Phase Diagrams and Alloy Classification

Iron-Carbon Phase Diagram

• The iron-carbon phase diagram is obtained for temperature and composition while maintaining a constant pressure of 1 atm.
• The diagram contains 1.3% carbon, which is a eutectic composition, and shows the presence of various phases, including α (ferrite), γ (austenite), and δ (delta ferrite) iron, as well as Fe3C (cementite).

Invariant Reactions

• There are three invariant reactions in the iron-carbon phase diagram:
  • Eutectic reaction at 4.3%C, 1130°C
  • Eutectoid reaction at 0.8%C, 723°C
  • Peritectic reaction at 1492°C

Microstructure of Steel

• Pearlite is a microstructure that forms through the eutectoid reaction, consisting of alternating layers of α (ferrite) and Fe3C (cementite).
• Hypoeutectoid steel has a microstructure that consists of ferrite and pearlite, while hypereutectoid steel has a microstructure that consists of pearlite and cementite.

Lever Rule

• The lever rule is used to find the proportions of the phases present in a two-phase region.
• The rule is applied by calculating the relative amounts of the phases based on the composition of the alloy.

Steel Classification

• According to AISI, plain carbon steel is defined as steel with a composition that does not exceed the following limits: Mn < 1.65%, Si < 0.6%, Cu < 0.6%.
• Alloy steel is defined as steel with a composition that exceeds these limits.

Types of Plain Carbon Steel

• Low carbon steel: 0.3%C, with a microstructure consisting of ferrite and pearlite, possessing good formability, weldability, and machinability.
• Medium carbon steel: 0.3-0.8%C, with a microstructure consisting of ferrite and pearlite, possessing good toughness, ductility, and machinability.
• High carbon steel: 0.8-2.0%C, with a microstructure consisting of pearlite and cementite, possessing good hardness and wear resistance.

Alloys

• An alloy is a metallic material consisting of two or more elements, with the principal element being a metal.
• Alloys can be formed in three ways: interstitial solid solution, substitutional solid solution, and intermediate compound formation.

Interstitial Solid Solution

• In an interstitial solid solution, atoms of the solute element occupy the interstitial sites of the solvent element's lattice.

Substitutional Solid Solution

• In a substitutional solid solution, atoms of the solute element replace atoms of the solvent element in the lattice.

Intermediate Compound

• An intermediate compound has a fixed composition, and its properties are different from those of the constituent elements.
• Example: Fe3C (cementite), with a weight percentage of carbon of 6.67%.