IRON-IRON CARBIDE Phase Diagram

Description of the phase diagram,invariant reactions of Fe-FeC.

Reference:Material Science and Engineering,William Callister

1. IRON-CARBON SYSTEM

2. IRON- IRON CARBIDE SYSTEM (Fe - Fe ₃ C DIAGRAM) Both steels and cast irons, primary structural materials in every technologically advanced culture, are essentially iron–carbon alloys. Iron-carbon phase diagram shown in figure is not a complete diagram. Part of the diagram after 6.67 wt % C is ignored as it has little commercial significance. MTE/III SEMESTER/MSE/MTE 2101 2 Fe-C Phase diagram

3. MTE/III SEMESTER/MSE/MTE 2101 3 Pic Courtesy: Material Science and Engineering, Callister Iron – Iron carbide phase diagram

4. The left vertical axis represents 100% iron and shows all the allotropic changes of pure iron with temperature. The temperature at which allotropic changes occur in iron changes when it is alloyed with different amounts of carbon. The right vertical axis does not represent 100% carbon but instead represents only 6.67% C by weight. This is because only a maximum of 6.67% C can be added to molten iron at which it becomes saturated. MTE/III SEMESTER/MSE/MTE 2101 4

5. Any further addition of carbon will not dissolve in iron but rather floats or gets blown away owing to its very low density. Iron when contains exactly 6.67% C by weight forms an intermediate phase called Cementite or Iron Carbide (Fe 3 C). Hence, the Iron-Carbon equilibrium diagram is actually called as the Iron – Iron Carbide Equilibrium Diagram with pure iron and pure Fe 3 C (Iron Carbide) forming the extremities. Actually, the phase Fe 3 C (Iron Carbide) is called meta stable state because it decomposes with passage of time. Here, the equilibrium phase diagram for iron and carbon assumes that Fe 3 C is stable with respect to time. Hence, it is not a true equilibrium diagram. MTE/III SEMESTER/MSE/MTE 2101 5

6. Characteristics of Fe-C Diagram The Fe-Fe ₃ C is characterized by five individual phases and three invariant reactions. Five phases that exist in the diagram are: α– ferrite (BCC) Fe-C solid solution , γ - austenite (FCC) Fe-C solid solution , δ - ferrite (BCC) Fe-C solid solution , Fe ₃ C (iron carbide) or cementite - an inter-metallic compound and liquid Fe-C solution . Three invariant reactions that cause transformations in the system are namely: eutectoid, eutectic, and peritectic . MTE/III SEMESTER/MSE/MTE 2101 6

7. When iron solidifies first, at 1538 o C, it is in the BCC  form called ‘Delta – Iron’. Further cooling at 1394 o C , a phase change occurs and atoms re-arrange themselves into FCC  form called as ‘Gamma Iron’, which is non-magnetic. When the temperature reaches 912 o C , another phase change occurs and atoms begin to re-arrange themselves into  form called as ‘Alpha Iron’ , which is non-magnetic. Finally at 768 o C, the  -iron becomes magnetic without a change in the crystal lattice structure. MTE/III SEMESTER/MSE/MTE 2101 7

8. MTE/III SEMESTER/MSE/MTE 2101 8 Cooling Curves of Fe – C Diagram

9. The diagram shows 3 horizontal lines (isotherms) which represent 3 invariant reactions: (a) The first isotherm at 1493 o C indicates the peritectic reaction. This region where the peritectic reaction takes place is called the Delta region . (b) The second isotherm at 1147 o C indicates the eutectic reaction. This region where the eutectic reaction takes place is called the Eutectic region . (c) The third isotherm at 727 o C indicates the eutectoid reaction. This region where the eutectoid reaction takes place is called the Eutectoid region . MTE/III SEMESTER/MSE/MTE 2101 9

10. Carbon is an interstitial impurity in iron and forms a solid solution with each of α and δ ferrites, and also with austenite, as indicated by the α,γ and δ single phase fields. α - Ferrite Phase (912ᵒC-768ᵒC): In the BCC α ferrite, only small concentrations of carbon are soluble; the maximum solubility is 0.022 wt % at 727 °C. The interstitial position in BCC, α – Ferrite is very small to accommodate carbon atoms. It is because of this reason the solubility is less . This particular iron–carbon phase is relatively soft, may be made magnetic at temperatures below 768 °C and has a density of 7.88 g/cm³. MTE/III SEMESTER/MSE/MTE 2101 10

11. γ – Austenite Phase (1394ᵒC-912ᵒC): The austenite, or γ phase of iron, when alloyed with carbon alone, is not stable below 727 °C. The maximum solubility of carbon in austenite, 2.14 wt %, occurs at 1147 °C . This solubility is approximately 100 times greater than the maximum for BCC ferrite, since the FCC interstitial positions are larger, and, therefore, the strains imposed on the surrounding iron atoms are much lower. Austenite is nonmagnetic. δ Phase (1538ᵒC-1394ᵒC): The δ ferrite is virtually the same as ferrite, except for the range of temperatures over which each exists. δ ferrite is stable only at relatively high temperatures, MTE/III SEMESTER/MSE/MTE 2101 11

12. Cementite ( Fe ₃ C ): It is formed when the solubility limit of carbon in α ferrite is exceeded below 727 °C (for compositions within the α+ Fe ₃ C phase region). Fe ₃ C will also coexist with the phase between 727 °C and 1341 ° C. Mechanically, cementite is very hard and brittle; the strength of some steels is greatly enhanced by its presence. MTE/III SEMESTER/MSE/MTE 2101 12

13. THE INVARIANT REACTIONS IN THE PHASE DIAGRAM THE DELTA REGION: The first horizontal line MB at 1495 o C indicates the Peritectic reaction. This region is called as the delta region because of the solid solution  . The point P is known as the peritectic point at 1495 o C & 0.18%C. The peritectic reaction may be written as: Cooling Liquid +   (Austenite) Heating The maximum solubility of carbon in BCC  Fe is 0.1% (point M). MTE/III SEMESTER/MSE/MTE 2101 13

14. (2) THE EUTECTIC REGION: The Eutectic reaction takes place at 1148 o C. The Eutectic point ‘E’ is at 1148 o C and at 4.33% C. Whenever an alloy crosses the line line CED, and Eutectic reaction takes place, giving rise to a fine mixture of Austenite + Cementite (  + Fe 3 C). This eutectic mixture is commonly called a Ledeburite . This reaction can be expressed as: Cooling Liquid (L) Austenite ( γ ) + Cementite ( Fe₃C ) Heating MTE/III SEMESTER/MSE/MTE 2101 14

15. (3) THE EUTECTOID REACTION: A small solid solution region to the left of the line GH, which represents  -Fe (Ferrite), is a solid solution with small amounts of carbon dissolved in BCC Fe. At 912 o C, the FCC  -Fe changes to BCC  -Fe. Line HJK represents the Eutectoid reaction taking place at 727 o C. The Eutectoid point is ‘J’ which has 0.8%C & 727 o C. Whenever an alloy crosses the line HJK, the Eutectoid reaction takes place giving rise to the fine Eutectoid mixture of Ferrite + Cementite (  + Fe 3 C) commonly known as Pearlite. The reaction is : Cooling Austenite ( γ ) Ferrite ( α )+ Cementite ( Fe 3 C) Heating MTE/III SEMESTER/MSE/MTE 2101 15

16. MTE/III SEMESTER/MSE/MTE 2101 16 PHASES PRESENT: PHASES STRUCTURE PROPERTY Liquid Phase (1538ᵒC) - - δ Phase (1538ᵒC-1394ᵒC) BCC Structure - γ – Austenite Phase (1394ᵒC-912ᵒC) FCC Structure Non-magnetic property, Fairly ductile α - Ferrite Phase (912ᵒC-768ᵒC) BCC Structure Non-magnetic, Ductile α - Ferrite Phase(768ᵒC) BCC Structure Magnetic REACTIONS TAKING PLACE: Peritectic : L+ δ=γ Eutectic: L= γ + Fe₃C Eutectoid: γ = α + Fe 3 C Three types of ferrous alloys: Iron: less than 0.008 wt % C in α−ferrite at room T Steels: 0.008 - 2.14 wt % C (usually < 1 wt % ): α-ferrite + Fe ₃ C at room T Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)