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 727oC.
 As the alloy crosses the eutectoid reaction line at 727oC, 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.
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2. 2. SLOW COOLING OF HYPO-EUTECTOID STEEL (LESSTHAN 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
y1.
 This line is known as the upper-critical-temperature line on the hypo-
eutectoid side and is labeled A3.
 The allotropic change from FCC to BCC iron takes place at 912oC for pure iron and
decreases in temperature with increasing carbon content, as shown by the A3 line.At
y1 ferrite 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.
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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
A3 line.
 Finally, the line HJ is reached at point y2.
 This line is known as lower-critical-temperature line on the hypo-
eutectoid side and is labeled A1.
 The A1 line is the eutectoid-temperature line and is the lowest temperature
at which FCC iron can exist under equilibrium conditions.
 Just above the A1 line, 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
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4. MTE/III SEMSTER/MSE/MTE 2101 4
 Lever rule is used in conjunction with a tie line that
extends from α-(α+Fe₃C)the phase boundary (0.022
wt% C) to the eutectoid composition (0.76 wt% C), in as
much as pearlite is the transformation product of
austenite having this composition.
 For example, let us consider an alloy of composition Co´.
Thus, the fraction of pearlite Wp, may be determined
according to
 The fraction of proeutectoid α,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 z1.
 This line is known as the upper-critical temperature line on the hyper-
eutectoid side and is denoted by Acm.
 The Acm line shows the maximum amount of carbon that can be
dissolved in austenite as a function of temperature.
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6.  Above the Acm line, austenite is an unsaturated solid solution.At Acm
line, at point z1, 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 Acm line towards
point J.
 Therefore, as the temperature decreases from z1 to z2, 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 z2.This line is
called the lower-critical temperature line on the hyper-
eutectoid side and is denoted by A3,1.
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7.  Just above A3,1 line, the microstructure consists of largely austenite,
with the excess proeutectoid cementite as a network surrounding the
austinite grains.
 The A3,1 line for hyper-eutectoid steel represents the beginning and the
end of the allotropic change from FCC austenite to BCC ferrite.
 At z2, the remaining austenite (containing 0.76%C) transforms to the
eutectoid mixture, pearlite.
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8. MTE/III SEMSTER/MSE/MTE 2101 8
 The Lever rule expression is:

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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
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