Cracked tubing repair (TIG)

[sarge338]

Good Evening,

I am an amateur TIG welder, so my experience is limited. I have some thin walled tubing (mild steel) that has a crack. I have no way of cleaning the back side of the tubing. Without being able to properly clean the back side, how do I TIG weld this crack without contamination/porosity? I have no idea how to do this without the weld ending up messed up from back side contamination.

Thanks for your help!!

[MAC702]

I've only heard of people really caring about this on stainless steels, but I do very little TIG welding myself so don't know how much difference it makes on mild steel. Are you able to back-purge the tubing with an inert gas? How big is the crack? Maybe you won't even have significant overpenetration to even worry about.

[ryanjones2150]

Sucking in nasties from the back and ending up with trouble during your weld is a real possibility. Your filler metal has stuff in it to help with that, but sometimes it isn’t enough. That’s why you keep some 309 stainless filler on hand. If your weld puddle goes to bubbling like a witches’ brew, dab a little 309 into the weld puddle and watch it smooth right out. You can even just weld the whole thing out with 309 and it’ll probably be fine. What’s the part on?

[MMW]

Another option is to pulse it. Just use the pedal to increase until you dab then back off almost all the way but still keep the arc. Kind of like lots of tacks.

309 is good option also as well as back purging. Really all depends on how critical it is?

[jjohn76]

Funny thing, one of the long time welders at work was talking about using 309 filler to calm down a puddle - he might have said he has used it on code work from time to time - just made sure not to let the CWI see the filler, and made sure to check for 309 filler once he became a CWI and super.

[Sberry]

Whats it on? Grill or a nuke?

[sarge338]

Thanks everyone! This is nothing critical. It is a table support in an RV. I’m less concerned about it’s structural integrity, but more concerned about the aesthetic when it’s done… since it will be visible to everyone hanging out in the RV. I do have 309, so I may give that a shot since I do not have a means of cleaning the inside of the tube. I’ll also give pulsing a shot to see if that helps, as well.

As for the question about how big the crack is, I’d say it’s around 4-5”. I think someone decided to sit on the end of the table.

[MAC702]

...As for the question about how big the crack is, I’d say it’s around 4-5”...

I should have been specific. I was wanting to know the width of the gap you are trying to fill to see how much exposure to the inside of the tube you'll have.

I don't see an issue since the appearance of the inside doesn't matter, and perfect structural strength isn't needed like a pressure pipe or roll cage. Any gap big enough to be an issue is going to let in shielding gas.

[Electric4Life]

309 for the win....I rarely use 70s2 anymore for making repairs or fabricating everyday weldments in mild steel...I even repaired a set of chromoly lower control arms on a TT using 309 (non heat treated of course)
I am a firm believer in running it (309) on any mild steel application less the ones that require the filler be to code.
Common use is for welding 304 to mild steel as well