Metal can warp in heat treatment. One way to prevent this
warping is to cool the part slowly and uniformly (bury it in ashes and wait a day).
Conversely, if you heat the steel and cool it NONuniformly, it can work the
metal according to a plan. The key
is to aim the cooling so that the warp goes the way you want.
A warmed piece of hard metal, if cooled by an ice cube, water spray, or CO2 extinguisher,
will shrink on the cold side and the internal strain will cause the other side to take
on a slight bend, concave toward the cold spot. After complete cooling, and maybe
a few light hammer taps, the strains are relieved and the new shape is stable.
It isn't necessary to soften the steel, just get it to circa tempering heat. Many
a frypan has had water applied, made creaking noises, and taken on a warp,
all at canola-oil-not-yet-burning temperatures.
Here's a video showing the procedure
***correction, here's the heat-straightening video*** I goofed the first time...