Cooling curves-

Cooling curves are very useful in depicting some of the important features of the extend of superheat, and this superheat indicates the total time available for the liquid metal to fill the cavity before solidification begins. The higher the superheat, the more is the time available for pouring.

Solidification occurs in two stages-

Nucleation and grain growth. Once a stable nucleus is formed in the liquid metal, the second phase of solidification, that is, grain growth begins by acquiring atoms from the liquid metal. Nucleation generally starts on the mould walls since the extraction of heat is maximum at the walls. In the next stage of solidification grain growth starts in a direction opposite to the direction of heat transfer, that is, the liquid metal begins to freeze on to the thin skin and the thickness of solidified metal begins to freeze on to the skin and the thickness of solidified metal begins to grow towards the centre. The grain growth rate depends on the cooling rate and only those grains which have a favourable orientation in the cooling direction grow.