Iron-Carbon Alloys: Components, Treatments, and Classification

Principal Components of Iron-Carbon Alloys

The main components of iron-carbon alloys include:

• Perlite: Thin films formed with ferrite (light gray) and cementite (dark gray). Carbon content is 0.89%. Ferrite is softer, while cementite is harder.
• Ferrite: Almost pure iron, very soft, ductile, and magnetic. Its structure is cubic.
• Cementite: Located on the right side of the eutectoid point due to its carbon composition (Fe3C). It is the hardest and most fragile constituent of steel.
• Austenite
• Martensite
• Ledeburite: Consisting of cementite and austenite.

Thermal Treatments for Steel

Thermal treatments modify the properties of steel:

• Tempering: Increases hardness, toughness, and yield strength, but also increases fragility. Methods include immersion in water, oil, airflow, or lead.
• Annealing: Given to tempered carbon steel to restore ductility, eliminate internal stresses, and reduce brittleness while maintaining hardness and strength.

Thermo-Chemical Treatments

Thermo-chemical treatments alter the surface composition of steel:

• Cementation
• Nitriding
• Carbonitriding
• Cyaniding
• Sulfinizing

Material Testing

Various tests are used to evaluate material properties:

• Static hardness testing
• Static testing (destructive)
• Dynamic mechanical tests
• Bend test
• Drawing test
• Forging test
• Shear test

Classification of Metals

Metals are classified into two main categories:

• Ferrous Metals: Metals containing iron.
• Non-Ferrous Metals: Metals that do not contain iron.

Ferrous Metals

• Iron: Exhibits allotropy, meaning it can adopt different internal atomic arrangements, forming different structures (liquid iron, delta iron, gamma iron, and alpha iron).
• Steel: Iron alloyed with a certain amount of carbon. Includes low carbon steel, medium carbon steel, high carbon steel, and stainless steel (alloy of iron and chromium, with higher chromium content providing greater oxidation resistance).
• Cast Iron

Non-Ferrous Metals

• Aluminum and its alloys
• Copper
• Zinc
• Tin
• Brass (copper-zinc alloy, ductile and malleable, with hardness and color varying with copper content)
• Bronze

Iron Allotropes

• Alpha Iron (Ferrite): Exhibits magnetism, crystallizes in a cubic system, and has a Curie temperature of 770°C.
• Gamma Iron (Austenite): Exists between 911°C and 1400°C, is non-magnetic, and crystallizes in a face-centered cubic system.
• Delta Iron: Has a larger crystal size than alpha iron and exists between 1400°C and 1539°C, above which iron is in a liquid state.