Pure iron has two crystalline forms, one BCC, commonly called α - iron which remains stable from low temperatures upto 910°C (1414°F) when it changes to FCC called γ – iron. The γ - iron remains stable upto 1394°C (2554°F), when it reverts to BCC form now called as δ - iron, which is stable upto the melting point of iron (1539°C or 2802°F).All the allotropic changes give off heat (exothermic) when iron is cooled and absorb heat (endothermic) when iron is heated.
Effect of pressure on allotropy of iron
Increse in pressure lowers the α - Fe to γ - Fe transition temperature and increses the γ - Fe to δ - Fe traqnsition temperature. This is according to the lechatlier’s principle as volume of FCC (γ - Fe) is lower than that of BCC. A volume change of FCC to BCC is 8.8%.
Iron – Iron carbide diagram
The temperature at which allotrophic change (critical temperatures) takes place is influenced by alloying elements. The curie temperature is not effected by alloying elements. Carbon is the most common alloying element in the iron which significantly affects the allotrophy, structure and properties of iron. Conventionally, the complete Fe – C diagram should extend from 100% Fe to 100 % carbon (graphite), but it is normally studied upto 6.67% carbon (Fe3C) because iron alloys of practical industrial importance contain not more than 5% carbon. Fe – Fe3C is not a true equilibrium diagram, since Fe3C (meta stable) decomposes into Iron and Carbon which take a very long time at room temperature and even at 700°C it takes several years to form graphite. When the carbon content becomes more than solubility limits of iron though carbon should be present as Graphite (lower free energy than cementite) , yet cementite forms beacause the formation of cementite is most probable kinetically i.e. it is easier to from it, as only 6.67% C has to diffuse to segregate to form cementite whereas 100% C segregation is required to nucleate graphite..
Definition of structures or phases in Fe – Fe3C diagram
Ferrite: it is an interstitial solid solution of carbon in α - iron (BCC). The maximum solubility of carbon in ferrite is 0.02 wt% at 727°C and the minimum is 0.00005 wt% at 20°C. the size of the largest atom that can fit in octahedral void is 0.19 A°, which is much smaller than carbon atom (0.71°A°). so the solubility is exremely limited. It is soft and ductile. Ferrite is ferromagnetic upto 768°C becomes paramagnetic above this temperature.
Ferrite: it is an interstitial solid solution of carbon in α - iron (BCC). The maximum solubility of carbon in ferrite is 0.02 wt% at 727°C and the minimum is 0.00005 wt% at 20°C. the size of the largest atom that can fit in octahedral void is 0.19 A°, which is much smaller than carbon atom (0.71°A°). so the solubility is exremely limited. It is soft and ductile. Ferrite is ferromagnetic upto 768°C becomes paramagnetic above this temperature.
Austenite: it is an interstitial solid solution of carbon in γ - iron (FCC). The maximum solubility of carbon is 2.1 wt% at 1146°C which decreses to 0.77 wt% at 727°C. the size of the largest atom that can fit in octahedral void is 0.52 A°. correspondingly the solubility is larger here compared to ferrite. It is soft, ductile, malleable, tough and non-magenetic. It is stable above 727°C in plain carbon steels but can be obtained even at room temperature by adding elements like Ni or Mn in steels.
δ - ferrite: it is an interstitial solid solution of carbon in δ - iron (BCC). The maximum solubility of carbon is 0.09 wt% at 1495°C. it is paramagnetic. It is high temperature version of α -iron.
Cementite (Fe3C): It is an interstitial intermetllic compound havinbg fixed carbon content of 6.67wt%. it has a complex orthorhombic structure, with 12 Fe atoms and 4 C atoms per unitcell. High hardness, brittle, very low tensile strenght and high compressive strength. It is the hardest phase that appears on the phase diagram.
Ledeburite: it is eutectic mixture of austenite and cementite. It contains 4.3wt% C and is formed at 1146°C. this is very fine mixture.
Pearlite: it is eutectoid mixture of ferrite and cementite containing 0.8wt% C and is formed at 727°C. it is a very fine platelike or lamellar mixture.
Invariant reactions in Fe – Fe3C diagram
1. Peritectic reaction
Composition wt% 0.09 0.53 0.17
2. Eutectic reaction
Composition wt% 4.3 2.11 6.67
3. Eutectoid reaction
Composition wt% 0.77 0.02 6.67
The temperatures at which phase transformations occurs during heating or cooling an alloy. certain symbols are used to denote the critical temperature in steels. The upper and lower critical lines under equilibrium are indicated by Ae3 and Ae1 etc.
It is found that in actual practice the critical line on heating and the critical line on cooling are not occur at same temperature. The critical line on heating is always higher than the critical line on cooling. The former is denoted by Ac and the later is denoted by Ar. A for arret (means arrest), C for chauffage (means heating), R for Refroidissement (means cooling), e for equilibrium.
If extremely slow rates of heating or cooling are employed then critical temperatures are nearly equal i.e. Ac1 = Ar1 = Ae1.
The curie temperature (magnetic to non-megnetic change) of cementite is called A0. Ae1 or A1 is eutectoid tempersture line (727°C). Ae2 or A2 is curie temperature line (768°C) and this is constant for all Fe – C alloys.
Hypo-eutectoid side
Upper critical temperature (Ae3 , Ac3 , Ar3): It is the temperature at which Austenite to ferrite transformation begins on cooling (or) at which ferrite to austenite transformation ends on heating. This is denoted by A3 line. (Ac3 > Ar3)
Lower critical temperature: It is the temperature at which Austenite to ferrite transformation ends on cooling (or) at which ferrite to austenite transformation starts on heating. (Ac1 > Ar1).
Hypo-eutectoid side
Lower critical temperature (Ae3,1 Ac3,1 Ar3,1) : It is the temperature at which precipitation of cementite from austenite ends uopn cooling (or) at which dissolution of cementite in austenite begins upon heating.
Upper critical temperature (Aem , Acm , Arm): It is the temperature at which precipitation of cementite from austenite begins uopn cooling (or) at which dissolution of cementite in austenite ends upon heating.
Effect of alloying elements on the Fe – C diagram
Ferrite stabilizers: some alloying elements tend to stabilize the ferrite phase in preference to austenite. Many of these elements have same crystal structure as ferrite (BCC). They reduce the extent of the austenite area on the equilibrium diagram by forming a gamma loop. Austenite is enclosed within the loop. eg: Cr, Si, Mo, W, V, Ti etc.
Austenite stabilizers: These enlarge the area of the austenite phase on the phase diagram. critical amount of these alloying elements results in Austenite even at room temperture. eg: Mn, Ni, C, N etc.
Effect on eutectoid temperature and composition
Ferrite stabilizers raises the eutectoid temperatute to above 727°C, Austenite stabilizers lowers the euctectoid tempersture to below 727°C. Both Ferrite and Austenite stbilizers decrease the eutectoid composition from 0.77% to lower values.