Steel Phase Transformations: Austenite to Pearlite, Bainite and Martensite

When steel is cooled, it undergoes various phase transformations, depending on the cooling rate and composition of the steel.

Austenite, the high-temperature phase of steel, can transform into several lower-temperature phases:

Pearlite

Pearlite is a microstructure that forms when steel is cooled relatively slowly. It is a mixture of ferrite (a low-temperature phase of steel) and cementite (a hard, brittle iron carbide).

Bainite

Bainite is a microstructure that forms when steel is cooled at a faster rate than pearlite. It is harder and stronger than pearlite, but not as brittle.

Martensite

Martensite is a microstructure that forms when steel is cooled very quickly. It is the hardest and strongest of the three, but also the most brittle.

In summary, Austenite to Pearlite transformation is a slow cooling process that results in a mixture of ferrite and cementite, Austenite to Bainite transformation is a faster cooling process that results in harder and stronger but not as brittle microstructure, and Austenite to Martensite transformation is a very fast cooling process that results in the hardest, strongest, but also brittle microstructure.

Steel Phase Transformations: Austenite to Pearlite, Bainite and Martensite

In addition to the phase transformations mentioned above, steel can also transform into other microstructures depending on the cooling rate and composition.

Ledeburite

Ledeburite is a microstructure that forms when steel containing a high carbon content is cooled slowly. It is a mixture of pearlite and cementite.

Spheroidite

Spheroidite is a microstructure that forms when steel containing a low carbon content is cooled slowly. It is a mixture of ferrite and pearlite, with the ferrite particles having a spherical shape.

Troostite

Troostite is a microstructure that forms when steel containing a low carbon content is cooled at a moderately fast rate. It is a mixture of ferrite and cementite, with the cementite particles having a needle-like shape.

Acicular ferrite is a microstructure that forms when steel containing a low carbon content is cooled at a very fast rate. It is a mixture of ferrite and cementite, with the ferrite particles having a needle-like shape.

In summary, the microstructure of steel depends on the cooling rate, composition and the heat treatment that is applied to the steel. The different microstructures have different properties that make them suitable for different applications.