Transformation Diagrams

The  kinetic aspects  of phase transformations are as important as  the equilibrium diagrams  for the heat treatment  of  steels.
The  metastable  phase  martensite  and  the  morphologically  metastable  microconstituent  bainite,  which  are  of  extreme
importance  to  the  properties  of  steels,  can  generally  form  with  comparatively  rapid  cooling  to  ambient  temperature,  that
is, when the diffusion of carbon and alloying elements is suppressed or limited to a very short range. Bainite is a eutectoid
decomposition that isa mixture offerrite and cementite. Martensite,the hardestconstituent, formsduring severe quenches
from  supersaturated  austenite  by  a  shear  transformation.  Its  hardness  increases  monotonically  with  carbon  content  up  to
about  0.7  wt%.  If  these  unstable  metastable  products  are  subsequently  heated  to  a  moderately  elevated  temperature,  they
decompose to more  stable  distributions  of  ferrite and  carbide. The  reheating process  is  sometimes known as  tempering or
annealing.
The  transformation  of  an  ambient  temperature  structure  like  ferrite-pearlite  or  tempered  martensite  to  the  elevatedtemperature structure of austeniteor austenite + carbideisalso of importance in the heat treatment of steel.
One  can  conveniently  describe  what  is  happening  during  transformation  with  transformation  diagrams.  Four  different
typesof such diagramscan be distinguished. These include:
·  Isothermal  transformation  diagrams  describing  the  formation  of  austenite,  which  will  be  referred  to  as
ITh diagrams
·  Isothermal  transformation  (IT)  diagrams,  also  referred  to  as  time-temperature-transformation  (TTT)
diagrams, describingthe decompositionof austenite
·  Continuous heatingtransformation (CHT) diagrams
·  Continuous cooling transformation (CCT) diagrams