The Map of Metallurgy
A phase diagram is a roadmap that tells a metallurgist which phases are stable at any given composition and temperature. From it you can read melting ranges, the result of alloying, and — most importantly — the transformations that heat treatment exploits. For steel, the iron-carbon diagram is the single most consulted phase diagram in all of engineering.
What a Phase Is
A phase is a region of material with uniform structure and composition — for example, liquid metal, or a particular solid crystal structure. A phase diagram plots composition (horizontal axis) against temperature (vertical axis) and divides the field into single-phase and two-phase regions, separated by boundary lines such as the liquidus (above which everything is liquid) and the solidus (below which everything is solid).
Reading a Binary Phase Diagram
Within any two-phase region, three pieces of information are available at a given point:
- Which phases are present — read directly from the region you are in.
- The composition of each phase — read from the ends of the horizontal tie line drawn at that temperature.
- The amount of each phase — calculated with the lever rule.
The Lever Rule
The lever rule gives the weight fraction of each phase in a two-phase region. Drawing a tie line at the temperature of interest, with the overall composition as the fulcrum, the fraction of each phase is proportional to the length of the opposite arm:
Fraction of phase A = (length of arm toward B) / (total tie-line length)
It is exactly the physics of a balanced seesaw: the phase whose composition is farther from the overall composition is present in smaller amount.
Invariant Reactions
Certain points on a diagram involve three phases in equilibrium at a fixed temperature and composition — invariant reactions:
| Reaction | On cooling |
|---|---|
| Eutectic | Liquid → Solid₁ + Solid₂ |
| Eutectoid | Solid → Solid₁ + Solid₂ |
| Peritectic | Liquid + Solid₁ → Solid₂ |
The eutectic gives the lowest melting point of an alloy system; the eutectoid is the all-solid analog and is the key to steel heat treatment.
The Iron-Carbon Diagram
The iron-carbon (Fe–Fe₃C) diagram governs steel and cast iron. As carbon dissolves in iron and temperature changes, several distinct solid phases appear:
| Phase | Structure | Max carbon | Character |
|---|---|---|---|
| Ferrite (α) | BCC | 0.022% | Soft, ductile, magnetic |
| Austenite (γ) | FCC | 2.1% | Soft, ductile, non-magnetic, high-temp |
| Cementite (Fe₃C) | Orthorhombic | 6.67% (compound) | Very hard, brittle |
| Pearlite | Ferrite + cementite layers | 0.76% (mixture) | Strong, moderately ductile |
Ferrite and Austenite
Ferrite (α-iron) is the body-centered cubic form stable at room temperature; it is soft, ductile, and holds very little carbon. Austenite (γ-iron) is the face-centered cubic form stable at high temperature; its more open FCC structure dissolves much more carbon (up to 2.1%). Heating steel into the austenite region — and controlling how it cools — is the essence of heat treatment.
Cementite and Pearlite
Cementite (Fe₃C) is an iron carbide — extremely hard and brittle. When austenite of eutectoid composition (0.76% C) cools through 727 °C, it undergoes the eutectoid reaction, transforming into alternating fine layers of soft ferrite and hard cementite. This lamellar mixture is pearlite, named for its pearly luster. The fine interleaving of a soft and a hard phase gives pearlite an excellent combination of strength and toughness.
The Eutectoid and Steel Classification
The eutectoid point (0.76% C, 727 °C) divides steels:
- Hypoeutectoid steel (< 0.76% C): on cooling, ferrite forms first (proeutectoid ferrite), then the remaining austenite becomes pearlite. More ferrite means more ductility.
- Eutectoid steel (= 0.76% C): transforms entirely to pearlite.
- Hypereutectoid steel (> 0.76% C): cementite forms first along grain boundaries, then pearlite — harder but more brittle.
Steel vs. Cast Iron
Carbon content draws the line between the two great families of iron alloys:
- Steel (up to ~2.1% C): stays within the austenite range when hot, so it can be forged, rolled, and heat-treated. The workhorse structural material.
- Cast iron (2.1%–4.5% C): high carbon lets it pass through the eutectic reaction at 1147 °C, giving a low melting point and excellent castability — but more cementite makes it harder and more brittle. Gray, ductile, and white cast irons differ in how the excess carbon appears (as graphite flakes, nodules, or cementite).
Why It All Matters
The iron-carbon diagram is not an abstraction — it predicts the microstructure you will get from a given steel composition and cooling path, and therefore its hardness, strength, and ductility. Combined with the time-dependent TTT and CCT curves used in heat treatment, it is the foundation on which the entire steel industry rests.