Download - 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
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