Characteristics of phases appeared in Fe-Fe3C phase diagram
The Fe-Fe3C phase diagram, also known as the iron-carbon phase diagram, describes the phases that can appear in iron-carbon alloys as a function of temperature and carbon content. The main phases that appear in this diagram are:
Ferrite (α): Ferrite is a body-centered cubic (BCC) phase of iron that is stable at low temperatures. It can dissolve a small amount of carbon, up to 0.022% at 727°C.
Austenite (γ): Austenite is a face-centered cubic (FCC) solid solution of carbon in iron. It is stable at high temperatures and can dissolve up to 2.11% carbon at 1147°C.Cementite (Fe3C): Cementite is a compound that consists of iron and carbon in a fixed ratio of 6.67% carbon. It has an orthorhombic crystal structure and is a hard and brittle phase.
Pearlite: Pearlite is a two-phase microstructure consisting of alternating layers of ferrite and cementite. It forms when austenite is cooled rapidly to a temperature between 727°C and 550°C, which is in the eutectoid region of the phase diagram.
Martensite: Martensite is a metastable phase that forms when austenite is rapidly quenched to a temperature below the eutectoid temperature. It has a highly distorted BCC structure and is extremely hard and brittle.
Liquid: At temperatures above the melting point of iron (1538°C), the alloy exists as a liquid phase.
These phases and their compositions can be used to predict the mechanical and physical properties of iron-carbon alloys.
Ferrite (α)
It is an interstitial solid solution of a small amount of carbon dissolved in α iron. The maximum solubility is 0.025%C at 723ºC and it dissolves only 0.008%C at room temperature. It is the softest structure that appears on the diagram.
Ferrite is ferromagnetic at low temperatures but loses its magnetic properties with the rise of temperatures with major loss at curies temperatures, 768ºC and above this temperature, it becomes non magnetic (paramagnetic).
The crystal structure of ferrite (α) is B.C.C
Tensile strength – 245 Mpa, Yield strength 118 Mpa
Elongation – 40-50% in 2 in.
Hardness - 95 VPN
Cementite (Fe3C)
Cementite or iron carbide, chemical formula Fe3C, contains 6.67%C by weight and it is a metastable phase.
It is typically hard and brittle interstitial compound of low tensile strength (35 Mpa) but high compressive strength and high hardness ~800VPN.
It is the hardest structure that appears on the diagram.
It has a complex orthorhombic crystal structure with 12 iron atoms and 4 carbon atoms per unitcell.
It is slightly ferromagnetic up to 210ºC and paramagnetic above it. Melting point around 1227ºC.
Pearlite (α+Fe3C)
Pearlite is the eutectoid mixture containing 0.80 %C and is formed at 723ºC on very slow cooling.
It is very fine platelike or lamellar mixture of ferrite and cementite. The fine fingerprint mixture called pearlite is shown in below figure.
The weight % of these two phases are thus in ratio 8:1
Tensile strength – 120,000 psi or 825 Mpa
Elongation – 20 percent in 2 in.
Hardness – HRC 20, HRB 95-100, or BHN 250-300
Austenite (γ)
It is an interstitial solid solution of a small amount of carbon dissolved in γ iron and has FCC crystal structure. The maximum solubility is 2.1%C at 1147ºC.
Austenite is soft, ductile tough and malleable (FCC structure) and non magnetic (paramagnetic).
Steels are commonly rolled and forged above about 1100ºC when they are in austenite state
due to its high ductility and malleability, which is also due to its FCC structure.
Tensile strength – 150,000 psi or 1035 Mpa
Elongation – 10% in 2 in.
Hardness - 395 VPN and Toughness is high.
Ledeburite (γ+Fe3C)
Ledeburite is the eutectic mixture of austenite and cementite. It contains 4.3%C and is formed at 1147ºC
Structure of ledeburite contains small islands of austenite are dispersed in the carbide phase.
Not stable at room temperature
The pearlite is resolved in some regions where the sectioning plane makes a glancing angle to the lamellae. The ledeburite eutectic is highlighted by the arrows. At high temperatures this is a mixture of austenite and cementite formed from liquid. The austenite subsequently decomposes to pearlite.
Ferrite (δ)
Interstitial solid solution of carbon in iron of body centered cubic crystal structure. (δ iron ) of higher lattice parameter (2.89Å) having solubility limit of 0.09 wt% at 1495°C with respect to austenite. The stability of the phase ranges between 1394-1539°C.
It is a high temperature phase and is a high temperature manifestation of α ferrite.
This is not stable at room temperature in plain carbon steel. However it can be present at
room temperature in alloy steel specially duplex stainless steel.
