If austenite containing about 0.80 percent carbon is slowly cooled through the critical temperature,
ferrite and cementite are rejected simultaneously, forming alternate plates or lamellae. This
microstructure is called pearlite. At temperatures just belot the A1, the transformation from
austenite.to pearlite may take an appreciable time to initiate and complete, but the product will be
lameller pearlite. As the transformation temperature is lowered, the time to initiate transformation shortens but the product is pearlite of increasing fineness, and at temperatures approaching 550°C it
cannot be resolved into its lamellar constituents. Further deerease in transformation temperature
causes a lengthening of the ncubation period and a change in structure of the product to a form known
as “bainite”.
If the temperature is lowered sufficiently, the diffusion controlled nucleation and growth modes of
transformation are suppressed completely and the austenite transforms by a diffusionless process in
which the crystal lattice effectively shears to a new crystallographic configuration known as
“martensite”. This phase has a tetragonal crystal structure and contains carbon in supersaturated solid
solution.