Understanding Steel: Properties, Manufacturing, and Iron-Carbon Alloys

Posted on Feb 16, 2025 in Geology

The properties of a metal depend on the size and arrangement of its grains (granular structure). This structure is very important.

Types of HSS Steel

Microalloyed steel, dual phase steels, rephosphorized steels, bake hardening steels, IF steels, TRIP steels.

Active Materials

The most remarkable property of active materials lies in their response to specific stimuli. These materials have varying properties and change automatically.

Physical Properties

  • Fusibility: The property of metals to liquefy under heat.
  • Specific Heat: The amount of heat needed to increase the temperature of a unit mass of a body from 0 ° to 1 °.
  • Dilatability: The property of bodies to increase their volume with heat.
  • Melting Temperature: The temperature at which a metal changes state due to heat.
  • Thermal Conductivity: The property of transmitting heat through its mass.
  • Electrical Conductivity: The ease with which metals transmit electric current through their mass.

Chemical Properties

  • Oxidation: A chemical combination of oxygen with metal elements, leading to corrosion or degradation.
  • Corrosion: Slow deterioration of a metal due to an external agent.

Mechanical Properties

  • Tenacity: The ability of metals to resist breakage or deformation.
  • Elasticity: The ability of metals to return to their original shape after deformation.
  • Plasticity: The ability of materials to acquire permanent deformation.
  • Malleability: The property of some metals to be shaped into plates by compressive stress.
  • Ductility: The property of some metals to be stretched by tensile stress.
  • Fatigue: The failure of metals when subjected to varying stress.
  • Breaking Strength: The maximum load a metal can withstand without breaking.
  • Resistance: The property of metals to oppose the reduction of their cross-section when subjected to tensile loading.
  • Hardness: The resistance of metals to penetration.
  • Fragility: The ease with which metals break under load.

Steel Fabrication

  1. Raw materials
  2. Minerals
  3. Fluxes
  4. Coke (coal)
  5. Blast furnace
  6. Pig iron
  7. Converter
  8. Steel
  9. Continuous casting
  10. Conventional casting
  11. Molding
  12. Lamination
  13. Forge
  14. Rolled pieces
  15. Bars
  16. Sheets
  17. Reels
  18. Tubes (seamless)
  19. Cut lengths
  20. Rolled products
  21. Forgings and stampings
  22. Castings

Process

Iron ore, fluxes, and fuel enter the blast furnace. The oven is loaded with iron ore, coke, limestone, and hot air. The heat from the coke smelts the ore, carbon monoxide removes oxygen from iron oxide, limestone mixes with sulfur and silicon dioxide to purify the iron, and hot air provides the oxygen needed to burn the coke. This yields pig iron, which is transported to a converter to obtain liquid steel. The liquid steel is poured into continuous or conventional casting processes, resulting in a semi-finished product or cast steel. These intermediate products are transformed into commercial products through shaping processes like rolling and hot forging.

Allotropic States of Iron

  • Alpha: Exists below 911 °C. Magnetic up to 768 °C.
  • Gamma: Between 911 °C and 1400 °C. Non-magnetic, denser, and more dilatable than alpha; can dissolve more carbon.
  • Delta: Between 1400 °C and 1539 °C (melting temperature). Weakly magnetic and dissolves low carbon.

Constituents Diagram Fe-C

  1. Molten liquid metal
  2. Austenite
  3. Ferrite + austenite
  4. Pearlite + ferrite
  5. Pearlite
  6. Austenite + liquid
  7. Austenite + cementite
  8. Pearlite + cementite
  9. Metal paste
  10. Ledeburite + austenite + cementite
  11. Pearlite + cementite
  12. Cementite + liquid
  13. Ledeburite
  14. Pearlite + cementite

Iron-Carbon Alloy

In these alloys, carbon may be in the form of cementite (iron carbide), dissolved in iron forming ferrite, austenite, and martensite, or as free graphite sheets.

Constituents

  • Ferrite: Soft, low strength, ductile, and magnetic.
  • Cementite: Harder and more brittle constituent, magnetic below 210 °C.
  • Pearlite: More resilient and harder than ferrite, but softer and more malleable than cementite.
  • Austenite: The densest, non-magnetic form. Easily worked due to its plasticity.
  • Martensite: Obtained from the rapid transformation of austenite to iron through quenching. Very hard and strong, but less ductile and malleable.