Recrystallization nucleation
The formation of strain-free crystallites in heavily deformed material through thermally activated boundary migration, thereby lowering the internal energy of the lattice.
Recrystallization happens when you heat heavily deformed or cold-worked material enough. New, stress-free grains form in the structure, making the material softer and tougher again. In welding, this is especially important in the heat-affected zone, where the welding heat anneals out the deformation from the base material. If you cool too quickly, the structure stays hard and becomes brittle. If you overdo the heat or time, the grain coarsens and the material loses toughness. So make sure cooling after welding isn't too fast, then the structure can settle properly and the weld won't be so prone to cracking.