Purge welding 321ss
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  1. #1
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    Default Purge welding 321ss

    When I weld header tubes together I have always purge welded them. I have some caps made up with a hole in one for the purge hose and the other with a grid of .050 holes. I set the purge flow meter to 8 wait a minute reduce flow to 3 and weld. For no reason I got to thinking about what is going on inside the tube. Does the air and argon mix and eventually become all argon as more argon flows in and the excess flows out the vented cap or does it stay separate? Argon on bottom/ air on top? Air on bottom Argon on top? What say you all?

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    Quote Originally Posted by moonlight machine View Post
    When I weld header tubes together I have always purge welded them. I have some caps made up with a hole in one for the purge hose and the other with a grid of .050 holes. I set the purge flow meter to 8 wait a minute reduce flow to 3 and weld. For no reason I got to thinking about what is going on inside the tube. Does the air and argon mix and eventually become all argon as more argon flows in and the excess flows out the vented cap or does it stay separate? Argon on bottom/ air on top? Air on bottom Argon on top? What say you all?
    argon is heavier than the oxygen rich atmosphere inside the tube so it pushes everything out but the Ar, or at least that's how i have always understood it

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    Quote Originally Posted by vanguard machine View Post
    argon is heavier than the oxygen rich atmosphere inside the tube so it pushes everything out but the Ar, or at least that's how i have always understood it
    That is the way I was taught as well. My thought is it like looking at a glass jar with oil and water, clearly a line where the oil lays on top of the water. If you could see the the argon and air is that what it would be like?

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    Ar and Air mix, the densities are different but not that different.
    Ar is 1.74 g/L and N is 1.25 g/L (O2 is 1.42g/L), pretty close together...
    You need to displace 7-10 times the internal volume to get the mix lower in O2. Sometimes more depending on what your part is.
    Semiconductor/medical orbital tube welds are down to part per million if not lower for a good weld. Parts for someones fancy exhaust on some overdone lawnmower, maybe not so much...
    O2 meters are a good thing if you are that worried about purge levels.
    316l-stainless-coupons.jpg

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    gasses do not "settle" like liquids and segregate by density due to the effect of diffusion. the molecules are always in constant energetic motion. a classic example is sulfur hexafloride in air won't stay segregated by gravity, even though it is over 6 grms per liter!

    oil and water don't mix to begin with, so nothing at all like Argon in air.

    having said that, the last fuel tank I welded (100 gal. diesel) I purged for about 20-30 min. with the Ar coming in low and slow (non turbulent as possible), the air- purge exiting out the top. might as well use the density difference for what it can accomplish, just don't expect much...

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    Quote Originally Posted by PlasmaOnTheBrain View Post
    Ar and Air mix, the densities are different but not that different.
    Ar is 1.74 g/L and N is 1.25 g/L (O2 is 1.42g/L), pretty close together...
    You need to displace 7-10 times the internal volume to get the mix lower in O2. Sometimes more depending on what your part is.
    Semiconductor/medical orbital tube welds are down to part per million if not lower for a good weld. Parts for someones fancy exhaust on some overdone lawnmower, maybe not so much...
    O2 meters are a good thing if you are that worried about purge levels.
    316l-stainless-coupons.jpg
    Overdone lawnmower???? A bit pompous are we?

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    A lot of the mixing that happens is due to turbulent flow too, using a large air silencer or similar on the argon tube reduces this, so it crates more of a fill displacing the air better, same approach a tig gas lens benefits from. Small high pressure jet will cause more turbulence and create more mixing hence you need more purge to hit the same numbers.

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    No, I was in a rush and couldn't think of another wingit that would need purged stainless piping... Coulda used wingit I suppose
    Most of the parts I make are thin stainless sheet and piping. They have to work for 8-12 hours and are buried deep in concrete never to be seen or heard of again...
    A lot of the purge welds involve just enough argon to keep things from getting "sugar" inside, that nasty grey spiky crap oxide, they could be grey otherwise and have no effect on service. I'll hook up the purge with Al foil/foil tape and 10 psi and wander down to the bathroom to take a leak, ready to go when I get back.
    I have had to work on stuff that needed super shiny internal welds as well... It all depends on end application and customer specification.
    I have a print article from the AWS journal (that I can't find online)that was all about purging parts. It was full of fun stuff about total gas volumes of the internal part, traps and internal features that can prevent purging, flow rates and mixed gases... ect ect... how far down the rabbit hole do you need to go?
    Sorry if I sounded like an ass, twas not my intent. 312 sounds like fun to weld, kinda jealous...

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    Oh... and the other part of my problem is that I'm not a car guy...
    I read the post in a rush and headers translated to parts for big steam boilers, not exhaust systems for cars... my bad.

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    [QUOTE=PlasmaOnTheBrain;2943331
    Most of the parts I make are thin stainless sheet and piping. They have to work for 8-12 hours and are buried deep in concrete never to be seen or heard of again... [/QUOTE]

    Hell I will bite, why bother with stainless, hot glue and cardboard tubes with a waxed inside would see that service life out. Must be for the Nuke industry there the only idiots with funds to burn for service applications like that :-(

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    you nailed it, nuclear garbage cans.

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    ^ Can't say im surprised, whilst were not a preferred supplier or approved by any means because were local we still do odd non safety critical bits for the local nukes once in a while. Its just a completely different nuts world. So much of its so over kill, then they fuck up badly on the basics like nearly letting the used fuel pool run dry (was leaking through the wash room bellow :-S) Some of the stories the old timers tell of the place are out right frigging scary too.

    Probably not fair for all but when it comes to nuke engineers, i have a feeling its a bit like the teaching profession, thoes that can't get a job in the commercial world go there.

    My all time favourites was they have the typical security pass they wear around there neck. Then some one had the bright idea they would also do extra safety cards they could also wear on the same set-up. 3 chaps came in a couple of years ago and they damn near had a 2" stack of safety and procedure cards on the strap around there necks. Can only assume they got to the point the weight was enough that some ones head fell off so they stopped doing it.

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  17. #13
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    Yeah... nuke people can be pretty special at times. Some of the "take 2 steps this way and you die" stories give a glimpse into why they go so overkill, but then they do things like call out for a 3/8" fillet weld on a 16gauge tank...


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