Recrystallization Nucleus
A locally strain-free crystal region that originates and grows within a work-hardened microstructure during heating. It serves to restore ductile material properties.
Recrystallization is what happens when you take a cold-worked material – one that has become hard and brittle from bending or forming – and make it soft again. When heated, new undeformed grains form in the structure, making the material ductile again. In welding, this is why the heat-affected zone becomes softer than the surrounding cold material. But if you get too hot or stay on it too long, you can also coarsen the grain, and then the part becomes brittle again – you don't want that. In practice, let the weld and the zone next to it cool slowly and evenly, and you'll get a clean, tough structure. If you're in a hurry and cool it with water or compressed air, you risk hardening cracks or an unnecessarily brittle structure.