1. MECHANICAL ENGINEERING MATERIALS
(1625304)
UNIT-2: Ferrous Metals and Alloys
Cooling Curve of Pure Iron, Phase equilibrium diagram for Iron and Iron Carbide
Introduction, Phase equilibrium diagram for Iron and Iron Carbide, Three Invariant
Reactions, Definitions, Solubility of Carbon in various Phases, Critical Temperature Lines &
their significance
Er. Prakash Kumar
Lecturer
Department of Mechanical Engineering
Government Polytechnic Katihar
Department of Science and Technology
Patna, Bihar
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2. MECHANICAL ENGINEERING MATERIALS (1625304)
• UNIT-1 : Engineering Materials and their Properties
• UNIT-2 : Ferrous Metals and Alloys
• UNIT-3 : Non Ferrous Metals and Alloys
• UNIT-4 : Heat Treatment of Steels
• UNIT-5 : Non Metallic Materials
• UNIT-6 : Powder Metallurgy & Nondestructive Testing
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3. MECHANICAL ENGINEERING MATERIALS (1625304)
UNIT-2 : Ferrous Metals and Alloys
18 Marks
• Module-1 : Characteristics and application of ferrous metals
• Module-2 : Phase equilibrium diagram for Iron and Iron Carbide.
• Module-3 : Flow diagram for production of Iron and Steel, Classification, composition
and uses of cast iron, effect of sulphur, silicon and phosphorous.
• Module-4 : Classification, composition and application of low carbon steel, medium
carbon steel and high carbon steel with their chemical composition.
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4. MECHANICAL ENGINEERING MATERIALS (1625304)
UNIT-2 : Ferrous Metals and Alloys
18 Marks
• Module-5 : Alloy Steels: - Low alloy steel, high alloy steel, tools steel & stainless steel.
Effect of various alloying elements such as – Chromium, nickel, manganese,
molybdenum, tungsten, vanadium.
• Module-6 : Tool Steels: - High speed Steels (HSS), Hot & cold Working dies, shear,
punches etc., properties & applications.
• Module-7 : Magnetic materials: - Properties & Applications of commonly used
magnetic materials (Permanent magnets and temporary magnets).
• Module-8 : Special Cutting Tool Materials – Diamond, Stelites & Tungsten Carbide.
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Cooling Curve of Pure Iron
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2
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1. Magnetic Transformation
Line
2. Crystal Structure
Transformation Line
3. Crystal Structure
Transformation Line
4. State Transformation Line
• α-Iron-The structural form
of pure iron at room
temperature is called
ferrite or α -Iron.
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• Change of phase means either there is a change in microstructure or there is change in
the lattice structure.
• A phase diagram has temperature as its ordinate and alloy composition as abscissa.
• Curie point (768°C): There is no change in phase and only magnetic properties are
disappearing (Ferro to Para).
• Paramagnetic Materials: Electrons are unpaired and such alloys exhibit colour.
• Diamagnetic Materials: Electrons are paired and such alloys are colourless.
• Ferromagnetic: Iron behave as paramagnetic as well as diamagnetic.
Cooling Curve of Pure Iron contd…
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Phase equilibrium diagram for Iron and Iron Carbide Introduction
Fe - Fe3C phase diagram is characterized by four term:
• α–ferrite (BCC) Fe-C solid solution.
• γ-austenite (FCC) Fe-C solid solution.
• δ-ferrite (BCC) Fe-C solid solution.
• Fe3C (iron carbide) or cementite –Greater than 6.67% Carbon.
FCC Structure - γ is called Austenite from 9100C to 14000C
BCC Structure- α is called ferrite at lower temperature upto 9100C.
δ is called ferrite at higher temperature above 14000C.
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Phase equilibrium diagram for Iron and Iron Carbide
Iron Carbon Equilibrium Diagram
• Hypo Eutectoid Steel
(α + Pearlite) - 0% C to 0.8% C
• Hyper Eutectoid Steel
(Fe3C + Pearlite) – 0.8% C to 2.14% C
• Hypo Eutectic Cast Iron
(γ + Ledeburite) – 2.14% C to 4.3% C
• Hyper Eutectic Cast Iron
(Fe3C + Ledeburite) – 4.3% C to 6.67% C
• A - Peritectic Point (14950C)
• B – Eutectic Point (11470C)
• C - Eutectoid Point (7230C)
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Three Invariant Reactions
• Peritectic reaction at 14950C and 0.16%C
δ-ferrite + L↔ γ-iron (austenite)
• Eutectic reaction at 11470C and 4.3 %C
L ↔ γ-iron + Fe3C (cementite) [ledeburite]
• Eutectoid reaction at 7230C and 0.8%C
γ-iron ↔ α– ferrite + Fe3C (cementite) [pearlite]
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• Eutectoid Reaction
When one solid upon cooling converts into two different solids
γ ↔ α + Fe3C
(S) (S1) + (S2)
γ-austenite is not stable at 7230C, So carbon diffuses from one interstitial site to another &
forms alternate plate like structure of α + Fe3C called Pearlite.
• Eutectic Reaction
When liquid converts into two different solids upon cooling called Eutectic Reaction
L ↔ γ + Fe3C [ledeburite]
(L) (S1) + (S2)
Similarly Eutectic decomposition produces alternate plates of austenite & cementite &
these micro structure are called Ledeburite.
Pearlite & Ledeburite are not phases, these are phase mixture.
• Five phase of Iron
(1) α– ferrite, (2) γ-austenite, (3) δ-ferrite,( 4) cementite, (5) Martensite (Hardest Steel)
Invariant Reactions Contd…
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Definitions
• α– ferrite - Interstitial solid solution of carbon in α-Iron which is pure form of Iron
having BCC structure with average no. of the atom equal to 2 having magnetic
character upto 7680C.
• γ-austenite- Interstitial solid solution of carbon in γ-Iron which is pure form of Iron
having FCC structure with average no. of the atom equal to 4 having non magnetic
character.
• δ-ferrite - Interstitial solid solution of carbon in δ-Iron which is pure form of Iron having
BCC structure with average no. of the atom equal to 2 having non magnetic character.
• Fe3C (iron carbide) or cementite - Fe3C is also called Iron carbide. It is 6.67% C & 93.3%
iron properties. It is hard and brittle.
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Solubility of Carbon in various Phases
• Solubility of carbon in δ-ferrite is
0.1% C
• Solubility of carbon in
• γ-austenite is 2.14% C
• Solubility of carbon in δ-ferrite is
0.02% C
• Solubility of carbon in α– ferrite
at room temperature is 0.008% C
• Solubility of carbon in cementite
is 6.67% C
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• A1 Line – Lower critical temperature line. This line signifies the transformation of
Pearlite into austenite upon heating of eutectoid steel at 7230C.
• A2 Line – It is called as Curie point temperature line. This line signifies the magnetic
to non magnetic transformation which takes place upon heating of iron carbon alloys
at 7680C.
• A3 Line - It is known as Upper critical temperature line for hypo eutectoid steel. This
line signifies the transformation of ferrite into austenite upon heating of hypo
eutectoid steel at 9100C.
• Acm Line - It is known as Upper critical temperature line for hyper eutectoid steel.
This line signifies the transformation of cementite into austenite upon heating of
hyper eutectoid steel at 11470C.
Critical Temperature Lines & their significance
