Iron-Carbon Diagram (Fe-C) — Complete Guide for Welders
Introduction
The Iron-Carbon equilibrium diagram (Fe-Fe₃C) is the fundamental tool of steel metallurgy. It shows the phases and constituents that form as a function of temperature and carbon content.
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Allotropy of Pure Iron
| Temperature | Structure | Designation |
|---|---|---|
| < 912°C | BCC | Ferrite α |
| 912 - 1,394°C | FCC | Austenite γ |
| 1,394 - 1,538°C | BCC | Ferrite δ |
| > 1,538°C | — | Liquid |
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Phases
- Ferrite (α): Solid solution of C in BCC iron. Max 0.022% C. Soft, ductile, magnetic (~300 MPa).
- Austenite (γ): Solid solution of C in FCC iron. Max 2.11% C. Non-magnetic.
- Cementite (Fe₃C): Iron carbide with 6.67% C. Very hard (650 HB), brittle.
- Pearlite: Mechanical mixture of ferrite (88.5%) + cementite (11.5%) in alternating lamellae. Forms at 727°C with 0.77% C.
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Important Reactions
| Reaction | Temperature | % C | Description |
|---|---|---|---|
| Peritectic | 1,495°C | 0.17 | L + δ → γ |
| Eutectic | 1,148°C | 4.3 | L → γ + Fe₃C |
| Eutectoid | 727°C | 0.77 | γ → α + Fe₃C (pearlite) |
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Critical Temperatures
| Designation | Description |
|---|---|
| A₁ (727°C) | Eutectoid temperature |
| A₃ (variable) | Upper boundary of α+γ field |
| Acm | C solubility limit in γ |
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Fe-C Alloy Classification
| % Carbon | Classification |
|---|---|
| 0 - 0.77 | Hypoeutectoid steel |
| 0.77 | Eutectoid steel |
| 0.77 - 2.11 | Hypereutectoid steel |
| 2.11 - 4.3 | Hypoeutectic cast iron |
| > 4.3 | Hypereutectic cast iron |
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Relevance to Welding
During welding, the HAZ experiences different peak temperatures:
| HAZ Sub-zone | Temperature | Effect |
|---|---|---|
| Grain growth | >1,100°C | Coarse austenite grain → low toughness |
| Grain refinement | 900-1,100°C | Fine grain → good properties |
| Intercritical | 727-900°C | Partial transformation |
| Subcritical | <727°C | Stress relief / overtempering |
More carbon = more tendency to form martensite in HAZ = poorer weldability.
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Mechanical Properties vs. % Carbon
| Steel | UTS (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|
| 0.10% C | 370 | 38 | 110 |
| 0.20% C | 440 | 36 | 131 |
| 0.40% C | 590 | 28 | 170 |
| 0.80% C | 1,010 | 11 | 293 |
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*Article based on Welding Handbook (AWS, Vols. 1-3), Tecnologia Mecânica (V. Chiaverini), Aços e Ferros Fundidos (V. Chiaverini). Published on WeldFC Academy.*