Iron carbon diagram

Fe-C diagram, eutectic point, eutectoid point

1. Material Science and Technology

Unit 1 Lecture 13
By:
Saumy Agarwal
Asst. Professor (MED)
BTKIT Dwarahat

2. Iron- Carbon Diagram
•The iron-carbon phase diagram is widely used to
understand the different phases of steel and cast
iron. Both steel and cast iron are a mix of iron
and carbon.
•The Fe–Fe3C diagram contains the following
solid phases: α ferrite, austenite (γ), cementite
(Fe
3C), and δ ferrite.
•In practice, all steels and cast irons have carbon
contents less than 6.7 wt% C

3. 6.70 wt% C corresponds to 100 wt% Fe
3C

4. Solid Phases in the Fe–Fe
3C Phase
Diagram
α-ferrite: This phase is an interstitial solid
solution of carbon in the BCC iron (α-Fe) crystal
lattice.
•Carbon is only slightly soluble in α-ferrite,
reaching a maximum solid solubility of 0.022%
at 727°C.
•The solubility of carbon in α ferrite decreases to
0.008% at 0°C.

5. Austenite (γ): The interstitial solid solution of
carbon in γ-iron is called austenite.
•Austenite has an FCC crystal structure.
•The solid solubility of carbon in austenite is a
maximum of 2.1% at 1148°C and decreases to
0.76% at 727°C
•It is soft, ductile, malleable and paramagnetic.

6. δ-ferrite: The interstitial solid solution of carbon
in δ iron is called δ-ferrite.
•It has a BCC crystal structure like α ferrite but
with a greater lattice constant.

Cementite: It is a metastable and intermediate
phase of this alloy with a fixed composition of
carbon content of 6.7% by weight..
•When heated at about 700
o
C, it decomposes
extremely slowly into Iron and carbon
(graphite).
•Fe
3C has Orthorhombic crystal structure.
•It is hard and brittle.

7. Invariant Reactions in the Fe–Fe
3C
Phase Diagram
Peritectic Reaction :
At the peritectic reaction point, liquid of 0.55% C
combines with δ-ferrite of 0.1% C to form γ-
austenite of 0.18% C. This reaction, which occurs
at 1493°C, can be written as

Liquid (0.55% C)+ δ (0.1% C) γ (0.18% C)
(No liquid trace)
1493
o
C

9. Eutectic Reaction: At the eutectic reaction
point, liquid of 4.3% forms γ austenite of 2.1% C
and the intermetallic compound Fe3C (cementite),
which contains 6.67% C. This reaction, which
occurs at 1148°C, can be written as

Liquid (4.3% C) γ (2.1% C) + Fe
3C (6.67% C)


1148
o
C

10. Eutectoid Reaction: At the eutectoid reaction
point, solid austenite of 0.76% C produces α-
ferrite with 0.022% C and Fe
3C (cementite) that
contains 6.67% C. This reaction, which occurs at
727°C, can be written as

γ (0.76% C) α-ferrite (0.022% C) + Fe
3C (6.67% C)
(Pearlite)


727
o
C