Too much heat?

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A few months ago I was lucky enough to acquire a never been used sync 250dx for next to nothing. I had never tig welded before, so I wanted to read as much as I could about the process before I started wasting time and money by fumbling around blindly. After months of reading on this and other forums as well as miller and lincoln literature I set up the sync and started practicing on some 1/8" hot rolled mild steel coupons. After a few days and about an hour of arc time I can finally put down a consistant looking fillet weld. Now I feel like I need some assistance. Are the welds supposed to be gray and dull on mild steel? Or are they supposed to be shiny like stainless? I test welded a couple pieces of 14 gauge 304 stainless with 1/16" tungsten and filler(308L) and after a couple of tries I figured out how keep the weld a shiny goldish color with some blues. I can't get the mild steel welds to stay shiny. The tack welds are nice and shiny however. That leads me to believe I am putting to much heat in to the piece. Also, when I go to weld the other side of the t joint it turns out like crap. It looks like there might be some scale developing during my previous weld. Is that normal? Are you supposed to be able to weld the other side without sanding it again? I know that wouldn't work on stainless without back purging but mild steel? Maybe my coupons are too small and there is no place for the heat to go? My coupons are on top of a huge aluminum heat sink when I am welding but that doesn't seem to change anything. I am dipping the filler in less than 1 second intervals. Is that too slow? Here are some pics. My starts and stops really suck. I haven't figure that out yet either. The craters were much more easily avoided on stainless. The first two are the front side of the t-joint and the last one is of the back side. Any help will be greatly appreciated. Thanks guys.oh yeah...settings....DCENAmp dial set to 140Foot pedal pressed about 75% sorunning maybe 100-120 amps?3/32 - 2% Thoriated3/32 ER70S-2 rod wiped with acetone#7 cup w/ gas lenseArgon @ 14cfh1/8" thick 3"x3" coupons mill scale removed by flap disk and wiped with acetone.Kory Attached Images
Reply:So I just read http://weldingweb.com/vbb/showthread.php?t=45697and I guess that answered some of my questions. I am definitely not letting the piece cool to room temperature before welding the other side. That can't be a good thing. Kory
Reply:The front side weld looks good, maybe a little undercut on the top toe.  There must be too much oxidation from that front weld that has screwed up the flow of the back side.
Reply:So is the oxidation on the other side common and I just need to let the part cool then reclean it? Or am I going too slow or something? Thanks for your time.Kory
Reply:From what I can see, the weld looks good, not too hot or too cold, good flat face, not convex or concave, and the toes are nicely wet into the base metal.  If the backside oxidizes then you need to clean it before welding.  The oxidation is dependent on the size of the front weld, and in turn the current and travel speed to produce that weld.Whether this weld is too hot or not depends on what the requirements for the weld are.  I know this is just practice, but suppose you where building this for a customer and working to a drawing that specified a weld exactly this size.  In this case you did it correctly to spec. and it would be difficult to make that size weld and not oxidize the backside.  If it were specified to weld both sides, then you'd have to figure out if removing the oxidation on the backside will work, or do you need to shield the backside.  Sometimes a copper or aluminum backup bar can be clamped on the backside of a joint like this and it will help avoid overheating and oxidizing the backside.Last edited by pulser; 10-19-2010 at 07:26 PM.
Reply:To the right it looks like you've got some SLAG CHASERS going on. In which some kind of crap on the metal jumps up to the top of the pool and floats around chasing your puddle and generally mucking up everything, and when you look at the weld afterwards it's got these weird lines radiating towards the edge of the dimes. Are you at least wirebrushing the fitting thoroughly before you weld?
Reply:Thanks pulser. I didn't really notice any oxidation on the other side until the arc started, and that is why I didn't bother to hit it with the flap disk again. I won't make that mistake again.