Recrystallization Threshold
The minimum degree of prior cold work and the required minimum temperature that must be exceeded for new grain formation to occur at all. Below this value, the microstructure remains stable.
Recrystallization means: the structure rebuilds itself after cold forming. For this to happen, you need two things: enough prior deformation and enough heat. If the temperature is below that, nothing happens – the grain stays distorted and the material becomes hard and brittle. In welding, this is important because the heat-affected zone is practically cold-worked and then heated again. If you don't watch out, you get a coarse, uneven grain and the part becomes prone to cracking. Rapid cooling is not always the solution – it can even worsen the structure. The right temperature depends on the material, but for normal structural steels, you don't need to treat the weld specially if you weld cleanly.