Cooling Curves

Quick
cooling curve - shows how the temperature of a material changes with time.


Details
The liquid is poured into a mold at the pouring temperature. The difference between the pouring temperature and the freezing temperature is the superheat. The liquid cools as the specific heat is extracted by the mold until the liquid reaches the freezing temperature. The slope of the cooling curve before solidification begins is the cooling rate, ΔT / Δt.

Cooling curve for the solidification of a pure material

If effective heterogeneous nuclei are present in the liquid, solidification begins at the freezing temperature. A thermal arrest, or plateau, is produced because of the evolution of the latent heat of fusion. The latent heat keeps the remaining liquid at the freezing temperature until all of the liquid has solidified and no more heat can be evolved. Growth under these conditions is planar. The total solidification time of the casting is the time required to remove both the specific heat of the superheated liquid and the latent heat of fusion. Measured from the time of pouring until solidifcation is complete, this time is given by Chvorinov's rule. The local solidification time is the time required to remove only the latent heat of fusion at a particular location in the casting; it is measured from when solidification begins until solidification is completed.

The figure shows a eutectic binary phase diagram to the right, and its corresponding cooling curve for the specific composition indicated by the dashed line in the eutectic diagram. There is a corresponding cooling curve for any composition in the eutectic phase diagram. The cooling curve for a hypoeutectic alloy is a composite of those for solid solution alloys and "straight" eutectic alloys. A change in slope occurs at the liquidus as primary α begins to form. Evolution of the latent heat of fusion slows the cooling rate as the solid α grows. When the alloy cools to the eutectic temperature, a thermal arrest is produced as the eutectic reaction proceeds. The solidification sequence is similar in a hypereutectic alloy.