Introduction to Iron-Carbon Equilibrium Diagram | Structures in Fe-C Diagram


Fig. shows, the  Fe-C equilibrium diagram in  which
various structure  (obtained during heating and
 cooling),  phases and  microscopic
constituents  of various kinds of steel and  cast
iron  are  depicted. The  main structures,
 significance of various lines and  critical points
are discussed as  under.

 Structures in

The  main microscopic constituents  of iron
 and  steel are  as  follows:
1.   Austenite
2.   Ferrite
3.   Cementite
4.   Pearlite

1 Austenite
Austenite is a solid  solution of free
 carbon (ferrite) and  iron  in  gamma
iron. On  heating the  steel, after upper critical
temperature, the  formation of structure completes into
 austenite which  is  hard, ductile
and  non-magnetic
. It is  able  to
 dissolve large amount of carbon. It is in  between
the  critical or  transfer ranges during heating
and  cooling  of steel. It is  formed when
steel contains carbon up to 1.8% at 1130°C. On cooling
below 723°C, it starts transforming into
 pearlite and  ferrite. Austenitic steels cannot be
hardened by usual heat treatment methods and  are

2 Ferrite
Ferrite contains very  little or no carbon in
 It is the  name given  to pure iron
 crystals which  are  soft and  ductile.
The  slow cooling  of low carbon steel below
 the  critical temperature produces ferrite
structure. Ferrite does  not  harden when cooled
 rapidly. It is  very  soft  and
highly magnetic.

3 Cementite
Cementite is  a  chemical compound of carbon with
iron  and  is  known as  iron
 carbide (Fe3C).  Cast iron  having 6.67%
 carbon is  possessing complete structure  of
cementite. Free cementite is found  in all steel
containing more  than 0.83% carbon. It increases with
increase in  carbon % as  reflected in  Fe-C
 Equilibrium diagram. It is  extremely hard. The
 hardness and   brittleness of  cast  
iron   is  believed  to  be  due
  to  the   presence  of  the  
cementite.  It decreases tensile strength.  This
  is  formed when  the   carbon forms
  definite combinations with iron  in  form
 of iron  carbides which  are  extremely
hard in  nature. The  brittleness and hardness of
cast  iron  is mainly controlled by the
 presence of cementite in  it.  It is magnetic
below 200°C.

4 Pearlite
Pearlite is a eutectoid alloy  of ferrite and
It occurs  particularly in medium
and low carbon steels in  the  form  of
mechanical mixture of ferrite and  cementite in
 the  ratio of 87:13.  Its hardness
increases with the  proportional of pearlite in ferrous
material. Pearlite is relatively strong, hard and
 ductile, whilst ferrite is  weak, soft  and
 ductile. It is  built up  of alternate light
and  dark plates. These layers are alternately ferrite
and  cementite. When seen with the   help   of
 a  microscope, the   surface has  
appearance like   pearl, hence it is  called
pearlite. Hard steels are  mixtures of pearlite and
 cementite while  soft  steels are
 mixtures of ferrite and  pearlite.

As the  carbon content increases beyond 0.2%  in
 the  temperature at which  the  ferrite
is first rejected from  austenite drop  until, at
or  above  0.8%  carbon, no  free
 ferrite is  rejected from  the  
austenite. This   steel is  called eutectoid steel,
and   it is  the   pearlite structure in
As iron  having various % of carbon (up  to 6%) is
heated and  cooled,  the  following phases
representing the  lines will  tell  the
 about the  structure of iron,  how  it

Significance of  Transformations

The  line  ABCD  tells that above  this
line  melting has  been  completed during
heating the iron.  The  molten metal is purely in the
liquidus form.  Below this line  and  above
 line  AHJECF the  metal is partially solid
 and  partially liquid. The  solid  metal
is known as  austenite. Thus the  line  ABCD
 represents temperatures at which melting is considered as
 completed. Beyond this line  metal is totally in
 molten state. It is not  a horizontal line  the
 melting temperature will  vary  with carbon

This  line  tells us that metal starts melting at
this temperature. This  line  is not
 horizontal and   hence the  melting
temperatures  will  change with carbon content.
Below  this line  and above  line  GSEC,
the  metal is  in  solid  form  and
 having austenite structure.

Line PSK
This   line   occurs   near 723°C  and
  is  a  horizontal line   and   is
 known as  lower   critical temperature line
 because transformation of steels starts at, this line.
 Carbon % has  not  effect on it that means
steel having different % of carbon will transforms at the
 same temperature. The  range above  the
 line  up  to GSE  is known as
 transformation range. This  line  tells us
 the steel having carbon up  to 0.8%  up
 to 0.8%  will  starts transforming from
 ferrite and  pearlite to  austenite during

Line  ECF
It is a line  at temperature 1130°C  which tells
that for cast  iron  having % of C from  2% to
 4.3%.  Below  this line  and  above
 line  SK,  Cast iron  will  have
austenite + ledeburite and cementite +  ledeburite.