Drawing Note Conflict on a 304L Pipe-to-Flange Joint
A 6-in Schedule 40 304L pipe-to-flange joint must be welded to a drawing that shows a single-bevel groove weld, arrow side, all around, with a 37.5° bevel and a 3 mm root gap. The tail carries a “G” and the note reads “root, fill, and cap GTAW.” The project specification requires purge oxygen of 50 ppm or less and a WPS qualified for 304L. The only approved WPS on site covers a 60° included angle, 2.4 mm root gap, 90–110 A, 9–11 V, 60–80 mm/min, and a maximum heat input of 1.0 kJ/mm. The shop purge meter reads 0.10% O₂ after 20 minutes. The fitter says he can open the root to 3.5 mm to get penetration. The welder wants to run 115 A at 11 V and 55 mm/min because the root is tight in places.
1. Explain what the drawing symbol and notes require, and list the first checks you make before striking an arc.
Read the worked answer
Symbol and notes:
- Single-bevel groove weld on the arrow side, welded all around.
- Tail “G” and the note mean GTAW for root, fill, and cap.
- The drawing calls for a 37.5° bevel and 3 mm root gap.
- Specification adds a purge oxygen limit of 50 ppm and requires a 304L-qualified WPS.
First checks:
- WPS revision and qualification - confirm the WPS is qualified for the actual joint geometry, thickness, and 304L. A 60° included angle WPS does not automatically cover a 37.5° bevel detail; verify essential variable coverage.
- Joint geometry - measure bevel angle, root face, gap, and misalignment with a gauge. Do not let the fitter open the gap to 3.5 mm without engineering approval and WPS coverage.
- Purge quality - 0.10% O₂ is 1,000 ppm, far above 50 ppm. Check purge dams, seals, flow, and sample point at the low point of the pipe.
- Consumables and cleanliness - 304L filler cert, tungsten type and grind, acetone-cleaned joint, no carbon steel contamination.
- Welder qualification - valid for GTAW on stainless pipe in the required position.
2. Calculate the heat input for 115 A, 11 V, and 55 mm/min. Compare it with the WPS limit and decide whether to approve the welder’s proposal.
Read the worked answer
Heat input formula:
HI (kJ/mm) = [60 × V × A] ÷ [1,000 × travel speed in mm/min]
Calculation:
- 60 × 11 × 115 = 75,900
- 1,000 × 55 = 55,000
- HI = 75,900 ÷ 55,000 = 1.38 kJ/mm
The WPS maximum is 1.0 kJ/mm, so the proposal is about 38% above the qualified maximum. Do not approve it as written. Raising current and slowing travel both increase heat input, which also raises the risk of sensitization and distortion in 304L.
Better response: correct the tight root by improving fit-up, using the qualified 2.4 mm gap, checking bevel and alignment, and using the WPS current range. If the drawing truly requires a 3 mm gap or a different bevel, the WPS must be revised and requalified, or engineering must approve a qualified alternative. A run at 100 A, 10 V, and 70 mm/min gives about 0.86 kJ/mm, which is within the WPS range.
3. The purge meter reads 0.10% O₂. What actions do you take before welding, and why is 50 ppm specified for stainless steel?
Read the worked answer
0.10% O₂ equals 1,000 ppm, which is 20 times the 50 ppm limit. Do not start the root pass. First check whether the sample is representative: sample at the lowest point of the pipe, allow the meter to stabilize, and confirm the meter is calibrated for low-oxygen ranges. Then improve the purge by sealing the pipe ends with dams or water-soluble film, reducing the purge volume, checking for leaks at the flange and tape joints, and increasing purge time or flow within a safe limit. Re-test until the reading is ≤50 ppm.
The 50 ppm limit protects the root from oxidation and heat tint. In stainless steel, oxygen at the root forms chromium oxide, depleting chromium from the weld surface and reducing corrosion resistance. A heavy tint or black oxide may require grinding or pickling, which is difficult inside a pipe. The meter reading is therefore a weld-quality control, not just a housekeeping check. Record the final reading and meter ID.