A710 Gr.A 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 follows the steel's Table 5.6 base-metal group: Table 5.7 marks AWS A5.18 carbon-steel wire N/A for Groups III, IV and V and lists AWS A5.28 low-alloy classifications such as ER80S-Ni1 (Group III), ER90S-D2 (Group IV) and ER100S-1 (Group V). 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 A710 Gr.A precipitation-hardened plate (0.07% max carbon, CE-IIW 0.32-0.38), GMAW with ER70S-6 at 260-280 A leverages the ultra-low carbon for exceptional weldability. Class 2 (age-hardened at mill) requires attention to avoid over-aging the HAZ. Class 3 (hardened after fabrication) allows more flexibility since final strengthening occurs post-weld.

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.

A710 Gr.A

ASTM A710 Grade A is a precipitation-hardened low-carbon steel plate achieving high strength through copper precipitation rather than carbon content. Class 2 (65 ksi yield, age-hardened at mill) and Class 3 (75 ksi yield, precipitation-hardened after fabrication) both feature very low carbon (0.07% max) producing a CE-IIW of approximately 0.32-0.38 — among the lowest of any high-strength steel. Table 5.11 lists it by class and thickness: with standard low-hydrogen processes, Category C covers Class 2 up to 2 in [50 mm] and Class 3 over 2 in [50 mm] and Category B covers Class 2 over 2 in [50 mm], while every class takes the reduced Category D preheat (32°F all thicknesses) with H8-certified consumables, reflecting the exceptional hydrogen cracking resistance of this ultra-low-carbon metallurgy. The precipitation hardening mechanism means weld thermal cycles can alter the strength in the HAZ depending on peak temperature and cooling rate, requiring attention to heat input control during procedure qualification. This 300°F (150°C) value applies only to A710 Grade A Class 3 (over 2 in [50 mm]); Class 2 at this thickness is Category B, and Class 1 is listed only under Category D with H8.

A710 Grade A Table 5.11 split

A710 Grade A is listed by class and thickness in AWS D1.1:2025 Table 5.11. The Category C value on this page applies only to Class 2 up to 2 in [50 mm] and Class 3 over 2 in [50 mm]. Class 2 over 2 in [50 mm] is Category B. Class 1, and Class 3 up to 2 in [50 mm], have no Table 5.11 row without H8 consumables and are listed only under Category D with H8, so this value does not apply to them. Confirm the class before using it.

Why This Preheat for A710 Gr.A with GMAW

Precipitation-hardened low-carbon plate with multiple category paths. 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. This 300°F (150°C) value applies only to A710 Grade A Class 3 (over 2 in [50 mm]); Class 2 at this thickness is Category B, and Class 1 is listed only under Category D with H8.

Typical Applications for A710 Gr.A

Found in naval hull plates, military vehicle armor brackets, offshore platform node connections, heavy-lift crane boom sections, and mine hoist drum shells. A710 Gr.A precipitation-hardened plate offers a rare combination of high strength and exceptional weldability at low carbon equivalent (CE-IIW approximately 0.32-0.38). The multiple thickness-dependent category paths (B, C, and D with H8) reflect its complex metallurgical response to different section sizes. Class 2 plate is age-hardened at the mill through a controlled thermal cycle, while Class 3 achieves higher strength through precipitation hardening after welding, which makes it particularly suitable for applications where extensive welding occurs before final strengthening. The distinction between Class 2 and Class 3 response to weld thermal cycles requires careful attention during procedure qualification. Plate thicknesses up to 6" are available but procurement requires extended lead times due to limited production volume.

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 A710 Gr.A

Category C in Table 5.11 applies to higher-strength steels where the combination of hardenability and residual stress requires elevated preheat. For A710 Gr.A 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. This 300°F (150°C) value applies only to A710 Grade A Class 3 (over 2 in [50 mm]); Class 2 at this thickness is Category B, and Class 1 is listed only under Category D with H8.

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

A710 Gr.A 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.

A710 Gr.A Welding Guides