A1066 Gr.70 Preheat for GMAW — 1-1/2" to 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.

A1066 Gr.70

ASTM A1066 Grade 70 (70 ksi yield, 85 ksi minimum tensile) is the highest-strength grade in this specification, produced as quenched-and-tempered or TMCP plate for demanding bridge and structural applications. It falls under Category C in Table 5.11 but notably does not qualify for any H8 reduced-preheat category — unlike Grades 50/60/65, Grade 70 must use full Category C preheat regardless of consumable hydrogen designation. This distinction exists because the higher alloying level needed for 70 ksi yield pushes the CE-IIW to approximately 0.48-0.54, a range where even H8 hydrogen control is not sufficient to offset the hardenability-driven cracking risk at reduced preheat. Fabricators working with A1066 Gr.70 should budget for full Category C preheating time on every joint, with no H8 shortcut available.

Why This Preheat for A1066 Gr.70 with GMAW

Highest-strength A1066 at 70 ksi with no H8 preheat reduction available. 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 225°F minimum with GMAW, the preheat ensures the cooling rate stays slow enough to prevent hydrogen-induced cracking in this higher-hardenability material. Category C steels demand careful attention to interpass temperature control throughout the weld sequence.

Typical Applications for A1066 Gr.70

Used in the most demanding bridge and structural applications: main girder flanges on record-span bridges, arch rib plates, suspension bridge stiffening truss chords, and heavy industrial crane runway girders. A1066 Gr.70 does not qualify for any H8 reduced preheat, so full Category C preheat must be applied regardless of consumable certification. This makes preheat management the primary production consideration for heavy-section joints where flange thicknesses of 2-4" are common. Fabrication bid costs for A1066 Gr.70 components must account for the full Category C preheat time in their labor estimates — typically 45-90 minutes per joint on plate over 2". Unlike Grades 50 and 60/65, there is no consumable-based path to reduce preheat duration on Gr.70 joints. Bridge fabrication shops working with this grade maintain dedicated preheat logs and temperature monitoring records for every CJP and PJP weld as part of their quality management system.

Why Preheat Matters at 1-1/2" to 2-1/2"

Heavy plate with significant restraint and thermal mass — preheat is critical to maintain slow cooling for hydrogen escape.

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

A1066 Gr.70 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.

A1066 Gr.70 Welding Guides