the iron–iron carbide (fe–fe3c) phase

8/9/2019 The Iron–Iron Carbide (Fe–Fe3C) Phase

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Materials Science I

Dr Ali Salami Golezani Page 1

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.


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Materials Science I


Phases in the Fe–Fe3C & Fee-Fe3C System

Phase C.Struc Description and Comments

Liquid - Liquid solution of C in Fe.

δ b.c.cRandom 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

γ(austenite) f.c.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

α (also called “ferrite”) b.c.cRandom interstitial solid solution of C in b.c.c. Fe. Maximumsolubility of 0.035 wt% C occurs at 723°C. Pure α Fe is the stable

polymorph below 914°C

Fe3C (iron carbide orcementite)

Complex A hard and brittle chemical compound of Fe and C containing 25atomic % (6.7 wt%) C.

Pearlite-

The composite eutectoid structure of alternating plates of α andFe3C 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 iswrong to say that Pearlite forms in grains: we say instead that it

forms in nodules.

Ledeburite .-

The composite eutectic structure of alternating plates of g andFe3C produced when liquid containing 4.3 wt% C is cooled below

1130°C. Again, not a phase! Ledeburite only occurs during thesolidification 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

Microstructures During the Slow Cooling of a Eutectoid Steel from the Hot Working

Temperature

microstructure of a 0.76%C steel


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Materials Science I


Microstructures During the Slow Cooling of a Hypoeutectoid Steel from the Hot Working

Temperature


Microstructures During the Slow Cooling of a Hypereutectoid Steel.



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Materials Science I


5-2-Austenite Transformation

5-3-Steels

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

Medium Carbon Steels

O.25 wt%< C


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Materials Science I


2. Often used in hardened and tempered

condition3. Cutting tools contain Cr, V, W, Mo carbide

formers

4.

Used in knives, razor blades, springs, wire Stainless Steels

1. Cr used for improving corrosion resistance

2. Cr content > 11wt%

3. Ni and Mo increase corrosion resistance

4.

Martensitic, ferritic, and austenitic stainlesssteels

5. Austenitic stainless steels not heat treatable


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Materials Science I


5-4-Cast Irons

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

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

White Iron

1 % Si, Carbon exists as cementite. Hard and

very brittle

Maleable Iron

Heat white iron to 800-900°C for long times

in neutral atmosphere. Cementite forms

graphite clusters. Strong and reasonably

ductile


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Materials Science I


the iron–iron carbide (fe–fe3c) phase

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