Engineering - TEDs - Iron Carbon Diagrams

Before we look at the Iron - Carbon T.E.D. we need to know a bit of information about Iron itself.

Iron is Allotropic. This means that Iron can exist in different forms.

As Iron cools in solid form it changes its crystal structure. The temperature where the changes occur are called Arrest Points. When iron is cooling from 1600°C it has a BCC crystal lattice structure and is called Ferrite, or Delta Iron, (). The first of these arrest points occurs at 1390°C where Iron changes to an FCC lattice. It is now called Austenite, or Gamma Iron, (). Because the FCC Unit Cell is more tightly packed there is a slight reduction in the volume of the Iron. Another arrest point occurs at 910°C and Iron changes back to a BCC lattice. Again it is called Ferrite, but Alpha Iron,().

As you can see, Iron can exist in three different forms, which are temperature dependant, and so Iron is said to be Allotropic.

The T.E.D. of Iron - Carbon at first looks complicated, but it is the same as a Partial Solubility alloy, however because of the importance of irons and steels in today's engineering, it needs to be looked at in more detail. We are only interested in the area up to an inclusion of 6% Carbon.

We will first take a look at the Iron - Carbon T.E.D. As was mentioned at the start of this section, an alloy is a combination of two or more materials at least one of which must be a metal. Carbon is not a metal.

The Solid Solution area in an Iron - Carbon T.E.D. is called Austenite, or Gamma Iron. The other Solid Solution formed is called Cementite, but is not shown on this T.E.D. as it is of little interest.

Notice that the lowest temperature at which an Iron - Carbon alloy can remain liquid is at 1140°C, (the Eutectic Point). This point occurs when 4.3% Carbon is contained in the alloy. To the left and right, respectively, of the Eutectic Point, we have pasty phases of Austenite and liquid, and Cementite and liquid.

The next diagram is extremely important in the heat treatment of steels.

It is generally accepted that Iron - Carbon alloys with a Carbon content of less than 2% are considered Steels, and those with a greater Carbon content are considered Cast Irons. In the above diagram all of the phases are solid but there are important differences to note.

Notice the Eutectoid Point in the diagram. A Eutectoid Point is where an alloy changes from a solid to a solid. The crystal structure changes.

Notice that different areas of the alloys have different names.

Ferrite, which we also know as Alpha Iron, can only dissolve a small amount of Carbon. It has a white appearance.

Cementite has a dark appearance.

At the Eutectoid Point the solid Austenite changes into the two solid phases called Ferrite and Cementite. These two solids combine to form a mixture called Pearlite, which contains alternating layers of Ferrite and Cementite. Because there is more Ferrite in the Pearlite it has a white/silver sheen.

Below is a representation of what these different metals look like, under a microscope.