The heat from the welding shrinks the metal as it cools in the heat affected zone. You will have to stretch the material out by hammer and dollying the welds. What kind of welder did you use? If MIG, the welds will most likely have to be ground down. MIG welds are hard and can't take a ton of hammering, they will crack. Plus, grinding them down helps in flattening them out. I usually use a cutoff wheel on a highspeed air motor. I have lots more control on the amount of material being removed. So, I would stretch the weld area first before trying to shrink anything with a torch and water, ice or dry ice. Good luck.