The iron-carbon phase diagram — formally the Fe–Fe₃C metastable diagram —
is the single most important diagram in ferrous metallurgy. It maps every phase that can exist
in a carbon steel or cast iron at any combination of temperature and carbon content, from pure
iron (0%C) to iron carbide Fe₃C (6.67%C). Every steel heat treatment — annealing, quenching,
tempering, normalising — is designed with reference to this diagram. Understanding it is the
absolute foundation of steel metallurgy.
KEY TAKEAWAYS
- The A1 line at 727°C is the eutectoid temperature — the most important horizontal line in steel metallurgy.
- Steel = Fe-C alloys with <2.14%C. Cast iron = 2.14–6.67%C. The boundary is set by eutectic reaction.
- The eutectoid point S is at 0.77%C, 727°C — austenite transforms to pearlite (ferrite + cementite) here.
- Austenite (γ-Fe, FCC) dissolves up to 2.14%C — this high solubility is why steels can be hardened.
- Ferrite (α-Fe, BCC) dissolves only 0.022%C max — the small BCC interstitial sites cannot accommodate carbon.
- Martensite, bainite, and pearlite are all austenite transformation products; none appear on the equilibrium diagram.
- The lever rule allows calculation of phase fractions at any composition and temperature in a two-phase field.
Phases in the Iron-Carbon System
The Fe-Fe₃C diagram contains five distinct phases or phase mixtures that engineers must recognise:
| Phase | Crystal Structure | Max Carbon Solubility | Hardness | Magnetic? | Key Characteristics |
|---|---|---|---|---|---|
| α-Ferrite | BCC | 0.022%C at 727°C | 60–90 HV | Yes (<770°C) | Soft, ductile, tough; dominant phase in low-C steels |
| γ-Austenite | FCC | 2.14%C at 1147°C | ~180 HV | No | Parent phase for all heat treatment; non-magnetic |
| δ-Ferrite | BCC | 0.09%C at 1493°C | Similar to α | — | High-temperature BCC; relevant to solidification and SS welds |
| Fe₃C (Cementite) | Orthorhombic | 100% C compound = 6.67%C | 800–1100 HV | Weakly ferromagnetic | Hardest phase; brittle; forms plates in pearlite, networks in hypereutectoid |
| Ledeburite | Eutectic mixture | 4.3%C eutectic composition | — | — | Austenite + cementite eutectic; forms in cast irons ≥2.14%C |
Critical Temperature Lines and Their Meanings
| Line | Temperature (pure iron) | Significance |
|---|---|---|
| A1 (eutectoid) | 727°C | Below A1: no austenite at equilibrium. Most important line for heat treatment. Above A1 = austenitising range. |
| A3 | 727–912°C | Upper boundary of ferrite+austenite two-phase field. Above A3 = fully austenitic for hypoeutectoid steels. |
| Acm | 727–1147°C | Boundary between austenite and austenite+cementite for hypereutectoid steels (0.77–2.14%C). |
| Eutectic (1147°C) | 1147°C | Liquid → austenite + cementite at 4.3%C. The casting/solidification reaction in white cast iron. |
| A4 | 1394°C | Upper stability limit of austenite; δ-ferrite above this. Rarely relevant to engineering heat treatment. |
Heating vs cooling temperature notation: During heating, actual transformation temperatures are slightly above the equilibrium values — designated Ac1 (c from French chauffage = heating) and Ac3. During cooling, they are below equilibrium — Ar1 and Ar3 (refroidissement = cooling). Industrial heat treatment specifications always use Ac temperatures (heating) for austenitising. The difference is typically 20–50°C depending on heating rate.
Steel Classification by Carbon Content
| Type | Carbon Range | Room-Temperature Microstructure (slow cooled) | Typical Applications |
|---|---|---|---|
| Low-carbon | 0–0.30%C | Ferrite (dominant) + small islands of pearlite | Structural sections, sheet, drawn wire, nails |
| Medium-carbon | 0.30–0.60%C | Ferrite + increasing pearlite fraction | Shafts, gears, connecting rods, springs |
| Eutectoid | 0.77%C | 100% pearlite (no proeutectoid phases) | High-tensile wire, rail head, piano wire |
| Hypereutectoid | 0.77–2.14%C | Pearlite + proeutectoid cementite network | Tool steels, bearing steels (52100/100Cr6) |
| Hypoeutectic cast iron | 2.14–4.3%C | Pearlite + ledeburite + cementite | Pipes, manhole covers, sanitary castings |
| Eutectic cast iron | 4.3%C | Ledeburite = austenite + cementite eutectic | White cast iron wear parts |
| Hypereutectic cast iron | 4.3–6.67%C | Primary cementite + ledeburite | Special wear applications |
The Lever Rule — Calculating Phase Fractions
In any two-phase field on the diagram, the relative proportions of the two phases at equilibrium are given by the lever rule. For a steel with bulk carbon content C₀ in a field bounded by phases of composition C_α and C_γ:
Fraction of phase α = (C_γ − C₀) / (C_γ − C_α)
Worked example — 0.40%C steel at 780°C (between A1 and A3):
C_α = 0.02% (carbon in ferrite at A1)
C_γ = 0.77% (carbon in austenite at A1, eutectoid)
Fraction austenite = (0.40 − 0.02) / (0.77 − 0.02) = 0.38 / 0.75 = 51%
Fraction ferrite = (0.77 − 0.40) / (0.77 − 0.02) = 0.37 / 0.75 = 49%
After slow cooling through A1 — fraction of pearlite:
F_pearlite = C₀ / 0.77 = 0.40 / 0.77 = 52% (lever rule at A1)
How the Diagram Guides Heat Treatment
Every standard steel heat treatment is positioned relative to the A1 and A3 lines:
- Hardening (austenitising + quench): Heat 30–50°C above A3 (hypoeutectoid) or just above A1 (hypereutectoid) → quench to form martensite
- Full annealing: Heat above A3 → slow furnace cool below A1 → coarse pearlite + ferrite; maximum softness
- Normalising: Heat above A3 → air cool → fine pearlite + ferrite; finer than full anneal
- Spheroidise annealing: Hold at 690–720°C (below A1) for extended time → cementite spheroidises → maximum machinability
- Stress relief: 550–650°C (well below A1) → no phase change; only residual stress reduction
- Tempering: Below A1 → no phase change; martensite → tempered martensite → reduces brittleness
📷 IMAGE: Steel Microstructure: Ferrite + Pearlite
Optical micrograph of normalised 0.4%C steel showing white ferrite grains and dark pearlite colonies (colonies of alternating ferrite and cementite lamellae). Nital etch, 200×. This microstructure corresponds to a hypoeutectoid steel slowly cooled through the phase diagram.
Search terms: optical micrograph ferrite pearlite normalized steel nital etch
Source:
https://en.wikipedia.org/wiki/Pearlite#/media/File:Pearlite_micrograph.jpg
Attribution: Image via Wikimedia Commons CC-BY-SA. Download from Wikipedia and upload to your WordPress Media Library.
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Frequently Asked Questions
Q: Why is 2.14%C the boundary between steel and cast iron?
A: 2.14%C is the maximum carbon solubility in austenite at the eutectic temperature (1147°C). Below this, steel passes through a fully austenitic phase field during solidification, enabling hot working (rolling, forging) and conventional heat treatment. Above 2.14%C, solidification involves the eutectic reaction producing ledeburite — an inherently brittle austenite+cementite mixture that cannot be hot worked. This processability boundary defines the engineering difference between steel and cast iron.
Q: What happens at exactly the eutectoid composition (0.77%C) on slow cooling?
A: At 0.77%C, the steel passes directly from single-phase austenite to the eutectoid reaction at 727°C: γ → α + Fe₃C simultaneously. This produces 100% pearlite — alternating lamellar colonies of ferrite and cementite — with no proeutectoid phases. The lamellar spacing of the pearlite depends on cooling rate: slower cooling → coarser pearlite (softer, ~200 HV); faster cooling → finer pearlite (harder, ~320 HV). Fine fully-pearlitic steel wire (patented wire) can reach tensile strengths above 2,000 MPa — the basis of piano wire and high-tensile bridge cables.
Q: What is the difference between the Fe-Fe₃C and Fe-graphite phase diagrams?
A: The standard Fe-Fe₃C diagram uses cementite (Fe₃C) as the carbon-rich end member — it is technically a metastable diagram because Fe₃C is not the thermodynamically stable equilibrium phase (graphite is). The Fe-graphite equilibrium diagram lies slightly to the left of the Fe-Fe₃C diagram, with the eutectic at slightly lower temperature. In practice, Fe₃C forms in all steels under normal cooling rates. Graphite only forms in cast irons when silicon is present (≥1.5–2.5% Si) or under extremely slow cooling, because silicon strongly destabilises cementite and promotes graphite precipitation.
Q: How is the lever rule used to predict steel properties?
A: The lever rule predicts the volume fractions of microstructural constituents after slow cooling, which directly affect mechanical properties. For example: a 0.25%C steel (S355/A36 class) has approximately 0.25/0.77 = 32% pearlite and 68% ferrite after normalising. Since pearlite contributes approximately 250 MPa above ferrite’s baseline strength, and ferrite is approximately 250 MPa YS, the steel YS ≈ 250 + 0.32 × 250 = 330 MPa — consistent with the actual specification of 355 MPa YS (higher than the simple prediction due to grain refinement and solid solution strengthening from Mn, Si).
References
- Callister, W.D. and Rethwisch, D.G., Materials Science and Engineering: An Introduction. 10th ed. John Wiley & Sons, 2018.
- Bhadeshia, H.K.D.H. and Honeycombe, R., Steels: Microstructure and Properties. 4th ed. Butterworth-Heinemann, 2017.
- Krauss, G., Steels: Processing, Structure, and Performance. 2nd ed. ASM International, 2015.
- ASM Handbook Vol. 1: Properties and Selection — Irons, Steels, and High-Performance Alloys. ASM International, 1990.
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