I've only heard of one person that was able to do it with any success. He heated the piece red hot and used a series of presses and custom made pieces that kept the piece from being stressed too much and cracking. I think he then let the pieces cool very slowly. He only described the process, so I never saw the finished work and he's no longer on the forum here. I've never heard of anyone else who was able to remove any warp from cast iron.