Thin wall tube chassis TIG welding tips?

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Hello!  I recently took up the task of welding up the chassis for my Formula SAE team at my university, and am looking for some tips on welding thin walled tubes.  The tubes range anywhere from .028" to .095" mild steel with ER70S rod.  I usually set the machine up to a little more than one amp per thou.  I've found that my biggest issue so far is heat/pedal control.  What is the proper heat affected zone for welding tubes of this size?  Whenever I show my practice welds to some of the other team members, I'll get different explanations regarding the HAZ.  Sometimes my beads will have a rainbow, blue, or even gold color on the bead itself with very little discoloration immediately around the bead.  Other times my beads will be a flat gray with discoloration extending out a quarter inch or so alongside the bead.  Some tell me the gray means I'm welding too hot, some say the rainbow indicates too much heat.  I'm really not sure at this point what I should be aiming for.  I just want my welds to be strong and dependable.  I'm still relatively new to TIG so I can't say exactly what all my settings are when I weld; I can find out details if you need more information. I'll also have pictures up as soon as possible if that will help explain better.  Sorry for the super long post
Reply:The most important part of welding thin tubes is the fit up before welding. As far as color goes, anything from gold to blue is okay and should still be shiny, dull grey means too hot and/or not enough sheilding gas. For a good example, check out post #20 by ZTFab here:  http://weldingweb.com/vbb/showthread.php...ighlight=ztfabIan TannerKawasaki KX450 and many other fine tools
Reply:Oxide colors and the HAZ are two different things.The HAZ (Heat Affected Zone) refers to the zone of the parent material that has undergone metallurgical changes due to the heating/cooling cycle. Pretty much the only way to see the exact extent of the HAZ is to cross-section and etch the weld, it is not otherwise visible on a finished weld. (On thicker material it is sometimes possible to see the HAZ after a bend test due to the HAZ deforming differently from the weld metal and unaffected base metal.)Oxide colors are what happens when bare, heated metal (in this case steel) is exposed to the atmosphere. When the oxide layer is thin it refracts different colors of light depending on it's thickness (just like a thin film of oil on water will make different colors), as it thickens, it no longer refracts but absorbs light giving it a grey to black color. On steel these colors are only on the surface and can only provide very general clues about approximately how hot that section of the metal was when it lost the coverage of the shielding gas and was exposed to the oxygen in the atmosphere. Taken out of specific context, the colors themselves are meaningless.Keep in mind that the vast majority of the heat from any weld is transfered away by the surrounding metal, relatively little is dissipated into the atmosphere. Therefore, on thin stuff the welds tend too cool relatively slowly since there isn't much thermal mass to 'suck' away the heat. Further, joint geometry and position have a large impact on how long the shielding gas covers any given part of the weld. On tube, you may be running the 'correct' amperage, speed, etc. but still get grey welds simply because you and the shielding gas from the torch are already on the other side of the tube before it has fully cooled.On steel grey welds do not always mean 'too hot', they simply means that they were still hot when they lost gas coverage. It's takes experience to determine why and if it's actually a problem, something I doubt your teammates have (especially if they think that the colors are the HAZ).Visit Tensaiteki.com
Reply:That clears things up immensely.  So I really shouldn't mind the colors too much so long as the weld is clean and has penetrated?  I've noticed the joint geometry making a very large affect on how the bead looks.  The welds I lay down in the tight corners always have a nice color to them, which I'm assuming is because the shielding gas shrouds the arc better due to the more confined space.  Is there anything I should be looking out for in particular, where say I look at a bead I just laid down, I should be thinking, "uh oh, that's not good."?  Other than the obvious stuff like holes, of course.Fortyonethirty:  Those tubes are beautiful.  Some of the tubes we get laser cut don't even fit up that nicely, haha.
Reply:color is not only predicted by heat and cover gas, but also by travel speed and material thickness. tensaiteki hit the nail on the head, color truely is meaningless!
Reply:Originally Posted by kromewldrcolor is not only predicted by heat and cover gas, but also by travel speed and material thickness. tensaiteki hit the nail on the head, color truely is meaningless!