A709 HPS50W Preheat for GMAW — 3/4" to 1-1/2"

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.

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, which is why it appears in Category B but not Category A. The 50°F minimum preheat with GMAW balances the steel's strength level and carbon equivalent against the controlled hydrogen input from the consumable. 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 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"

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