Foundry Metallurgy

Iron Carbon Phase Diagram: A Complete, Proven Guide for the Foundry Floor

Every casting we pour crosses the iron carbon phase diagram twice: once on the way down from the furnace to the mold, and again in the heat-treat oven. Metallurgy students memorize the chart for an exam. Foundry people use it as a working map, because it explains why a part freezes the way it does, why one section machines like butter while another chips a cutting insert, and what temperature a casting has to reach before heat treatment will do anything at all.

On the floor in Hattiesburg, reading it comes down to a few things: the lines that matter, the phases you will hear named, the two points every heat treater knows by heart, and why the line between steel and cast iron sits just above 2% carbon.

What the Iron Carbon Phase Diagram Actually Shows

The chart has two axes. Carbon content runs left to right, from pure iron at 0% to iron carbide (cementite) at 6.67% carbon by weight. Temperature runs bottom to top. Each line on the chart is a boundary: cross it while heating or cooling and a phase either appears or disappears.

Two caveats keep the chart honest. First, the version most people learn is technically the iron to iron carbide (Fe-Fe₃C) diagram, which describes carbon locked up as carbide. Gray and ductile irons follow a closely related “stable” version where carbon comes out as graphite instead. Second, the diagram assumes equilibrium, meaning cooling so slow that atoms have all the time they need to rearrange. Real castings cool faster than that, especially in thin walls, and that gap between theory and the mold is where most foundry problems live.

The Four Structures Every Foundry Works With

Metallurgy reports on castings keep coming back to the same four names. In plain shop terms:

1. Ferrite (α)

Ferrite is iron with a body-centered cubic crystal structure. It is soft, ductile, and magnetic, and it can hold almost no carbon: about 0.022% at most, right at 727°C. A fully ferritic matrix is what you want when a casting has to machine easily or take a hit without cracking.

2. Austenite (γ)

Austenite is the high-temperature, face-centered cubic form of iron. Its crystal lattice has room for far more carbon, up to about 2.14% at 1148°C. Austenite is the starting point for nearly every heat treatment, because you have to dissolve carbon into it before you can control what happens on the way back down.

3. Cementite (Fe₃C)

Cementite is iron carbide, a compound that is 6.67% carbon. It is extremely hard and brittle, and when it dominates a structure the fracture face looks bright and silvery. That is where the name “white iron” comes from.

4. Pearlite

Pearlite is not a single phase. It is a layered mix of ferrite and cementite plates that forms when austenite cools through 727°C. Under a microscope it looks like a fingerprint. Pearlite gives gray and ductile irons much of their strength, and how fine or coarse those layers are depends on how quickly the casting cooled.

The Two Points That Matter Most: Eutectic and Eutectoid

The Eutectic: 4.3% Carbon, 1148°C

Pure iron melts at about 1538°C. Add carbon and the melting point drops steadily until it bottoms out at the eutectic, roughly 4.3% carbon and 1148°C. At that composition the liquid freezes all at once, at a single temperature, into a mix of austenite and either cementite or graphite.

For a foundry, that low freezing point is a gift. Iron near the eutectic stays fluid several hundred degrees below what steel demands, so it fills thin sections and fine detail easily. It also freezes over a very narrow range, which avoids the mushy zone where shrinkage porosity likes to hide. In graphitic irons there is an extra bonus: graphite takes up more room than the liquid it came from, and that expansion can offset much of the shrinkage, so near-eutectic irons need far less feeding from risers.

The Eutectoid: 0.76% Carbon, 727°C

The eutectoid is the solid-state version of the same idea. Austenite at about 0.76% carbon, cooled through 727°C, transforms entirely into pearlite. That 727°C line is called the lower critical temperature, or A1. The sloped line above it on the low-carbon side is the A3, and the one climbing toward 2.14% carbon is the Acm. Heat treaters live on these three lines, because they define how hot a part must get before its structure resets.

Why Cast Iron Sits Above 2% Carbon

The 2.14% mark on the iron carbon phase diagram is the most carbon that solid austenite can dissolve. Below that number, molten iron can freeze completely into austenite, and the result is a steel that can be rolled, forged, and welded with relative ease. Above it, there is too much carbon to fit, so the last liquid has to freeze as a eutectic. That eutectic is the signature of cast iron.

In practice, most engineering cast irons run between about 2.5% and 4% carbon, plus 1% to 3% silicon. Silicon matters because it pushes the eutectic to lower carbon levels, so foundries track a carbon equivalent (commonly carbon plus one-third of the silicon and phosphorus) instead of carbon alone. A typical gray iron at 3.2% carbon and 2.0% silicon lands at a carbon equivalent near 3.9%, just shy of eutectic. Ductile irons usually run at or slightly above the eutectic, commonly around 4.3% to 4.6%.

That wedge is the diagram’s stable-versus-metastable split made visible. The same chemistry can freeze gray or white depending on cooling rate and alloy content. Silicon and slow cooling favor graphite. Fast cooling and carbide formers such as chromium favor cementite. A thin flange that chills white by accident can come out hard enough to chip a cutting insert on its first pass, which is one reason casting and precision CNC machining work best when the same team controls both.

Sometimes white iron is exactly the goal. Our high-chrome cast iron wear parts are deliberately white irons, with chromium forming hard carbides that stand up to rock and abrasive slurry far longer than gray iron could. Our gray iron work for municipal, highway, and marine castings follows ASTM A48 Class 35, which calls for a graphitic structure instead.

Using the Iron Carbon Phase Diagram for Heat Treatment

The chart’s biggest job after the pour is telling you where to set the oven. To harden, normalize, or fully anneal a steel, you first heat it above the A3 (or the Acm for high-carbon grades) so the structure turns into austenite. What happens next depends on cooling speed:

• Furnace cooling (annealing) produces coarse pearlite and ferrite: soft, stress-free, and easy to machine.
• Air cooling (normalizing) produces finer pearlite and a stronger, more uniform part.
• Quenching skips pearlite and forms martensite, a very hard structure that does not appear on the equilibrium diagram at all. For that, metallurgists turn to time-temperature-transformation curves.

Cast irons use the same map with their own recipes. Stress relieving stays well below 727°C so the matrix does not change. A ferritizing anneal holds gray or ductile iron long enough to break pearlite down into ferrite and graphite for better machinability. High-chrome white irons go the other way: they are held at high temperature so secondary carbides come out of the austenite, then cooled so the remaining matrix transforms to martensite. That cycle is how our foundry and heat treatment services bring chrome iron wear parts to a 600 to 700 HB hardness range.

A caution: alloying shifts the lines. Chromium, nickel, molybdenum, and silicon all move the critical temperatures and the carbon limits, sometimes by a lot. The plain iron-carbon chart is the foundation, and alloy-specific diagrams and hardness testing fill in the rest.

Reading the Diagram on a Real Job

When a new print comes across our desk, the metallurgy question usually follows the same short path:

1. Find the carbon. For steel, use the carbon content. For cast iron, calculate the carbon equivalent so silicon is accounted for.
2. Locate the freeze. Is the chemistry near the eutectic? If so, expect good fluidity and modest feeding. If not, plan risers and gating accordingly.
3. Check the section sizes. Thin walls cool fast and push iron toward carbides. Thick sections cool slowly and coarsen the structure.
4. Mark the critical temperatures. A1, A3, and Acm set the floor for any heat treatment that is meant to change the structure.
5. Verify, do not assume. Hardness readings and a quick microstructure check confirm that the part actually did what the chart predicted.

For a deeper academic treatment of how phase diagrams are built and read, the University of Cambridge’s DoITPoMS teaching library on phase diagrams is free and well worth the time.