A36 Preheat for GMAW — 1-1/2" to 2-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A36 welded with GMAW at 1-1/2" to 2-1/2" is 150°F (65°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 A36 structural steel (36 ksi yield, 0.26% max carbon), GMAW spray transfer with 0.035" ER70S-6 at a minimum of 210 A or 0.045" at minimum 260 A per D1.1:2025 Table 5.3 provides high deposition for shop fillet welds. Use 75/25 Ar/CO2 shielding at 35–45 CFH. A36 plate at 3/4" and under qualifies for Category A (non-LH) or Category B.
Typical values for reference — always verify against your approved WPS and electrode manufacturer data.
Why GMAW for A36 at 1-1/2" to 2-1/2"
Why GMAW for A36 at 1-1/2" to 2-1/2"? GMAW delivers 8-12 lb/hr deposition — compared to <a href="/welding/preheat-calculator/a36/saw/1-1-2-to-2-1-2-inch/">SAW</a> at 15-40 lb/hr. Position capability: all positions (not GMAW-S). Suitability: primarily shop.
A36
ASTM A36 is the most commonly specified structural steel in North America, with a minimum yield strength of 36 ksi and 58-80 ksi tensile range. It appears in both Category A (non-low-hydrogen SMAW) and Category B (low-hydrogen processes) of Table 5.11. A36 is available as plate (up to 8" thick), W-shapes, channels, angles, and bars from virtually every domestic mill. Its moderate carbon content (0.26% max for shapes, 0.25% max for plate up to 3/4") and typical carbon equivalent of 0.35-0.42 give it good weldability across all prequalified processes. A36 plate thicker than 1-1/2" carries a slightly higher carbon limit of 0.29%, while plate from 3/4" to 1-1/2" stays at 0.25% max.
Why This Preheat for A36 with GMAW
Widely used structural carbon steel with 36 ksi yield and 0.26% max carbon. With low-hydrogen GMAW, this combination falls under Category B rather than Category A — the continuous solid wire and gas shielding in GMAW produce inherently low hydrogen levels, typically 2-4 mL/100g. The 150°F minimum preheat is lower than what non-low-hydrogen SMAW would require at the same thickness because GMAW significantly reduces the driving force for hydrogen-induced cracking in the heat-affected zone.
Typical Applications for A36
Common in angle-to-gusset fillet welds, beam web clip angles, stiffener plates, base plate bearing connections, light bracing members, stair stringers, handrail posts, and miscellaneous steel fabrication. A36 plate is the default choice for connection elements such as shear tabs, moment end plates under 36 ksi demand, and simple beam-to-column seated connections. In retrofit and renovation, A36 angles and channels are standard for reinforcement brackets and framing infill. Typical shop drawing callouts include 3/8" and 1/2" A36 plate for gussets, 5/16" fillet welds on clip angles, and partial joint penetration groove welds on base plate stiffeners. A36 is so ubiquitous that most structural steel shops maintain permanent inventory in multiple thicknesses from 1/4" through 2" plate. Fillet weld sizes on A36 connections typically range from 3/16" minimum to 5/8" for heavy gusset-to-column welds, with E70XX electrodes providing significant overmatching strength.
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"
| Steel | Category | Preheat |
|---|---|---|
| A53 Gr.B | B | 150°F (65°C) |
| A633 Gr.E | C | 225°F (110°C) |
| A709 HPS70W | C | 225°F (110°C) |
| A710 Gr.A | C | 225°F (110°C) |
A36 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.
A36 Welding Guides
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A36 plate in the 1-1/2 to 2-1/2 inch range with GMAW is the heavy-section production combination — plate girder primary fillet runs in shop conditions, heavy A36 connection plates on transfer girders, transmission-tower base plates, and walkway-bracket structural fabrication on heavy plate where the gas-shielded process suits the controlled environment of shop work and the 150°F preheat floor is binding.
Pre-weld notes
Crossing the 1-1/2 inch threshold changes the preheat-extent rule per Clause 7.6 — for base metal 1-1/2 inch and greater, the heated zone shall extend at least equal to the base-metal thickness, but not less than 3 inches. Real preheat infrastructure is required at this thickness — induction blankets, electric resistance pads, or oxy-fuel rosebud burners; torch passes alone are unreliable on a 2-inch A36 section. Through-thickness preheat verification is the binding inspection effort: surface temperature on the heated face overstates the through-thickness reading by minutes during heat-up.
What a CWI verifies
A CWI on A36 GMAW heavy-section work verifies (1) preheat through-thickness, not just surface — typical method is sampling 3-6 inches from the arc on the back side, where surface temperature lags through-thickness by the longest interval, (2) the heated-zone extent against Clause 7.6 (at least equal to base-metal thickness, not less than 3 inches), (3) interpass temperature held above 150°F with a contact pyrometer between pass groups, and (4) shielding gas conforms to Table 5.10 with flow-rate sampled at the torch. Heat-input pacing matters more at heavy section — over-rapid GMAW deposition into a 2-inch A36 section can leave a brittle HAZ that the preheat floor exists to prevent.