Nucleation rate

Nucleation determines how fine or coarse the structure turns out after welding. If you cool too fast, the structure becomes hard and brittle, then the weld will crack. If you cool too slowly, you get coarse grain and the material becomes tough but soft. So you have to control the cooling to get a medium, fine structure that is neither too hard nor too soft. Adjust the heat input and cooling rate to the material, then it'll be fine.