Phases in the Fe-Fe₃C Diagram
Ferrite is a solid solution of carbon in body-centred cubic (BCC) iron and is a major phase in steels and cast irons. It is known for its softness, ductility, and magnetic properties and plays a crucial role in determining the mechanical behavior of iron-carbon alloys.
Ferrite forms when austenite (γ-Fe) transforms below 912°C.
Exists in hypoeutectoid steels (0.022 - 0.76% C) at room temperature.
Below 727°C, remaining austenite transforms into pearlite (α + Fe₃C).
Reactions Involving Ferrite
1️⃣ Austenite → Ferrite
γ→α (Below 912°C in pure iron)
γ→α+Fe3C (Eutectoid reaction at 727°C)
2️⃣ Delta Ferrite (δ-Fe) Formation
High-temperature phase, stable above 1394°C.
✅ Mild Steel (Low-Carbon Steel): Construction materials, pipelines, car bodies.
✅ Transformer Cores: Used due to ferrite’s magnetic properties.
✅ Stainless Steels: Ferritic stainless steels (e.g., SS 430, SS 409).
✅ Welding & Heat Treatment: Helps improve toughness and formability.
Austenite is a solid solution of carbon in face-centered cubic (FCC) iron (γ-Fe). It is an important phase in steels and cast irons, playing a key role in heat treatment processes such as quenching and annealing.
Pure iron transforms into austenite at 912°C (from ferrite).
Austenite exists between 912°C and 1394°C in pure iron.
In steels, austenite is present at high temperatures and transforms into other phases during cooling.
1️⃣ Ferrite (α) → Austenite (γ)
Occurs above 912°C in low-carbon steel.
2️⃣ Austenite → Ferrite + Cementite (Fe₃C) (Eutectoid Reaction at 727°C)
γ→α+Fe3C (Forms pearlite in eutectoid steel).
3️⃣ Austenite → Martensite (During Rapid Cooling/Quenching)
Forms hard, brittle martensite, used in heat treatment.
Applications of Austenite
✅ Heat Treatment Processes: Quenching produces martensite from austenite.
✅ Stainless Steels: Austenitic stainless steels (e.g., 304, 316) have high corrosion resistance.
✅ High-Temperature Alloys: Austenitic steels are used in boilers, turbines, and heat exchangers.
✅ Non-Magnetic Applications: Used in industries requiring non-magnetic materials, such as medical implants and cryogenic applications.
Cementite (Fe₃C) is an iron carbide compound that plays a crucial role in the microstructure and mechanical properties of steels and cast irons. It is a hard, brittle phase that contributes to strength and wear resistance but reduces ductility.
Cementite forms in steels and cast irons during solidification and cooling. It appears in different forms based on carbon content and heat treatment.
1️⃣ Eutectoid Reaction (727°C, 0.76% C)
Austenite (γ) → Ferrite (α) + Cementite (Fe₃C) (Forms Pearlite)
γ→α+Fe3C
2️⃣ Eutectic Reaction (1147°C, 4.3% C)
Liquid (L) → Austenite (γ) + Cementite (Fe₃C) (Forms Ledeburite)
L→γ+Fe3C
3️⃣ Formation in Hypereutectoid Steels (>0.76% C)
Cementite forms at grain boundaries before pearlite transformation.
✅ Increases Hardness and Strength:
Found in tool steels, wear-resistant alloys, and white cast iron.
✅ Reduces Ductility and Toughness:
Excess cementite makes steel brittle and difficult to machine.
✅ Forms Pearlite for Balanced Strength and Toughness:
Pearlite (ferrite + cementite) provides a good mix of strength and ductility.
✅ Key Role in Heat Treatment:
Can dissolve, spheroidize, or transform under different heat treatments.
✅ Cutting Tools & Dies: High-speed steels contain cementite for hardness.
✅ Railway Tracks & Gears: High-strength steels rely on pearlitic cementite.
✅ Wear-Resistant Cast Irons: White cast iron (Fe₃C-rich) is used in crusher plates and grinding balls.
✅ Heat-Treated Steels: Used in hardened and tempered components.
Pearlite is a two-phase microstructure composed of alternating layers of ferrite (α-Fe) and cementite (Fe₃C). It forms in steels and cast irons during slow cooling and provides a balance of strength, hardness, and ductility.
Pearlite forms at 727°C during the eutectoid reaction in steel.
🔹 Austenite (γ) → Ferrite (α) + Cementite (Fe₃C)
γ→α+Fe3C
In hypoeutectoid steels (0.022 - 0.76% C): Forms along with proeutectoid ferrite.
In eutectoid steels (0.76% C): Fully pearlitic microstructure.
In hypereutectoid steels (0.76 - 2.14% C): Forms along with proeutectoid cementite.
✅ Strength and Hardness
Fine pearlite is harder and stronger than coarse pearlite.
Common in railway tracks, springs, and tool steels.
✅ Ductility and Toughness
Coarse pearlite provides better toughness.
Used in structural steels and automotive parts.
✅ Wear Resistance
Pearlite enhances wear resistance, making it useful in bearings and gears.
✅ Heat Treatment Influence
Slow cooling favors pearlite formation.
Faster cooling leads to bainite or martensite formation.
✅ Rails & Springs: Fine pearlite gives high strength.
✅ Automobile & Structural Steel: Coarse pearlite provides better toughness.
✅ Cutting Tools & Wear Parts: Pearlite enhances hardness & durability.
✅ Wire Drawing: Used in high-carbon steel (e.g., piano wire, tire cords).
Ledeburite is a eutectic mixture of austenite (γ-Fe) and cementite (Fe₃C) that forms in high-carbon alloys (cast irons, tool steels). It is extremely hard and brittle and is commonly found in white cast iron and high-carbon steels.
Ledeburite forms at 1147°C during the eutectic reaction in high-carbon iron alloys (>2.14% C).
🔹 Liquid (L) → Austenite (γ) + Cementite (Fe₃C)
🔹 L→γ+Fe3C
At 727°C, austenite (γ) transforms into pearlite (α + Fe₃C).
The final structure consists of pearlite + cementite (in slowly cooled cast irons).
✅ Extreme Hardness
Used in abrasion-resistant materials (e.g., crushing rollers, grinding balls).
✅ Brittleness
Excess cementite makes ledeburite very brittle.
✅ Wear Resistance
Common in wear-resistant cast irons.
✅ Difficult to Machine
Requires special heat treatment or alloying elements to improve machinability.
✅ White Cast Iron: Used in wear-resistant applications like crusher plates and grinding tools.
✅ Tool Steels: High-carbon steels with ledeburite provide excellent hardness.
✅ Rolling Mills & Dies: Require high wear resistance.
✅ High-Chromium Cast Irons: Used in mining and cement industries.
Delta ferrite (δ-Fe) is a high-temperature phase of iron with a body-centered cubic (BCC) structure. It is stable at very high temperatures (above 1394°C) and appears in certain stainless steels and high-temperature alloys to improve weldability and resistance to hot cracking.
Delta ferrite forms at very high temperatures and exists between 1394°C and 1538°C in pure iron.
1️⃣ During Heating (Pure Iron):
Ferrite (α) → Austenite (γ) → Delta Ferrite (δ) at 1394°C.
Delta Ferrite (δ) → Liquid (L) at 1538°C (melting point of iron).
2️⃣ In Stainless Steels:
Delta ferrite is stabilized by elements like Cr, Mo, and Si in high-temperature stainless steels.
Prevents weld cracking and improves high-temperature stability.
In stainless steels, delta ferrite is retained at room temperature depending on alloy composition.
✅ Prevents Hot Cracking in Welding
Delta ferrite reduces the risk of solidification cracking in welded stainless steels.
✅ Improves High-Temperature Stability
Used in heat-resistant alloys for furnaces, boilers, and exhaust systems.
✅ Enhances Corrosion Resistance
Forms a protective oxide layer in stainless steels (e.g., 304, 316, duplex steels).
✅ Reduces Toughness in Some Cases
High amounts of delta ferrite can lead to brittle behavior at low temperatures.
✅ Welded Stainless Steels (e.g., 304L, 316L): Prevents hot cracking.
✅ Duplex Stainless Steels: Contains a mixture of delta ferrite and austenite for high strength and corrosion resistance.
✅ Heat-Resistant Components: Used in furnaces, exhaust manifolds, and gas turbines.
✅ Cryogenic Applications: Controlled delta ferrite helps maintain toughness at low temperatures.