Recrystallization nucleation
The formation of new, strain-free crystallites within a cold-worked microstructure at elevated temperature to relieve stored energy.
Cold-formed metal is internally stressed and hardened. When you heat it strongly during welding, the structure can reform. This is called recrystallization. The material becomes softer and tougher again, and the hardness from cold forming is lost. Keep this in mind: When welding cold-formed sheet metal, you lose exactly the strength in the heat-affected zone that you gained from forming. Grain size and properties depend heavily on how hot the spot gets and how long it stays hot. In practice, after welding cold-formed parts, check whether the component strength is still adequate. If not, only heat treatment helps, or you must design the part thicker. And if you don't properly straighten the cold area before welding, stress relief during heating can cause distortion.