Phase diagram of Fe_C

Iron carbon system

It is important binary alloy. The primary structural material in every technologically advanced culture is essentially Fe – C alloy. So the study of phase diagram related to its heat treatment microstructure and mechanical properties are important.

Iron-Iron-Carbide (Fe - \(Fe_3C\)) phase diagram:

1. A portion of the iron-carbon phase diagram is shown in figure. A pure iron upon heating experiences to change in structural before it melts.
2. At room temperature stable form of iron called ferrite or \(\alpha\)-iron has bcc structure.

3. Ferrite experiences polymorphic transformation to \(\gamma\)-iron, which has fcc crystal and called ‘Austenite’. The transformation is at \(912^\circ\)C.
4. This crystal structure reverts back to bcc at temperature \(1394^\circ\)C known delta ferrite.
5. The delta ferrite melts at \(1538^\circ\)C. All this changes are visible in the left vertical axes of phase diagram.

Compositional form:

1. The horizontal line represents the composition of carbon. Due to the mixing of carbon with iron at low temperature cementite. The phase of \(\alpha\)-ferrite and cementite exist below, \(7276\circ\)C.
2. In higher concentration of carbon the presence of cementite increases with increase in temperature.
3. The Fe –C system is divided into the parts:
4. Iron rich portion
5. Carbon rich portion (not shown in diagram )
6. In practice all steel and cast iron has carbon contain led than 6.70 wt % of carbon.
7. In all form of Fe –C alloy carbon exist as interstial impurity.
8. In BCC \(\alpha\)-ferrite only small concentration of carbon are soluble. Maximum solubility is 6.022 wt % of carbon at \(7276\circ\)C.
9. The limited solubility is explain by shape and size of BCC interstial position which make it difficult to accommodate carbon atoms.
10. Below \(78^\circ\)C Fe- C phase relatively soft and it is ferromagnetic.
11. At \(1147^\circ\)C the maximum solubility of carbon in Austenite is 2.14 wt % of carbon.
12. \(\delta\)-ferrite is virtually same as \(\alpha\)-ferrite. Here, \(\delta\)-ferrite is stable at high temperature. And it has no technological importance.
13. Cementite (\(Fe_3C\)) forms when solubility limit of \(\alpha\)- ferrite is expected below \(727^\circ\)C. for \(\alpha+ Fe_3C\) and below \(1147^\circ\)C for \(\gamma+fe_3C\).
14. The two phase region exists at different temperature. One eutectic exist for iron-iron-carbide system 4.30 wt % of carbon and \(1147^\circ\)C. for this eutectic reaction,

$$L \Rightarrow \gamma+Fe_3C$$The liquid solidified to form Austenite and cementite phase.

15.The properties and various classification of steel is studied with the help phase diagram and stress- strain diagram. This signified the importance of phase diagram of Fe – C system.

It may be noted that a eutectoid invariant point exists at a composition of 0.76 wt% C and a temperature of \(727^\circ C (1341^\circ \)F). This eutectoid reaction may be represented by

Or, upon cooling, the solid \(\gamma\) phase is transformed into \(\alpha\)-iron and cementite. The eutectoid phase changes described by above equation are very important, being fundamental to the heat treatment of steels, as explained in subsequent discussions

For Ferrous alloys, iron is the prime component, but carbon as well as other alloying elements may be present. For the division of ferrous alloys based on carbon content, there are three types: iron, steel, and cast iron. Pure iron contains less than 0.008 wt% C for commercial field. In the phase diagram, it is composed almost exclusively of the ferrite phase at room temperature. The iron–carbon alloys that contain between 0.008 and 2.14 wt% C are classified as steels where the microstructure consists of both \(\alpha\) and Fe3C phases. . In practice, carbon concentrations rarely exceed 1.0 wt%. Irons are classified as ferrous alloys that contain between 2.14 and 6.70 wt% C. However, commercial cast irons normally contain less than 4.5 wt% C.

When temperature decreases to room temperature, an alloy within this composition range must pass through at least a portion of the \(\gamma\)-phase field. Though, a steel alloy may contain as much as 2.14 wt% C.

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