Cementite

Iron carbide

Iron carbon phase diagram

Cementite, also known as iron carbide, is a chemical compound with the formula Fe₃C. It is a hard, brittle material that is usually found as a component of steel and cast iron. Cementite plays a crucial role in the microstructure of carbon steels and cast irons, influencing their mechanical properties such as hardness, strength, and toughness.

Composition and Structure

Cementite contains 6.67% carbon by weight, making it one of the principal forms in which carbon is found in alloyed iron. The structure of cementite is orthorhombic, with a crystal structure that is complex and contains 12 iron atoms and 4 carbon atoms in its unit cell. This structure is significantly different from the body-centered cubic structure of ferrite (α-iron) and the face-centered cubic structure of austenite (γ-iron).

Formation and Stability

Cementite forms through a reaction between ferrite and carbon, where carbon atoms diffuse into the iron lattice. This process occurs during the cooling of carbon steels and cast irons, typically in the temperature range where the austenite phase starts to transform. The stability of cementite is influenced by temperature and the presence of other alloying elements. It is stable at room temperature but decomposes into iron and carbon (graphite) at temperatures above 727°C in a process known as graphitization.

Role in the Iron-Carbon Phase Diagram

In the iron-carbon phase diagram, cementite is a critical phase that defines the boundary of the phase fields. The diagram shows the conditions under which cementite forms and coexists with other phases such as ferrite, austenite, and graphite. The presence of cementite in the microstructure of steels and cast irons is a key factor in determining their mechanical properties.

Influence on Mechanical Properties

The hardness and strength of steel and cast iron are significantly increased by the presence of cementite. However, this increase in hardness and strength comes at the expense of ductility and toughness. The distribution and morphology of cementite in the microstructure—whether it forms as a continuous network or is dispersed as individual particles—have a profound effect on the material's overall properties.

Heat Treatment and Microstructural Control

Through heat treatment processes such as annealing, normalizing, and tempering, the size, shape, and distribution of cementite in the microstructure can be controlled. These heat treatments are used to optimize the mechanical properties of steel and cast iron for specific applications. For example, spheroidizing is a heat treatment process that converts cementite into spherical particles, which improves the ductility and machinability of high carbon steels.

Applications

Cementite is found in a wide range of steel and cast iron products, from high-strength structural steels to wear-resistant cast irons. Its presence is particularly important in tool steels, where a high hardness is required for cutting or shaping other materials.

Cementite Resources
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