Iron Carbon Diagram

Only available on StudyMode
  • Topic: Phase diagram, Austenite, Steel
  • Pages : 28 (7780 words )
  • Download(s) : 93
  • Published : February 28, 2013
Open Document
Text Preview
The Iron-Carbon Diagram
There are two iron-carbon equilibrium diagrams: - stable iron-graphite Fe-Gr - metastable iron-cementite Fe-Fe3C The stable condition usually takes a very long time to develop. The metastable diagram is of more interest. Fe3C iron carbide called cementite because it is hard.

Following phases exist on Fe-Fe3C diagram: - liquid solution of iron and carbon (L) - ferrite ( ) – an interstitial solid solution of carbon in Fe (bcc). At room temperature ferrite is ductile but not very strong. - austenite - an interstitial solid solution of carbon in Fe (fcc). - cementite (Fe3C) hard and brittle compound with chemical formula Fe3C. It has metallic properties.

On a base of Fe-Fe3C diagram we can divide iron-carbon alloys into: - steels, - cast steels, - cast irons.

Steel is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content up to 2% intended for wrought products or semi products.

Cast iron is an alloy of carbon and iron and other alloying elements (e.g. Mn, Si) with carbon content over 2% intended for castings.

Now, we consider only a part of Fe-Fe3C diagram referring to steel. Perlite is a structure (i.e. consists of two phases) consists of alternate layers of ferrite and cementite in the proportion 87:13 by weight. Perlite is formed from austenite at eutectoid temperature (A1) 727°C upon slow cooling. There are three groups of steels according to carbon content: - hypoeutectoid steels containing less than 0.76% C - eutectoid steel with carbon content about 0.76% - hypereutectoid steels contain more than 0.76% C (up to 2% C).

MSE 300 Materials Laboratory Procedures

Iron-Carbon Phase Diagram (a review) see Callister Chapter 9

University of Tennessee, Dept. of Materials Science and Engineering

1

MSE 300 Materials Laboratory Procedures

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% Carbon.

University of Tennessee, Dept. of Materials Science and Engineering

2

MSE 300 Materials Laboratory Procedures

Phases in Fe–Fe3C Phase Diagram -ferrite - solid solution of C in BCC Fe • Stable form of iron at room temperature. • The maximum solubility of C is 0.022 wt% • Transforms to FCC -austenite at 912 C -austenite - solid solution of C in FCC Fe • The maximum solubility of C is 2.14 wt %. • Transforms to BCC -ferrite at 1395 C • Is not stable below the eutectic temperature (727 C) unless cooled rapidly (Chapter 10) -ferrite solid solution of C in BCC Fe • The same structure as -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 -Fe and C (graphite) at 650 - 700 C Fe-C liquid solution University of Tennessee, Dept. of Materials Science and Engineering 3

MSE 300 Materials Laboratory Procedures

A few comments on Fe–Fe3C system C is an interstitial impurity in Fe. It forms a solid solution with phases of iron Maximum solubility in BCC -ferrite is limited (max. 0.022 wt% at 727 C) - BCC has relatively small interstitial positions Maximum solubility in FCC austenite is 2.14 wt% at 1147 C - FCC has larger interstitial positions Mechanical properties: Cementite is very hard and brittle can strengthen steels. Mechanical properties also depend on the microstructure, that is, how ferrite and cementite are mixed. Magnetic properties: -ferrite is magnetic below 768 C, austenite is non-magnetic Classification. Three types of ferrous alloys: • Iron: less than 0.008 wt % C in ferrite at room T • Steels: 0.008 - 2.14 wt % C (usually < 1 wt % ) -ferrite + Fe3C at room T (Chapter 12) • Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %) University of Tennessee,...
tracking img