How do you weld steel without warping flanges?

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So I was messing around with some exhaust piping and decided to weld some 3" steel to a flange.Well, I heated it up pretty good and the edges of the flange started to pull back some. So now the flange isn't really flat.How do you prevent this/fix this?
Reply:Welding will always warp the parts being welded.  What process are you using?  To fix this, try running smaller rod/wire diameters and do multiple passes.  Weld in shorter passes and alternate around the flange.  Allow time for it to cool every couple of beads.  Clamp everything down before welding.
Reply:I like to bolt or clamp the flange to a much thicker piece of plate prior to weldling.  A thick slab of alluminum works great.
Reply:That is an excellent method.
Reply:Well, here is the situation. There was a flange welded to a pipe. The pipe went throught the flange at a slight angle. This created a barrier for airflow. It's hard to explain. Normally you would be able to look straight through a flange. However, this one had a pipe at an angle so the edges blocked air.I attempted to weld the inside of the flange to get rid of these edges and it messed up the flange itself. I wasn't welding on the pipe side, I was welding on the open side. So I don't think I would have been able to clamp it to anything. Oh well. Lesson learned. Thanks for the tips.
Reply:I do a lot of custom headers and I try to get as many junk heads for whatever bolt pattern or flange Im doing and I bolt down the new flanges to the junk heads and weld from there It usually keeps them perfect I always just run them on a belt sander just to ensure theyre absolutely flat and will seal properly