Annotation of Steel Phase Diagram

hurdaji — Sat, 28 Jan 2012 15:36:01 +0000

Iron-carbon alloys, containing up to 2,06% of carbon, are called steels. Alloys, containing from 2,06% to 6,67% of carbon, experience eutectic transformation at 1147 ºC. The eutectic concentration of carbon is 4.3%. In practice only hypoeutectic alloys are used. These alloys are called cast irons. When temperature of an alloy from this range reaches 1147 ºC, it contains primary austenite crystals and some amount of the liquid phase. The latter decomposes by eutectic mechanism to a fine mixture of austenite and cementite, called ledeburite.

Fe-Fe3C Phase Diagram, Materials Science and Metallurgy, 4th ed., Pollack, Prentice-Hall, 1988

Figure above shows the equilibrium diagram for combinations of carbon in a solid solution of iron. The diagram shows iron and carbons combined to form Fe-Fe3C at the 6.67%C end of the diagram. The left side of the diagram is pure iron combined with carbon, resulting in steel alloys. Three significant regions can be made relative to the steel portion of the diagram. They are the eutectoid E, the hypoeutectoid A, and the hypereutectoid B. The right side of the pure iron line is carbon in combination with various forms of iron called alpha iron (ferrite), gamma iron (austenite), and delta iron. The black dots mark clickable sections of the diagram.

Allotropic changes take place when there is a change in crystal lattice structure. From 1538º-1400ºC the delta iron has a body-centered cubic lattice structure. At 1400ºC, the lattice changes from a body-centered cubic to a face-centered cubic lattice type. At 760ºC, the curve shows a plateau but this does not signify an allotropic change. It is called the Curie temperature, where the metal changes its magnetic properties.

Two very important phase changes take place at 0.83%C and at 4.3% C. At 0.83%C, the transformation is eutectoid, called pearlite.

gamma (austenite) –> alpha + Fe3C (cementite)

At 4.3% C and 2066ºF, the transformation is eutectic, called ledeburite.

L(liquid) –> gamma (austenite) + Fe3C (cementite)

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Annotation of Steel Phase Diagram