When welding stainless steel tubes, the outside of the seam is only half the story – the root inside the tube needs protection from oxygen, otherwise heat tint forms and corrosion resistance collapses. That is what backing (root purging) is for: flooding the tube interior with shielding gas before and during welding.
Why heat tint is more than looks
Stainless steel owes its resistance to the chromium oxide passive layer. From about 250 °C, under the influence of oxygen, this layer grows uncontrolled and depletes the chromium beneath – visible as heat tint from straw yellow through brown to blue. The darker the colour, the stronger the chromium depletion and the more susceptible the zone to pitting corrosion. In pharma and semiconductor plants the rule is therefore usually: root metallically bright, at most light straw yellow.
The residual oxygen question
Practical rules of thumb: at 50 ppm O₂ the root of most austenitic steels stays free of heat tint, below 20 ppm you are on the safe side, semiconductor and pharma specifications often demand ≤ 10 ppm. Above 200–500 ppm, clear discolouration appears. Measure with a residual oxygen meter at the outlet – not the inlet, because what counts is what actually arrives at the weld.
Purge time: calculate, don't guess
From air (209,000 ppm O₂) down to 20 ppm takes around 9.3 volume exchanges – for a DN50 tube over 10 m that is roughly 20 litres of purge volume. At 10 l/min of backing gas that means: purge for nearly 20 minutes before striking the first arc. Start after 5 minutes and you are welding at several thousand ppm. The Purge Time Calculator for Gas Lines computes this for any tube geometry; for checking moisture in the backing gas, the Dew Point to ppm Calculator (Moisture in Gases) helps.
Which backing gas?
Argon 4.6 is the all-rounder – heavier than air, fills from below. Nitrogen-hydrogen mixtures (90/10) are cheaper and the hydrogen acts reducing, but they are unsuitable for ferritic and duplex steels (hydrogen embrittlement) and for titanium. Pure nitrogen works for austenitic steels but is taboo for nitrogen-sensitive materials. When in doubt: argon.
The most common purging mistakes
- Welding too early: purge time not waited out or never calculated – the classic
- Flow too high: turbulence sucks air in through the root gap; backing means gentle flow (rule of thumb 5–12 l/min depending on cross-section) with slight overpressure, not blasting
- Switched off too early: keep purging until the seam has cooled below approx. 250 °C – otherwise heat tint forms after welding
- Leaky dams: paper and adhesive-tape dams instead of soluble dams or purge chambers let air stream back in
- O₂ measured in the wrong place: always at the outlet near the weld; only weld once the target value is stable
Conclusion
Backing is applied purge arithmetic plus patience: determine the volume, calculate the volume exchanges, measure residual oxygen, keep the gas flowing after welding. Follow these four points and you get reproducibly bright roots – and save yourself pickling or, in the worst case, cutting out the seam.
