The Iron-Carbon Phase Diagram: Phases, Critical Points and What It Means for Steel
Send an enquiryThe iron-carbon phase diagram is a chart of temperature against carbon content that shows which phases are stable in iron-carbon alloys at each combination. It is the map on which the heat treatment of steel is planned. The form in engineering use is the metastable iron-iron carbide (Fe-Fe3C) diagram, which runs from pure iron to 6.67 percent carbon, the carbon content of iron carbide. Its key points are the eutectoid at 727 degrees Celsius and 0.76 to 0.77 percent carbon, where austenite transforms to pearlite, and the eutectic at 1,147 degrees Celsius and 4.3 percent carbon, where liquid freezes to ledeburite. Steels are the alloys up to about 2.1 percent carbon, the most carbon that austenite can dissolve; cast irons lie above that. The diagram describes equilibrium, which slow cooling approaches. It does not show martensite or bainite, the structures that form when steel is cooled fast.
The metastable and the stable diagram
Two versions of the diagram exist. In true equilibrium, carbon separates from iron as graphite. Iron carbide, Fe3C, known as cementite, is metastable: it decomposes to iron and graphite only over very long times at high temperature, and in steel it does not decompose at all under normal processing. The iron-iron carbide diagram therefore describes steel, and it is the diagram meant when the iron-carbon diagram is named without qualification. The stable iron-graphite diagram matters for grey and ductile cast irons, in which silicon promotes graphite. Carbon contents on the diagram are percentages by mass. Handbooks differ by a degree or two and by a few hundredths of a percent in the values below, which is the precision to which the diagram is known.
The phases of the iron-carbon system
Pure iron changes its crystal structure twice before it melts. Below 912 degrees Celsius it is body-centred cubic alpha iron, called ferrite. Between 912 and 1,394 degrees Celsius it is face-centred cubic gamma iron, called austenite. From 1,394 degrees Celsius to the melting point at 1,538 degrees Celsius it is body-centred cubic again, called delta ferrite. Carbon dissolves in the gaps between the iron atoms, and the two structures hold very different amounts. The face-centred cubic lattice of austenite has larger gaps and holds up to about 2.1 percent carbon. Ferrite holds at most about 0.02 percent. Carbon beyond what the iron can dissolve forms cementite.
| Phase or structure | Crystal structure | Carbon, % | Where it exists | Character |
|---|---|---|---|---|
| Ferrite (alpha iron) | Body-centred cubic | About 0.02 maximum, at 727 degrees Celsius | Below 912 degrees Celsius | Soft and ductile; magnetic below about 770 degrees Celsius |
| Austenite (gamma iron) | Face-centred cubic | About 2.1 maximum, at 1,147 degrees Celsius | 912 to 1,394 degrees Celsius in pure iron; down to 727 degrees Celsius at the eutectoid | Soft, ductile and non-magnetic; the starting structure for hardening |
| Delta ferrite | Body-centred cubic | Low | 1,394 to 1,538 degrees Celsius in pure iron | Forms only at high temperature in plain carbon steel |
| Cementite (Fe3C) | Iron carbide compound | 6.67, fixed | Wherever carbon exceeds what the iron can dissolve | Hard and brittle iron carbide |
| Pearlite | Layers of ferrite and cementite | 0.76 to 0.77 overall | Below 727 degrees Celsius | Stronger and harder than ferrite; finer when cooled faster |
| Ledeburite | Austenite and cementite (eutectic) | 4.3 overall | Below 1,147 degrees Celsius; its austenite becomes pearlite below 727 degrees Celsius | Hard and brittle; the matrix of white cast iron |
| Martensite (not on the diagram) | Body-centred tetragonal | That of the austenite it forms from | Forms when austenite is cooled fast | The hardest structure of steel; brittle until tempered |
Pearlite and ledeburite are not phases but mixtures of two phases, formed together in one reaction. They appear on the diagram because they are the structures a metallurgist sees under the microscope.
Key points and lines: A1, A3 and Acm
The lines of the diagram separate the phase fields, and three of them govern the heat treatment of steel. A1 is the horizontal line at 727 degrees Celsius, below which austenite cannot exist in a plain carbon steel. A3 falls from 912 degrees Celsius at zero carbon to 727 degrees Celsius at 0.76 percent; above it, a low-carbon steel is fully austenitic. Acm rises from the eutectoid point to 1,147 degrees Celsius at about 2.1 percent carbon; above it, all the carbon of a higher-carbon steel is dissolved in austenite. A2, the Curie point of ferrite at about 770 degrees Celsius, marks the loss of magnetism and no change of phase. Older references give 723 degrees Celsius and 0.8 percent carbon for the eutectoid. At the top left of the diagram, close to the melting point, lies a peritectic, where delta ferrite and liquid react to form austenite; it bears on the solidification of low-carbon steel and not on its heat treatment.
| Point | Temperature, degrees Celsius | Carbon, % | Reaction or meaning |
|---|---|---|---|
| Melting point of pure iron | 1,538 | 0 | Delta ferrite melts |
| Delta to gamma in pure iron | 1,394 | 0 | Delta ferrite becomes austenite |
| Eutectic | 1,147 | 4.3 | Liquid freezes to austenite and cementite (ledeburite) |
| Maximum carbon in austenite | 1,147 | About 2.1 | Boundary between steel and cast iron |
| Alpha to gamma in pure iron (A3 at zero carbon) | 912 | 0 | Ferrite becomes austenite |
| Curie point of ferrite (A2) | About 770 | Any ferrite | Ferrite loses its magnetism |
| Eutectoid (A1) | 727 | 0.76 to 0.77 | Austenite transforms to pearlite |
| Maximum carbon in ferrite | 727 | About 0.02 | Solubility limit of alpha iron |
Hypoeutectoid and hypereutectoid steels
A hypoeutectoid steel has less than 0.76 percent carbon. On slow cooling through A3, ferrite forms at the austenite grain boundaries. The remaining austenite grows richer in carbon until, at 727 degrees Celsius, it reaches the eutectoid composition and transforms to pearlite. The finished structure is ferrite and pearlite, in proportions set by the carbon content: a 0.4 percent carbon steel such as EN8 is about half pearlite. Most structural and engineering steels are hypoeutectoid.
A hypereutectoid steel has between 0.76 and about 2.1 percent carbon. On cooling below Acm, cementite forms first, along the austenite grain boundaries, and the remaining austenite transforms to pearlite at 727 degrees Celsius. A continuous network of grain-boundary cementite makes the steel brittle. Bearing and tool steels near 1 percent carbon are therefore spheroidised before machining, so that the cementite gathers into rounded particles.
Why steel stops at about 2.1 percent carbon
The boundary between steel and cast iron is the greatest amount of carbon austenite can dissolve, about 2.1 percent at 1,147 degrees Celsius. An alloy below that limit can be heated into a field where it is entirely austenite, a single ductile phase that can be rolled and forged. An alloy above it cannot. Its last liquid freezes as eutectic, either ledeburite with cementite or, in grey iron, a mixture with graphite, and the eutectic stays brittle at every temperature below the solidus. The eutectic at 4.3 percent carbon also has the lowest melting point in the system, 1,147 degrees Celsius against 1,538 degrees Celsius for pure iron. This is why cast irons are made near that composition and shaped by casting, as described in the melting point article.
What the diagram does not show
The diagram shows only equilibrium, the state reached when time allows carbon to move to where it belongs. Real cooling is faster. When austenite is cooled faster than a critical rate, carbon has no time to leave, and the lattice shears into martensite, a body-centred tetragonal structure with the carbon trapped inside. At intermediate rates, or on holding at an intermediate temperature, austenite forms bainite, a fine mixture of ferrite and carbide. Neither structure appears on the equilibrium diagram. Their formation is read from time-temperature-transformation (TTT) diagrams, measured for a steel held at constant temperature, and continuous cooling transformation (CCT) diagrams, measured for a steel cooled at a steady rate.
The diagram also describes only iron and carbon. Alloying elements move its lines. Nickel, manganese and nitrogen widen the austenite field; enough of them keeps austenite stable at room temperature. This is why austenitic stainless steel is non-magnetic in the annealed state, while martensitic stainless steels harden by quenching in the same way as carbon steel. For an alloy steel, the transformation temperatures of the grade itself govern its heat treatment.
How heat treatment uses the diagram
Every heat treatment of carbon and low-alloy steel is a path across the diagram. Full annealing heats a hypoeutectoid steel above A3 and cools it slowly in the furnace, which gives coarse ferrite and pearlite, the softest condition. Normalising heats above A3, or above Acm for a hypereutectoid steel, and cools in still air, which gives a finer pearlite and a more uniform grain. Hardening heats a hypoeutectoid steel above A3 into austenite and quenches it to martensite. A hypereutectoid steel is usually hardened from between A1 and Acm, not from above Acm, so that part of its cementite stays undissolved. Tempering reheats the hardened steel below A1 to exchange some hardness for toughness. Spheroidising holds the steel just below A1. The temperatures, cooling media and results for each process are set out in the heat treatment article.
The diagram in the mill
Laxcon Steels supplies hot rolled round bar from 16 to 125 mm soft annealed, spheroidise annealed, normalised, or quenched and tempered, the routes this diagram explains for carbon, alloy and martensitic stainless grades. Bright bar from 5 to 115 mm is supplied as drawn or annealed after drawing. The laboratory examines the microstructure of the bar on the optical microscope, and the certificate names the heat treatment condition delivered.
Frequently asked questions
What is the eutectoid point of steel?
The eutectoid point is 727 degrees Celsius and 0.76 to 0.77 percent carbon. At that point austenite transforms, on slow cooling, into pearlite, a layered mixture of ferrite and cementite. Older references give 723 degrees Celsius and 0.8 percent carbon.
Why is steel limited to about 2.1 percent carbon?
About 2.1 percent is the most carbon austenite can dissolve, reached at 1,147 degrees Celsius. Below that limit a steel can be made entirely austenitic and hot worked. Above it, part of the alloy freezes as a brittle eutectic that no heat treatment removes, and the alloy is a cast iron.
Is martensite shown on the iron-carbon diagram?
No. Martensite forms only when austenite is cooled too fast for carbon to diffuse, and the iron-carbon diagram shows equilibrium phases only. Martensite and bainite are read from time-temperature-transformation and continuous cooling transformation diagrams.
What is the difference between ferrite and austenite?
Ferrite is body-centred cubic iron, stable below 912 degrees Celsius in pure iron, dissolving at most about 0.02 percent carbon and magnetic below about 770 degrees Celsius. Austenite is face-centred cubic iron that dissolves up to about 2.1 percent carbon and is non-magnetic. Steel is hardened by heating it to austenite and quenching it.
Sources
- ASM Handbook, Volume 3, Alloy Phase Diagrams: the iron-carbon system.
- ASM Handbook, Volume 4, Heat Treating: annealing, normalising, hardening and tempering of steel.
- Materials Science and Engineering: An Introduction, the chapter on phase diagrams and the iron-iron carbide system.
- The heat treatment routes listed on the Laxcon Steels product pages.
