2. PHASE DIAGRAMS, FERROUS METALS AND ITS ALLOYS
2.1 Isomorphs, Eutectic and Eutectoid Systems
Isomorphous Systems: In an isomorphous system, the two components are completely soluble in each other in the liquid and solid states. An example is the Cu-Ni (Copper-Nickel) alloy system. The phase diagram of an isomorphous system shows a single phase field at all compositions.
Eutectic System: A eutectic system is one in which a mixture of two elements, when cooled from a liquid state, forms two solid phases at a specific composition and temperature. The eutectic point represents the lowest temperature at which the liquid phase can coexist with the two solid phases. In a phase diagram, the eutectic point is marked as the composition and temperature where the solidification occurs at a fixed temperature.
Example: The Pb-Sn (Lead-Tin) alloy system is a classic eutectic system. At the eutectic composition (Pb:Sn = 61.9%:38.1%), the alloy solidifies at a constant temperature of 183°C.
Eutectoid System: A eutectoid system involves a solid-to-solid phase transformation at a specific temperature and composition. When cooled from the liquid phase, a single solid phase transforms into two distinct solid phases at the eutectoid composition and temperature.
Example: In the Fe-C (Iron-Carbon) system, the eutectoid composition is 0.8% C at 727°C, where the phase transformation from austenite (γ-phase) to a mixture of pearlite (ferrite + cementite) occurs.
Phase Diagram for Eutectic System (Pb-Sn Alloy):
2.2 Iron-Carbon Binary Diagram
The Iron-Carbon Phase Diagram is one of the most important phase diagrams for understanding the behavior of steel and cast iron alloys. It shows the phases present at different temperatures and carbon compositions.
- Key Phases in the Iron-Carbon Diagram:
- Ferrite (α-iron): A body-centered cubic (BCC) structure, which can dissolve only a small amount of carbon (up to 0.022% C).
- Austenite (γ-iron): A face-centered cubic (FCC) structure, capable of dissolving more carbon (up to 2.11% C at 1147°C).
- Cementite (Fe₃C): A hard, brittle compound of iron and carbon, consisting of 6.67% carbon by weight.
- Pearlite: A mixture of ferrite and cementite formed at the eutectoid composition (0.8% C).
- Important Regions in the Diagram:
- Eutectoid Point (0.8% C, 727°C): The temperature and composition where austenite transforms into pearlite.
- Eutectic Point (4.3% C, 1147°C): The point at which liquid iron solidifies into a mixture of austenite and cementite.
Iron-Carbon Phase Diagram:
2.3 Iron and Carbon Steels
- Iron and Carbon Steels are alloys of iron with carbon as the primary alloying element. Based on the carbon content, these can be classified as follows:
- Low Carbon Steel (0.05-0.3% C): Malleable and ductile, used for structural components, automotive bodies.
- Medium Carbon Steel (0.3-0.6% C): Balanced properties of strength and ductility, used for gears, shafts.
- High Carbon Steel (0.6-1.0% C): Hard and strong, used for cutting tools, springs.
Carbon content affects the properties of the steel, such as strength, hardness, and ductility. Higher carbon content increases hardness but reduces ductility.
2.4 Flow Sheet for Production of Iron and Steel
The production of iron and steel typically involves the following steps:
- Extraction of Iron Ore: Iron ore is mined from the earth.
- Blast Furnace: Iron ore, coke, and limestone are added to the blast furnace. The coke acts as a reducing agent, and limestone helps remove impurities as slag.
- Pig Iron: The molten iron produced in the blast furnace is called pig iron. It has a high carbon content (3-4%).
- Steelmaking: The pig iron is then refined in a steelmaking furnace (e.g., basic oxygen furnace or electric arc furnace) to reduce carbon and other impurities.
- Casting and Rolling: The refined steel is cast into various shapes and rolled into the desired thickness.
Flow Chart for Iron and Steel Production:
2.5 Iron Ores
The primary iron ores used in the production of iron are:
- Hematite (Fe₂O₃): The most common ore, which is rich in iron.
- Magnetite (Fe₃O₄): A magnetic iron ore with a high iron content.
- Limonite (FeO(OH)·nH₂O): A lower-grade ore.
- Siderite (FeCO₃): An iron carbonate ore.
2.6 Pig Iron
Pig Iron is the crude iron produced directly from the blast furnace.
- Classification: Based on carbon content:
- White Pig Iron: Low in impurities, but very hard and brittle.
- Gray Pig Iron: Contains graphite flakes, which make it more machinable.
- Classification: Based on carbon content:
Composition: Pig iron typically contains:
- Carbon: 3-4%
- Silicon: 1-3%
- Manganese: 0.3-1.5%
- Phosphorus and Sulfur: Trace amounts (impurities).
Effects of Impurities:
- Phosphorus: Makes iron brittle.
- Sulfur: Reduces strength and ductility.
- Manganese: Improves hardness and strength.
2.7 Cast Iron
Classification: Cast iron is classified based on its carbon content and structure:
- Gray Cast Iron: Contains graphite flakes, good machinability.
- White Cast Iron: Contains cementite and is very hard.
- Malleable Cast Iron: Heat-treated to improve ductility.
- Ductile Cast Iron: Contains nodular graphite, offering improved ductility.
Composition:
- Carbon: 2-4%
- Silicon: 1-3%
- Manganese: 0.5-1.5%
Properties and Uses:
- Good castability and machinability.
- Used for engine blocks, pipes, machinery.
2.8 Wrought Iron
Properties: Wrought iron is pure iron (99.5% Fe), malleable, ductile, and corrosion-resistant.
Uses/Applications:
- Historically used in construction, fencing, and railings.
- Replaced by mild steel in most applications but still used for decorative work.
2.9 Comparison of Cast Iron, Wrought Iron, Mild Steel, and High Carbon Steel
| Property | Cast Iron | Wrought Iron | Mild Steel | High Carbon Steel |
|---|---|---|---|---|
| Carbon Content | 2-4% | 0.02-0.08% | 0.05-0.25% | 0.6-1.5% |
| Strength | Moderate | Low | Moderate | High |
| Ductility | Low | High | High | Low |
| Uses | Pipes, machinery parts | Fencing, decorative | Structural components | Cutting tools, springs |
2.10 Standard Commercial Grades of Steel (BIS and AISI)
BIS (Bureau of Indian Standards): In India, steel grades are classified according to IS codes (Indian Standard). Examples include IS 2062 (mild steel).
AISI (American Iron and Steel Institute): AISI standardizes steel grades with a numbering system like AISI 1020 (low carbon steel) or AISI 304 (stainless steel).
2.11 Alloy Steels – Types and Uses
Alloy steels contain additional elements such as chromium, nickel, or molybdenum to improve properties.
Types of Alloy Steels:
- Low Alloy Steels: Contain up to 5% of alloying elements (e.g., Ni, Cr).
- High Alloy Steels: Contain more than 5% of alloying elements.
Uses: Alloy steels are used for tools, aerospace, automotive, and industrial applications.
2.11.1 Stainless Steels – Types and Uses
Types of Stainless Steels:
- Austenitic Stainless Steel (e.g., 304, 316): Non-magnetic, excellent corrosion resistance, used in food processing, and medical equipment.
- Ferritic Stainless Steel (e.g., 430): Magnetic, good oxidation resistance, used in automotive exhaust systems.
- Martensitic Stainless Steel (e.g., 410, 420): Hard and strong, used for cutting tools, knives.
Uses: Stainless steel is used in kitchenware, medical devices, aerospace, and marine industries due to its corrosion resistance.
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