I'm almost embarrassed that I don't get this, except I'm an old, uneducated, dumb welder and I've looked way more stupid than this on many occasions. I've used the "draw" to straighten things for years and the other day I was showing a college kid helper in my shop how to straighten a piece of metal warped from a weld. He ask me why that worked and I couldn't tell him, at least the truth (although that generally doesn't stop me). OK, so the heated metal is expanding. The cooler metal is restricting it. At the "yield point", the heated metal is still expanding. If nothing is lost, does it thicken?
The main reason parts bend after you weld them is primarily due to reduction of stress or stress relief in the parts. That is why they warp. Nothing is lost. Matter cannot be created or destroyed.
What you are doing is relieving the stresses that were put into the metal when it was made at the steel mill. When the steel is processed you can have residule tensile stresses praimary at the surface, or compressive, or neutral. If you have a part which has tensile stresses on the surface and you weld one side you put the stresses on that side you weld to neutral. The stresses on the other side of the part are still there and that is what causes the part to warp.
Hope this helps
kam
It does thicken. You can confirm this with a caliper by measuring the thickness of a piece of carbon steel before performing your "flame straightening" technique, and measuring after. One of the best examples of this the Seattle Space Needle, those long curved structural shapes were not rolled, they used "flame straightening" to put the curve in.
Regards, Brad
I think an esy way to explain it is that "all" materials expand when heat is applied and contract when it is taken away. Not only metals. This goes down way further than the grain structure. It occurs at the molecular, and even down to the subatomic level or the atoms themselves. The molecules expand when heated and contract when cooled. This accounts for the shrinkage. The metal doesn't go anywhere. The molecules simply become tighter. That gives the metal its refined grain structure.
Thanks Brad, and all of you. I've got it.