In this article we are going to learn about cooling curve for pure iron.
What is a Cooling Curve ?
A Cooling Curve is a graphical plot of the changes in temperature with time for a material over the entire temperature range through which it cools.
Cooling Curve graph for Pure Metals is shown below.
COOLING CURVE FOR PURE IRON
The melting point of pure iron is 1535°C. Iron may exist in several allotropic forms, e.g., a, b, y, 8 in the solid state. The existence of one form to the other depends upon the temperature to which the iron is heated. Fig. 6.1 illustrates the cooling curve for pure molten iron plotted in time vs temperature coordinates.
The first horizontal step appears on this curve at a temperature of 1535°C. It indicates that the transformation from the liquid state takes place at a constant temperature. Upon freezing the melt, delta iron is formed. This has a body centered cubic lattice constant a=2.938. (The lattice constants are measured in angstrom units, abbreviated Å and 1 Å =10-10 m).
The second temperature effect occurs at 1400° C (Ar) on the iron cooling curves and corresponds to the transformation of delta iron into a new allotropic form (Gamma iron). It has a face centered cubic lattice with = a a constant of 3.63 Å. Gamma iron is very weakly magnetic (paramagnetic), and it can dissolve carbon reaching a maximum value of 2 per cent at 1,130° C.
The third temperature effect occurs at 910° C (Ar). Here gamma iron is transformed into alpha iron with a body centered cubic lattice having a constant a=2.9 Å. It is nonmagnetic.
The last and the fourth retardation is observed at 768° C (Ar). It corresponds to alpha iron. The alpha form of iron is highly magnetic and is one that exists at room temperature. It can dissolve only a very small amount of carbon, the maximum being about 0.025 per cent at 723° C. At 20° C, the solubility is only 0.0025 per cent.
Thus it is evident from the illustration, there are actually two allotropic forms of iron-the low temperature modification, alpha iron; and the high temperature form, gamma iron. These changes are reversible, i.e., same allotropic forms are observed during heating.
This was the detail about cooling curve for pure iron.
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