Leaking welds on small tubes... precision tig welding help needed

[USAeng]

Hey guys,
We are welding some small 316 SS tubes into some 1018 carbon steel with 309L Tig wire... there are 4 pictures attached to show the welds... the welds have to hold pressure and most are leaking. It appears most are leaking on the area between the tubes as the welder is having a hard time welding inside that area... also some of the welds are cracking. Anyone have any advice? See pictures... Thanks for any help!

 

[USAeng]

another pic... only lets me put 1 pic in each post I guess...

 

[USAeng]

another pic

 

[USAeng]

Last pic

 

[StrykerTECH]

Could you attach a pic of what it looks like in your fixture, prior to welding? Also is this entire assembly closed? Or does it have open end to keep pressure from building up from heat.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[USAeng]

to pressure test we close off both ends that you see in the pictures... then we put a few pounds of air in just to see if there is any leaks... then we put 200lb of oil pressure... the air is leaking so we dont bother with the oil until the air has passed.

He is just tig welding around the outsides of the tubes and then trying to get in between them...

 

[stanweld]

You've created a real accessibility problem for your welder. Have you considered brazing? or other nonconventional welding processes?

 

[Duwe6]

Concur with stanweld: High-temp braze with a "silver solder" like BAg-7 would solve the weldability problem, and will withstand a hydro test in 10,000 psig range. There will be a pretty large surface area [braze] to internal cross-section [pressure] ratio on something this small.

Other alternative is to get the *best* Tube-Welder from the Boilermakers.

 

[Kiwi2671]

USAeng,
Are you purging the s/s tubes ?
The reason I ask about the purge is you may be able to perform a very small autogenous weld first to "seal" the joint and then any additional weld put in with filler metal is purely reinforcement.
Similar procedure to socket welding with SMAW. Put a small, hot E6010 to "seal" the joint, grind and then one run with an E7018.
Weld similar to how the boiler tube welders do to get into the tight areas - extend the tungsten and weld as far as you possibly can from one side and then hit the slope down button (this negates trying to tail off when the ceramic is jammed between two tubes)Repeat the procedure from the other side and hopefully the two halves will overlap and you will have a nicely sealed joint.
Are the tubes sitting flush with the end of the 1018 or are they recessed into a hole in the 1018 ?
Hope that helps,
Regards,
Kiwi

 

[berkshire]

If you are not already doing so, use a gas lense on the tig torch. In still air conditions, this will allow the welder to extend his tungsten out far enough, to get in, between the tubes to complete his weld.
B.E.

 

[StrykerTECH]

May I suggest a slight design change. I would create a plate that has two holes in it, then weld from the other side. then weld that plate on to the other piece. That will allow you to get in between easier.
See attached pic.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[unclesyd]

I like the tubesheet approach posted by StrykerTECH

Some of the problems shown in your pictures are the presence of cold laps probably caused by "daub welding". with filler rod that is too big. I also see too much weld metal being deposited.

In addition to the advice given in the posts above I would use a Micro-Tig welding process if you have very many pieces to fabricate.

http://www.arc-zone.com/index.php?main_page=index&cPath=13_18_39

 

[USAeng]

I like the micro torch...

I did note that the tube thickness is only .049". This would cool very rapidly compared to the thicker solid pieces on each end and in the middle... would this cause centerline cracking? That seems to be the biggest problem causing the leaking. Thanks a lot.

 

[MikeHalloran]

You could machine the end pieces so they have integral nipples, tapered on the outside, so you then have to make two thin wall butt welds...



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

One caveat in using Micro-Tig is that you have to maintain the electrode shape.
Here two suppliers of Micro-Tig welding wire.

I would look at 312 SS welding wire.

http://www.novatechinc.com/default.html

http://www.microweldingproducts.com/stainless_steel_alloys.html