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


IRON-CARBON  EQUILIBRIUM  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  Fe-C-diagram
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  non-magnetic.

2 Ferrite
Ferrite contains very  little or no carbon in iron.  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  cementite. 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 composition.
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 charges.

Significance of  Transformations  Lines

Line ABCD
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 content.

Line AHJECF
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 heating.

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.

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