IRON-IRON CARBIDE Phase Diagram

Development of Microstructure in Fe-FeC Phase Diagram

Reference: Material Science and engineering,William Callister

1. DEVELOPMENT OF MICROSTRUCTURE IN FE-C
ALLOYS
1. SLOW COOLING OF EUTECTOID STEEL (0.76% C -STEEL)
The figure shows the slow cooling of 0.76% C eutectoid steel.
In the austenite range, this alloy consists of a uniform interstitial solid
solution.
Each grain of contains 0.76% C dissolved in the spaces of the FCC iron
lattice structure.
Nothing happens to austenite until the alloy is cooled to the eutectoid
temperature of 727
o
C.
As the alloy crosses the eutectoid reaction line at 727
o
C, austenite
undergoes the eutectoid reaction to form the eutectoid mixture pearlite.
The room temperature microstructure reveals the presence of uniform
alternate layers of ferrite and cementite in each grain of pearlite.
MTE/III SEMSTER/MSE/MTE 2101 1

2. 2. SLOW COOLING OF HYPO -EUTECTOID STEEL (LESS THAN 0.76% C -
STEEL)
The figure depicts the slow cooling of 0.2% C hypo-eutectoid steel.
In the austenite range, this alloy consists of a uniform interstitial solid solution.
Each grain of contains 0.2% C dissolved in the spaces of the FCC iron lattice
structure.
Upon slow cooling, the austenite phase is retained until the line GJ is crossed at point
y
1.
This line is known as the upper-critical-temperature line on the hypo-
eutectoid sideand is labeled A
3.
The allotropic change from FCC to BCC iron takes place at 912
o
C for pure iron and
decreases in temperature with increasing carbon content, as shown by the A
3line. At
y
1ferrite must begin to form at the austenite grain boundaries.
Since ferrite can dissolve very little carbon, carbon atoms must come out of the
solution where ferrite is forming. Fe atoms rearrange themselves into BCC.
MTE/III SEMSTER/MSE/MTE 2101 2

3. The carbon which comes out of the solution dissolves in the remaining
austenite.
Carbon content in austenite gradually moves down to the right along the
A
3line.
Finally, the line HJ is reached at point y
2.
This line is known as lower-critical-temperature line on the hypo-
eutectoid sideand is labeled A
1.
The A
1line is the eutectoid-temperature line and is the lowest temperature
at which FCC iron can exist under equilibrium conditions.
Just above the A
1line, the microstructure consists of approximately 25% 
(austenite) and 75% α(ferrite).
The remaining about 25% of the total material and containing 0.8% C
now experiences the eutectoid reaction
MTE/III SEMSTER/MSE/MTE 2101 3

4. MTE/III SEMSTER/MSE/MTE 2101 4
Leverruleisusedinconjunctionwithatielinethat
extendsfromα-(α+Fe₃C)thephaseboundary(0.022
wt%C)totheeutectoidcomposition(0.76wt%C),inas
muchaspearliteisthetransformationproductof
austenitehavingthiscomposition.
Forexample,letusconsideranalloyofcompositionCo´.
Thus,thefractionofpearliteWp,maybedetermined
accordingto
Thefractionofproeutectoidα,Wα

5. 3. SLOW COOLING OF HYPER -EUTECTOID STEEL (GREATER
THAN 0.76% C-STEEL)
The figure shows the slow cooling of a hyper-eutectoid steel containing
1% C.
In the austenite range, this alloy consists of a uniform FCC interstitial solid
solution.
Each grain of contains 1% C dissolved in the spaces of the FCC iron lattice
structure.
Upon slow cooling nothing happens to austenite until the line CJ is crossed
at z
1.
This line is known as the upper-critical temperature line on the hyper-
eutectoid sideand is denoted by A
cm.
The A
cmline shows the maximum amount of carbon that can be
dissolved in austenite as a function of temperature.
MTE/III SEMSTER/MSE/MTE 2101 5

6. Above the A
cmline, austenite is an unsaturated solid solution. At A
cm
line, at point z
1, the austenite is saturated in carbon.
As the temperature is decreased, the maximum amount of carbon that
can be dissolved in austenite, moves down along the A
cmline towards
point J.
Therefore, as the temperature decreases from z
1to z
2, the excess
carbon above the amount required to saturate austenite is precipitated
as cementite primarily along the grain boundaries.
Finally, the eutectoid temperature line is reached at z
2. This line is
called the lower-critical temperature line on the hyper-
eutectoid sideand is denoted by A
3,1.
MTE/III SEMSTER/MSE/MTE 2101 6

7. Just above A
3,1 line, the microstructure consists of largely austenite,
with the excess proeutectoidcementite as a network surrounding the
austinitegrains.
The A
3,1 line for hyper-eutectoid steel represents the beginning and the
end of the allotropic change from FCC austenite to BCC ferrite.
At z
2, the remaining austenite (containing 0.76%C) transforms to the
eutectoid mixture, pearlite.
MTE/III SEMSTER/MSE/MTE 2101 7

8. MTE/III SEMSTER/MSE/MTE 2101 8
The Lever rule expression is:

9. MTE/III SEMSTER/MSE/MTE 2101 9

10. Determination of Relative Amounts of Ferrite,
Cementite, and Pearlite Micro constituents
1.) For a 99.65 wt% Fe–0.35 wt% C alloy at a
temperature just below the eutectoid, determine the
following:
(a) The fractions of total ferrite and cementite phases
(b) The fractions of the pro-eutectoid ferrite and
pearlite
(c) The fraction of eutectoid ferrite
MTE/III SEMSTER/MSE/MTE 2101 10