Purge Gas GTAW penetration question

[rustbuster]

We had a lot of difficulty with our final tie-in welds with concave root and some cracking with Inco 117 filler on 304H and a needle purge on a project.

We are attempting to weld up some test pieces of 800H with 117 (Needle purge) in our shop and discovered that this too resulted in significant concavity without backing gas for the hot pass.

Our second attempt was with a full purge (Argon) and even with a hotter hot pass we were successful.

My question is how would maintaining a purge during the hot pass positively affect the root vs. no purge during the hot pass.

 

[Ron]

The old timers called it "suck back". More prevalent with SMAW. Purging with argon, in continuous purge, allows hot argon to moderate the cooling of the weld and there's less "suck back"

 

[qcrobert]

I agree with Ron, however maintaining a continuous purge during the second pass (hot pass) does not guarantee that you wont suck back the root pass.

I have been in situations where it is not possible to maintain a purge during the hot pass. In these cases you need to turn down the amps of the second pass so that the root pass is not in a molten (boiling) state during the second pass.

Always maintain an interior purge during hot pass if at all posible.

Robert

 

[Duwe6]

Additionally, the major purpose of the ID purge was to prevent the oxidation of the ID side of the root. Without a purge during the hot-pass, significant oxidation does occur. If your welders are required to complete the root and hot-pass before they can leave the weld, this all becomes academic. 1 purge for tacks & root & hotpass, cheaper and ultimately faster. Faster due to the time required to establish a good purge every time it has to be reestablished.