Crystallization nucleation rate

The nucleation rate determines how quickly new crystals form as the melt cools. It depends strongly on the cooling rate and ultimately determines the grain size and microstructure of the weld metal. If you cool too fast, the microstructure becomes hard and brittle, which can quickly lead to cracks. If you cool too slowly, you get coarse grains and the material becomes soft and tough. So you have to control the cooling so that the microstructure is right. For higher-strength steels, this means carefully controlling the cooling time in the critical range to avoid entering the dangerous hardening range.