Ferrite, Pearlite and Cementite: Mechanical Properties by Carbon Content
Introduction
The mechanical behaviour of steel is largely determined by the relative proportion of ferrite, pearlite and cementite in its microstructure. These phases form during cooling from austenite, according to the Fe-Fe₃C equilibrium diagram.
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Ferrite (α)
Solid solution of carbon in BCC iron. The softest and most ductile phase in steels.
| Property | Typical value |
|---|---|
| Tensile strength | ~300 MPa |
| Yield strength | ~170 MPa |
| Elongation | 40–50% |
| Hardness | 80–100 HB |
| C solubility | max. 0.022% at 727°C |
| Magnetic | Yes (below 768°C) |
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Pearlite
A constituent formed by decomposition of eutectoid austenite (0.77% C) at 727°C. Alternating lamellae of ferrite (88.5%) and cementite (11.5%).
| Property | Fine pearlite | Coarse pearlite |
|---|---|---|
| Tensile strength | 700–1,000 MPa | 500–700 MPa |
| Yield strength | 400–600 MPa | 300–450 MPa |
| Elongation | 10–20% | 20–30% |
| Hardness | 250–350 HB | 180–250 HB |
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Cementite (Fe₃C)
Iron carbide with 6.67% C — the hardest and most brittle phase in the Fe-C system.
| Property | Value |
|---|---|
| Hardness | ~800–900 HV (≈ 65 HRC) |
| Elongation | 0% |
| Melting point | 1,227°C |
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Mechanical Properties by Carbon Content
| % C | Microstructure | UTS (MPa) | YS (MPa) | El. (%) | HB |
|---|---|---|---|---|---|
| 0.10 | Ferrite + pearlite (13% P) | 370 | 250 | 38 | 110 |
| 0.20 | Ferrite + pearlite (26% P) | 440 | 345 | 36 | 131 |
| 0.40 | Ferrite + pearlite (52% P) | 590 | 375 | 28 | 170 |
| 0.60 | Ferrite + pearlite (78% P) | 775 | 420 | 18 | 229 |
| 0.77 | 100% Pearlite | 1,010 | 525 | 11 | 293 |
| 0.80 | Pearlite + cementite network | ~1,050 | — | ~8 | ~300 |
*Source: Chiaverini, V. — Tecnologia Mecânica, Vol. 1, 2nd ed.*
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HAZ Impact during Welding
| Cooling rate | Microstructure | Properties |
|---|---|---|
| Very slow (furnace) | Ferrite + Coarse pearlite | Soft, ductile |
| Slow (air) | Ferrite + Fine pearlite | Strong |
| Fast (quench) | Martensite | Hard, brittle |
| Intermediate | Bainite | Strength + toughness |
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