Root Oxidation On Closure Joint of Stainless Steel

Why not use a flanged joint for your final connection ?

it is to be used for the He gas service which may contain tritium.

Since it is Helium and helium is a decent shielding gas, perhaps if the flow could be throttled sufficiently low, (or intermittent puffs as small as possible small as possible, of course stopping welding for each puff so your arc doesn't get blown around), you could just purge with the service gas...

...of course it would still need to be purged with clean gas when everything is done.

If you can't set up dams there are a few other options.

.

1. Using backing flux and fluxed filler. The downside if you can't access the interior post weld, is the flux chips off and contaminates what is in the pipe. Also the spaces created in the chipped flux can cause problems.

.

2. Backing ring insert. With a reasonable fit, an acceptable weld is easy to achieve. The problem with this is the inherent crevice.

.

3. Finesse with local purging and techniques to minimizes necessary time and heat applied. This includes varying local purge gas temperature and application to minimize O2 content locally inside the pipe. Also pulsed GTAW, should be used to minimized the heat necessary for complete penetration as well as reducing time required to complete root pass.

.

4. RDM. Regulated Metal Deposition. This process uses a venturi effect to draw shield gas smoothly to the back side of the weld pool. Miller markets some technology using this process specifically for welding stainless pipe without a back purge.

.

Hope that helps.

.

Thankyou for valuable infirmation, I was doing it by local purging. I will also work on the 4th method suggested by you.