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Phases and constituents of steel

Short answer

Ferrite: BCC interstitial solid solution of C in Fe, max 0.022% C, soft. Austinite: FCC, up to 2.14% C, ductile. Cementite: Fe3C, very hard and brittle. Pearlite: lamellar eutectoid mixture of ferrite and cementite (0.76% C), forms at 727°C. Martensite: diffusionless shear product, BCT with supersaturate C, extremely hard and brittle.

Write short notes on ferrite, austenite, cementite, pearlite and martensite.

ConstituentNatureStructureProperties
Ferrite (α\alpha)Interstitial solid solution of C in α\alpha-FeBCC, max 0.022% CSoft, ductile, magnetic below 768 °C, tensile strength about 280 MPa
Austenite (γ\gamma)Solid solution of C in γ\gamma-FeFCC, up to 2.14% CSoft, ductile, tough, non-magnetic; stable above 727 °C (retained in austenitic stainless steel)
Cementite (Fe3_3C)Intermetallic compound, 6.67% COrthorhombicVery hard (about 800 HV), brittle
PearliteEutectoid mixture of ferrite and cementite (0.76% C)Alternate lamellae of ferrite and cementiteHardness about 200 HB, tensile strength about 800 MPa; fine pearlite is harder than coarse
MartensiteSupersaturated solid solution of C in ironBody-centred tetragonal (BCT), needle/plate shapeExtremely hard (up to 65 HRC) and brittle; formed by rapid quenching without diffusion

Notes

  • Pearlite forms when austenite of 0.76% C cools slowly through 727 °C: γ→α+Fe3C\gamma \rightarrow \alpha + \text{Fe}_3\text{C}. Lamellar spacing decreases with faster cooling.
  • Martensite forms by a diffusionless shear transformation when austenite is cooled faster than the critical cooling rate, below the MsM_s temperature. Carbon is trapped, distorting the lattice into BCT, which gives hardness. It is tempered to improve toughness.
  • Other constituent: bainite (fine ferrite + carbide formed between about 250 and 550 °C), ledeburite (eutectic of austenite and cementite in cast iron).

✓ Answer checked by Claude Opus 5.5 (high effort)

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