What Is Cementite? Fe₃C — the Phase That Gives Steel Its Hardness

Short Answer: Cementite = Iron Carbide (Fe₃C)

Cementite is the intermetallic compound of iron and carbon: Fe₃C — three iron atoms to one carbon atom, 6.67% carbon by weight. It is one of the hardest constituents of steel microstructure: roughly 800 HV (about ten times harder than ferrite). On its own it is brittle as glass; but combined properly with soft ferrite it becomes the real source of steel's strength and wear resistance. What gives steel its "steel" character is largely the amount, shape and distribution of cementite.

Where Do You Meet It? The Five Forms of Cementite

The same Fe₃C phase takes utterly different forms depending on thermal history — and each form produces a different material character:

Form	Appearance	How it forms	Effect
Pearlite lamellae	Alternating thin plates with ferrite	Eutectoid transformation (~727 °C, ~0.77% C)	Strength + ductility balance — the structure of normalised steel
Secondary cementite	Network along grain boundaries	Above 0.77% C on slow cooling	As a network it embrittles — unwanted
Spheroidised cementite	Round particles in ferrite	Spheroidise annealing (~700 °C, long hold)	Best machinability — the condition before machining
Tempering carbides	Very finely dispersed particles	Precipitate from martensite during tempering	The toughness + strength balance of Q&T steel
Ledeburite carbide	Coarse, continuous carbide network	In white cast iron (2.1%+ C, fast cooling)	Extremely hard + extremely brittle — grinding/wear parts

The Hardness-Brittleness Balance: Why Both Power and Danger?

The essence of steel design is this balance: ferrite is soft and ductile (~80 HV), cementite is hard and brittle (~800 HV). By adjusting the ratio and geometry of the two, the same Fe-C system yields both deep-drawing sheet and bearing races:

• Little cementite, fine dispersion → ductile, formable steel (low-carbon grades like C15)
• ~0.45% C, pearlitic structure → strength/ductility balance (C45 — the backbone of machine building)
• ~1% C, spheroidised carbides → maximum hardness + wear resistance (100Cr6 bearing steel — its heat-treated structure is sub-micron spheroidal cementite dispersed in martensite)
• 2.1%+ C, continuous carbide network → no longer steel but cast iron territory; white iron's extreme hardness and its brittleness both come from this network — detailed in our cast iron vs cast steel guide

To see carbon ranges grade by grade use the Chemical Composition tool; for hardness conversions, the Hardness Converter.

Heat Treatment = the Art of Reshaping Cementite

Most of heat treatment is really about changing the shape of cementite:

• Spheroidise annealing: Long holding at ~700 °C turns lamellar/network cementite into spheres → hardness drops, machinability peaks. High-carbon steels enter machining in this state (covered in detail in our annealing guide).
• Quenching: Traps carbon inside martensite — gives cementite no time to form. That is where the hardness comes from.
• Tempering: Precipitates the trapped carbon in a controlled way as very fine cementite particles → brittle martensite becomes the tough Q&T structure.

So the toughness of a quenched-and-tempered shaft and the wear resistance of a bearing are different choreographies of the same phase.

A Curious Detail: Cementite Is Magnetic Too (but Only up to 210 °C)

Cementite is ferromagnetic — but its Curie temperature is far lower than that of pure iron (~770 °C): about 210 °C. This difference is even used in laboratory phase analysis. For steel's full magnetic behaviour, see our article Is Steel Magnetic?

What Is It Good for in Practice?

Understanding cementite answers three concrete material-selection questions:

1. "Why did we choose C45 and not C15?" — More carbon = more cementite = higher strength (but harder welding).
2. "Why did we order annealed material?" — Spheroidised cementite directly improves tool life and surface finish.
3. "Why are grinding balls so wear-resistant?" — The grinding performance of our cast and forged balls is the work of a martensite + carbide structure.

Supply from Aktif Çelik

Aktif Çelik supplies material from every point of the cementite balance: from low-carbon case-hardening grades to carbon steels, from hot-rolled bars to high-carbon grinding balls. Our technical team supports you in choosing grade and delivery condition (annealed/normalised/Q&T) for your application — contact us for 3.1 certificates and analysis reports.