TIG multipass Vs. single pass

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Hi, I am new to this forum so i figured i would post a question that has been bothering me for a while. It seems to me that there has been a big arguement (not on this forum) between single pass tig and multipass. the dissagreement is wether 2 or even 3 passes tig welding is really stronger than one pass. specificly dealing with role cages or chassis made from 1.75 OD .120 (or thinner) DOM or 4130 tubing. i have heard that the first pass is for a hot root pass without filler to fuse the 2 pieces together then a second pass adding filler and sometimes even a 3rd to blend everything together and the weld ends up looking like a weave. on the other hand i have heard that a single pass on material that thin is sufficent and with 2 or more passes the HAZ becomes a issue. i have talked to a friend who is a blacksmith currently and has been tig welding longer than i have been alive and has worked with many aerospace companies and he says that a single pass is the best process. but then why does it seem that the majority of offroad race chassis are welded with multiple passes?
Reply:Offroad race chassis welded as multipass?  Name one.  There isn't a single drag car, circle track car, trophy truck, baja car, etc I know of that welds tubing with mutliple passes on a single joint.Whoever is telling you to dry wash/fuse your base materials without filler first, or those telling you that multipass welds are required in .120 wall or thinner tubing, well, I suggest you keep them as far away from your vehicle as possible, because they don't have a f*cking clue, to put it nicely.
Reply:robby gordon's trophy truck and camburg's new Trophy truck just to name a few and there are quite a few moreLast edited by krawlercj7; 02-09-2009 at 04:48 PM.
Reply:There's a lot of variables that affect the answer to your question...In the case of offroad rollcages made with Chro-moly alloy steel, the less heat you put into the metal the better off you'll be.  The heat of welding induces changes in the HAZ that result in a reduction of yield strength.  The more heat you push into the material, the large and more potentially damaging these changes will be.  That is, unless you can heat-treat the entire structure after welding.  Since this is usually impractical, 1 pass, properly made, is best.I can only speculate why welders would use 2 or 3 passes.  In the case you described, where the first pass is autogenous, without filler metal, I can't see why you'd do this unless the welder was concerned about incomplete penetration to the root in a zero gap joint.I could see using 2 passes if the welder severely undercut the toelines in the first pass and needed to repair this defect.  Undercut is a great stress concentrator, and it may be that the added deterioration of the metal properties in the HAZ is a smaller risk than a crack started at a line of undercut on the weld.  Somebody smarter than I would have to give you a concrete answer to that question...A third pass to weave over the joint is purely cosmetic.  I can't see any good reason to do this with Chrom-moly.With mild or low carbon steel, multiple passes is less of an issue.  The added heat will still cause more distortion, but properties of the material aren't as strongly influenced by surplus heat.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:.120 tube needs a .120 fillet.   That would be .204 across the flat.  Anything more is a waste or worse.David Real world weldin.  When I grow up I want to be a tig weldor.
Reply:Fusing without filler will leave your material susceptible to suckback and centerline cracking, and there is absolutely no beneftit to doing so.Minimizing the overall heat input is the key with 4130.  For governing the strength of the actual weld, your throat thickness is your main concern.  As long as you have an adquate throat thickness, increasing the size of your fillets its purely detrimental.  There is absolutely no reason that anyone who knows what they're doing cannot achieve an adequate throat size in a single pass fillet weld on .120 wall tubing.  Those who experienced failures with a single pass, were likely these self proclaimed "pro" TIG guys who put in the tiniest, pretty little hairline bead they could muster up which look great, but are grossly undersized and can result in higher hardness values in the weld from the lack of dilution with the mild filler.
Reply:Originally Posted by krawlercj7Snip.... and sometimes even a 3rd to blend everything together and the weld ends up looking like a weave.
Reply:Krawler, I followed this on RDC....there are 2 thoughts to why exactly you might do 2 pass: #1. Many, in fact most tig welds done are rather small in actual size; they are made small to minimize heat input, and more commonly, for astetics- right or wrong. If you were to compare sheer strength on many single pass tig welds that are very dainty and pretty, they actually won't have the maximum grab, not enough cross section of weld bead. In that case, a second pass, WITH FILLER, is a better option. Especially when they are adding the filler, weaving the weld, and getting the full throat of the weld bead. If they are not adding filler for the second pass, they are adding heat, without the de-oxidizing properties of the filler material....and that is double bad.A single pass with adequate filler, adequate heat, and adequate penetration is as good as the weld will EVER get. I agree with your blacksmith friend, and the guys. A second pass is fine, but for the right reasons, and done the right way.And then, after so much work...... you have it in your hand, and you look over to your side...... and the runner has run off. Leaving you holding the prize, wondering when the runner will return.
Reply:When I went to a tig welding school some 20 plus years ago we never went back over a tig weld unless there was a problem with the weld and that would be only after the problem area was cutout or ground away. The goal was to complete the weld in one pass but than again we only welded thin material.Miller Syncrowave 200
Reply:ok, this clears up alot of questions i had, i appreciate all the answers guys. thanks again
Reply:Originally Posted by A_DAB_will_doThere's a lot of variables that affect the answer to your question...In the case of offroad rollcages made with Chro-moly alloy steel, the less heat you put into the metal the better off you'll be.  The heat of welding induces changes in the HAZ that result in a reduction of yield strength.  The more heat you push into the material, the large and more potentially damaging these changes will be.  That is, unless you can heat-treat the entire structure after welding.  Since this is usually impractical, 1 pass, properly made, is best.I can only speculate why welders would use 2 or 3 passes.  In the case you described, where the first pass is autogenous, without filler metal, I can't see why you'd do this unless the welder was concerned about incomplete penetration to the root in a zero gap joint.I could see using 2 passes if the welder severely undercut the toelines in the first pass and needed to repair this defect.  Undercut is a great stress concentrator, and it may be that the added deterioration of the metal properties in the HAZ is a smaller risk than a crack started at a line of undercut on the weld.  Somebody smarter than I would have to give you a concrete answer to that question...A third pass to weave over the joint is purely cosmetic.  I can't see any good reason to do this with Chrom-moly.With mild or low carbon steel, multiple passes is less of an issue.  The added heat will still cause more distortion, but properties of the material aren't as strongly influenced by surplus heat.
Reply:Whole roll cages aren't usually heat treated.  They're too big/too expensive to heat treat in most cases.  This is why it's important to keep the Chro-moly welds as low heat as reasonable.  The HAZ around the weld does not have the same strength as the rest of the tube or weld joint.  The greater the heat input, the larger the HAZ, the greater the likelihood of a problem.... Originally Posted by uninformedsome of the suspension parts do get heat treated after welding the get final machining... but i would not have thought the whole chassis was treated...Serg
Reply:If you can make a proper crown with full pen then knock yourself out. If your worried about heat there is heat gels and sprays that pull the heat out. I blow a root in it, then cap it off. alot of ways to skin a cat ppl.
Reply:Originally Posted by A_DAB_will_doThere's a lot of variables that affect the answer to your question...In the case of offroad rollcages made with Chro-moly alloy steel, the less heat you put into the metal the better off you'll be.  The heat of welding induces changes in the HAZ that result in a reduction of yield strength.  The more heat you push into the material, the large and more potentially damaging these changes will be.  That is, unless you can heat-treat the entire structure after welding.  Since this is usually impractical, 1 pass, properly made, is best.I can only speculate why welders would use 2 or 3 passes.  In the case you described, where the first pass is autogenous, without filler metal, I can't see why you'd do this unless the welder was concerned about incomplete penetration to the root in a zero gap joint.I could see using 2 passes if the welder severely undercut the toelines in the first pass and needed to repair this defect.  Undercut is a great stress concentrator, and it may be that the added deterioration of the metal properties in the HAZ is a smaller risk than a crack started at a line of undercut on the weld.  Somebody smarter than I would have to give you a concrete answer to that question...A third pass to weave over the joint is purely cosmetic.  I can't see any good reason to do this with Chrom-moly.With mild or low carbon steel, multiple passes is less of an issue.  The added heat will still cause more distortion, but properties of the material aren't as strongly influenced by surplus heat.
Reply:Originally Posted by krawlercj7Hi, I am new to this forum so i figured i would post a question that has been bothering me for a while. It seems to me that there has been a big arguement (not on this forum) between single pass tig and multipass. the dissagreement is wether 2 or even 3 passes tig welding is really stronger than one pass. specificly dealing with role cages or chassis made from 1.75 OD .120 (or thinner) DOM or 4130 tubing. i have heard that the first pass is for a hot root pass without filler to fuse the 2 pieces together then a second pass adding filler and sometimes even a 3rd to blend everything together and the weld ends up looking like a weave. on the other hand i have heard that a single pass on material that thin is sufficent and with 2 or more passes the HAZ becomes a issue. i have talked to a friend who is a blacksmith currently and has been tig welding longer than i have been alive and has worked with many aerospace companies and he says that a single pass is the best process. but then why does it seem that the majority of offroad race chassis are welded with multiple passes?
Reply:Originally Posted by Fat BastardI have a question: in the years before the electric welding became the norm most if not all aircraft with 4130 space frames were welded using O/A no? The total heat input from this process O/A would be much greater and the HAZ would be less abrupt, in my opinion. The O/A process would  if not input more total heat energy the amount would be less defined, no? I would think it is less about the total heat but more about the concentration and application of that heat, and the rate of cooling.   The procedure I have always followed,when using GTAW,  make the weld and then to wrap it so as to allow it to cool more slowly. The time to room temperature is increased many times over (7-10 times) If it took several passes to make the joint (poor fit or many pipes joining at one point like the tail section of many small air craft) then some cooling time with wrapping would be employed between welds.  When using O/A process, the joint was made and no effort to control cooling was employed. (save, keeping it from quenching)In the end when in doubt prove the joint (weld) you intend on using is sufficient for the stresses you intend inflicting on it. TEST YOUR WELDS