2. Iron – Carbon Phase Diagram

Iron – Carbon Phase Diagram

Some basic terminology

Phase: A phase may be defined as a homogeneous portion of a system that has uniform physical and chemical characteristics. Every pure material is considered to be a phase; so also is every solid, liquid, and gaseous solution.

Component: Components are pure metals and/or compounds of which an alloy is composed.

Phase Diagram

“A phase diagram is a way to represent the various phases of a substance and the conditions under which each phase exists. ”

Water phase diagram

IMPORTANCE OF PHASE DIAGRAMS

There is a strong correlation between microstructure and mechanical properties, and the development of microstructure of an alloy is related to the characteristics of its phase diagram.

Also phase diagrams provide valuable information about melting, casting, crystallization, and other phenomena.

Fe-C Phase Diagram

Strucutres in the Diagram - Ferrite

Ferrite Also known as α iron. BCC solution of C in Fe Stable at room

temperature Maximum solubility of

Carbon – 0.022% at 727 °C

Maximum solubility of C at room temperature – 0.008%

Maximum temperature of existence – 912 °C

Is Magnetic upto 768 °C

Softest phase Maximum ductility Low tensile stength

Structures in the diagram - Austenite

Austenite Also known as γ iron FCC solution of C in Fe Not stable at room

temperature Maximum solubility of

Carbon – 2.14% at 1147 °C

Temperature range of existence – 727 °C – 1493° C

Is non magnetic Hard phase Low ductility High toughness

Structures in the diagram - δ iron

δ Iron BCC solution of C in Fe Temperature range is 1394 – 1538 °C Maximum solubility of C – 0.1 % Non magnetic Properties similar to Ferrite

Structures in the diagram - Cementite

Cementite It is a compound of Fe

and C – Fe3C The wt% of C in

cementite is 6.70 % Orthorhombic crystal

structure Exists at room

temperature The hardest structure

in the diagram The most brittle

structure Lowest tensile

strength High compressive

strength

Structures in the diagram - Pearlite

Pearlite Contains 0.76 % C Formed by slow

cooling at 727 °C. Very fine lamellar

mixture of ferrite and cementite

The white background is ferrite containing thin plates of cementite.

High tensile strength Medium ductility Medium hardness

Development of microstructure – Pearlitic steels

Formation Of Pearlite

Development of microstructure – Hypoeutectoid steels

Microstructure of hypoeutectoid steels

Photomicrograph of a 0.38 wt% C steel having a microstructure consisting of pearlite and proeutectoid ferrite. 635X

Development of microstructure – Hypereutectoid steels

Microstructure of hypereutectoid steels

Photomicrograph of a 1.4 wt% C steel having a microstructure consisting of a white proeutectoid cementite network surrounding the pearlite colonies. 1000X

What should you know?

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