Question

(Chapter 10)

Q: Explain the difference between coarse and fine pearlite?

Answer 1

Pearlite is basically a microstructure that occurs in steel, when it is cooled from approx 1340^oF to roughly 1000^oF

It consists of an alternating layers of ferrite as well as Cementite. It is basically a ductile form of microstructure which is used for the manufacture of anything where sudden failure is undesirable. There are two types of pearlite viz., Fine pearlite and Coarse pearlite.

The main differrence between them is thickness of the layers of pearlite.

Fine pearlite has finer grain and thinner layer,and is formed due to slow diffusion (as diffusion is hindered at low temperatures) of steel at lower temperatures (closer to 1000^oF )

On the other hand, Coarse pearlite is formed at higher temperatures (closer to 1340^oF) due to faster diffusion of steel and has larger grains and thick layers.

As fine pearlite is formed at minimal temperatures , it needs to be cooled so down quickly and can be done by letting it cool down in air outside the furnance. To form coarse pearlite, it needs to be maintained at 1100^oF to 1300^oF, which is done by taking it to the another furnance and letting it cool down very slowly.

As the thicker layers of Coarse pearlite can easily expand and stretch it is a lot more ductile. However, as the layers of fine pearlite are thinner, they resist slipping relative to each other and as such are much more stronger.