Iron-Carbon

Confey College, Leixlip.

The Iron - Carbon Diagram

The Structure and Properties of Plain Carbon Steels.

The ALLOTROPY of Iron (Fe)

Definition
Allotropy is the ability of some elements to exist in different physical forms (differing in color ,hardness, melting point etc.).

Iron is allotropic. At room temperature pure iron exists in the Body Centered Cubic (BCC) crystal form. However, on heating transforms to a Face Centered Cubic (FCC) crystal. The temperature that this first transformation takes place is known as a critical point and it occurs at 910 degrees Celsius.

This change in crystal structure is accompanied by a shrinkage in volume, sine the atoms in the face centred crystal are more densely packed together than in the body centred cubic crystal.
At the second critical point the F.C.C crystal changes back to an B.C.C crystal and this change occurs at 1390 degrees Celsius.

Body Centered Cubic

Face Centered Cubic

Body Centered Cubic

	*Iron above 1390 degrees is known as delta iron (d)*
	Iron between 1390 and 910 degrees is known as gamma iron (g)
	Iron below 910 degrees is known as alpha iron (a).

The Steel Section of the Iron - Carbon Diagram.

Shown here is the steel part of the iron carbon diagram containing up to 2% Carbon. At the eutectoid point 0.83% Carbon, Austenite which is in a solid solution changes directly into a solid known as Pearlite which is a layered structure consisting of layers of Ferrite and Cementite.

The steel part of the iron- carbon diagram.

In order to fully understand the changes that occur in these different alloys of steels we will look at individual microstructures of common steel alloys.

Here we see the various microstructures that exist in phases up to 2% Carbon content. (The red dots represent carbon dissolved in austenite, in the real structure these would be invisible).

Looks pretty confusing doesn't it. Well do not worry it's really very simple, the easiest way to work on these diagrams is to take an example. Let's take 3 steels containing Carbon as our examples shall we?

	0.4 % Carbon
	0.83% Carbon
	1.2 % Carbon

0.4 % Carbon

0.83 % Carbon

1.2 % Carbon

Original source and acknowledgement to: Michael Walshe

For further information follow:

http://www.ul.ie/~walshem/fyp/mainmenu.htm