A500 Gr.B/C Preheat for GMAW — 3/4" to 1-1/2"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A500 Gr.B/C welded with GMAW at 3/4" to 1-1/2" is 50°F (10°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 A500 Grade B/C cold-formed HSS (46–50 ksi yield), GMAW with 0.035" ER70S-6 at 210–250 A handles tube-to-plate fillet welds and tube-to-tube connections. The cold-formed corners have elevated hardness — avoid placing weld starts or crater stops within 1/2" of corner radii to prevent heat concentration at these work-hardened zones. Use 75/25 Ar/CO2 at 35–40 CFH for standard shop fillet.
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.
A500 Gr.B/C
ASTM A500 Grade B (46 ksi [315 MPa] minimum yield) and Grade C (50 ksi [345 MPa] minimum yield), as AWS D1.1:2025 Table 5.6 lists them in Group I, cover cold-formed welded and seamless structural tubing — round, square, and rectangular HSS sections. These are the standard tubular members in building frames, trusses, and signage structures, produced in wall thicknesses from 16 gauge (0.065") up to 5/8" for rectangular and 1/2" for round. Table 5.11 assigns both Category A and B preheat requirements. The cold-forming process work-hardens the corners, producing corner radii with higher hardness (up to 20% increase) and reduced ductility compared to the flat faces. This affects welding behavior at corner locations, particularly on heavily loaded connection details where weld starts or stops near corner radii can create initiation points for fatigue cracks.
Why This Preheat for A500 Gr.B/C with GMAW
Cold-formed structural tubing for HSS sections in frames and trusses. 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 50°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 A500 Gr.B/C
Standard for HSS columns in office buildings, hollow section trusses in warehouse roofs, exposed architectural tube steel, sign structures, solar panel racking frames, greenhouse frames, and guard rail posts. A500 tube-to-tube moment connections require through-plate or diaphragm detailing to transfer forces across the closed section. Slotted gusset connections into HSS bracing members are a signature fabrication detail requiring careful fit-up and beveling of the gusset plate. Common sizes include HSS 6x6x3/8 and HSS 8x8x1/2 for columns, HSS 4x4x1/4 for bracing, and HSS 10x6x3/8 for rectangular beam applications. The cold-formed corner radius creates a heat-affected zone consideration that differs from hot-rolled shapes when planning multi-pass weld sequences. Round HSS pipe columns are also common, typically HSS 6.625x0.280 through HSS 12.750x0.500 for mezzanine posts and canopy supports. Fillet welds connecting HSS to cap plates and base plates are the most frequent weld joint detail.
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"
| Steel | Category | Preheat |
|---|---|---|
| A516 Gr.55/60 | B | 50°F (10°C) |
| A633 Gr.E | C | 150°F (65°C) |
| A709 HPS70W | C | 150°F (65°C) |
| A710 Gr.A | C | 150°F (65°C) |
A500 Gr.B/C 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.
A500 Gr.B/C Welding Guides
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A500 Grade B/C heavy-wall hollow structural sections in the 3/4 to 1-1/2 inch wall range with GMAW is the controlled-shop combination for major HSS-to-plate connections, large lattice-tower main legs, primary HSS column connections at moment-frame buildings, and HSS shop fabrication where the gas-shielded process delivers cleaner branch-to-main connections than self-shielded variants and the wall thickness pushes the 50°F preheat floor into the binding-constraint position.
Pre-weld notes
Heavy-wall HSS GMAW work stacks tubular geometry constraints on standard mid-thickness GMAW disciplines. First, the 50°F preheat floor needs active verification per Clause 7.6 — the heated zone must extend at least twice the wall thickness from the welding point. Second, the HSS connection geometry must match a prequalified tubular detail under Clause 10 — branch-to-main fit-up and prequalified joint detail limits are tighter on heavy-wall HSS than on plate. Third, axial heat conduction along the tube and the heat-sink effect of the hollow section make through-thickness preheat lag larger than on plate. Fourth, shielding-gas integrity per Table 5.10 is critical because the open-architecture nature of HSS connections means gas-flow rate at the torch and cup cleanliness drive porosity at start-of-arc.
What a CWI verifies
A CWI on heavy-wall A500 HSS GMAW mid-thickness work verifies (1) preheat at multiple positions around the connection circumference with a contact pyrometer — uneven preheat is more common on tubular work than plate, (2) the connection geometry matches a prequalified tubular detail under Clause 10, (3) shielding gas conforms to Table 5.10 with flow-rate sampled at the torch, and (4) the welder qualification covers tubular position welding under Clause 6.11. The 50°F floor is binding at this wall thickness.