Iron carbon phase diagrams

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The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram

In their simplest form, steels are alloys of Iron (Fe) and Carbon (C).

The Fe-C phase diagram is a fairly complex one, but we will only consider the steel part of the diagram, up to around 7% C b d Carbon.

Phases present α-ferrite, γ-ferrite, δ-ferrite, Fe3C (iron carbide or cementite)
Fe-C liquid solution

School of Mechanical and Building Sciences, VIT University, Vellore


Phases in Fe–Fe3C Phase Diagram α-ferrite - solid solution of C in BCC Fe
• Stable form of iron at room temperature.
• The maximum solubility of C is 0.022 wt%
• T
Transforms t FCC γ-austenite at 912 °C f to t it t γ-austenite - solid solution of C in FCC Fe

Fe3C (iron
…show more content…

Horizontal line represents the eutectic temperature line and y g whenever an alloy crosses the line must undergo the eutectic reaction •

Any liquid that is present when this line is reached must solidify now into very fine intimate mixture of two phases namely austenite
(γ) and cementite (Fe3C).

The eutectic mixture has been given with the name LEDEBURITE and the equation is given as
(4.3% C)
0.18% C

The eutectic mixture is not usually seen in the microscope because the austenite is not stable at room temperatures and must undergo another reaction during cooling
School of Mechanical and Building Sciences, VIT University, Vellore


Development of Microstructure - Fe–Fe3C System

An alloy of eutectoid composition (0.76 wt% C) as it is cooled from a temperature within the phase region, say, 800 °C—that is, beginning at point a and moving down the vertical line xx`.

Initially, the alloy is composed entirely of the austenite p phase having a composition of 0.76 wt% C ( g g p
(Figure a).
As the alloy is cooled, no changes will occur until the eutectoid temperature (727 °C) is reached.
Upon crossing this temperature to point b, the austenite transforms to α and Fe3C) .

This microstructure, represented schematically in point b, is called pearlite (alternating layers or lamellae of α and Fe3C.

School of Mechanical and Building Sciences, VIT University,