A106 Gr.B Preheat for GMAW — up to 3/4"
Per AWS D1.1:2025 Table 5.11, the minimum preheat for A106 Gr.B welded with GMAW at up to 3/4" is 32°F (0°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.
Have a preheat question? Ask Flux
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.
GMAW on pipe requires specialized gun angles and travel speed control for circumferential joints. Pulsed GMAW is increasingly popular for pipe root passes as an alternative to SMAW. The absence of slag between passes eliminates inter-pass cleaning time, significantly improving productivity on multi-pass pipe joints.
GMAW Tips for Pipe and Tube Steels
For A106 Grade B seamless pipe (35 ksi yield, high-temperature service to ~750°F), GMAW with 0.035" ER70S-6 at 210–250 A handles fill passes on roll-welded shop spools. The 100% seamless construction means no ERW seam concerns at adjacent weld HAZs. Preheat to 32°F minimum (Category B) on thicknesses up to 3/4"; thicker Schedule 160 and XH walls require 150°F at 3/4"–1-1/2" and 225°F above.
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.
A106 Gr.B
ASTM A106 Grade B covers seamless carbon steel pipe for high-temperature service up to about 750°F, with 35 ksi minimum yield and 60 ksi minimum tensile strength. Produced in sizes from 1/4" through 30" NPS, it is the standard pipe material for power plants, refineries, and process piping where elevated temperature and pressure coexist. Chemistry limits (0.30% max carbon, 0.29-1.06% manganese) give it a weldability profile that matches common structural grades with Category A and B preheat requirements. A106 is exclusively seamless, which eliminates the ERW seam concern present in A53. Grade C (40 ksi yield) exists but Grade B handles the vast majority of process piping service.
Why This Preheat for A106 Gr.B with GMAW
Seamless pipe rated for high-temperature service up to 750°F. 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 32°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 A106 Gr.B
Used in boiler steam headers, refinery piping racks, process plant branch connections, heat exchanger nozzle welds, power plant main steam lines, petrochemical reactor feed piping, and high-temperature manifolds. A106 Gr.B butt welds in process piping require joint preparation to B31.1 (power piping) or B31.3 (process piping) depending on service classification. Socket welds on small-bore A106 drain lines and instrument take-offs are common in utility stations, typically 1/2" to 2" NPS. Typical wall thicknesses range from Schedule 40 (0.237" wall on 2" NPS) to Schedule 160 (0.500" wall on 4" NPS) in critical high-energy service. Weld procedure qualification often includes side bend tests and tensile tests specific to the pipe diameter and wall thickness being joined. Field welds at pipe-to-flange connections and valve set-on joints require portable preheating equipment when wall thickness exceeds 1/2".
Why Preheat Matters at up to 3/4"
Thin material sheds heat quickly, allowing hydrogen to escape the HAZ readily — lowest preheat tier in Table 5.11.
Other Steels with GMAW at up to 3/4"
| Steel | Category | Preheat |
|---|---|---|
| A36 | B | 32°F (0°C) |
| A633 Gr.E | C | 50°F (10°C) |
| A709 HPS70W | C | 50°F (10°C) |
| A710 Gr.A | C | 50°F (10°C) |
A106 Gr.B 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.
A106 Gr.B Welding Guides
Primary sources
D1.1:2025 reference data. Not affiliated with AWS.
Application context
A106 Grade B seamless pipe at or below 3/4 inch wall with GMAW is the controlled-shop structural-pipe combination for refinery and industrial pipe-support fabrication — pipe support brackets, pipe-to-plate fittings, walkway brackets on piping systems, and structural members built from A106-B mechanical pipe where the welding falls under D1.1 (structural) rather than ASME B31.x (process piping). Shop conditions stabilize gas integrity for the gas-shielded process.
Pre-weld notes
Same scope question on every A106 weld: D1.1 vs ASME B31.1/B31.3. D1.1 governs structural welding, ASME B31.x governs pressure piping. For D1.1 structural work at this wall thickness, the 32°F preheat floor is satisfied by ordinary shop ambient. Pipe geometry adds heat-sink and axial-conduction effects that change start-of-arc behaviour relative to plate. Shielding-gas integrity per Table 5.10 (Ar/CO₂ 75–90 / 10–25, Ar/O₂ 95–98 / 2–5, or 100% CO₂ for ER70S-X / E70C-X electrodes per A5.18/A5.18M) holds.
What a CWI verifies
A CWI on A106-B GMAW thin-wall structural work first verifies the WPS scope by reading the WPS title block — D1.1 structural vs ASME B31.x pressure piping. For D1.1 structural work, the inspection covers shielding gas conformance to Table 5.10 with flow-rate sampled at the torch (not the regulator), the matching-strength filler classification against Table 5.7 Group I, and the prequalified GMAW limits in Table 5.3. Welder qualification for tubular work under Clause 6.11 applies if the joint is around the pipe circumference.