Basic butt weld question...

Recently I was modifying a dolly into a welding cart for my oxy/acetylene bottles. I was trying to extend the bottom plate to better support the bottles. It is 1/8 steel about 6" X 12". I attempted to (gas) butt weld a 1.5" piece to the 12" end of it. Of course, it did not work. The 1.5" wide piece cooled quicker that the 6" piece, causing it to crack my welds with a loud POP! I then tried small tacks spread apart 3 inches, allowing everything to cool between each tack, but it didn't help, they still popped. I was able to complete the job by welding 1 X 2" strips to the underside of the bottom plate and just do some 1/2" tack welds in between them. I presume that worked because the lap welds on these strips are stronger and put more weld in a smaller area.
My question after that long-ass set up is, it there a way to complete a butt weld that long(12") on two unequal sized pieces? The weld I did looked really nice, that is, until the POP!

not speaking from experience here, i'm a amateur, but what about heating the small piece with a propane torch while the big piece cooled? try to get them to cool down evenly.

Assuming that the welds were really burned in, ie yellow orange color, and equal on both pieces of plate, the only thing I can come up with is that the plate on the dolly was heat treated, (tempered) to keep it from bending. I would really like to see pictures of broken weld. Starting in from each side about an inch at a time should have done it. The only other question is what you are using for filler rod?

I agree whith Bonehead. The problem you described sounds more like welding disimilar metals together. Than a butt or lap weld problem.

Thanks for the replies! I'm pretty sure my welds were hot enough, probably bordering on too hot at times. I'm a total amateur and occasionally I get too much heat in everything and I lose control and have to back off. I didn't think of the possibility of the metal being heat treated or different. I suppose that is very possible. The dolly is a Chinese job and I got the 1.5" piece from Menard's. I was using a standard copper coated filler rod I got at the welding supply. I don't have it here so I don't know the number.
I don't have any pictures. But I had some reference marks on both pieces before the welding. After the welds popped, the marks were probably 3/16" misaligned. That's how I know uneven shrinkage/expansion caused the failure. It seemed like the last weld I did would be the one that held while the others popped, until I moved on and then it popped. It always left a fine hairline crack right down the center of the weld.

bonehead nailed. tempered and non-tempered metal joining will cause similar problems like you mention. try and pre-heat both with your torch and go from there.
no matter what keep doing something. Best way to learn brother.

Sounds like different steels.Anything out of China is highly suspect.

Bolting would be a better idea.