preheat floor. First, shielding-gas integrity per Table 5.10 (Ar/CO₂ 75–90 / 10–25, Ar/O₂ 95–98 / 2–5, or 100% CO₂ for ER70S-X / E70C-X electrodes per A5.18/A5.18M) — porosity at start-of-arc on bridge-grade work has tighter rejection criteria than general structural. Second, for exposed bare HPS50W applications the electrode must conform to Table 5.9 per Clause 5.6.3 (filler metals with B2L, G, Ni1, Ni2, or Ni3 analysis per 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). Third, bridge owners frequently specify supplementary CVN testing for HPS members per their project documents — verify whether the WPS is prequalified under Clause 5 or qualified under Clause 6.
What a CWI verifies
A CWI on A709 HPS50W GMAW thin-section work verifies (1) the bridge-grade material certification against the WPS-cited steel, (2) electrode classification matches Table 5.9 for exposed bare applications and any project-specific CVN deposit requirements, (3) shielding gas conforms to Table 5.10 with flow-rate sampled at the torch (not the regulator), and (4) travel speed against the WPS bounds. The 32°F preheat floor rarely binds at this thickness; consumable selection, gas integrity, and parameter conformance dominate.