The diagram below shows the phases present when Fe C alloys C up to 7 are

The diagram below shows the phases present when fe c

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The diagram below shows the phases present when Fe-C alloys (C up to 7%) are cooled from liquid to solid. The left side of the diagram represents pure iron and the right hand of the diagram represents an alloy with 6.67% C which results on cooling in the formation of Cementite. This is an intermetallic compound (iron carbide-Fe 3 C) which although not 100% stable is to all practical purposes a stable phase. The phase diagram shown is therefore a metastable phase. Note: Different reference sources indicate the Eutectoid point at 0.8% C and 0.77% C. Iron Forms Iron can exist in three forms: α... BCC crystal with crystal dimension a = 2.86 Angstrom exists at temperatures up to 910 °C. γ... FCC crystal with crystal dimension a = 3.65 Angstrom exists at temperature range 910 °C to 1403 °C. δ... BCC crystal with crystal dimension a = 2.93 Angstrom exists at temperature range 1403 °C to 1535 °C.
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~ Page 23 of 73 ~ Solid Phases The phased diagram includes four solid phases: α Ferrite This is the solid solution of carbon in iron. At 0% C this is pure iron BCC crystal structure. The maximum solubility of carbon in iron is 0.02 to 0.03% at 723 °C. At 0 °C temperature the solubility falls to 0.008%. The carbon atoms are located in the crystal interstices. Austenite The solid solution of carbon in γ iron is called austenite. This has a FCC crystal structure with a high solubility for carbon compared with α ferrite. The solubility reaches a maximum of 2.08% at 1148 °C. The solubility decreases to 0.8% at 723 °C . The carbon atoms are dissolved interstitially. The difference in solubility between the austenite and α Ferrite is the basis for the hardening of steels. Cementite This is an intermetallic compound which contains 6.67% C and 93.3% Fe. Cementite is a hard brittle compound with an orthorhombic crystal structure. Each unit cell has 12 Fe atoms and 4 C atoms. δ Ferrite This is a solid solution of carbon in iron and has a BCC crystal structure. The maximum solubility of C in Fe is 0.09% at 1495 °C. This has no real practical significance in engineering. Pearlite This is a lamellar structure of α ferrite and cementite formed by the transformation of austenite of eutectoid composition (0.8% C) at 723 °C .
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~ Page 24 of 73 ~ Lever Rule The lever rule can be applied to any phase region and provides an indication of the proportions of the constituent parts at any point on the phase diagram. Applying the lever rule to the eutectoid point (0.8% C at 723 °C) Wt% Ferrite = 100 (6.67 -0.8)/ 6.67- 0.02) = 88% Wt% Cementite = 100 (0.8- 0.02) /6.67- 0.02) = 12%
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~ Page 25 of 73 ~ Steels If the carbon content of the cooled solid is less than Eutectoid (about 0.8% C) the solid is identified as a hypoeutectoid steel: most steels are of this form. If the carbon content is more than 0.8% then the solid is a hypereutectoid steel. Hypereutectoid steels with carbon content over 1.2% C are very brittle. Few steels are made with carbon contents over 1.2%.
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