Steel Microstructure Constituents: Properties and Characteristics
Steel Microstructure Constituents
Ferrite
Ferrite is a solid solution of carbon in alpha-iron with very low solubility. Because its composition is close to pure iron, its magnetic and mechanical properties are relatively low, making it one of the softest constituents of steel.
Cementite
Cementite is a chemical compound of iron and carbon with the formula Fe₃C (iron carbide). Its melting point is approximately 1250 °C. At low temperatures, it is ferromagnetic, but it loses these properties above 217 °C. While it possesses high hardness, its plasticity is practically zero.
Perlite
Perlite is an eutectoid constituent formed by alternating layers of ferrite and cementite. It contains approximately 0.8% carbon and exhibits balanced mechanical properties, being harder and more resistant than ferrite, yet more tenacious than cementite. The layer spacing depends on the cooling rate:
- Coarse Perlite: Large layer spacing, 200 Brinell hardness, obtained by slow cooling.
- Normal Perlite: Medium layer spacing, 220 Brinell hardness.
- Fine Perlite: Very small layer spacing, 300 Brinell hardness.
Austenite
Austenite is a solid solution of carbon in gamma-iron. It is non-magnetic, soft, very ductile, and tenacious. In hypoeutectoid steels, austenite transforms into a mixture of ferrite and perlite during cooling, while in hypereutectoid steels, it transforms into cementite and perlite.
Martensite
Martensite is a supersaturated solid solution of carbon in alpha-iron, obtained by rapid cooling of steel from high temperatures. It is a non-equilibrium constituent with a body-centered tetragonal crystal structure. It is characterized by being extremely hard and very fragile.
Bainite
Bainite forms during the cooling of austenite when the bath temperature is between 250 °C and 600 °C. Two types are distinguished:
- Upper Bainite: Feather-like structure.
- Lower Bainite: Needle-like (acicular) structure.
Phase Diagram Reference Points
Key composition points (Carbon % at 727 °C):
- B: 0.022%
- I: 0.77%
- KJ: 6.67%
- G: 2.11%
- E: 4.30%