backpurging while welding

[Parker]

What do you guys use for a back purging setup? I have never done this before and would like to try it out. I was thinking of rubber stopper caps, but I'm still not sure what to do about the purge line and flow rate on the purge.

[Parker]

What do you guys use for a back purging setup? I have never done this before and would like to try it out. I was thinking of rubber stopper caps, but I'm still not sure what to do about the purge line and flow rate on the purge.

[Boston]

what is back purging? filling the tube you are welding with argon?

[Jersey Tom]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">what is back purging? filling the tube you are welding with argon? </div></BLOCKQUOTE>

Yep. I don't bother with chromoly steel. With stainless, particularly exhaust tubing, its a really good idea for final welding. Tacking you can do without if your heat control is good. I guess final welding you could do without a backpurge if your heat and penetration control was real good.

Last year we had an alumni do our exhaust at his place and he had his own purge setup. You need a 2nd tank and regulator or a regulator with a splitter, which we now have on campus so I'll probably just use that. We just tape off the ends with a small inlet and outlet and some tube that's not really anything special. Not sure about flowrate.

[drivetrainUW-Platt]

I have just used a regulator with 2 outputs and 2 guages/adjustment knobs. One goes to your torch like normal, the other just put a hose on it and run it in the tube. Masking tape is sufice for sealing the ends. It only needs 15psi like your torch does.

You could probably just Y off a single regulator if you put a valve on it.

[Erich Ohlde]

If your tube/part ends don't get particularly hot, you can use latex gloves. They work great.

[KU_Racing]

I use masking tape. Then again, I never seem to be welding very close to the end of a tube. On stainless,you can weld within an inch or so of the tape before the adhesive goes soft (if you know what you are doing)

As far as other materials go, I would not recommend using any chromoly weldments that are not backpurged.... No offense to Tom- we have gone back and forth on enough fabrication threads that I would just about trust you to weld on my car at this point- but you might want to do a little research on the effects of nitrogen contamination on the backside of a chromoly weld before you decide to not backpurge those thinwall 4130 control arms (or whatever)

[Jersey Tom]

I'll admit the ins and outs of welding are not my forte. Machining is my background, been doing it part and full time learning from a 40-year experienced trade machinist for the past 3 years now.

I made the big mistake of having someone show me how to set up the TIG last year, after having sworn to never learn how to weld, and since then I've practiced enough to be stuck welding the frame, a-arms, fuel tank, dry sump tank.. everything critical... on top of machining the hubs, uprights, yatta yatta.

My training in welding is informal at best, limited to practicing by myself and reading forums here and there. I can lay a decent looking bead in Steel, Stainless, or Aluminum with good penetration, no undercut, good bead size and without excessive heat input, but beyond that.. I don't know what's best practice. Everything I've had to weld has worked great (so far!). Wish I had the time / money for classes or a pro welder to give some feedback. Oh well.

[KU_Racing]

Tom, just so you know, I in no way meant to insult you or anything like that. I would be the first to say that while I can weld just about anything that you put in front of me, I have trouble machining bushings on a manual lathe.

The thing about welding chromoly is that the point of using chromoly in the first place is the higher yield strength and other material properties. When 4130 is welded without an inert backing, the nitrogen in the air combines with the elements that make 4130 4130 and basically turn the weld bead into an area with the properties of 1020 steel, or worse if the heat control is bad. So you can imagine what can happen if someone calculates loading and stress based on the properties of 4130, then constructs a weldment where the most highly stressed areas (usually the welded joints) have yield properties far inferior to what was planned for. Do I think someone is going to die if you make a frame with chromoly and then dont purge it when its welded?? No. Are you wasting your time by spending the extra money and fabrication difficulty on chromoly and then losing its properties when you weld it?? In my opinion, yes. The way that the higher level FSAE teams are headed these days, with lower vehicle weights and safety factors shrinking by the minute, I would guess that a lot of failures have been cause by the above scenario: designing a chromo part with a tight safety factor, then letting the QC on the produced part slack off a little.

[Drew Price]

KU,

Do you use chromoly frame and suspension tubing at Kettering? If so, do you just get creative backpurging the frame joints as best you can, or do you just use experience building the spaceframe to follow a build sequence that will allow you to keep the tubing purged correctly?

I am just trying to visualize how you would go about backpurging joints that meet closed tubing sections, like welding onto roll-hoops.

Best,
Drew

[Drew Price]

Just occured to me that you could just have an argon line stuck down the open end of a tube feeding argon in against the closed tube, filling the tube towards the open end, but what about when you weld tubing shut, attaching to both ends?

Best,
Drew

[John Stimpson]

There is no reason to backpurge 4130 if you know what you're doing.

Keep the heat down using a combination of low amperage, keeping the electrode CLOSE to the work, and by using filler rod (filler rod cools the puddle down).

DO backpurge your stainless. You can get by without it if you are only fusion welding your 304, but nearly any use of filler rod will cause overpenetration and "rock candy" carbide precipitates on the backside.

[KU_Racing]

The last frame had a removeable rear 'subframe' that was made out of 4130 tube, both round and square. Each tube was purged with a continuous flow of argon until the section was closed. Once this happens, think of it as a cell containing argon, which can be welded onto without any trouble. Tubes that were added to the closed sections were welded on one end, then backfilled with argon, then welded shut.

[Analogue]

I've been meaning to try Solar Flux on the backside of steel weldments. Before you go bashing the use of flux with TIG, do a little reading first.

I have always backpurged stainless exhaust parts and aluminum fuel tanks, but not steel spaceframes. They are already more than strong enough if they have adequate stiffness.

Purging suspension pieces would be a good idea, but can be difficult. This is where I plan to try out the flux.

As for capping ends, try using the fancy UL rated aluminum duct tape-- the stuff judges demand holes in firewalls be sealed with. It stays pretty sticky when it gets warm. If it gets too hot, you'll have to wipe off the adhesive with acetone, but that's no big deal. At least it stays put while welding.

Keith

[KU_Racing]

Keith, I have used solar flux before, and it is good stuff. Very useful when making stainless merge collectors, as the complex joints that result from hand-fabbing a merge collector can be hard to purge. Very cheap, very easy to use... I would recommend it to anyone who doesnt want to spend the extra time and money to argon purge ss or chromo pieces. One word of warning though: the flux fuses into a hard, flaky, glass-like coating on the back side of the weld bead, which will flake off over time. It is not a big deal on structural parts or normally aspirated exhausts, but I would NOT recommend it be used on the joints of a turbo header- the flux will flake off and send very small, very hard and sharp particles through the turbo.