The document discusses the iron-carbon phase diagram which is used to understand the different phases of steel and cast iron. The diagram shows the solid phases of α-ferrite, austenite (γ), cementite (Fe3C), and δ-ferrite that exist in steels and cast irons containing less than 6.7% carbon. It describes the crystal structures and carbon solubility ranges for each phase. Invariant reactions on the diagram are also summarized, including the peritectic, eutectic, and eutectoid reactions that occur between phases.

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
(Fe3C), 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% Fe3C

4. Solid Phases in the Fe–Fe3C 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 oC, it decomposes
extremely slowly into Iron and carbon
(graphite).
• Fe3C has Orthorhombic crystal structure.
• It is hard and brittle.

7. Invariant Reactions in the Fe–Fe3C
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)
1493oC

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) + Fe3C (6.67% C)
1148oC

10. Eutectoid Reaction: At the eutectoid reaction
point, solid austenite of 0.76% C produces α-
ferrite with 0.022% C and Fe3C (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) + Fe3C (6.67% C)
(Pearlite)
727oC