Black on the side of the tig bead- Mtbr.com
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  1. #1
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    Black on the side of the tig bead

    It's been awhile since there's been a weld critique post...
    Any and all advice would be greatly appreciated.
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    Here's what I do:
    To clean: first i scrub with 80 grit emery cloth, then spray with simple green and scrub the tube inside and out a lot, then rinse with water, blow dry with air compressor, wipe with acetone and again blow dry. I also wipe the filler rod with the solvent before using. I used to grind the tungsten using a disc sander but i recently got a TechSouth handheld orbital grinder (it's worth every penny IMO).
    For welding: I pre-flow the argon for 1 sec at around 18cfh and the post-flow is set at 10 seconds. I use a size 7 cup and a gas lens. I also backpurge at less than 5cfh but not really sure how much of it gets to the head tube from the plug in the BB. No HT heatsink.
    So, i'm confused as to why I sometimes get this charcoal-like residue on the edge of the bead. When this happens while I'm welding, there's almost a sizzling sound - the black seems like it's burnt something...but i don't feel like I'm putting too many amps on the joint but the inside does seem like i'm getting it a tad hot?
    Black on the side of the tig bead-insideht-welds-sm.jpg
    The black line doesn't happen everywhere, just occasionally. So, it must be something I'm doing wrong. Thanks for any help!

  2. #2
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    I would say your purge isn't doing a thing.. but I wouldn't worry about that with Chromolly anyway.
    And you don't have to clean that much, just wasting time. IMHO.

    You could use a bigger gas lens if you want a shinier weld, and set up your purge better so you don't get the black on the inside, crank it up to 10 if you want. It flows just like water so block everything except what is past where you are welding so it runs past where you are welding and then out the remaining hole.


    Because Chromolly creates an oxide layer on the surface when it gets hot, (the black stuff) it protects all the metal underneath.


    But if your welding stainless, your ***ing it.

  3. #3
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    you might have too much flow (in terms of velocity) for that gas lens/cup combo at that particular area....meaning.....you could be getting turbulent air pattern and thus not enough argon coverage/mixed argon coverage.

    Try reducing you flow a few clicks.

    I had similar issues using the large gas lens and a #12 cup, reduced flow and it made a big improvement.

  4. #4
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    I think your over cleaning a little. Skip the simple green and the blow drying. I also think that 3wfab is on track with your flow rate being to high. And your wasting money back purging cro-moly, IMO. I've never seen a weld procedure in industry that called for back purging cro-moly, not sure why some frame builders think it's necessary. Judging by the inside of that paragon head tube your moving pretty slow. Remember there is no such thing as to hot, just to slow!

  5. #5
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    I've had that happen from time to time, I think it's a flow issue as well but maybe more in a coverage sense.
    Looking at the dt-ht weld going towards the tt its good coverage and pretty even heat on both sides.

    On the TT going the same direction where the black is it looks like your cup was hard against the ht pointing down and blocking the flow in that area. The turbulence from that may be sucking air into the program and leaving the soot.

    I've gotten the same sooty stuff from a backcap o-ring being cracked and pulling air and also a torch hose with a small hole in it, that's why I think air is the culprit.

  6. #6
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    Backpurging cromoly...

    -Cuts down on distortion, meaning less work reaming/facing/chasing.
    -Massively reduces oxidization/slag on the inside of the weld, meaning less work reaming/facing/chasing and less wear and tear on cutting tools
    -Makes it considerably harder to blow holes in thin walled tubes

    And it costs something on the order of $1/frame. Totally worth it, IMO, as long as you're set up for it. Is it mandatory? Of course not.

    -Walt

    Quote Originally Posted by customfab View Post
    I think your over cleaning a little. Skip the simple green and the blow drying. I also think that 3wfab is on track with your flow rate being to high. And your wasting money back purging cro-moly, IMO. I've never seen a weld procedure in industry that called for back purging cro-moly, not sure why some frame builders think it's necessary. Judging by the inside of that paragon head tube your moving pretty slow. Remember there is no such thing as to hot, just to slow!

  7. #7
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    Here is my backpurge setup:

    https://3.bp.blogspot.com/-ltcOk5Brw...0/100_0606.JPG

    It always helps to have the flow as close to the welding as possible. You can tell when the argon does its job, as the inside weld area is devoid of oxidization and even the heat distress coloring.

    Its my opinion that the inside of the weld needs to be shielded to prevent oxidization and its resulting embrittlement. This embrittlement will cause a reduced fatigue life on thin bike frame tubes, especially in highly stressed areas like the head tube area and the seat stay/ seat tube weld areas:

    Krauczu at non in Olsztyn, Poland - photo by highjump - Pinkbike.com

  8. #8
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    Looks like that green frame has some crazy undercuts too. Or did the tube wrinkle adjacent to weld?

    It's funny. Every weldor outside of the bike industry that I talk to that is familiar with what we are doing and/or works with some of the more exotic materials says you would be crazy to not back purge considering the tube thicknesses we are working with. But if you just say "4130" without explaining anything else, they'll usually say you don't really need to.

    The purged heatsinks help me keep melt-through (like you see inside the HT) to a minimum. I'm mostly just getting a little discoloration in the HT, ST, and BB now and only a few little spots of melt-through. And as Walt said, once I started back purging, the puddle was a lot more stable and I wasn't blowing holes in my test pieces.
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  9. #9
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    Thanks all for your expertise!
    It's a relief to hear I'm over-cleaning it, and I will reduce the flow to more like 15. I think the coverage issue could be a big part of it - the cup was likely angled more down towards the DT and not shielding the HT as much. A bigger cup may help but really I just need to get better at rotating the torch (and moving my head to see what's going on better) while moving around the tube. That, and moving quicker over the joint...and maybe also use a larger filler rod to cool the puddle down more as i go.

    All great recommendations, thanks again.

  10. #10
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    I'm of the opinion that you can not over-clean prior to TIG welding. But I'm new to the process and I don't want any unnecessary handicaps as I learn.
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  11. #11
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    Back Purge

    Thanks guys for the info about back purging the 4130.
    I have welded alot of Chrommolly pipe and never back purged it because the Embrittlement is only on the surface. But on thin tube the "surface" makes up quite a percentage of the wall therefore requiring protection from the atmosphere.

    Thanks guys Makes sense.

  12. #12
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    Quote Originally Posted by zank View Post
    Looks like that green frame has some crazy undercuts too. Or did the tube wrinkle adjacent to weld?

    It's funny. Every weldor outside of the bike industry that I talk to that is familiar with what we are doing and/or works with some of the more exotic materials says you would be crazy to not back purge considering the tube thicknesses we are working with. But if you just say "4130" without explaining anything else, they'll usually say you don't really need to.
    I'd like to see the AWS do an article about this. Might put some of the shade tree impressions to rest.

    Quote Originally Posted by zank View Post
    I'm of the opinion that you can not over-clean prior to TIG welding. But I'm new to the process and I don't want any unnecessary handicaps as I learn.
    Scotch bright/emery cloth, acetone, weld.

  13. #13
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    Interesting take

    I look at it this way - I do some serious work to make sure that grease/dirt/gunk isn't in/on my tubes before welding them, because all of those things are going to react with the metal when it gets hot - and the resulting slag won't be strong (or pretty).

    To me, anything getting to the weld when it's hot but a noble gas or a vacuum is just a contaminant. That means, in our atmosphere, there are a lot of contaminants, oxygen being arguably the worst. Sure, you can't see the atmosphere, but that's a pretty lame excuse for ignoring it.

    Cut up a non-backpurged frame made from thin-wall tubes and look at the back side of the weld. If the front side looked all grey and slagged like that, you'd throw a fit! Why let that happen when you can prevent it for $1 or less per frame?

    Even if the backpurge doesn't improve overall yield strength or fatigue life (and I have no evidence that it does, though it seems quite plausible), there are other salutatory effects that I've mentioned before - harder to damage fragile bb threads, distort head tubes, blow holes in thin tubes, etc.

    So worst case scenario, the backpurge does nothing for the frame and saves you some hassle and wear on your cutting tools, and you're out a few bucks. There is no plausible scenario where the presence of argon is going to hurt anything - and really no reason not to do it except laziness.

    -Walt

    Quote Originally Posted by customfab View Post
    I'd like to see the AWS do an article about this. Might put some of the shade tree impressions to rest.

    Scotch bright/emery cloth, acetone, weld.

  14. #14
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    I think it may have something to do with the simplegreen. I would just prep with a 3m pad or similar. Try running a bead on some scrap with and without cleansers. It dosent look like enough of a containment to cause a problem with the weld though.

  15. #15
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    I did some more welding this week and found that the upper part of some welds still turned black - so it was still happening even with lowering the flow to around 15cfh, using a size 8 cup, and getting a good backpurge. I also made damn sure the torch was NOT directed more at one tube or another. But I still am pretty sure it's a coverage issue.

    I saw this from Drew on the Framebuilder's Google group and I think it may one of the main reasons I'm getting this result. I tend not to weld with the torch facing down all the time:

    "The other trick to tungsten stick out is you ALWAYS have the torch point
    down (since Argon is heavier than air) and kick the pedal away from the work
    and allow for a few seconds of pre flow that will fill the area with argon.
    This is how I get some rather long stick out for welding the underside seat
    stays. Anytime you want more tungsten just hit the pedal and allow for lots
    of pre flow. I hit the pedal and allow for a pre flow beyond the setting on
    the machine about 90% of the time but with the seat stays I wait even
    longer."

  16. #16
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    Quote Originally Posted by Hudnut View Post
    I think it may have something to do with the simplegreen. I would just prep with a 3m pad or similar. Try running a bead on some scrap with and without cleansers. It dosent look like enough of a containment to cause a problem with the weld though.
    +1

    The only times I have ever gotten this carbon stuff is when I neglect to mechanically clean my dropouts enough. (there is oil residue that burns off leaving carbon)

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