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Short answer: Check the inside of the weld as carefully as the outside. In ultra-high-purity (UHP) lines, the ID bead has to be fully penetrated, smooth, consistent and free of heavy heat tint, because the gas only touches the inside. Inspect the OD bead shape, alignment and discoloration, the purge and coupon records, and traceability to the weld schedule. Use the acceptance criteria in your governing spec, such as ASME BPE or SEMI F78 and SEMI F81, or the customer's own spec.
Why orbital welds get special attention
Orbital GTAW welders weld thin-wall 316L electropolished (EP) tubing autogenously, without filler, with a closed head and an inert ID purge. The process repeats well, but only if the inputs are controlled: tube end prep, fit-up, purge gas quality, the weld schedule and the electrode condition. A bad ID can shed particles or trap moisture for the life of the tool, and nobody sees it from outside.
Inspection steps
1. Before welding, confirm the inputs.
- Square, burr-free tube ends from a facing tool.
- Matching heat numbers or material certs as your spec requires. Sulfur content differences between heats can shift the weld pool. This is why some specs control heat-to-heat matching.
- A weld schedule qualified for that diameter, wall and material.
- A sharp, correctly sized and positioned electrode.
2. Make a coupon (test weld) at the start of each shift or setup change, as your procedure requires, and cut it open to inspect the ID.
3. Inspect the OD bead.
- Even width all the way around.
- No undercut, cracks, porosity or arc strikes.
- Concavity or convexity within the spec limits.
4. Inspect the ID bead with a borescope on the production weld, or by sectioning a coupon.
- Full penetration around 360°.
- Consistent bead width with no "meander" (wandering bead).
- No suck-back (concavity), excess drop-through or lack of fusion.
- Discoloration no worse than the acceptance level in your spec. Specs often use comparison charts for heat tint.
5. Check alignment and mismatch. Tube-to-tube misalignment shows up as a step at the ID. Specs limit it.
6. Check the records. Each weld should log its weld ID, operator, machine, schedule, coupon reference and inspection result on the weld map.
Worked example
A gas-line installer welds a run of 1/4 in. 316L EP tubing between a manual valve and a VCR gland.
- The morning coupon shows full penetration, but the bead wanders slightly at the 6 o'clock position. The welder finds a slightly worn electrode tip, replaces it and re-runs the coupon. The second coupon is uniform.
- During production, the borescope on weld W-07 shows a light straw tint at the ID. The project spec's discoloration chart accepts that level, so the weld is accepted and recorded.
- Weld W-09 shows a dark blue band at the ID. The purge log shows a cylinder change just before that weld. The installer cuts out the weld, verifies purge gas quality and re-welds. The original weld is logged as rejected, with the reason.
The pattern matters: a defect almost always traces back to an input (electrode, purge, fit-up or material), not to bad luck.
Common mistakes
- Inspecting only the OD. A beautiful OD bead can sit over incomplete penetration or heavy ID oxidation.
- Skipping coupons after changing a tube size, head or cylinder.
- Starting the weld before the purge has cleared oxygen. This leads to ID discoloration.
- Poor end prep. Sawn or burred ends give gaps and inconsistent penetration.
- Mixing heats without considering the effect on penetration where your spec controls it.
- Weak traceability. No weld map, or weld IDs that don't match the drawing, makes later failure analysis guesswork.
Design-side notes
- On weldment and gas-line drawings, number every weld and show the weld map. Reviewers and QA can then check each one.
- Leave head clearance. Orbital weld heads need radial and axial space around the joint, and tight layouts sometimes force a less repeatable manual weld.
- Prefer prefabricated, qualified components (for example, tube stubs on valves) so the field welds are simpler.
Quick inspection record template
| Field | Example entry |
|---|---|
| Weld ID | W-07 (matches weld map on drawing) |
| Tube/component | 1/4 in. 316L EP, heat no. per cert |
| Welder / machine / head | ID per shop records |
| Weld schedule | Schedule ID, revision |
| Coupon reference | Coupon C-03, accepted |
| OD visual | Accept |
| ID borescope | Accept, discoloration within spec chart |
| Disposition | Accept / Reject + reason |
Keeping this record per weld makes audits and later failure analysis straightforward.
FAQ
Is some ID discoloration acceptable?
Often a light level is, but the limit comes from your governing spec or the customer's spec, usually defined with reference charts. Don't assume a limit. Read it from the spec.
Do I need a borescope for every weld?
It depends on the spec. Many UHP projects require ID inspection of production welds or a set sampling plan plus coupons.
Why autogenous, without filler?
Thin-wall tube can be fused cleanly without filler. That gives a smooth ID and avoids bringing in a different material.
What standards apply?
Common references are ASME BPE (bioprocessing, widely used for hygienic tube welds), SEMI F78 and F81 for semiconductor gas systems, and AWS D18.1 (welding of austenitic stainless steel tube and pipe in sanitary/hygienic applications) with AWS D18.2 (guide to weld discoloration levels on the inside of austenitic stainless steel tube). Your contract defines which one governs.