Why is cementite metastable?
Because if you heat cementite (the most common iron carbide, ) and keep it in temperature for a sufficient amount of time it will inevitably decompose into ferrite and graphite. Any metastable phase is a phase with momentary stability or, in other words, which, given enough time, will change.
What is cementite in iron-carbon diagram?
Cementite is a metastable phase of this alloy with a fixed composition of Fe3C. It decomposes extremely slowly at room temperature into Iron and carbon (graphite). This decomposition time is long and it will take much longer than the service life of the application at room temperature.
What is Fe-Fe3C phase diagram?
In heat treatment processes of steel the very important role plays the Fe-Fe3C phase equilibrium diagram. It enables the selection of the temperature of austenitisation in respect to carbon content in steel as well as to predict the microstructure composition of annealed steel.
What is cementite structure?
Fe3CCementite / Formula
Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure.
Why is the Fe-Fe3C phase diagram a metastable phase diagram instead of a true equilibrium phase diagram?
Why is the Fe-Fe3C phase diagram a metastable phase diagram instead of a true equilibrium phase diagram? The phase diagram is a metastable phase diagram rather than a true equilibrium phase diagram because the compound iron carbide ( ) formed, cementite, is not a true equilibrium phase.
Is cementite a BCC or FCC?
The alpha phase is called ferrite. Ferrite is a common constituent in steels and has a Body Centred Cubic (BCC) structure [which is less densely packed than FCC]. Fe3C is called cementite and lastly (for us), the “eutectic like” mixture of alpha+cementite is called pearlite.
What are the five individual phases in the iron cementite phase diagram explain each phase in brief?
The Iron-Iron carbide (Fe-Fe3C) is defined by five individual phases and four invariant reactions. Five phases are- α-ferrite (BCC) Fe-C solid solution, γ-austenite (FCC) Fe-C solid solution, δ -ferrite (BCC) Fe-C solid solution, Fe3C (iron carbide) or cementite – an inter- metallic compound and liquid Fe-C solution.
What are the five individual phases in the iron cementite phase diagram?
Five phases are- α-ferrite (BCC) Fe-C solid solution, γ-austenite (FCC) Fe-C solid solution, δ -ferrite (BCC) Fe-C solid solution, Fe3C (iron carbide) or cementite – an inter- metallic compound and liquid Fe-C solution.
Why β phase is not appeared in the Fe-Fe3C phase diagram?
The alpha iron which exist between 768 to 910 o C is paramagnetic in nature is called beta iron, just it is non magnetic in nature while iron- iron carbide is drawn for temp v/s wt % carbon so no need to show magnetic nature of aplha iron.
How cementite is formed?
Cementite also forms when a mixture of iron and graphite heated under a pressure of less than 5 GPa at about 1000°C, (Figure) (Tsuzuki:1984). Cementite powders have been made traditionally by electrochemical extraction from steel containing cementite (Rokhmanov:1997).
What are the metastable states?
metastable state, in physics and chemistry, particular excited state of an atom, nucleus, or other system that has a longer lifetime than the ordinary excited states and that generally has a shorter lifetime than the lowest, often stable, energy state, called the ground state.
How is metastable state created?
Metastable State in Lasers In a number of gas lasers, helium atoms are excited into metastable states by an electrical discharge. During collisions with other atoms (e.g. neon in a helium-neon laser), they can then transfer or exchange the excitation energy to those atoms.
Why are metastable phases not part of the Fe-Fe3C diagram?
Fe-Fe3C is not considered an equilibrium phase diagram, because Fe3C is behaving metastable in iron. It is not really a metallurgical reason but a thermochemical reason, in an equilibrium phase diagram, there are no metastable phases.
Is iron a BCC?
Between room temperature and 912oC, iron has the BCC structure, and is a tough, hard metal (“tough as nails”). Above 912oC, pure iron switches over to the FCC (austenite) structure, which is much more ductile.
What are the invariant reactions occur in iron-carbon phase diagram?
The invariant eutectoid reaction in Fe-Fe3C diagram is given by equation: i.e. during cooling, austenite of 0.77%C at constant eutectoid temperature, 727°C undergoes eutectoid transformation to form a mixture of ferrite (e = 0.02%) and cementite i.e. there are alternate lamellae of ferrite and cementite.
What are the phases in iron iron carbide diagram?
1. Solid phases in the Fe-Fe3C phase diagram: Four solid phases, namely α-Ferrite, Austenite, Cementite (Fe3C), and δ-Ferrite.
What means cementite?
Definition of cementite : a hard brittle iron carbide Fe3C that occurs in steel, cast iron, and iron-carbon alloys.
What is the maximum amount of cementite in a metastable system?
The microstructure in the metastable system can therefore consist of a maximum of 100 % cementite. Since the carbon content in the cementite (F e 3 C) is 6.67 %, the metastable iron-carbon phase diagram ends at this concentration. Figure: Complete iron-carbon diagram of the metastable system
What is the iron-carbon diagram?
The iron-carbon diagram. It should first be pointed out that the normal equilibrium diagram really represents the metastable equilibrium between iron and iron carbide (cementite). Cementite is metastable, and the true equilibrium should be between iron and graphite.
Why is Fe3C iron carbide called cementite?
The metastable diagram is of more interest. Fe3C iron carbide called cementite because it is hard. Following phases exist on Fe-Fe3C diagram: – liquid solution of iron and carbon (L) – ferrite (α) – an interstitial solid solution of carbon in Feα (bcc).
What is the carbon content of cementite in phase diagram?
Since the carbon content in the cementite ( F e 3 C) is 6.67 %, the metastable iron-carbon phase diagram ends at this concentration. If only the range of the phase diagram above a carbon content of 2.06 % is considered, the fundamental difference between steels and cast iron in the solidification process becomes apparent.