Pad welding Reheater Tubes without Argon

[Gem1]

Hi All,

I'm hoping that someone might be able to point me to some information about pad welding re-heater tubes. The standard route is to Argon flush the tubes for pad welding, but we are trying to explore the possibility of doing the pad welding without argon, as long as there is no burn through to the tube's ID. The tubes are made from Super 304H and the damage (grinding damage) on the OD is shallow, but the walls are only 3.2mm.

Has anyone got experience with pad welding reheater tubes with/without Argon?

Thanks,
Gemma

 

[metengr]

Yes. In my years of working Power generation and pad welding waterwall tubes, superheater tubes, reheater tubes there is no need for backing gas or purging the ID surface with Argon because you should not be pad welding over holes. Pad welding should be done on tube surfaces where enough material is present to avoid burn-through. You are welding with backing.

 

[Gem1]

Thanks metengr, those are my thoughts exactly. The repair in this case is just shallow grinding marks from 'uncontrolled' grinding to remove shields on neighbouring tubes. However, i can find very little information in papers or on the net to support this position, was hoping to use it to give the station more confidence.

 

[metengr]

Gem1;
Present it to site personnel this way…. backing gas or purging of the ID surface is only required for open root butt welds, where the weld metal is exposed to both the OD and ID surfaces during welding of the root pass. In pad welding with wall thickness that prevents burn-through all you need is shielding gas because the weld metal is deposited over base material (which prevents exposure to the ID surface).

 

[Kiwi2671]

Gemma,
If possible try replicating the repairs to prove you can pad weld without purge - use an offcut of pipe if available and put some notches in with a grinder. 3.2 mm is pretty thin WT and if you reduce that thickness even further with a notch it is possible you may get oxidation inside the tube.

metengr,
In all my years as a pipewelder and then inspector I have never seen a WPS for S/S that allows you to turn the purge off until after the second weld pass has been completed.
Is that not the same situation here ? - you purging to ensure no oxidation on the ID surface possibly due to localised overheating.

Regards,
Kiwi

 

[metengr]

Kiwi;
Your statement is correct if either welding on thin austenitic stainless steel material or completion of a hot pass for an open root butt weld. In the above case, there is wall thickness to weld on and no purge is needed to protect the weld or even base metal. I can't tell you how many times we have pad welded eeroded stainless steel reheater, superheater and other tubes with no issues and no back purging. Even if you had some surface oxidation no problem, a reheater tube is exposed to gas side temperatures well above 1500 deg F with steam on the ID. I see no issues.