1
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |

2
Keywords   :   Ferrite   (

),   Austenite   (

),   Ferrite   (d):   three   different   forms   of   iron,   solubility   of   carbon   in   different   forms   of   iron,   cementite,   graphite,   Fe   –   Fe
3
C   meta ‐ stable   phase   diagram,   pearlite,   ledeburite,   hypo ‐ eutectoid   steel,   hyper ‐ eutectoid   steel,   hypo ‐ eutectic   white   cast   iron,   hyper ‐ eutectic   white   cast   iron
Introduction
The   last   five   modules   were   devoted   to   the   solidification   behavior   of   binary   alloys.
This   is   best   represented   by   its   phase   diagram.
We   are   now   familiar   with   a   wide   range   of   simple   and   complex   phase   diagrams.
However   while   introducing   this   we   only   considered   hypothetical   cases.
Now   let   us   consider   a   specific   binary   alloy   of   considerable   importance.
We   are   going   to   talk   about   iron  ‐  carbon   system.
Iron   can   exist   in   two   different   crystalline   forms.
These   are   BCC   and   FCC.
This   is   known   as   allotropy.
Carbon   too   has   several   allotropic   forms.
However   the   form   which   is   of   relevance   to   this   system   is   graphite.
It   has   a   hexagonal   structure.
This   is   considered   to   be   the   most   stable   state   of   carbon.
An   atom   of   carbon   atom   is   significantly   smaller   than   that   of   iron.
Therefore   it   can   be   accommodated   within   the   interstitial   sites   of   iron   lattice   resulting   in   an   interstitial   solid   solution.
Solubility   carbon   is   likely   to   be   limited.
Apart   from   this   iron   has   a   strong   affinity   to   form   carbide.
The   most   common   form   is   cementite.
Its   chemical   formula   is   Fe
3
C.
This   is   truly   a   meta ‐ stable.
However   the   rate   of   decomposition   of   Fe
3
C   into   Fe   and   C   is   extremely   slow.
In   most   of   the   common   grades   iron   –   carbon   alloy   excess   carbon   is   present   as   cementite.
Let   us   first   look   at   the   Fe ‐ Fe
3
C   meta ‐ stable   phase   diagram   and   in   a   later   module   we   shall   discuss   about   iron   –   graphite   phase   diagram   as   well.
The   alloys   belonging   to   this   system   is   popularly   known   as   steel   or   cast   iron.
We   would   soon   learn   about   the   difference   between   the   two.
Solidification   of   pure   iron:
Fe : crystal structure
T
1539
1394
910
770


Liquid
Solubility of carbon in Fe = f(structure, temperature)
BCC
FCC
BCC
Where is the carbon located in iron lattice?
Paramagnetic
Ferromagnetic
 time
Slide   1
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |

The   sketch   in   slide   1   is   a   typical   cooling   curve   of   pure   iron.
Solidification   begins   with   nucleation   and   growth   of   crystals   of   iron   at   1539°C.
It   is   BCC   (body   centered   cubic).
At   1394°C   it   transforms   into   FCC
(face   centered   cubic)   structure.
This   is   stable   till   910°C   where   it   again   transforms   into   BCC.
Each   of   these   transformations   appears   as   steps   on   the   cooling   curve.
Apart   from   this   there   is   another   transformation   which   may   not   get   detected   by   thermal   analysis.
This   is   the   transformation   from   paramagnetic   to   ferromagnetic   state.
It   occurs   at   770°C.
This   is   known   as   its   Curie   temperature.
The   property   which   is   most   sensitive   to   detect   it,   is   magnetic   permeability.
The   three   different   forms   of   iron   are   known   as   ferrite   (

),   stable   until   910°C,   austenite   (

),   stable   from   910° ‐ 1394°C   and   ferrite   (

),   stable   from1394°–
1539°C.
Note   that   the   BCC   form   of   iron   is   known   as   ferrite.
Therefore   in   order   to   distinguish   between   the   two,   the   high   temperature   form   is   termed   as   delta   ferrite.
If   carbon   atoms   are   introduced   into   iron   these   are   likely   to   occupy   the   interstitial   sites   because   the   atoms   carbon   are   much   smaller   than   those   of   iron   atoms.
The   interstitial   sites   in   BCC   and   FCC   are   shown   in   slide   2.
The   solubility   of   carbon   in   iron   is   a   function   of   temperature   and   crystal   structure.
Solubility   of   carbon   is   higher   in   BCC   or   FCC?
c
Interstitial sites in iron lattice c
3
Slide   2   b b a
BCC a
FCC
Lattice site
Interstitial site
Slide   2   shows   the   lattice   sites   occupied   by   iron   atoms   and   possible   interstitial   sites   for   carbon   atoms   in   both   BCC   &   FCC   structures.
The   interstitial   sites   shown   here   are   known   as   octahedral   sites.
The   sketches   in   slide   2   show   only   the   positions   of   iron   atoms   and   the   interstitial   sites   in   one   unit   cell.
The   sites   located   within   the   cell   belong   exclusively   to   a   unit   cells   but   those   lying   on   the   faces,   the   edges   or   the   corners   are   shared   by   the   neighboring   unit   cells   as   well.
BCC   has   eight   corner   sites   for   iron   atoms.
Each   of   these   is   shared   by   8   neighboring   unit   cells.
The   contribution   of   corner   site   is   thus   1/8.
The   atom   at   the   centre   belongs   exclusively   to   this   unit   cell.
Therefore   the   number   of   iron   atoms   /   unit   cell   =   8   x   (1/8)   +   1   =   2.
Let
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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4
us   use   the   same   approach   to   estimate   the   number   of   interstitial   sites   /unit   cell   for   BCC   structure.
The   sketch   in   slide   2   shows   the   locations   of   interstitial   sites.
There   are   6   sites   at   the   centers   of   6   faces.
Each   face   is   shared   by   two   unit   cells.
There   are   12   sites   at   each   of   the   12   edges.
Each   of   these   is   shared   by   4   neighboring   cells.
Therefore   the   number   of   interstitial   sites   /   unit   cell   in   BCC   crystal   =   12   x   (1/4)   +   6   x
(1/2)   =   6.
Note   that   it   is   3   times   the   number   of   Fe   atoms   in   a   unit   cell.
Look   at   the   sketch   for   FCC   unit   cell   in   slide   2.
Count   the   number   of   lattice   sites.
Follow   the   same   approach   to   show   that   the   number   of   Fe   atoms   /   unit   cell   =   6   x   (1/2)   +   8   x   (1/8)   =   4.
In   the   same   way   the   number   of   interstitial   sites   /   unit   cell   in
FCC   structure   =   1   +   12   x   (1/4)   =   4.
Note   that   in   FCC   structure   the   ratio   of   the   number   of   interstitial   site   to   the   number   of   lattice   sites   =   1.
Now   that   we   know   about   the   possible   sites   carbon   atoms   could   occupy   is   it   possible   to   guess   which   of   these   is   likely   to   have   higher   solubility?
F
D   C
F
O
A   B
O
D
Fig   1
E
A   C
B
(b)
E
(a)
Fig.
1(a):   Shows   the   lattice   and   the   interstitial   sites   in   a   BCC   unit   cell.
Circles   are   the   lattice   sites.
The   black   dots   are   the   interstitial   sites.
In   a   lattice   several   of   such   cells   are   stacked   one   after   the   other.
F   represents   the   body   center   site   in   a   unit   cell   just   above   the   one   shown   in   the   sketch.
The   figure   ABCDEF   formed   by   joining   the   lattice   sites   as   illustrated   is   an   octahedron.
There   is   an   interstitial   site   at   its   center   marked   as   O.
All   the   interstitial   sites   are   identical.
Note   that   AB   =   BC   =   CD   =   DA   =     =   the   lattice   parameter.
The   other   edges   of   the   octahedron   AE   =   BE   =   CE   =   DE   =   AF   =   BF   =   CF   =   DF   = √3/2 .
It   is   not   a   regular   octahedron.
The   interstitial   site   in   BCC   structure   is   not   symmetrical.
The   gap   along   EF   is   shorter   than   that   along   AC   or   DB.
It   is 2 √3 /2 .
The   lattice   parameter   of   BCC   iron   called   ferrite   is
0.286nm.
The   shortest   gap   is   equal   to   0.038nm.
Fig.
1(b):   Shows   the   lattice   and   the   interstitial   sites   in   a   FCC   unit   cell.
Circles   are   the   lattice   sites.
The   black   dots   are   the   interstitial   sites.
In   a   lattice   several   of   such   cells   are   stacked   one   after   the   other.
The   figure   ABCDEF   formed   by   joining   the   lattice   sites   as   illustrated   is   an   octahedron.
There   is   an   interstitial   site   at   its   center   marked   as   O.
All   the   interstitial   sites   are   identical.
Note   that   AB   =   BC   =   CD   =   DA   =   AE   =
BE   =   CE   =   DE   =   AF   =   BF   =   CF   =   DF   =
√
,   where     =   the   lattice   parameter.
It   is   a   regular   octahedron.
The   interstitial   site   in   FCC   structure   is   symmetrical.
The   gap   along   EF   is   equal   to   that   along   AC   or   DB.
It   is 2 √2 /2 .
The   lattice   parameter   of   FCC   iron   called   austenite   is   0.362nm.
The   interstitial   gap   is   equal   to   0.106nm.
It   is   much   larger   than   the   interstitial   gap   in   ferrite.
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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FCC   has   higher   packing   density   than   BCC.
Yet   the   solubility   of   carbon   is   FCC   is   higher.
This   is   because   the   interstitial   sites   in   FCC   are   bigger   than   that   in   BCC.
There   are   two   types   of   interstitial   sites   known   as   octahedral   and   tetrahedral.
In   FCC   the   tetrahedral   sites   are   surrounded   by   four   close   packed   atoms.
The   gap   is   extremely   small,   whereas   in   BCC   the   four   atoms   are   more   widely   spaced.
The   tetrahedron   is   not   symmetric.
The   shortest   dimension   (size)   of   tetrahedral   sites   in   BCC   lattice   was   derived   in   module   5.
This   is   nearly   same   or   marginally   lower   (0.036nm)   than   that   of   the   octahedral   site   (0.038nm).
The   number   of   such   sites   is   much   more   than   that   of   the   octahedral   sites.
Although   the   packing   density   in   BCC   is   lower   than   that   of   FCC,   the   gaps   are   equally   distributed   between   two   types   of   interstices   making   these   significantly   smaller   than   that   of   FCC.
This   is   why   the   solubility   of   carbon   which   is   an   interstitial   solute   is   significantly   lower   in   ferrite   (BCC)   than   that   in   austenite   (FCC).
Apart   from   this   the   interstitial   site   in   BCC   is   asymmetric.
When   a   carbon   atom   goes   into   the   interstices   the   atoms   along   one   of   the   axes   are   pushed   further   apart.
This   results   in   a   tetragonal   distortion.
We   shall   talk   about   it   in   one   of   the   subsequent   modules.
Phases   in   iron   –   carbon   binary   system:
Iron   can   exist   in   three   different   crystalline   forms   each   having   limited   solubility   of   carbon.
The   stability   of   these   depends   on   temperature   and   composition.
The   two   high   temperature   forms   of   iron   are

ferrite   which   is   BCC   (stable   above   1394°C)   and   austenite   (
 stable   above   910°C)   which   is   FCC.
The   room   temperature   form   of   iron   is

ferrite   which   is   BCC.
The   solubility   of   carbon   in   ferrite   is   limited.
The   maximum   solubility   is   around   0.025wt%   as   against   this   the   solubility   of   carbon   in   austenite   is   a   little   more.
It   is   about   2wt%.
Apart   from   this   iron   carbon   system   may   have   iron   carbide   (Fe
3
C)   called   cementite.
It   has   6.67%   carbon.
It   is   considered   as   an   inter ‐ metallic   compound   having   relatively   more   complex   crystal   structure   than   those   of   ferrite   and   austenite.
It   is   a   meta ‐ stable   phase.
It   may   exist   for   indefinite   periods   of   time   at   room   temperature.
However   on   prolonged   thermal   exposure   at   600°C   or   beyond   it   transforms   into   ferrite   and   graphite.
Therefore   iron   carbon   alloys   of   commercial   importance   may   be   considered   as   a   binary   alloy   of   iron   and   cementite.
Let   us   first   look   at   its   phase   diagram.
It   is   also   known   as   iron   cementite   meta ‐ stable   phase   diagram.
Although   it   is   a   binary   system   there   are   5   different   phases   including   the   liquid.
This   is   likely   to   have   more   than   one   invariant   reaction   involving   3   phases.
5
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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6
Iron – cementite phase diagram
1539

1394

+


+ L

910
Peritectic isotherm

+ L
L
Eutectic isotherm
L + Cm
Hypoeutectic Hyper-eutectic
727

 
+ Cm
0
Hypo eutectoid
0.8
Hyper eutectoid
2.0
Eutectoid isotherm
% C
4.3
1148
6.67
Slide   3
Slide   3   gives   a   schematic   Fe ‐ Fe
3
C   phase   diagram.
It   has   3   invariant   reactions   (transformation).
These   are   given   in   slide   4.
The   one   occurring   at   1495°C   is   the   peritectic   reaction.
The   delta   ferrite   reacts   with   liquid   to   form   austenite.
The   one   at   1148°C   is   known   as   the   eutectic   reaction   where   the   liquid   transforms   into   a   mixture   of   austenite   and   cementite.
The   eutectic   is   known   as   Ledeburite.
The   one   at   727°C   is   known   as   eutectoid   transformation   where   austenite   decomposes   into   a   mixture   of   ferrite   and   cementite.
This   is   known   as   Pearlite.
On   the   basis   of   this   diagram   iron   –   carbon   alloys   having   less   than   2.0%   carbon   are   known   as   steel,   whereas   those   having   more   than   2.0%   carbon   are   known   as   cast   iron.
This   classification   is   based   on   their   ability   to   undergo   large   plastic   deformation.
Steel   is   ductile   but   cast   iron   is   brittle.
Slide   4
Steel:
It   is   an   iron   carbon   alloy   where   most   of   the   carbon   is   present   as   meta ‐ stable   iron   carbide   called   cementite.
The   upper   limit   of   carbon   content   is   2%.
Phase   diagram   helps   us   guess   the   structure   of   alloys   and   their   properties.
Let   us   look   at   what   kinds   of   structure   steel   could   have   depending   on   its   composition.
We   would   only   consider   the   structure   that   develops   under   equilibrium   rate   of   cooling.
The   steel   on   solidification   is   expected   to   have   fully   austenitic   structure.
It   may   be   assumed   to   be
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R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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7
homogeneous   since   the   rate   of   cooling   is   considered   to   be   slow.
Depending   on   its   composition   we   may
have   3   types   of   structures.
(i)   %   carbon   <   0.02
(ii)   0.02
<   %   carbon   <   0.8
(iii)   0.8
<   %   Carbon   <   2.0.

L
1539
L
L   +

1495
1394

+   L
L

+   L



T°C
910

T



Fig   2

727
 
+   Cm


+   Cm

+   Cm
0   0.02
0.8
2.0
Wt%   C time
(i) Figure   2   explains   the   solidification   behavior   of   steel   having   less   than   0.02%   carbon   with   the   help   of   a   set   of   schematic   diagrams.
The   sketch   on   the   left   shows   a   part   of   the   equilibrium   diagram   (Fe ‐ Fe
3
C)   with   the   location   of   the   alloy   by   a   vertical   dotted   line.
It   intersects   the   liquidus,   solidus,   and   a   set   of   solvus   curves.
These   are   projected   on   to   the   cooling   curve   shown   on   the   right   with   the   help   of   a   set   of   horizontal   lines.
The   cooling   curve   exhibits   inflection   points   at   each   of   these   intersections.
Solidification   begins   with   precipitation   of   a   few   grains   of

ferrite.
The   top   most   microstructure   corresponds   to   this   stage.
The   solidification   takes   place   by   nucleation   and   growth.
The   composition   of   the   liquid   and   the   solid   keeps   changing   during   this   stage.
When   solidification   is   complete   the   entire   liquid   is   replaced   by

ferrite   having   the   same   composition   as   that   of   the   alloy.
This   is   shown   by   the   second   schematic   structure   from   the   top   in   Fig   2.
The   structure   remains   unchanged   until   the   temperature   crosses   the   boundary   between

/

phase   fields.
Thereafter   austenite   precipitates   from

ferrite.
The   grain   corners   and   boundaries   are   the   preferred   sites   where   grains   of   austenite   nucleate.
The   third   microstructure   from   the   top   in   Fig   2   represents   its   main   features.
It   consists   of   grains   of

(white)   and   a   few   grains   of

(grey).
There   is   partition   of   carbon   between   these   two   phases.
Bulk   of   the   carbon   goes   into   austenite.
The   composition   of   the   two   keeps   changing   as   the   temperature   drops.
The   volume   fraction   of

increases   at   the   cost   of

.
When   the   %carbon   in   austenite   becomes   equal   to   that   of   the   steel

ferrite   disappears.
The   structure   now   consists   of   100%   austenite.
Note   the   main   features   of   the   fourth   microstructure   from   the   top   in   Fig   2.
The   grain   size   is   finer   than   that   of   100%

ferrite.
The   structure   remains   as   100%   austenite   until   the   temperature   drops   below   the   line   representing   the   boundary   between

and

phase   fields   of   the   equilibrium   diagram.
This   is   where

ferrite   starts   precipitating   from   austenite.
The   grain   boundaries   and   the
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R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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8
grain   corners   are   the   preferred   sites   for   precipitation.
The   fifth   sketch   from   the   top   of   Fig   2   is   a   typical   representation   of   its   microstructure   at   this   stage.
Ferrite   grains   are   shown   as   white   and   austenite   grains   are   shown   as   grey.
This   continues   through   nucleation   of   new   grains   and   growth   of   the   existing   ones   until   the   temperature   drops   below   the   line   between

and

phase   fields   of   the   phase   diagram.
At   this   stage   the   structure   is   100%   ferrite   (

).
The   6 th
sketch   in   Fig   2   is   a   typical   representation   of   the   microstructure.
This   remains   unchanged   till   the   temperature   drops   below   the   solvus.
At   this   stage   excess   carbon   precipitates   as   cementite.
The   last   sketch   in   Fig   2   is   a   typical   representation   of   its   microstructure.
The   amount   of   cementite   keeps   increasing   as   the   room   temperature   drops.
It   can   be   estimated   by   lever   rule.
From   the   phase   diagram   it   is   evident   that   the   steel   at   room   temperature   would   consist   of   ferrite   with   a   few   specks   of   cementite.
If   %   carbon   in   the   steel   is   0.01
the   amount   of   cementite   is   given   by   (0.01/6.67)x100   =   0.15%.
The   grains   are   relatively   finer   than   that   after   solidification.

L
L
T°C
1539
1495
1394

+   L
L

+   L
T
L



Fig   3
910
 


727

+   Cm



+   Cm
Pearlite

0   0.02
0.8
2.0
Wt%   C time
(ii) Figure   3   explains   the   solidification   behavior   of   steel   having   greater   than   0.02%   carbon   but   less   than   0.8%   carbon   with   the   help   of   a   set   of   schematic   diagrams.
The   sketch   on   the   left   shows   a   part   of   the   equilibrium   diagram   (Fe ‐ Fe
3
C)   with   the   location   of   the   alloy   by   a   vertical   dotted   line.
It   has   around   0.18%   carbon.
It   intersects   the   liquidus   and   a   set   of   lines   denoting   either   phase   field   boundaries   or   isotherms   representing   3   phase   equilibrium.
These   are   projected   on   to   the   cooling   curve   shown   on   the   right   with   the   help   of   a   set   of   horizontal   lines.
The   cooling   curve   exhibits   inflection   points   or   steps   (discontinuity)   at   each   of   these   intersections.
Solidification   begins   with   the   precipitation   of   a   few   grains   of

ferrite.
The   first   microstructure   from   the   top   corresponds   to   this   stage.
Solidification   takes   place   by   the   nucleation   of   new   grains   and   the   growth   of   the   existing   ones.
The   composition   of   the   liquid   and   the   solid   keeps   changing   during   this   stage.
When   the   temperature   reaches   1495°C   peritectic   reaction   sets   in.
This   is   an   invariant   reaction.
The   liquid   having   0.55%   carbon   reacts   with

ferrite   having   0.08%   carbon   to   form   austenite   (

)   having   0.18%   carbon.
The   amount   of
 is   just   enough   to   consume   the   remaining   liquid.
The   alloy   on   solidification
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9
consists   of   austenite   (

)   having   0.18%   carbon.
This   is   shown   by   the   second   schematic   structure   from   the   top   in   Fig   3.
The   structure   remains   unchanged   until   the   temperature   crosses   the   boundary   between

/

phase   fields.
Thereafter   ferrite   (

)   precipitates   from
(

austenite.
The   grain   corners   and   boundaries   are   the   preferred   sites   where   grains   of   ferrite   nucleate.
The   ferrite   precipitating   from   austenite   is   known   as   pro ‐ eutectoid   ferrite.
The   third   microstructure   from   the   top   in   Fig   3   represents   its   main   features.
It   consists   of   grains   of   pro ‐ eutectoid

(white)   and   a   few   grains   of

(grey).
There   is   partition   of   carbon   between   these   two   phases.
Bulk   of   the   carbon   goes   into   austenite.
The   composition   of   the   two   keeps   changing   as   the   temperature   drops.
The   volume   fraction   of

increases   at   the   cost   of

.
When   %   carbon   in   austenite   reaches   0.8%   eutectoid   reaction   sets   in.
This   is   an   invariant   reaction.
As   long   as   it   continues   the   temperature   remains   constant.
During   this   stage   both   cementite   and   ferrite   start   precipitating   from   austenite   at   the   same   time.
The   product   is   an   intimate   mixture   of   two   phases.
It   is   known   as   pearlite.
It   consists   of   parallel   layers   (plates)   of   ferrite   and   cementite.
Figure   4   gives   a   series   of   sketches   showing   various   stages   involved   in   the   formation   of   pearlite.
When   a   cementite   plate   nucleates   as   in   (Fig.
4(a))   at   an   austenite   boundary   it   takes   carbon   from   its   neighboring   area.
This   results   in   a   change   in   local   carbon   profile   as   shown   in   the   sketch.
Just   beside   cementite   %   C   =   0.02
whereas   at   a   distance   far   from   this   it   approaches   0.8
%   C   (this   is   the   composition   of   the   eutectoid).
When   this   becomes   low   enough   a   ferrite   plate   nucleates.
Figure   4   (b)   illustrates   this.
Note   the   change   in   carbon   concentration   as   a   function   of   distance.
Figure   4   (c)   shows   the   arrangement   of   ferrite   cementite   plates   in   a   colony   of   pearlite.
The   width   of   the   two   is   approximately   proportional   to   %   ferrite   and   %   cementite   in   eutectoid   steel   having   0.8%   carbon.
Using   lever   rule   %   cementite   in   pearlite   =100   x   (0.8
‐ 0.025)   /   (6.67
‐ 0.025)   =   12%
(approximately).
Therefore   %   ferrite   =   88%.
The   ratio   of   the   two   is   approximately   equal   to
1:7.
This   means   the   width   of   the   ferrite   plate   will   be   seven   times   the   width   of   cementite   plate.
On   completion   of   the   eutectoid   reaction   the   structure   consists   of   pro ‐ eutectoid   ferrite   and   pearlite.
Note   the   main   features   of   the   fourth   microstructure   from   the   top   in   Fig   3.
When   the   temperature   drops   below   the   eutectoid   temperature,   excess   carbon   from   ferrite
(

)   precipitates   as   cementite.
This   is   known   as   tertiary   cementite   (Why   this   is   called   tertiary   will   be   clear   from   subsequent   part   of   this   module).
However   the   change   in   structure   below
the   eutectoid   temperature   is   too   little   to   detect.
Steels   having   carbon   within   0.02
–   0.8
consist   of   ferrite   and   paerlite.
They   are   known   as   hypo ‐ eutectoid   steel.
6.67
6.67
0.8
0.02
Fe
3
C
%   C    x
0.8
0.02
0.8
%   C
0.02
Fe
3
C   x
0.8
0.02
Ferrite
(a)
Austenite   boundary
(b)
Austenite boundary
Fig   4
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |    
Pearlite
Fe
3
C    colony
(c)
Austenite   boundary
(d)   Structure

10
1539
1394
Hyper-eutectoid steel (0.8-2.0% C)
L L
L+

L

  
910





 
723

Cm




P
Cm
Slide   5
0 0.8
% C
2.0
Time
Cm
(iii) Slide   5   explains   the   solidification   behavior   of   steel   having   greater   than   0.8%   carbon   but   less   than   2.0%   carbon   with   the   help   of   a   set   of   schematic   diagrams.
Such   steels   are   known   as   hyper ‐ eutectoid   steel.
The   sketch   on   the   left   shows   a   part   of   the   equilibrium   diagram   (Fe ‐
Fe
3
C)   with   the   location   of   the   alloy   as   a   vertical   dotted   line.
It   has   around   1.0%   carbon.
It   intersects   the   liquidus,   solidus,   solvus   and   the   eutectoid   reaction   isotherm   representing   3   phase   equilibrium.
These   are   projected   on   to   the   cooling   curve   shown   on   the   right   with   the   help   of   a   set   of   horizontal   lines.
The   cooling   curve   exhibits   inflection   points   or   a   step
(discontinuity)   at   each   of   these   intersections.
Solidification   begins   with   precipitation   of   a   few   grains   of

austenite.
The   top   most   microstructure   corresponds   to   this   stage.
The   solidification   takes   place   by   nucleation   of   new   grains   and   growth   of   the   existing   ones.
The   composition   of   the   liquid   and   the   solid   keeps   changing   during   this   stage.
When   the   temperature   reaches   that   of   the   solidus   the   composition   of   the   solid   becomes   equal   to   that   of   the   steel.
The   alloy   on   solidification   consists   of   100%   austenite   (

)   having   1.0%   carbon
(say).
This   is   shown   by   the   second   schematic   structure   from   the   top   in   slide   5.
The   structure   remains   unchanged   until   the   temperature   crosses   the   solvus,   the   boundary   between

/

+
Cm   phase   fields.
At   this   stage   cementite   starts   precipitating   from   austenite.
It   grows   at   the   cost   of   austenite.
The   %   carbon   in   austenite   keeps   decreasing   as   the   amount   of   cementite   increases.
The   grain   boundary   is   the   most   favored   site   for   precipitation.
The   fourth   structure   from   the   top   in   slide   5   gives   a   typical   structure   of   steel   at   this   stage.
When   %   carbon   in   austenite   decreases   to   0.8%   eutectoid   reaction   sets   in.
This   is   an   invariant   reaction.
As   long   as   it   continues   the   temperature   remains   constant.
During   this   stage   both   cementite   and
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R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |

11   ferrite   start   precipitating   from   austenite   at   the   same   time.
The   product   is   an   intimate   mixture   of   two   phases.
It   is   known   as   pearlite.
The   details   about   the   pearlitic   structure   are   shown   in   Fig   4.
Cementite   is   a   hard   and   brittle   phase.
The   presence   of   a   continuous   network   of   cementite   makes   steel   extremely   brittle.
There   are   methods   to   avoid   the   formation   of   such   a   network   to   make   it   suitable   for   engineering   applications.
We   shall   learn   about   in   one   of   the   subsequent   modules.
Estimation   of   the   amount   of   micro ‐ constituents   in   steel   from   phase   diagram:
The   micro ‐ constituents   in   steel   are   ferrite,   austenite,   cementite   and   pearlite.
Out   of   these   austenite   is   not   stable   at   room   temperature.
When   we   look   at   microstructures   of   steel   we   get   an   idea   about   the   distribution   of   various   constituents.
If   the   composition   or   %   carbon   in   steel   is   known   we   can   estimate   the   volume   fraction   (weight   fraction)   of   each   the   likely   constituents.
T
910
(x)

(y)

0
0.02
0.8
a   b
2.0
c


+
+
Cm
Cm
L
4.3
d
1145
Fig   5:   Shows   how   to   estimate   the   amounts   of
727
6.67
e
micro ‐ constituents   in   steel   at   room   temperature.
The   phase   diagram   gives   approximate   %   C   for   various   points   of   interest.
These   are   denoted   as   a,   b,   c,   d,   &   e.
%   C   in   a   hypo ‐ eutectoid   steel   is   x   and   that   in   hyper ‐ eutectoid   steel   is   y.
Figure   5   gives   the   relevant   part   of   Fe ‐ Fe
3
C   phase   diagram.
For   practical   purpose   we   may   assume   that   the   solubility   of   carbon   in   ferrite   is   negligible.
The   general   expressions   for   the   amount   of   Pearlite   (P),
Cementite   (Cm),   Pro ‐  eutectoid   Ferrite   &   Total   ferrite   (

)   in   a   hypo ‐ eutectoid   steel   having   x%   C   are   as   follows.
% 100
.
100
% 100
.
.
100
% 100
.
100
%
.
100
.
.
100
This   truly   gives   weight   %   of   various   constituents.
When   you   look   at   the   microstructure   you   get   an   estimate   of   volume   %.
However   if   the   density   of   the   phases   are   known   weight   %   can   be   converted   to   volume   %.
In   the   case   of   steel   the   density   of   ferrite   is   7.87
g/cc   and   that   of   cementite   is   7.662.
Although   the   difference   is   not   much   the   amount   of   Cm   seen   in   a   microstructure   (volume   fraction)   will   be   a   little   more   than   weight   fraction   obtained   from   the   lever   rule.
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |

Microstructure   of   Fe ‐ Fe3C   eutectic:
The   slide   6   explains   the   solidification   behavior   of   Fe ‐ Fe
3
C   eutectic.
This   is   the   binary   iron ‐ carbon   alloy   having   the   lowest   melting   point.
It   solidifies   as   eutectic   at   1148°C.
It   is   known   as   Ledeburite.
Structure of eutectic (Ledeburite)
T
T L
 cm
 cm
Slide   6
12   a b c
%
 in eutectic at T
E d
= (de/ce) x100 e
% P in eutectic at RT = (de/be) x100 time
%   carbon   in   the   eutectic   is   4.3.
Solidification   occurs   at   a   constant   temperature.
Immediately   after   solidification   the   alloy   consists   of   a   mixture   of   austenite   and   cementite.
%   austenite   =   100   x   (e ‐ d)/(e ‐ c)   =
100   x   (6.67
–   4.3)/(6.67
‐ 2.0)   =52%   (approx).
The   solubility   of   carbon   in   austenite   decreases   with   decreasing   temperature.
At   the   eutectoid   temperature   the   solubility   of   carbon   in   austenite   is   only   0.8%.
Therefore   as   the   alloy   cools   cementite   precipitates   from   the   austenite   in   the   eutectic.
%   austentite   at   the   eutectoid   temperature   (just   before   the   decomposition   of   austenite   starts)   =   100   x   (e  ‐  d)/(e   –   b)   =
100   x   (6.67
–   4.3)/(6.67
–   0.8)   =   41%   (approx).
When   the   eutectoid   reaction   sets   in   the   austenite   having
0.8%   carbon   transform   into   pearlite.
The   eutectic   after   this   transformation   is   known   as   transformed
Ledeburite.
It   approximately   consists   of   41%   Pearlite   in   a   matrix   of   cementite.
Figure   6   shows   the   main   features   of   the   eutectic   that   forms   at   1148°C.
It   is   initially   a   mixture   of   cementite   and   austenite.
On   cooling   the   %   cementite   increases   and   the   carbon   content   of   austenite   decreases   till   it   reaches   727°C.
This   is   where   the   austenite   having   0.8%   carbon   transforms   into   pearlite.
Thereafter   as   per   the   phase   diagram   %   ferrite   within   the   pearlitic   region   would   decrease   as   some   cementite   would   precipitate   out   of   it.
This   is   due   to   the   decrease   in   the   solubility   of   carbon   in   ferrite   with   decreasing   temperature.
However   the   change   is   too   little   to   be   detected   from   the   microstructure.
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R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |

13
Cm

Fig.
6:    Shows   a   schematic   representation   of   the
eutectic   consisting   of   cementite   as   white   matrix
and   austenite   as   grey   particles   at   1145°C.
It   is   called   ledeburite.
Cm
Pearlite
Fig.
7:   Shows   the   structure   of   the   eutectic   Fe ‐ Fe
3
C   at   room   temperature.
Note   that   volume   fraction   of   austenite   has   decreased   and   it   has   transformed   into   pearlite.
It   is   called   transformed   ledeburite.
White   cast   iron:
If   %   carbon   is   greater   than   2.0
but   less   than   6.67
the   Fe ‐ Fe
3
C   alloy   is   known   as   white   cast   iron.
Such   an   alloy   can   be   easily   cast   into   various   shapes   but   it   has   poor   ductility.
The   fracture   surface   has   silvery   white   luster.
This   is   why   it   is   known   as   white   cast   iron.
On   the   basis   of   its   microstructure   there   are   two   types   of   white   cast   iron.
These   are   (i)   hypoeutectic   white   cast   iron   (2.0
<   %   C   <   4.3)   and   (ii)   hypereutectic   white   cast   iron   (4.3
<   %C   <   6.67).
Cementite   (Fe
3
C)   is   the   hardest   constituent   in   the   Fe   –   Fe
3
C   system.
Its   hardness   is   of   the   order   of   1000HV   (Vickers   Hardness   Number).
This   is   why   it   has   excellent   wear   resistance.
Recall   that   the   %   carbide   in   eutectic   is   around   60%.
Hard   materials   are   known   to   be   brittle.
There   is   no   wonder   that   white   cast   iron   has   poor   ductility.
Let   us   look   at   the   evolution   of   microstructure   in   hypo   &   hyper   eutectic   white   cast   iron.
This   has   been   explained   with   the   help   of   a   set   of   sketches   given   in   Fig   8   –   9.
1539
1495
1394
T°C
910

+   L

727

+

Cm
0   0.02
0.8
L

+   L   x
2.0
Wt%   C
3.0
4.3

+
L   +Cm
Cm
6.67
T
L



Cm
+

Cm

+
L

L
Pearlite time
Ledeburite
Transformed
Ledeburite
Fig   8
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Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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14
Figure   8   describes   the   evolution   of   the   microstructure   in   a   hypoeutectic   cast   iron.
The   vertical   dotted   line   marked   x   is   the   location   of   the   alloy   with   respect   to   the   Fe ‐ Fe
3
C   phase   diagram.
The   sketch   on   the   right   gives   its   cooling   curve.
Solidification   begins   with   nucleation   and   growth   of   austenite   grains.
As   it   cools   the   amount   of   austenite   increases   at   the   cost   the   liquid.
The   composition   of   the   austenite   and   the   liquid   keep   changing.
At   the   eutectic   temperature   the   composition   of   liquid   is   4.3
whereas   that   of   the   austenite   is   2.0.
If   %C   in   the   cast   iron   is   3.0,   %   austenite   at   this   temperature   =100   x   (4.3
‐ 3.0)/(4.3
‐ 2.0)   =
57%.
The   balance   43%   is   liquid.
On   cooling   this   it   solidifies   as   a   mixture   of   austenite   and   cementite.
The   eutectic   is   known   as   ledeburite.
The   austenite   that   forms   before   eutectic   transformation   is   called   pro ‐ eutectic   austenite   or   primary   austenite.
On   subsequent   cooling,   the   volume   fraction   of   cementite   increases   at   the   cost   of   austenite.
At   the   eutectoid   temperature   the   primary   austenite   and   the   austenite   in   the   eutectic   transform   into   pearlite.
The   final   structure   would   consist   of   relatively   large   nodule   of   pearlite   (originating   from   primay   austenite)   and   a   fine   dispersion   of   tiny   nodules   of   pearlite   (originating   from   the   austenite   in   the   eutectic)   in   a   matrix   of   cementite.
1539
1495
1394

+   L
T°C
910
727



+   Cm
0   0.02
0.8
L

+   L
2.0
Wt%   C
4.3
x

5.0
+
L   +Cm
Cm
6.67
T
L


Cm
+
Cm


+
Cm
L
Cm
Cm time
L
Ledeburite
Transformed
Ledeburite
Fig
Figure   9   describes   the   evolution   of   the   microstructure   in   a   hyper ‐ eutectic   white   cast   iron.
The   vertical   dotted   line   marked   x   is   the   location   of   the   alloy   with   respect   to   the   Fe ‐ Fe
3
C   phase   diagram.
The   sketch   on   the   right   gives   its   cooling   curve.
Solidification   begins   with   nucleation   and   growth   of   primary   (pro ‐ eutectic)   grains   of   cementite.
As   it   cools   the   amount   of   cementite   increases   at   the   cost   the   liquid.
Note   that   cementite   has   a   fixed   composition.
However   the   composition   of   the   liquid   keeps   changing.
At   the   eutectic   temperature   the   composition   of   liquid   is   4.3.
If   %C   in   the   cast   iron   is   5.0,   %liquid   at   this   temperature   =100   x   (6.67
‐ 5.0)/(6.67
‐ 4.3)   =   70%.
On   further   cooling   it   solidifies   as   a   mixture   of   austenite   and   cementite.
The   eutectic   is   known   as   ledeburite.
The   cementite   that   forms   before   eutectic   transformation   is   called   pro ‐ eutectic   cementite   or   primary   cementite.
On   subsequent   cooling,   the   volume   fraction   of   cementite   increases   at   the   cost   of   austenite   within   the   eutectic.
The   cementite   that   forms   during   this   stage   is   called   secondary   cementite.
At   the   eutectoid   temperature   the   austenite   in   the   eutectic   transforms   into   pearlite.
The   final   structure   consists   of   relatively   large   primary   cementite   and   a   fine   dispersion   of   tiny   nodules   of   pearlite   (originating   from   the   austenite   in   the   eutectic)   in   a   matrix   of
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |
9

15   cementite.
The   eutectic   in   cast   iron   at   room   temperature   is   known   as   transformed   ledeburite.
The   cementite   present   in   iron   carbon   system   has   several   distinct   morphologies.
In   hyper ‐ eutectic   white   cast   iron   has   all   of   theses.
Cementite   precipitating   from   the   liquid   is   called   primary   cementite,   that   precipitating   from   austenite   is   called   secondary   cementite,   and   those   precipitating   from   ferrite   below   eutectoid   temperature   is   called   tertiary   cementite.
Apart   from   these   cementite   is   also   present   as   thin   plates   within   nodules   of   pearlite.
Summary:
In   this   lecture   we   have   learned   Fe ‐ Fe
3
C   meta ‐ stable   phase   diagram.
It   gives   a   graphical   representation   of   the   stability   of   different   phases   one   may   come   across   in   such   a   system   over   a   range   of   temperature   (0° ‐
1550°C)   and   compositions   (0 ‐ 6.67%C).
It   has   3   invariant   reactions.
These   are   peritectic,   eutectic   and   eutectoid.
Barring   pure   iron,   cementite   and   the   eutectic   all   alloys   belonging   to   this   system   do   not   have   a   definite   melting   point.
We   have   learnt   how   to   draw   or   guess   the   cooling   curve   of   an   alloy   if   its   composition   were   known.
It   helps   in   predicting   the   type   of   structure   that   evolves   during   solidification.
Iron   carbon   alloys   having   less   than   2%   carbon   are   known   as   steel   whereas   those   having   carbon   more   than   2%   but   less   than   6.67%   are   known   as   cast   iron.
At   room   temperature   all   iron   carbon   alloys   are   made   of   ferrite   and   cementite.
However   the   distribution   and   the   amount   of   these   may   vary   widely.
On   the   basis   of   the   distribution   of   the   two   phases   we   can   have   3   types   of   steel   and   two   types   of   cast   iron.
These   are   (i)   terminal   solid   solution   having   only   ferrite   or   at   best   extremely   small   amounts   of   tertiary   carbides   (ii)   hypo ‐ eutectoid   steel   (iii)   hyper ‐ eutectoid   steel   (iv)   hypo   ‐  eutectic   white   cast   iron   and   (v)   hyper ‐  eutectic   white   cast   iron.
How   the   structure   evolves   in   these   during   solidification   has   been   explained   with   illustrations.
The   iron   cementite   phase   diagram   at   the   end   of   this   lecture   gives   an   overview   of   the   structural   features   of   each   of   the   5   types   of   alloys   belonging   to   this   system.
However   while   talking   about   the   evolution   of   structure   we   do   assume   that   cooling   rate   is   slow.
This   indeed   is   a   major   limitation   of   phase   diagram.
We   have   already   seen   the   effect   of   non ‐ equilibrium   cooling   on   segregation   during   liquid   to   solid   transformation.
In   Fe ‐ Fe
3
C   system   we   do   have   a   eutectoid   transformation   where   the   effect   of   cooling   rate   is   likely   to   be   much   more   severe.
We   shall   talk   about   it   in   a   subsequent   lecture.
Exercise:
1.
FCC   is   a   more   close   packed   structure   yet   solubility   of   carbon   in   austenite   which   is   FCC   is   higher   than   that   in   ferrite   which   is   BCC.
Why   it   is   so?
2.
Sketch   the   microstructure   of   0.2%   C   steel.
Calculate   %Pearlite   %   cementite,   %   proeutectoid   ferrite   and   %   total   ferrite.
3.
Estimate   the   ratio   of   the   widths   of   ferrite   and   cementite   plates   in   lamellar   pearlite.
4.
Sketch   the   temperarture   –   time   diagram   during   the   heating   cycle   of   a   0.8%   C   steel.
Use   standard
Fe ‐ Fe3C   phase   diagram.
5.
Estimate   %Cm   in   Ledeburite   just   below   eutectic   and   just   above   eutectoid   temeratures.
What   is   its   structure   at   room   temperature?
Answer:
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R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
|    |

16
1.
There   are   2   types   of   interstitial   sites   octahedral   &   tetrahedral.
In   FCC   the   former   is   significantly   larger   than   the   latter.
Whereas   in   BCC   these   are   nearly   same.
The   total   open   space   is   shared   by   more   number   of   sites.
Therfore   interstitial   gap   in   BCC   is   much   smaller   than   that   of   FCC.
This   is   why   carbon   which   occupies   interstitial   site   has   higher   solubility   in   austenite   (FCC).
2.
The   stucture   would   consist   of   proeutectoid   ferrite   and   pearlite.
Assume   solubility   of   carbon   in
.
ferrite   is   negligible.
Refer   to   phase   diagram   to   get   %   Pearlite   in   0.2%   carbon   steel   =
.
100 25   Balance   75%   is   proeutectoid   ferrite.
%   Cementite   =
.
.
100 3%   and   total   ferrite   =   97%.
3.
Assume   density   of   ferrite   and   cementite   to   be   same   (Note   that   in   reality   density   of   cementite   is
.
a   little   higher   than   that   of   ferrite).
%   ferrite   in   pearlite   =
.
.
100 88 .
Balance   12%   is   cementite.
Therefore   the   ratio   of   the   widths   of   the   two   =   12/88.
This   is   approximately   equal   to
1:7.
4.
0.8
%   carbon   corresponds   to   an   eutectoid   steel.
It   would   first   transform   into   austenite   and   then   it   starts   melting   when   the   temperature   reaches   its   solidus   temperature.
It   becomes   totally   liquid   when   temperature   goes   beyond   its   liquidus.
Shematic   heating   curve   is   as   follows:
T   liquid
L+



+cm:   Pearlite
Time
5.
Assume just
%   above
C
in   eutectic eutectoid
=
=
4.3.
.
.
%
.
.
Cm   just
100
below   eutectic
60%
temperature
Between   eutectic
= to
.
.
.
.
eutectoid
100
49% and temperatures proeutectoid   cementite   precipitates   from   austenite   in   eutectic.
On   subsequent   cooling   the
austenite   transforms   into   pearlite.
At   room   temperatutre   what   we   have   is   transformed   eutectic.
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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17
Iron   –   Cementite   Meta ‐ stable   Binary   Phase   diagram
1539
0.08
T°C
1495


+   L
1394
0.18

910
727   

+


Cm

+   L   =

=
+

P

+
0.55
Cm
Cm

+
+Cm
P

+   L
P
L

+
+   Cm
TLD
L   =
+

+   Cm
LD
+Cm

+   C m    Hypo ‐ eutectoid   steel
Hyper ‐ eutectoid   steel
0
Fe
0.02
0.8
2.0
Wt%   C
Hypo ‐ eutectic   white   cast   iron
4.3
Cm+
Cm   +
L
+Cm
LD
TLD
Hyper ‐ eutectic   white   cast   iron
1550
1146
6.67
Cm
727
The   above   sketch   is   a   little   enlarged   version   of   the   phase   diagram.
Fe   &   Fe
3
C   (also   known   as   cementite)   are   the   two   components.
Nomenclature:   P   denotes   Pearlite   the   product   of   eutectoid   reaction   in   iron   carbon   system.
It   is   a   mixture   of   ferrite   and   cementite.
Ferrite   =

,

=   austenite,

=   high   temperature   ferrite,   LD   =   Ledebutite:   the   product   of   the   eutectic   reaction.
Note   that   austenite   can   form   either   by   peritectic   reaction   between

&   L   and   also   by   direct   precipitation   from   liquid   when   %   C   >   0.55.
TLD   =   transformed   Ledeburite.
Ledeburite   is   a   eutectic   mixture   of   austenite   and   cementite.
At   room   temperature   austenite   is   not   stable.
It   transforms   into   Pearlite.
TLD   therefore   consists   of   tiny   nodules   of   pearlite   in   cementite   matrix.
It   also   describes   the   main   structural   features   of   five   types   of   iron   carbon   alloys.
Low   carbon   steel   consists   of   ferrite   and   tertiary   cementite.
Hypo ‐ eutectoid   steel   has   ferrite,   pro ‐ eutectoid   cememtite   and   pearlite.
Hyper ‐ eutectoid   steel   has   pro ‐ eutectoid   cementite   (secondary)   and   pearlite.
Hypo ‐ eutectic   cast   iron   has   pearlite,   transformed   ledeburite   and   pro ‐ eutectoid   (secondary)   cementite.
Hyper ‐ eutectic   cast   iron   has   primary   cementite   and   transformed   ledeburite.
So   far   evolution   of   microstructure   that   takes   place   during   solidification   of   iron   –   carbon   alloys   has   been   explained   with   the   help   of   schematic   microstructure.
The   following   page   gives   a   few   typical   microstructures   of   steel   and   cast   iron.
These   were   prepared   by   some   of   my   students   and   colleagues   at   I   I   T   Kharagpur   and   CSIR ‐ NML
Jamshedpur.
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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Typical   microstructures   of   steel   and   cast   iron
Fig   A1:   Microstructure   of   0.15%   carbon   steel   consisting   of   ferrite   (bright)   and   pearlite   (dark).
Lamellar   feature   pearlite   is   not   visible.
(Curtsey   G
Das   CSIR ‐ NML   Jamshedpur)
Fig   A2:   Microstructure   of   medium   carbon   steel   consisting   of   ferrite   (bright)   and   pearlite   (dark).
Amount   of   pearlite   increases   with   %   carbon.
(Curtsey   G   Das   CSIR ‐ NML   Jamshedpur).
Fig   A3:   Microstructure   of   a   near   eutectoid   steel   having   a   little   pro ‐ eutectoid   cementite   in   a   matrix   of   pearlite.
In   some   of   regions   finer   details   of   pearlitic   (lamellar)   structure   is   visible.
Fig   A4:   SEM   image   of   0.84%   C   steel   having   nearly
100%   pearlite.
Lamellar   feature   is   clearly   visible.
The   dark   region   is   pro ‐ eutectoid   cementite.
(Curtsey   G   Das   CSIR ‐ NML   Jamshedpur)
18
Fig   A5 :   Microstructure   of   hyper ‐ eutectic   white   cast   iron   showing   primary   carbides   (cm).
The   rest   of   the   region   consists   of   a   mixture   of   pearlite   and
Fig   A6:   Microstructure   of   grey   cast   iron   having   graphite   flakes   in   a   matrix   consisting   of   mostly   ferrite   and   some   amount   of   pearlite.
(Curtsey   G
cementite   or   transformed   eutectic   (ledeburite).
Das   CSIR ‐ NML   Jamshedpur)
NPTEL   Phase   II   :   IIT   Kharagpur   :   Prof.
R.
N.
Ghosh,   Dept   of   Metallurgical   and   Materials   Engineering    ||   |
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