1. Why Welds Decide the Performance of an Electropolished System
An electropolished tube arrives with a smooth, chromium-enriched bore. Every orbital weld replaces a narrow band of that surface with as-solidified weld metal and a heat-affected zone (HAZ) whose passive film has been destroyed. The bead is not electropolished afterwards. In a WFI loop or bioprocess line with hundreds of welds, the welds, not the tube, set the system's resistance to rouge, pitting and microbial attachment.
Three things decide weld quality: the oxygen content of the internal purge, the metallurgy of the HAZ, and the geometry of the bead. Each can be controlled and verified.
2. Process: Autogenous Orbital GTAW
Thin-wall hygienic tube, typically up to 0.065 in (1.65 mm) wall and often up to 0.109 in (2.77 mm), is joined by autogenous orbital gas tungsten arc welding: a single pass, no filler metal, with the arc rotating around a fixed tube in an enclosed weld head. Ends are prepared square, not bevelled, by facing so the two tubes butt with no gap and no misalignment. Square, burr-free, clean ends are a precondition for consistent fusion.
Heavier-wall pipe for process and utility service is different: it is bevelled and welded in multiple passes with filler, and pre-bevelled buttweld fittings are the right components there. Do not mix the two approaches: a bevelled fitting cannot be welded autogenously to sanitary tube. For hygienic systems use orbital weld fittings with extended tangents that fit the weld head and keep the weld away from the fitting radius.
Weld schedules are developed per size, wall and heat lot, then locked: pulsed current, travel speed and arc gap are qualified on coupons and recorded. ASME BPE expects sample welds (coupons) at the start of each shift and whenever a parameter, operator or heat changes, so that the production weld can be compared with a known-good reference.
3. Inert Backing Purge: Argon Purity and Oxygen Control
Heat tint is an oxide formed on the hot root by oxygen and moisture in the purge. Its colour runs from pale straw through gold, blue and purple to grey as oxide thickness increases. Beneath a heavy tint the surface is depleted in chromium, because chromium oxidises preferentially, so the metal immediately under the oxide has less chromium than the base alloy and a weaker passive film. That is where rouge and pitting start.
Purge practice that keeps the root bright:
- Gas: high-purity argon, commonly specified at 99.996% or better, with 99.998% widely used for high-purity work. Check that hoses and fittings do not admit air: a perfect cylinder is wasted on a leaking polymer hose.
- Measure, do not estimate: fit an oxygen analyser at the purge exit and strike the arc only when the reading is stable at the target. For product-contact welds in electropolished systems, many fabricators work to single-digit or low double-digit ppm oxygen; the ASME BPE acceptance itself is written in terms of permitted discolouration, so qualify the ppm level that achieves it with your process.
- Volume and time: displace several volumes of the purged section before welding; long spools need purge dams to reduce the volume.
- Pressure: keep internal purge pressure low and steady. Excess pressure pushes the molten root outward and leaves a concave ID; too little lets air in at the joint.
- Moisture: wet gas lines produce tint as surely as air; purge new lines thoroughly before use.
AWS D18.1 and D18.2 provide colour references for heat tint on austenitic stainless steel, and ASME BPE uses discolouration acceptance criteria for product-contact welds. In practice, high-purity systems accept only the lightest straw colour on the ID.
Need tube and fittings matched for orbital welding?
We supply electropolished tube with square-cut ends, heat-number traceability on every length and orbital weld fittings to ASME BPE dimensions.
4. HAZ Metallurgy: Avoiding Chromium Carbide Precipitation
Austenitic stainless steel held between roughly 425 °C and 870 °C allows carbon to combine with chromium at grain boundaries as chromium carbide. The adjacent zone is depleted in chromium and becomes susceptible to intergranular corrosion: sensitisation. A weld's HAZ inevitably passes through that range; the question is whether it stays there long enough for carbides to form.
- Use L grades. 304L and 316L are limited to 0.030% carbon maximum, which slows carbide precipitation enough that a normal weld thermal cycle does not sensitise the HAZ.
- Keep heat input low. Pulsed current and adequate travel speed limit the time at temperature. Autogenous single-pass welds on thin wall are inherently low heat input.
- Control interpass temperature on multi-pass pipe welds, commonly at or below about 150 °C, so later passes do not hold earlier HAZs in the sensitisation range.
- Control ferrite where relevant. Filler-metal welds on thicker pipe are designed for a small amount of delta ferrite to resist hot cracking; autogenous welds on thin tube take their composition from the base metal, which is one more reason to buy tube with controlled chemistry.
5. Sulphur Matching and Weld Pool Behaviour
Sulphur, even at a few hundredths of a percent, changes the surface tension of molten stainless steel and therefore how the weld pool flows. Higher sulphur produces a deeper, narrower pool; lower sulphur a wider, shallower one. When two tubes of different sulphur content are welded, the pool shifts toward the lower-sulphur side and penetration becomes uneven, sometimes incomplete on one side of the root.
ASME BPE therefore restricts sulphur in 316L tube and fittings to a controlled band, typically 0.005% to 0.017%, so that welds made with one schedule penetrate consistently across heats. Specify that range when you buy ASME BPE electropolished tubes and matching fittings, and keep heat records so the welding team can see which heats meet in each joint.
6. Bead Geometry and Internal Ra Continuity
The weld bead cannot match the Ra of the electropolished tube on either side of it; it has an as-solidified surface. What can be controlled is how smoothly the bead blends with the bore. ASME BPE sets limits for ID concavity and convexity, bead width variation, meander, misalignment and lack of penetration for product-contact welds. A bead that is fully penetrated, flush and uniform, with only light straw tint, drains and cleans almost like the tube itself. A bead with concavity, excess convexity or misalignment creates a step that holds product, CIP solution and micro-organisms.
Misalignment is controlled before welding: tube ovality and wall tolerance decide how well two ends match. Buying welded electropolished tube or seamless tube with consistent OD and wall from one source simplifies fit-up and reduces rejected welds.
7. Post-Weld Borescope Inspection and Documentation
Every product-contact weld that cannot be seen directly is examined with a borescope or video probe. The project specification states the percentage, often 100% for critical systems or a statistical sample with coupon comparison for others. Acceptance compares the weld with ASME BPE criteria and the shift coupons: full penetration, bead width and alignment within limits, no cracks, porosity or undercut, and discolouration no worse than the agreed reference.
A defensible weld record includes a weld map with a unique number for each weld, the operator, the weld schedule, the heat numbers of both components, the purge oxygen reading, the borescope image or video reference and the inspector's acceptance. That record, together with the material certificates, is what the validation team reviews during installation qualification.
8. After Welding: Cleaning and Passivation
Field welds cannot be electropolished. After installation the system is cleaned and chemically passivated to restore the passive film on welds and any handling marks, following ASTM A380 and A967 and the owner's procedure. Heavily tinted welds are cut out and remade, not passivated over: passivation does not remove heat tint or the chromium-depleted layer beneath it. The differences between passivation, pickling and electropolishing are covered in our article on passivation versus electropolishing.
Procurement: Tube and Fittings Prepared for Orbital Welding
Sakshi Forge supplies electropolished tube and orbital weld fittings from Mumbai with heat-number traceability on every length, EN 10204 3.1 certificates, PMI records and measured Ra reports, so your weld map can reference every heat that meets in a joint. Send your sizes, wall thicknesses, sulphur requirements and quantities for a quotation; dispatch and transit are confirmed in writing.