Wind, Drizzle, and a Choice of Two Procedures
A fabrication crew is installing stiffener plates on an outdoor structural steel frame. The base metal is 3/8 in. A36; each joint is a 6 in. long fillet in the 3F and 4F positions. Ambient temperature is 40°F with wind gusting to 22 mph and intermittent drizzle. The only power source is a 300 A engine-driven machine capable of CC and CV output. Two procedures are on hand: GMAW short-circuit using 0.035 in. ER70S-6 with 75% Ar/25% CO2, and FCAW self-shielded using 0.072 in. E71T-11. The applicable code is AWS D1.1 for a statically loaded structure, and the engineer states that no notch-toughness testing is required. The inspector will visit only at the end of the shift.
1. Which of the two procedures should be selected for the production fillets, and which specific conditions drive that choice?
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Select the self-shielded FCAW procedure.
Reasoning:
- GMAW short-circuit depends on an external gas shield. Wind above roughly 5 mph disperses the shielding gas and produces porosity and incomplete fusion. With gusts to 22 mph, the GMAW option cannot be protected reliably without an enclosure.
- Self-shielded FCAW generates its own shielding from the flux core and tolerates far more air movement, though it is not immune; the manufacturer's wind limit still applies.
- On 3/8 in. material in the 3F and 4F positions, FCAW gives higher deposition and better penetration than short-circuit GMAW, which is prone to lack-of-fusion on thicker plate.
- E71T-11 is not typically qualified for notch toughness, but the engineer has stated that toughness is not required. That removes the main technical objection to the self-shielded wire.
The GMAW unit should remain available only if a windbreak can hold air movement below the gas-shielded limit.
2. What minimum preheat and weather controls must be verified before the first arc is struck?
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- Preheat: The steel is at 40°F ambient and may be colder. For A36 at 3/8 in. thickness, AWS D1.1 requires a minimum preheat of 50°F. Verify the actual base metal temperature at the joint with a contact thermometer or temperature-indicating crayon, and heat until it is at least 50°F before welding.
- Moisture: Drizzle and condensation leave the surface wet. Dry the joint and at least 2 in. of adjacent base metal; welding on wet steel promotes porosity and hydrogen cracking.
- Wind: Even self-shielded FCAW has a manufacturer's wind limit. Erect windbreaks or reposition the work so that the limit is respected, and confirm by inspecting the first welds for porosity.
- Electrode storage: Keep the FCAW wire clean and dry, and protect the coil from rain.
The preheat and dryness checks belong on the record before production welding, not after the inspector arrives.
3. Mid-shift the crew runs out of self-shielded wire and proposes switching to the GMAW procedure under a tarp. What must be established before that switch is allowed?
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A tarp is not automatically adequate. Before switching:
- Measure or otherwise confirm that air movement at the arc stays below the gas-shielded limit, typically about 5 mph. A loose tarp in a 22 mph gust will not do this.
- Confirm the GMAW procedure is qualified for 3/8 in. thickness and for the 3F and 4F positions being welded.
- Make a short test weld and inspect it for porosity before committing production welds.
- Re-verify preheat after any delay, since the plate will cool toward ambient.
If air movement cannot be controlled, the correct decision is to stop and obtain more self-shielded wire rather than risk a shift's worth of unacceptable welds.