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Topic: Ionic bond  (Read 725 times)

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Offline shubham139401

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Ionic bond
« on: April 21, 2020, 01:57:52 AM »
I am going through the Iron-Carbon diagram(metal metallurgy), I have come across a question related to chemical engineering if anyone could help?

before we start
- Iron has a BCC structure that can hold(means through diffusion since the atomic size of carbon is less) 0.02% carbon within its interatomic spaces below 723-degree Celcius, but above that temperature Iron structure change to FCC, it can hold 2.11 percent of maximum carbon.

-The Fe3C( known as cementite) is a compound within Iron lattice which Breaks into Fe and C separate and the Carbon diffuses within FCC iron structure(not a bond) and remaining  again forms Cementite(Fe3C)

question-
-At which temperature Iron-Carbon bond breaks?
-Considering it Must be breaking below 723-degree Celcius, why does it prefer diffusion and remaining Carbon again forms Cementite (Fe3C)  at a temperature above 723 degrees and iron and carbon doest not exist separately at such higher temperature and rather again form the same bond.

Offline Enthalpy

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Re: Ionic bond
« Reply #1 on: April 22, 2020, 05:20:37 AM »
The Fe3C( known as cementite) is a compound within Iron lattice which Breaks into Fe and C separate and the Carbon diffuses within FCC iron structure(not a bond) and remaining again forms Cementite(Fe3C)
I don't grasp. Are you describing the diffusion of C through ferrite, from one cementite site to an other? I don't see neither how to tell "not a bond" between C and Fe. This must be badly hard to prove experimentally, and I can't really imagine any atom making no bonds in a compact metal matrix. Alone the proximity must force bonds to establish.

At which temperature Iron-Carbon bond breaks?
I don't believe the bonds break. As C or any atom diffuses through the metal matrix, it leaves former neighbours and gets new ones. The number of neighbours stays roughly constant.

The one thing that change when C leaves cementite is that it is bonded is a Fe matrix not organized for it, while in Fe3C the atoms rearranged themselves more favourably. This energy difference is accessible to modest heat.

Don't forget neither that diffusion often takes place along crystal defaults, not through places where stacking is perfect. This is much easier for the C atoms, already because they find some room. But this is badly difficult to model.

At which temperature Iron-Carbon bond breaks?
The mere non-zero solubility of C in ferrite already tells that some C leaves cementite at room temperature. In small amounts, but diffusion happens in minutes or years, while elementary processes take picoseconds, so small amounts suffice.

Considering it Must be breaking below 723-degree Celcius, why does it prefer diffusion and remaining Carbon again forms Cementite (Fe3C)  at a temperature above 723 degrees and iron and carbon doest not exist separately at such higher temperature and rather again form the same bond.
I don't understand clearly the question. Carbon is little soluble in ferrite, more so in austenite, but the main difference is the crystal shape of Fe, not any Fe-C bond. Above few %, C makes precipitates in austenite too.

Somehow the FCC lattice of Fe dissolves C more easily. This is not a matter of density. Maybe the "bond" directions in FCC, or the number of nearest neighbours, fits carbon better.

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