Fusion welding stainless steel

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At least I think that is what it is called? When  you butt two pieces of thin stainless steel (piping) together and just fuse it together using a tig torch. A friend of mine has been welding for a few years on lots of import cars. Mainly intercooler pipes and SS exhaust kind of stuff. Every now and then I will help him get caught up on some stuff. However when I go to weld stainless tubing together, I usually fuse it together with no filler rod (16-18 gauge). I was under the impression that this is fine. He insists that I use filler rod when doing it. That's fine, it is his shop, I'll do it his way BUT... Is it wrong to just fuse it together? Is it actually stronger to use filler rod?
Reply:Fusion welding is a commonly accepted term for joining metals without added filler material, but a more technical term for this is "autogeneous" welding.  Joining thin (< 1/8") SS autogeneously is very common with orbital TIG machines on tubing in the semiconductor, dairy, brewery, etc., industries.  With excellent square edge, tight fitting joints, an autogenous weld is the way to go.  Here is an old thread on the subject.  http://weldingweb.com/vbb/showthread.php...=tig+stainless
Reply:Thank you pulser. That is what I was thinking. autogeneous pronounced auto-jen-ee-us? He said he has had some pieces that he has actually had to repair. Customer brought back a cracked weld. I'm willing to bet it is because the SS was never back purged though, causing oxidizing on the back side of the weld.
Reply:Originally Posted by DiabolicZThank you pulser. That is what I was thinking. autogeneous pronounced auto-jen-ee-us? He said he has had some pieces that he has actually had to repair. Customer brought back a cracked weld. I'm willing to bet it is because the SS was never back purged though, causing oxidizing on the back side of the weld.
Reply:Originally Posted by dave powelsonWhy don't you LOOK at some fusion welds on thin SS?-undercut is typical--crater cracks more likely than not-the lack any decently faired fillet makes for weak welds, made worse by the lack of filler-hand fusion welders typically slam in too much heat-no technique, no decent welder parameters, making big time melt thruBackpurging won't fix any of the above
Reply:Originally Posted by DiabolicZWhen I made my stainless turbo manifolds, I fused them all together with no filler rod. After than I went over every fused weld and added filler rod from there.
Reply:Originally Posted by sn0border88What on earth was the purpose of doing that?
Reply:Double heating of the same area isn't going to be a good thing for stainless.Miller Syncrowave 350Millermatic 252/ 30A spoolgunMiller Bobcat 225g w/ 3545 spoolgunLincoln PowerArc4000Lincoln 175 Mig  Lincoln 135 Mig Everlast 250EX TigCentury ac/dc 230 amp stickVictor O/AHypertherm 1000 plasma
Reply:Aw-taj-in-usTwo turn tables and a microphone.
Reply:Originally Posted by DougAustinTXDouble heating of the same area isn't going to be a good thing for stainless.
Reply:Originally Posted by DiabolicZCan you explain why? I ask because I do not know. That is why I joined a welding forum. Any monkey can go out and melt two pieces together (as I have). But I would like to eventually know the majority of what is the right way to do something and the wrong way.
Reply:Originally Posted by DiabolicZCan you explain why? I ask because I do not know. That is why I joined a welding forum. Any monkey can go out and melt two pieces together (as I have). But I would like to eventually know the majority of what is the right way to do something and the wrong way.
Reply:I can't think of anyplace where bead reinforcement would be called for with SS, you're still stuck with the area at the weld edge being an achilles heel in the weldment.Matt
Reply:Ok, so what I think I have gathered is this for the next time I do this. 1. Autogeneous welding is hard to actually do correctly by hand. If you cannot do it correctly, then it is better to just add the filler rod. Filler rod should be a grade higher than the stainless you are working with correct?2. Weld the stainless with as little heat as you can get away with as to not increase your HAZDave, that blue and straw coloring that you mentioned... It is better to have the yellow straw coloring right? Basically, you want the chrome, not the carbon. Blue is too much?
Reply:Originally Posted by dave powelsonThe heat tinting in SS shows this separation with carbon (blue)and chrome (yellow/straw).
Reply:As said heat tint/ the pretty colours are the result of surface oxidation, the 'thickened' oxide layer refracts light differently. Heat tint in of itself doesn't mean a great deal as shielding quality will effect things i.e. can cook the crap out of a stainless weld and get no colour if the shielding is excellent (trailing cup/inert chamber). 'Colour' can be a big deal for certain critical applications as it effects corrosion resistance which is why critical parts are pickled and/or passivated after fabrication. An exhaust manifold on the other hand, entire thing will be covered in surface oxides the moment the engine is run in anger...Carbide precipitation can't be seen by the naked eye. The connection with weld 'colour' is because of the temp range that carbide precipitation occurs in. Unfortunatly everyone seems to forget that a source of carbon is needed to react with the Cr...1. the low carbon grades of stainless don't have enough carbon in them for it to be a big issue during welding2. The 'stabilised' grades contain elements that have a higher affinity for carbon than chromium does (say Ti in 321 for example) i.e. you get Ti carbides instead of chromium carbidesA graph charting the times needed for harmful levels of precipitation to form versus C content... http://www.staff.ncl.ac.uk/s.j.bull/mmm373/WELDMAT/sld016.htmAnother damaging mechanism that's related to temp is the transformation of delta ferrite to 'sigma phase'  above 550'C which is very brittle. This is a huge deal with the duplex alloys as sigma phase can form in only a few minutes. Can take thousand of hours with the more common austenitic alloys such as 304 as there's very little delta ferrite- welds initially solidify as delta ferrite and transform mostly to austenite as they coolMulti pass welds are made on stainless alloys every day. Who's going to weld a sch80 butt in one pass  Granted an initial autogenous run is at best a waste of time but there are things like paying attention to interpass temperature to help avoid overheating thingsA exhaust manifold operates WITHIN the temp range that carbide precipitation occurs
Reply:Thank you for wording that much better than I did.It escapes me at the moment, but as far as the formation of precipitates goes, how does 347 react?  I know it is stabilized with tantalum and colmbium, and for some reason I seem to recall that it will not form any precipitates under any circumstances.Have we all gone mad?
Reply:I'm not sure although we're heading into areas that make my head hurt. I believe that columbium works better than Ti with regards to stabilisation and IIRC 347 is more resistant to the loss of columbium during welding than 321 is to the loss of Ti. As to whether it's completely immune or just 'better' than 321 in this regard i don't know
Reply:Originally Posted by sn0border88Wow... Really? Seriously? Color or "Temper" bands on steels are not determined by some kind of band of alloy running beneath them.  It is representative of the temperature of that part of the material at the time it was exposed to oxygen.  Other factors can come into play, such as surface finish, material grade, and time exposed.Color Formed Approx Temperature Cpale yellow 290straw yellow 340dark yellow 370brown 390purple brown 420dark purple 450blue 540dark blue 600 The temperatures for carbon steels are slightly lower for the same color.
Reply:Autogeneous welding of stainless exhaust pipes is O.K.   Bottom line, strength is not an issue.  Those that don't backpurge quickly find out that adding enough heat to use 1/16 filler on a 16 gauge tube leads to ugly gray welds.  If they could see the backside they would have quit welding long ago.  If you fussion weld, with low enough heat and good enough fitup, the welds maintain the straw color, at the expense of full penetration.  The cover pass with filler is just for looks.  You can sand it down and blend/polish the pipes afterwards.As for required strength.  Everytime I put 4 tacks into an exhaust pipe I'm always amazed on how tough it is to break them.  We are talking extremely low pressure and only one side of the pipe is constrained - the other side has rubber bushings of sorts.Its a false economy though - You can get dual flowmeter for about $150 (Smith).  Everytime I buy stainless it seems to be $200 - $300 for 20ft (I think the low quantity online pirates want closer to $30/ft).  That regulator pays for itself quickly.  Oh, and argon is $28 per 80 right now.  Bottom line, its a false economy to autogeneous weld.  And somewhere around 18-20 gauge (or thinner) its pretty much useless.   Very difficult to keep the heat low enough that you don't melt all the way through - and you run the risk of gaps opening up due to distortion in the base metals.Con Fuse!Miller Dynasty 350Millermatic 350P-Spoolmatic 30AMiller Multimatic 200Hypertherm PowerMax 1000G3Miller Maxstar 200DX