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

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1 The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram
Steels: alloys of Iron (Fe) and Carbon (C). Fe-C phase diagram is complex. Will only consider the steel part of the diagram, up to around 7% Carbon.

2 Phases in Fe–Fe3C Phase Diagram
a-ferrite - solid solution of C in BCC Fe Stable form of iron at room temperature. The maximum solubility of C is wt% Transforms to FCC g-austenite at 912 C g-austenite - solid solution of C in FCC Fe The maximum solubility of C is 2.14 wt %. Transforms to BCC d-ferrite at 1395 C Is not stable below the eutectic temperature (727  C) unless cooled rapidly (Chapter 10) d-ferrite solid solution of C in BCC Fe The same structure as a-ferrite Stable only at high T, above 1394 C Melts at 1538 C Fe3C (iron carbide or cementite) This intermetallic compound is metastable, it remains as a compound indefinitely at room T, but decomposes (very slowly, within several years) into a-Fe and C (graphite) at C Fe-C liquid solution

3 Comments on Fe–Fe3C system
C is an interstitial impurity in Fe. It forms a solid solution with a, g, d phases of iron Maximum solubility in BCC a-ferrite is wt% at727 C. BCC:relatively small interstitial positions Maximum solubility in FCC austenite is 2.14 wt% at 1147 C - FCC has larger interstitial positions Mechanical properties: Cementite (Fe3C is hard and brittle: strengthens steels. Mechanical properties also depend on microstructure: how ferrite and cementite are mixed. Magnetic properties:  -ferrite is magnetic below 768 C, austenite is non-magnetic Classification. Three types of ferrous alloys: Pure Iron: < wt % C in a-ferrite at room T Steels: up to 2.14 wt % C (usually < 1 wt % ) a-ferrite + Fe3C at room T (Chapter 12) Cast iron: wt % (usually < 4.5 wt %)

4 Eutectic and eutectoid reactions in Fe–Fe3C
Eutectic: 4.30 wt% C, 1147 C L   + Fe3C Eutectoid: wt%C, 727 C (0.76 wt% C)   (0.022 wt% C) + Fe3C Eutectic and Eutectoid reactions are important in heat treatment of steels

5 Microstructure in Iron - Carbon alloys
Microstructure depends on composition (carbon content) and heat treatment. Assume slow cooling  equilibrium maintained Microstructure of eutectoid steel (I)

6 Microstructure of eutectoid steel (II)
Pearlite, layered structure of two phases: a-ferrite and cementite (Fe3C) Alloy of eutectoid composition (0.76 wt % C) Layers formed for same reason as in eutectic: Atomic diffusion of C atoms between ferrite (0.022 wt%) and cementite (6.7 wt%) Mechanically, properties intermediate to soft, ductile ferrite and hard, brittle cementite. In the micrograph, the dark areas are Fe3C layers, the light phase is a-ferrite

7 Microstructure of hypoeutectoid steel (I)
Compositions to the left of eutectoid ( wt % C) hypoeutectoid (less than eutectoid -Greek) alloys.    +    + Fe3C

8 Microstructure of hypoeutectoid steel (II)
Hypoeutectoid contains proeutectoid ferrite formed above eutectoid temperature and eutectoid perlite that contains ferrite and cementite.

9 Microstructure of hypereutectoid steel (I)
Compositions to right of eutectoid ( wt % C) hypereutectoid (more than eutectoid -Greek) alloys    + Fe3C   + Fe3C

10 Microstructure of hypereutectoid steel (II)
Hypereutectoid contains proeutectoid cementite (formed above eutectoid temperature) plus perlite that contains eutectoid ferrite and cementite.