The Iron-Carbon Alloys And Fe-C Phase Diagram

The iron-carbon phase diagram and the iron-carbon systems are the most important binary systems in engineering. Steels, cast irons, and various kinds of applications are dependent on iron-carbon systems. We also explained the tin-lead phase diagram and nickel-copper phase diagrams which are also very important in metallurgy and engineering. In this article, we will explain the iron-carbon phase diagram.

Fundamentals of Modern Manufacturing: Materials, Processes and Systems, Seventh Edition

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What Is Iron-Carbon Phase Diagram?

Iron-Carbon(Fe-C) phase diagram.

The iron-carbon phases and phase diagram is slightly complex, compared with other diagrams. The horizontal axis of the Fe-C diagram shows the percentage of carbon inside the Fe-C systems. The solubility limit of carbon inside the iron is very low. So the diagram above shows up to 6.11% of carbon content which is also the solubility limit of carbon inside the iron. The two vertical axes are showing the temperature on which the right side is Celsius, the left side is Fahrenheit.

If we take a look at the pure iron from the left side, there are lots of phases. The first phase that starts from the ambient temperature of pure iron is called the alpha solid phase which is also called ferrite. At 912 Celsius degrees of temperature, the austenite phase of iron is formed. At 1394 celsius degrees, the delta phase occurs up to the melting point of pure iron which is 1539 Celsius degrees.

If we take a look at the lattice structures of the phases, the alpha and beta phases of pure iron have a BCC lattice structure, and the gamma phase which is between these phases has an FCC lattice structure.

As we explained in another article, if the carboın content of the iron-carbon alloy is between 0-2.07%, this alloy is called steel. Steel is one of the most important metals in industry.

At the 0.77% carbon and 723 Celsius degrees point, called eutectoid composition. Steels that have carbon content bigger than 0.77% are called hyper-eutectoid and other ones are called hypo-eutectoid steels. There is another point is called the eutectic point that is occurring at 4.3% of carbon and 1130 Celsius degrees temperature.

If the carbon content is between 2.07-6.11%, the alloy is called cast iron.

There is an F3C phase in the diagram above that is prominent at low temperatures called cementite. Cementite is the hard and brittle phase of iron-carbon alloy which is solid also.


There general explanation of the iron-carbon phase diagram can be like above. Leave your comments and questions about the ‘iron-carbon phase diagram’ below!

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