What is cementite (iron carbide)?

What is cementite (iron carbide)?

Cementite, also known as iron carbide (Fe3C), is a compound of iron and carbon. It is one of the primary constituents in steel and cast iron. Cementite forms when carbon atoms combine with iron atoms at very high temperatures. The resulting compound has a distinct crystal structure that gives it unique properties.

Cementite has a unique crystal structure that consists of layers of iron atoms separated by layers of carbon atoms. This structure makes it very hard and brittle, which makes it useful in the production of cutting tools and other industrial applications. In addition to its physical properties, cementite also plays an important role in the heat treatment process used to strengthen steel.

It is commonly used in the production of cutting tools, as well as in machinery parts that require high wear resistance. Due to its hardness and brittleness, cementite can also be used as an abrasive material.

How cementite appears in steel microstructure?

The appearance and distribution of cementite can be analyzed through microscopic examination using techniques such as optical microscopy or electron microscopy. Cementite appears as dark lines on an optical microstructure. In the below picture, black area is Cementite while white is Ferrite.

In order for cementite to appear in steel, the carbon content must exceed 0.8%. This can be achieved through various methods such as adding carbon-rich materials or reducing oxygen levels during production. Once the proper carbon content is reached, the steel must be heated above 723°C (1333°F) to allow for the diffusion of carbon into austenite – a solid solution of iron with dissolved carbon.

As the steel cools down at a controlled rate, austenite transforms into different microstructure phases depending on its composition and cooling conditions.

Hardness range of cementite microstructure

The hardness range of cementite microstructure depends on various factors such as the amount and distribution of carbon within the structure, the cooling rate from austenitizing temperature, and the presence of other alloying elements.

The hardness range of cementite microstructures can be quite broad, ranging anywhere from 600 to over 1,000 vickers. Cementite typically falls within the range of 65-80 HRC, making it one of the hardest microstructures in steel alloys.

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