A709 HPS70W Preheat for GMAW — over 2-1/2"

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.

On high-strength plate, GMAW wire selection must meet the specified minimum tensile to satisfy weld metal matching. ER80S-D2 or ER100S-G wires serve higher-strength steels. The inherently low hydrogen input of solid wire makes GMAW particularly well-suited for crack-sensitive TMCP grades where minimizing diffusible hydrogen is the primary fabrication objective.

GMAW Tips for High-Strength and TMCP Steels

For A709 HPS70W (70 ksi yield, Category C, Q&T plate), GMAW requires higher-strength wire — ER80S-G or ER100S-G to achieve tensile matching at 70+ ksi weld strength. Note D1.1:2025 Table 5.11 sets both minimum AND maximum preheat/interpass for HPS70W: maximum 400°F up to 1-1/2", 450°F above. Exceeding the maximum interpass temperature risks over-tempering the Q&T microstructure.

Typical values for reference — always verify against your approved WPS and electrode manufacturer data.

Why GMAW for A709 HPS70W at over 2-1/2"

Why GMAW for A709 HPS70W at over 2-1/2"? GMAW delivers 8-12 lb/hr deposition — compared to SAW at 15-40 lb/hr. Position capability: all positions (not GMAW-S). Suitability: primarily shop.

A709 HPS70W

ASTM A709 HPS70W is the highest-strength weathering bridge steel with 70 ksi minimum yield and 85-110 ksi tensile range, used in long-span bridges and heavily loaded members where weight reduction is critical. Produced as quenched-and-tempered plate in thicknesses up to 4", its high strength places it in Category C of Table 5.11 with correspondingly higher preheat requirements. Table 5.11 footnote (b) sets a maximum preheat/interpass of 400°F for thickness up to 1-1/2” and 450°F for thicker material — this upper limit is unusual in D1.1 and exists because the Q&T microstructure can be degraded by excessive heat. The chemistry includes copper (0.25-0.40%), nickel (0.65-1.00%), and chromium (0.40-0.65%) for atmospheric corrosion resistance, with 0.19% max carbon keeping the CE-IIW in the 0.46-0.52 range.

Why This Preheat for A709 HPS70W with GMAW

Highest-strength 70 ksi weathering bridge steel for long-span applications. The higher strength level of this steel places it in Category C of Table 5.11, which carries elevated preheat requirements compared to Category B grades. At 300°F minimum with GMAW, the continuous solid wire and gas shielding in GMAW produce inherently low hydrogen levels, typically 2-4 mL/100g, but the preheat must still ensure the cooling rate stays slow enough to prevent hydrogen-induced cracking in this higher-hardenability material.

Typical Applications for A709 HPS70W

Reserved for long-span bridge main girder flanges, cable-stayed bridge edge girders, arch rib plates, high-load interchange ramp girders, and situations where reducing member depth saves clearance or reduces dead load. HPS70W enables weight savings of 20-30% versus conventional Gr.50 designs, allowing shallower girder sections that reduce embankment costs on grade-separation bridges. Flange thickness often exceeds 2”, making preheat and interpass control critical at every CJP splice and web-to-flange joint. Note that Table 5.11 footnote (b) sets maximum preheat and interpass limits of 400°F for thicknesses up to 1-1/2" and 450°F above. This upper limit is unusual in D1.1 and requires monitoring both minimum and maximum interpass throughout the welding sequence. Fabricators must use dual-readout temperature monitoring to ensure the joint stays within the qualified band. Filler metals must match the 70 ksi minimum tensile while providing weathering-compatible chemistry for exposed joints.

Why Preheat Matters at over 2-1/2"

The heaviest sections demand the highest preheat in Table 5.11. Multi-pass sequences require maintaining interpass temperature throughout.

Category C Preheat for A709 HPS70W

Category C in Table 5.11 applies to higher-strength steels where the combination of hardenability and residual stress requires elevated preheat. For A709 HPS70W at over 2-1/2", the 300°F minimum preheat slows the weld cooling rate to prevent formation of crack-susceptible martensite in the heat-affected zone. Maintaining interpass temperature at or above this minimum is especially critical for multi-pass welds on restrained joints.

Other Steels with GMAW at over 2-1/2"

A709 HPS70W 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 HPS70W Welding Guides