IRON-CARBON EQUILIBRIUM DIAGRAM: Austenite, Ferrite, Cementite, Pearlite, Critical Temperatures

 IRON-CARBON EQUILIBRIUM DIAGRAM

Below 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 s
tructures in Fe-C-diagram.
The main microscopic constituents of iron and steel are
as follows:


1. Austenite
2. Ferrite
3. Cementite
4. Pearlite


Fe-C  equilibrium diagram
Fe-C  equilibrium diagram


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.


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.



 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.


 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.


Critical Temperatures

The temperatures at which changes
in structure takes
place is known
as critical temperatures, these are as follows:

1.The temperature
along
GSE is known
as upper critical temperature
. The temperature along GS during
heating as (upper
critical temperature) 
where  austenite+ alpha iron changes into
austenite and vice versa.
2. The temperature along
GS during cooling as A
3 where austenite
changes into austenite + alpha
iron and vice versa during
heating.
3. The temperature along line SE during
heating as Acm changes into
austenite from austenite + cementite
and vice versa.
4. The temperature
along
PSK is known as lower  critical 
temperature 
when pearlite changes into austenite on heating as denoted, by A1.


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