CV welding auto exhaust

Got a question for the real welders from an old farm boy who owns a welding machine and pretends to weld sometimes.
It was time for a new exhaust system on my GMC Suburban, so I purchased the
parts including two catalytic convertors, 4 O2 sensors, new muffler, and new tailpipe. The original system was welded together at the factory, and had to be cut into two pieces to get it out. The new system went back in fine, except that the tailpipe and muffler had a joint which needed to be clamped or welded.
To convince myself that I could weld the pipe with my 100A Sears MIG using flux core, I took the old piece that I'd cut apart with a sawzall and welded it back together (Butt weld). The pipe thickness was about 1/16" (.065) so I figured that I'd need about 3/4 of the 100 amp welders capacity. Now the Sears welder doesn't have amperage settings, so I set it to 3/4 of full voltage and a wire speed of ~1/2 of full speed. This has worked well in the past, and in fact worked beautifully on the old pipe. I got a beautiful flat weld with full penetration, and no holes.
Piece of cake, so far. BTW, no shielding gas, just flux core wire.
The new tailpipe slipped over a smaller pipe at the output of the muffler, and they fit tightly, so I hit it with the flapper disk to remove the anti-corrosion coating and started welding. This time I couldn't get the weld to lay down flat at all, and ended up with an ugly (but functional) bead. At least it sealed the pipe, but I wouldn't want to show it to anyone. In fact, I'm tempted to grind it down and re-weld it even though its under the truck and real awkward to get to.
I'm thinking that the second weld didn't have enough current, but I don't want to blow through the thin pipe and compound my problems. The first weld was a butt weld, and the second weld was an overlapping seam.
What did I do wrong?
TIA,
Don W.
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Well, its been two days and no one has replied, so I guess I'll grind down the weld and just experiment. I was kinda hoping that Ernie, or Randy Z, or someone would jump in and tell me something like "Oh for a lap weld versus a butt weld you need 1.25x the current, so either up the voltage or crank up the wire speed"
Been thinking about buying a couple of pipes to practice on cuz it sure is a pain welding underneath the truck.
Don W.
Don W wrote:

<snip>
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Don, It sounds like maybe the exhaust system is stainless steel or aluminized steel. I know the automotive industry uses 409 stainless for cat converters.

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Its not stainless. It does have some kind of passivation coating (could be a paint) that keeps it from rusting, but that comes right off with the flapper disk revealing the bright steel underneath. Also, note that I was able to weld it, but I did it late at night, in an uncomfortable position underneath the truck. What had been a piece of cake weld on the old exhaust tubing turned into a mother on the new system, and I wasn't at all pleased with the result. It did seal, though, as shown when I shoved a rag up the tailpipe with the engine running.
I was planning on grinding it down today and experimenting with the welding setup, but ended up going out to the airport to look at an airplane a guy was trying to sell. That was a lot more fun than laying under the truck in 98F weather with a grinder blowing sparks in my face ;-).
Don W.
MES wrote:

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Don W wrote:

Miller sells some nice little welding calculators (slide chart) for stick, MIA and TIG, it's about $5 for the full set. You can order them from their site http://www.millerwelds.com
Pete C.
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I also suspected a stainless alloy in the 400 series. I have had bad experience with MIG on exhaust systems. A tractor muffler comes to mind. I kept getting brittle fractures next to the bead. It was not good for customer relations when the farmer kept coming back and the shop was not equipped for TIG.     If you sealed the thing I would leave well enough alone. Someone who welds exhaust regularly might have more input. 400 alloys are magnetic to some degree unlike 300. Randy
Don W wrote:

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I'll try a magnet on it. The parts were factory OEM and should have been the same metal as the originals that I took out which welded fine.
I could also hook up the argon tank and try doing it dual-shield. Guess I'll just experiment with it. I can always grind it down and do it over unless I start blowing big holes in the tubing. I don't have CO2, Helium, or mix available unless I make a run into town.
The butt weld that I made in the old tubing as a practice weld would have made an inspector happy. The new overlapping weld--less so ;-)
Thanks for your thoughts Randy,
Don W.
Randy Zimmerman wrote:

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