Materials Science I
6-Iron and Steel
6-1-The Iron–Iron Carbide (Fe–Fe3C) Phase
The iron-carbon phase diagram is important in engineering as it provides the basis for understanding all cast
irons and carbon steels and their heat treatment. For structural and mechanical applications, steels and other
alloys based on iron (the ferrous alloys) are the dominant engineering alloys. They are intrinsically stiff,
strong and tough, and mostly low cost.
Dr Ali Salami Golezani
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Materials Science I
¾
Phases in the Fe–Fe3C & Fee-Fe3C System
Phase
C.Struc
Description and Comments
Liquid
-
Liquid solution of C in Fe.
δ
b.c.c
γ(austenite)
f.c.c.
α (also called “ferrite”)
b.c.c
Fe3C (iron carbide or
cementite)
Complex
Pearlite
-
Ledeburite
.-
Dr Ali Salami Golezani
Random interstitial solid solution of C in b.c.c. Fe. Maximum
solubility of 0.08 wt% C occurs at 1492°C. Pure δ Fe is the stable
polymorph between 1391°C and 1536°C
Random interstitial solid solution of C in f.c.c. Fe. Maximum
solubility of 1.7 wt% C occurs at 1130°C. Pure γ Fe is the stable
polymorph between 914°C and 1391°C
Random interstitial solid solution of C in b.c.c. Fe. Maximum
solubility of 0.035 wt% C occurs at 723°C. Pure α Fe is the stable
polymorph below 914°C
A hard and brittle chemical compound of Fe and C containing 25
atomic % (6.7 wt%) C.
The composite eutectoid structure of alternating plates of α and
Fe3C produced when γ containing 0.80 wt% C is cooled below
723°CPearlite nucleates at γ grain boundaries. It occurs in low,
medium and high carbon steels. It is sometimes, quite wrongly,
called a phase. It is not a phase but is a mixture of the two separate
phases α and Fe3C in the proportions of 88.5% by weight of α to
11.5% by weight of Fe3C. Because grains are single crystals it is
wrong to say that Pearlite forms in grains: we say instead that it
forms in nodules.
The composite eutectic structure of alternating plates of g and
Fe3C produced when liquid containing 4.3 wt% C is cooled below
1130°C. Again, not a phase! Ledeburite only occurs during the
solidification of cast irons, and even then the g in ledeburite will
transform to a + Fe3C at 723°C
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Materials Science I
Microstructures During the Slow Cooling of Pure Iron from the Hot Working Temperature
¾ Microstructures During the Slow Cooling of a Eutectoid Steel from the Hot Working
Temperature
microstructure of a 0.76%C steel
Dr Ali Salami Golezani
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Materials Science I
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Microstructures During the Slow Cooling of a Hypoeutectoid Steel from the Hot Working
Temperature
microstructure of a 0.38%C steel
¾ Microstructures During the Slow Cooling of a Hypereutectoid Steel.
microstructure of a 1.4%C steel
Dr Ali Salami Golezani
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Materials Science I
5-2-Austenite Transformation
5-3-Steels
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Low Carbon Steels
C < 0.25 wt% (may include HSLA with Cu, V, Ni,
Mo additions)
1. Most steels are low carbon steels
2. Least hardenable
3. Relatively soft/weak but tough/ductile
¾ Medium Carbon Steels
O.25 wt%< C <0.60 wt%
1. Low hardenability improved by Cr, Ni
2. Heat treat by normalising and tempering
3. Stronger but less ductile/tough than low
carbon steels
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High Carbon Steels
O.6 wt% < C < 1.4 wt%
1. Strongest but least ductile
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Materials Science I
2. Often used in hardened and tempered
condition
3. Cutting tools contain Cr, V, W, Mo carbide
formers
4. Used in knives, razor blades, springs, wire
¾ Stainless Steels
1. Cr used for improving corrosion resistance
Dr Ali Salami Golezani
2. Cr content > 11wt%
3. Ni and Mo increase corrosion resistance
4. Martensitic, ferritic, and austenitic stainless
steels
5. Austenitic stainless steels not heat treatable
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Materials Science I
5-4-Cast Irons
¾
Gray Iron
C - 2.5 - 4%, Si - 1.0 - 3%, Silicon is a
graphite stabilizer, Weak and brittle in tension,
as graphite flakes act as cracks. graphite flakes,
weak & brittle in tension, stronger in
compression, excellent vibrational dampening,
wear resistant
¾ Ductile Iron
Mg or Ce causes the graphite to form nodules,
or spheres. Can have pearlite or ferrite
surrounding. Can have high strength and
reasonable ductility. raphite as nodules not
flakes, matrix often pearlite– tronger but less
ductile
¾
White Iron
1 % Si, Carbon exists as cementite. Hard and
very brittle
¾ Maleable Iron
Heat white iron to 800-900°C for long times
in neutral atmosphere. Cementite forms
graphite clusters. Strong and reasonably
ductile
Dr Ali Salami Golezani
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Materials Science I
Dr Ali Salami Golezani
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