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Up Topic Welding Industry / General Welding Discussion / Info on root pass TIG on stainless
- - By thetraininglink (*) Date 12-15-2008 15:44
I'm looking for advice on joint preparation and back purging for welding the root pass with TIG on stainless.  What kind of nose and root gap dimensions are you using?  I most often hear of a knife edge on the nose and equal to the filler rod diameter for the root gap, usually 3/32".  Any other ideas?  Also, is there any standard formulas or techniques for back purging?

Thanks for your help in advance!
Parent - By robgreg1978 (**) Date 12-15-2008 18:31
i do not use any landing. i try to use as less gap as possible when tigging any alloy. if your gap closses up i just take a metabo and open it up alittle bit to get your wire through. I always put some tape around most of the pipe open up about a 1/4 at a time. weld the bottom half first it will help keep the purge up ontop when welding top half. purging i would just stick an open ended hose with a rag taped to it to help dispurse the argon,then tape up both ends of pipe. fell over the gap that is to be welded when a cool draft is comming through it should be good. the weld should be shiney, if it is gray or black it will be good but you need a better purge. if it is sugaring you will notice right away there is something wrong with the purge
Parent - - By Shooter71 (**) Date 12-15-2008 18:33
I run knife edge and depending on the pipe 5/32 to 3/16 gap with 5/32 to 1/8 wire as it closes up. Back feed the wire just washing it off the end. the gap looks scary but it will close up. 3/32 will slam shut and then you're screwed. You want the wire to melt and form a tongue as it flows to the bevels. it everything is right it will be smooth and shiny on the inside.
Parent - - By ZCat (***) Date 12-15-2008 19:24
For the purge, I always like it high enough that I can still feel it coming out the bleeder hole when I pull the tape off the spot where I'm gonna weld.
Parent - - By sqiggy (**) Date 12-15-2008 20:01
Be careful with a 3/32 gap. Some places will not allow you to use a grinder on the test and it will close up on you.
First thing you want to do, find out what the QC wants. Then go from there.
Second, everything depends on the size and thickness of the pipe being welded. Normally, I like to start off with a 3/16 gap using 1/8 filler wire. By the time you weld up half of it, you will have to go with a 3/32 filler wire or I turn the heat up a bit and stay with 1/8 and let it just fall in.
On the purge, I like to use a gas lense at one in, tape it up real good and tape the other end completely and then poke a couple of holes in the tape. As you close up your joint, the excess purge will escape out of those holes and not thru the root bead.
Parent - - By zackhodge (*) Date 12-15-2008 20:54
what about tacking it up how do you keep the gas in?
Parent - By OBrien (***) Date 12-15-2008 21:57
Carefully lol.  No if its big enough and you got someone who is willing to help you out have them hold the purge hose on the inside of the pipe facing where your going to tack with the argon flowing.  If not tape it up get it into position start the purge wait then pull the tape off where your going to tack.   Thats just how I do it.
Parent - By robgreg1978 (**) Date 12-15-2008 22:40
you can use bridge tacks. don't tie in to the landing. when you sart welding, weld up to within 1/2 inch or so of the bridge tack then cut the bridge tack out and keep on welding you wont need a purge to do this. the tacks that is.
Parent - - By sqiggy (**) Date 12-15-2008 22:45
If you have a fab table or something simular, most have a piece of angle iron welded to it. I lay my joint in it to make sure I get a good fit. If so, I tear off some maskin tape long enough to go completly around my joint. Lay tape down, sticky side up, then lay my pipe on it, set my gap, bring the remainder of the tape on around, then peal back just what I need for a tack.
Parent - - By pypLynr (***) Date 12-16-2008 00:33
I always use bridges . That way it eliminates any sugar possibilities. If you are doing big bore - I put together a screen. I take a hose connection and a nut that will fit and a old hole saw blade. I stuff the blade with steel wool and hold it in place with a piece of TIG wire as a pin . I mount the connection in the center of my end cap and attach the blade on the inside and the hose on the outside. Kick the 'airgon' on and wait a bit .
Parent - - By thetraininglink (*) Date 12-16-2008 06:37
What kind of average cfh flow rates for back purging? 
Parent - By pypLynr (***) Date 12-16-2008 13:04
On the fill, I usually kick it up to 25-30 . And for the weld , I drop it to 15 - then on the close , I drop it to 8-10 .
Parent - - By Kix (****) Date 12-16-2008 13:21
Like rob said you can use bridge tacks.  They come in real handy for the test you will have to take an no clamp is provided.  They can come in handy out in the field as well sometimes, but usually you will have dogs or an ultraclamp to hold the pipe in place while you tape it off and get a purge going.  When I feel the purge is going good, I'll hold a lighter up to the vent hole and depending on how the flame goes out will tell you when all the air is vented.  Only do this if no one is looking. ;-)  Usually they will have o2 meters on hand if the contractor is worth a sh!t.  K, now back to the pipe.  Tack the pipe up, remove the clamp, and go to a burnin and and a earnin.
Parent - - By thetraininglink (*) Date 12-16-2008 16:56
What do you guys usually use for cfh flow rate for the purging on the inside of the pipe?
Parent - - By Kix (****) Date 12-16-2008 17:40
Kind of depends on the size of the pipe, how big a vent you have, and how much pipe you have to purge.  So you usually start out high anywhere from 10cfh for small pipes to 40 and 50 cfh fpr big pipes.  Like said above I like to put some kind of diffuser on the end of the purge hose so it's not turbulent inside the pipe.  When you have the pipe purged, you back it down till you can feel a bit of flow coming out of the root opening on your cheek or finger.  As you put your root in and close the gap you can back it down even more.  If the flow is to much, as you close up the pipe, it will push your root out and in some cases blow it out at the tie in.  When you're getting close to closing up your root you will be able to feel the air pissin out of the open root very rapidly if your cfh is to high.  5-10 CFH should be good when you're closing it up.  Always vent at the highest location and quarter your pipe so you finish the root at the top of the pipe if it's in the 5g or 6g.
Parent - By js55 (*****) Date 12-16-2008 18:02
With bigger pipe I like to use multiple hoses. This way you can turn down the flow and still maintain volume getting the gas to lay gently on the bottom, minimizing turbulence, and push the air out of the top.
Up Topic Welding Industry / General Welding Discussion / Info on root pass TIG on stainless

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