Welcome to ModelEngineMaker !If you have problems registering or logging in, please use the contact menu option to request assistance.
I would strongly suggest grinding the corners of the square off and get as close as you can to the desired circle before trying to turn it. Actually, I would have pressure chucked it...
That's interesting how much warpage you had. Do you think some of that might have been from applying heat from one side? I remember cutting 1/8" steel with a torch when I rebuilt a steel Commercial Salmon Troller I owned. It warped like crazy. I ended up cutting the rest of it with a Sawzall using metal cutting blades.