Dislocation Climb

Dislocations are lattice defects in the metal. If they get a lot of heat during welding and point defects join in, they can start to climb instead of glide. This strains the structure and makes the material prone to cracking. Especially when the weld cools slowly, the structure has time to transform in a way that makes it brittle. So you want to get the heat out quickly so the dislocations don't have a chance to get tangled. But you don't need to worry about the exact cooling time; the welding process takes care of that.