Phase Equilibrium in FeC System

1
Phase Equilibrium in Fe-C System

• Fe-C system is the basis for various steels and
  cast irons - primary structural materials
• the useful section is between 0-6.7 wt C
• engineering steels contain up to 1 C
• cast irons contain 2-5 C
• Fe-C system involves some basic reactions which
  can be manipulated through heat treatment to
  bring about a number of useful microstructures

• You need to be familiar with principles studied
  under phase diagrams as well as those just
  learned related to phase transformation
• One of the lab experiment is about
  microstructures of a steel material

Reading 9.13-9.15 (5th ed) and 9.17-9.19 (6th ed)
2
Phase Equilibrium in Fe-C System

• The Fe-Fe3C phase diagram

100 cementite
2
1
Peritectic L d g
Eutectic L g Fe3C
3
Eutectoid g a Fe3C
3
Phase Equilibrium in Fe-C System

• Main phases
• a called ferrite
• BCC structure
• existing up to 912C (in pure Fe)
• forming solid solution with C up to 0.022 at
  727C (at other temperatures, the solubility is
  lower)
• g called austenite
• FCC structure
• existing from 912-1394C (in pure Fe)
• forming solid solution with C up to 2.14 at
  1147C (at other temperatures, the solubility is
  lower)
• non-magnetic

• d called d-ferrite
• BCC structure
• existing up to 1394-1538C (in pure Fe)
• not of technogical importance
• Fe3C called cementite
• formed when too much C is added into a or g
• orthorhombic structure
• hard and brittle - its amount, shape and
  distribution affect greatly mechanical properties
• Engineering materials
• pure iron lt 0.008 C
• steels 0.008-2.14 C
• Cast iron gt 2.14 C

4
Phase Equilibrium in Fe-C System

• Equilibrium microstructures in Fe-C alloys
• Alloy of the eutectoid composition
• g grains in the single g region (e.g. point a)
• at 727C, eutectoid reaction
• g a Fe3C
• the amount of a and Fe3C can be calculated using
  the lever law (note Fe3C has 6.7 wt C, not shown
  here)
• the resulting eutectoid structure consisting of
  alternating lamellae of a and Fe3C, called
  pearlite
• little change on further cooling

0.76 wt C
Eutectoid composition
5
Phase Equilibrium in Fe-C System

• formation of the pearlite structure
• nucleating at g grain boundaries
• growth by diffusion of C to achieve the
  compositions of a and Fe3C (with structural
  changes)
• a lamellae much thicker

• microstructure of the pearlite under optical
  microscope

Fe3C
a
pearlite grain or colony
6
Phase Equilibrium in Fe-C System

• Alloys of hypoeutectoid compositions (0.022-0.76
  C)
• point c single g grains
• point A a starts to form (at g grain boundaries)
  just under A
• point d more a grains form (both in number and
  in size)
• composition of a follows the yellow dots and that
  of g the red dots (lever law for amount of each)
• at 727C g of composition O undergoes eutectoid
  reaction to form pearlite while a of composition
  N remains unchanged
• below 727C little further changes

A
727C
formed before the eutectoid reaction
HW calculate the fraction of the pearlite and
that of the proeutectic a at just below 727C.
7
Phase Equilibrium in Fe-C System
Fe-0.38C, optical microscopy
pearlite
eutectoid Fe3C
proeutectoid a
Fe-0.44C, scanning electron microscopy
8
Phase Equilibrium in Fe-C System

• Alloys of hypereutectoid compositions (0.76-2.14
  C)
• point g single g grains
• point B Fe3C starts to form (at g grain
  boundaries) just under B
• point h more a grains form (both in number and
  in size)
• composition of g follows the red dots and that of
  Fe3C does not change (6.7 lever law for amount
  of each)
• at 727C g of composition O undergoes eutectoid
  reaction to form pearlite while Fe3C of
  composition (6.7) remains unchanged
• below 727C little further changes

B
727C
formed before the eutectoid reaction
HW calculate the fraction of the pearlite and
that of the proeutectic cementite at just below
727C.
9
Phase Equilibrium in Fe-C System

• optical microstructure of an Fe-1.4C alloy

pearlite
proeutectoid cementite
10
Phase Equilibrium in Fe-C System

• Example 9.4

Find various fractions at this temperature for an
alloy of Fe0.35C
amount of a in the pearlite
0.35