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Backpurging SS for Exhaust Tacks


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Based on my limited research it's advisable to backpurge everything including tacks for SS like you'd do for critical operations in a nuclear plant or similar where they xray all of the welds, look for tungsten inclusions etc. For automotive applications it seems pre-turbo you'd want to backpurge everything as well to limit the chances of cracking over extreme heat cycles. Post turbo is less critical. 

 

I fully backpurged my downpipe from tacks to welding but man that took a lot of argon. 

 

For the midpipe back does it really matter so much? Specifically for tacks, I'd still be doing backpurge on the full weld-out

Edited by Dat73z
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@DuffyMahoney yeah thinking about it more as I own my argon bottle and get discounts at my local gas supplier...another $100 of argon won't break the budget when I'm already a few hundred in stainless tube. 

 

I am curious how many actually backpurge their tacks for exhaust work. Some youtube vids show most don't. I can understand why for the food industry you wouldn't want and defects or crevices in the welds at all

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18 hours ago, Dat73z said:

@DuffyMahoney yeah thinking about it more as I own my argon bottle and get discounts at my local gas supplier...another $100 of argon won't break the budget when I'm already a few hundred in stainless tube. 

 

I am curious how many actually backpurge their tacks for exhaust work. Some youtube vids show most don't. I can understand why for the food industry you wouldn't want and defects or crevices in the welds at all

It's a better weld for sure, it's easy to see if it is.  Is the weld raised? Or is it flush or sunken slightly?  I would say most wouldn't care, but any high end exhaust would be purge welded.  

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Posted (edited)
17 minutes ago, DuffyMahoney said:

It's a better weld for sure, it's easy to see if it is.  Is the weld raised? Or is it flush or sunken slightly?  I would say most wouldn't care, but any high end exhaust would be purge welded.  

 

I have been backpurging everything for the full weldouts. 

 

I would say the welds aren't underfilled, but I'm also an amateur so to be honest Im not sure. Before moving and starting back in on my car this past year I hadn't tig welded for at least 10 years...so I've been re-teaching myself how to tig on critical components of my car 😂. I was focusing more on full penetration and heat control. 

 

For the downpipe I backpurged all of the tacks and welds. For the midpipe I started tacking without backpurge and got a couple of tiny dots of sugaring which I'm still conflicted if it matters for an exhaust maybe 3 feet post-turbo. I plan to backpurge everything for the full weld outs. 

 

Edit: Good call on the weld bead profile. I took a look and some are flush, some are raised. I am somewhat consistent but not consistent enough. I think I need to focus on that as well and perhaps push more rod into the puddle as I go. 

 

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Edited by Dat73z
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  • 2 weeks later...

I tack my exhaust pieces usually laying under the car and never back purge.  It's just not practical or necessary.  I have back purged the full welds but usually use solar flux instead as it's much cheaper and easier.  No real risk in after turbo exhaust.  Pre-turbo, back purge only.   If you are building an extremely high end vehicle maybe back purge everything.

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Yeah I had some time to think about this and research more as I was waiting on filler. 

 

I'm going to backpurge the rest since I've already gone down that path but I'm not going to go crazy like the downpipe where I backpurged the tacks and let the purge run for like 10 mins until there was 0 air left in the pipe...I burned through like 1/2 of a 145cu ft tank just on that part alone. 

 

From my research some air left in the pipe is fine. There will be some discoloration like blues and whatnot but no sugaring and not critical on the exhaust post turbo

 

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You can do a few experiments on scrap pieces and it becomes pretty apparent that back-purging on tacks isn't really necessary.  As long as the fit is decent and thus the tack is pretty quick, you won't see any significant "sugaring" on the back of the tack.

 

But when fully welding all the seams, I think back-purging is critical and I never fail to do it.  Again, experiment on scrap.  You will see the porous gonad-looking growths that appear on the back of a stainless weld if you don't.  Not only are these likely places for cracks to start, but if every seam is welded in this matter, it would be hugely detrimental to exhaust flow as well.

 

There are occasional welds wherein it is either impossible or impractical to back purge.  In those places I either use Solar Flux, or clamp a piece of aluminum on the back of the joint to serve as a heat sink/argon trap.  If none of this is possible....I just grind off the sugaring when I am done welding.

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Posted (edited)

Yeah I did some test pieces over the weekend and any sugaring from the small tacks is consumed on full penetration. 

 

Really I shouldn't be penetrating through to the other side on tacks but that is mostly due to alignment (ex partially cut mandrel radii where the tube ends arent perfectly concentric so there's only 1/2-2/3rds wall thickness joint to joint at some areas) and my lack of skill. 

 

Since I'm just a hobbyist I'm still trying to find ways to save any gas especially with the way prices are nowadays. High priced gas prices to drive to get more high priced argon adds up.

 

I've been thinking perhaps for the longer sections where the welds are close to the ends of the tube I could crumple an AL ball with some gaps around the edges and shove it into the pipe to produce a smaller argon "chamber" as opposed to filling the whole pipe every time. Either way I'm about 1/2 done with my first full custom SS exhaust project now so I may as well keep experimenting and learning

 

As an aside another reason I've been going through so much gas is I've found SS can really move around a lot when welding. So most of the backpurge gas I've used is due to purging the pipe, welding a bit, then walking away to let the part cool. I've found the part doesn't move much when I do this. If I could weld an entire section in one go instead of constantly coming back and repurging then I could save a lot of gas

Edited by Dat73z
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On 4/25/2022 at 7:09 AM, Dat73z said:

Yeah I did some test pieces over the weekend and any sugaring from the small tacks is consumed on full penetration. 

 

Really I shouldn't be penetrating through to the other side on tacks but that is mostly due to alignment (ex partially cut mandrel radii where the tube ends arent perfectly concentric so there's only 1/2-2/3rds wall thickness joint to joint at some areas) and my lack of skill. 

 

Since I'm just a hobbyist I'm still trying to find ways to save any gas especially with the way prices are nowadays. High priced gas prices to drive to get more high priced argon adds up.

 

I've been thinking perhaps for the longer sections where the welds are close to the ends of the tube I could crumple an AL ball with some gaps around the edges and shove it into the pipe to produce a smaller argon "chamber" as opposed to filling the whole pipe every time. Either way I'm about 1/2 done with my first full custom SS exhaust project now so I may as well keep experimenting and learning

 

As an aside another reason I've been going through so much gas is I've found SS can really move around a lot when welding. So most of the backpurge gas I've used is due to purging the pipe, welding a bit, then walking away to let the part cool. I've found the part doesn't move much when I do this. If I could weld an entire section in one go instead of constantly coming back and repurging then I could save a lot of gas

We use tin foil over the end, then poke a few holes, let it purge/ fill for 10 or so minutes for a longer piece.  Keep the gas going while welding.  The joints should be clean and as close as possible.  No filler rod

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If any amateur welders like me were wondering in the future...what I found this past couple of days was if I shove a piece of foil into the tube and poke some holes into it I can fill just that section within a couple of minutes. 

 

You can see in this pic that the sugaring from the non-purged tacks is basically fully consumed when I penetrated through. They're still there as little black dots but that is because I was going conservative on the penetration because these pipes were not fully concentric and I was concerned about overdoing it as an amateur. On some test pieces I was able to fully dissolve the sugaring where my non xray eyes couldn't differentiate anything at least.

 

For an exhaust this is probably fine and I don't need things to be pretty just to hold. 

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On 4/25/2022 at 7:09 AM, Dat73z said:

As an aside another reason I've been going through so much gas is I've found SS can really move around a lot when welding. 

 

When welding tubing...like an exhaust...you will find that nothing moves around much if your fitup is perfect or very good.  I always face the ends of the tube on a disc sander, so that they fit together with no gap.  If you do this you will get little or no movement when you final weld it.

 

If your fitup is good you can "fusion" weld it together, with no filler, but I personally never do.  I just feel better about the weld if I add some .035" or .045" filler when final welding.  I realize a properly designed exhaust isn't under tremendous stress, and it's not like welding an airplane structure, but I always use a bit of filler regardless.  

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16 minutes ago, Ironhead said:

 

When welding tubing...like an exhaust...you will find that nothing moves around much if your fitup is perfect or very good.  I always face the ends of the tube on a disc sander, so that they fit together with no gap.  If you do this you will get little or no movement when you final weld it.

 

If your fitup is good you can "fusion" weld it together, with no filler, but I personally never do.  I just feel better about the weld if I add some .035" or .045" filler when final welding.  I realize a properly designed exhaust isn't under tremendous stress, and it's not like welding an airplane structure, but I always use a bit of filler regardless.  

So the pictures I posted earlier was a part that did move a bit...my fitup was perfect no gaps and I did 4 autogenous tacks. When I welded the vband in one go one of the tacks broke loose due to the thermal stress/strain and a 1mm gap appeared. When it fully cooled the gap shrank to something like 1/4 of a mm. I was able to retack and weld it back no problem but 1mm on that end of the tube moves the other end of the long tube a couple mm. I think I need to do like 8 or more tacks and work on my heat controk a bit more

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That is a good point. I actually pringled out one vband early on but fortunately I had a spare as I only needed one side for the turbo. I think I kept the warpage for them all to a minimum but worst case I'll get them skimmed by my local machinist.

 

I feel like I've had to re-teach myself tig fundamentals through this project but I guess there's no better way to learn than by doing

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A bit off topic but maybe someone searching someday may find useful. 

 

One thing that I've found really helpful on SS tube is just cranking up the post flow. I think I was hesitant to try it earlier as I was attempting to save gas but to my untrained eyes I've noticed even when I run further the quality of the final weld appearance seems to improve as well. 

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