PEARLITIC TRANSFORMATION


Pearlite :

Pearlite is a mixture of two phase which consists of alternate layers of ferrite and cementite. It is an important constituent of steel which present in all types of steel that slowly cooled.  Pearlite is formed by cooling austenite of eutectoid composition just below the critical temperature where containing 88%ferrite and 12%pearlite.

Pearlitic Transformation

● Pearlite transformation consists of two different phase that is
Ferrite : It has very low carbon content and for all practical purpose. It can be present almost pure iron or steel.
Cementite : It is an intermetallic compound of iron with 6.67% weight carbon. So the transformation is possible only by the redistribution of carbon atoms of austenite on cooling which takes place by diffusion of carbon atoms. Thus the pearlitic transformation proceeds by nucleation and growth mechanism.

● Here ferrite and cementite phases nucleates first. The nucleus which is formed first is referred to as active nucleus for pearlitic transformation which provided to transformed product. It has lattice orientation relationship with parent austenite.

Mechanism of Transformation :

The pearlite is the active nucleus for austenite to pearlite transformation is cementite. Some of the reasons in favour of this is :

1. Orientation relation between pearlitic ferrite and parent austenite is different from that of proeuectoid ferrite and parent austenite. Therefore ferrite cannot be active nucleus. This is contrast to bainitic transformation where ferrite has an orientation relation with parent austenite.

2. Formation of pearlite is affected by the presence of undissolved cementite particles where as the presence of ferrite dose not exhibit any such effect.

3. Proeutetiod cementite as well as pearlite cementite platelets are in general, parallel to high index or low atomic density plane of austenite.

Kinetics of Transformation :

● At lower temperature the free energy of austenite is equal to the free energy of pearlite. Therefore at this temperature transformation of pearlite form austenite will completed infinite time. The austenite to pearlite transformation rate will be zero for all practical purpose at this temperature. So it essential to undercook the austenite below the equilibrium temperature.

● Below the lower critical temperature the free energy of pearlite is lower than that of austenite and so the pearlite is thermodynamically stable. Free energy of pearlite will be less at lower temperature hence the stability of pearlite can be increased by increasing the degree of supercooling or by lowering the transformation temperature.

● The decomposition of austenite to pearlite proceeds by the redistribution of carbon atoms of austenite in to ferrite and cementite and is essentially a diffusion controlled process. The rate of diffusion of carbon atoms decreases exponentially with the decreasing temperature. This implies that the lower transformation temperature retards the rate of transformation, for which the diffusion rate for carbon atoms may be too small to result in the diffusion controlled transformation. The rate of carbon diffusion is negligible below 200℃
Effect of austenite transfirmation - pearlitic transformation
Effect in temp. on rate of austenite transformation

● So the rate of transformation in two ways:
1. Increased degree of undercooling increases the transformation rate by providing a greater difference in free energies of austenite and pearlite (curve b).
2. Increased degree of undercooling reduces the transformation rate by lowering the rate of carbon diffusion (curve a). And the combined effect is curve c shown in fig. Of rate of transformation of austenite.
Effect of undercooling on rate of nucleation - pearlitic transformation
Effect of undercooling on the rate of nucleation and growth.

● The rate of nucleation is expressed as total number of nuclei appearing per unit volume of untransformed austenite. Both the rate of nucleation and the rate of growth are zero at eutectoid temperature. They tends to be zero below 200℃.

Effect of Alloying Elements on Transformation :

● Transformation of austenite to pearlite in alloy steel is more complicated than in carbon steels. Almost all alloying elements whether ferrite stabilizers or austenite stabilizers, except cobalt.

● Carbon alloying elements partion to ferrite and carbide phase on decompression of austenite. The partitioning approaches equilibrium value at higher transformation temperature in the vicinity of eutectoid temperature.

● As transformation temperature decreases the amount of carbon and alloying elements present in ferrite and carbide phases deviate from equilibrium partitioning. Stable carbide possess higher metal to carbon ratio.







References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.
Metallurgist.




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