tig welding 4" dia. 316L pipe elbows

By Niekie3 (***)

Date 12-04-2002 18:01

All that I can add is that you must make sure that your back purge is done correctly.

Regards
Niekie Jooste

By QA Ron Date 12-11-2002 17:25

You must also take into account that is autogenous welding (welding without filler) and will require a PQR and WPS. I don't know the service, but any time you weld without filler the HAZ will be sensitized and will be susceptible to corrosion attack.

Hope this helps
QA Ron

By Niekie3 (***)

Date 12-12-2002 18:11

Hi QA Ron

I am not sure why you state that the weld will be autogenous? At any rate, the mere fact that you are performing an autogenous weld, will not result in sensitization of the HAZ. The only difference in structure will be in the weld metal itself, because 316 filler metals would typically have a slightly different composition than the base material. This is to ensure that there is some ferrite in the weld, and that the loss of alloy elements during welding is compensated for.

Thin walled tubing is often welded autogenously without any significant detramental effects. Thicker walled materials are however seldom welded autogenously, for the reasons mentioned by some of the other respondents.

Just a thought: Does it make sense to perform a two-pass autogenous weld?

Regards
Niekie Jooste

By TimGary (****)

Date 12-04-2002 22:23

If your piping is to be ASME Code compliant, I think that if you were to make the root pass by simply fusing with a tig torch, you would most likely wind up with too much internal concavity to be code compliant. Using filler metal during your root pass would help prevent the concavity.
Also, if you need to fill out and cap the bevel with only two GTAW passes, your elbows would have to be pretty thin, like sch 10 or maybe 20 for instance.
Even on sch 10 material, I would be hesitant to try to fill and cap the bevel with only one pass after fusing the root because the heat input required to lay down that thick of a pass would probably cause burn through problems.
Of course I may be wrong and you may have a talented enough welder to pull it off. I suggest that you try it out on a couple of disposable elbows first.
Tim

By jwright650 (*****)

Date 12-05-2002 14:10

I agree with Tim on the practice run with some extra elbows. Cheap insurance to make sure the joint will be acceptable in the inside as well as the cap.
Good Luck, Let us know how it goes, so we can archive the results you have.- for future reference
John Wright

By Weldmedic (*)

Date 12-11-2002 18:58

I think that your customer might be requiring 2 passes minimum for pressure retaining welds. This is a common requirement in many specifications and I think is also a requirement of some of the more common welding codes. I dont think that their intention was to limit your field weld to two passes. Autogenous welding of the root portion of these welds will result in internal root concavity which will affect the section thickness of the weldment. You can either do a an open root with a purge and fillermetal or use a consumable insert and purge. What welding code is governing the work and what is the schedule/thickness of the elbows being assembled?

Steve