Hi All,
I have a follow up metallurgical question after that tempering thread from a couple weeks back. Allow me to preface it by saying that my understanding of metallurgy is extremely narrow, and only a whisker above "rank beginner." So this question is going to be real basic, and I'm convinced that I only have it because I've missed something else.
But without further ado:
When the steel crystals are in a body centered cubic (bcc) configuration, my understanding is that there are eight iron atoms--forming the eight corners of the cube--and one carbon atom, located at the center of the cube. When the steel crystals are in a face centered cubic (fcc) configuration, my understanding is that there are those same eight iron atoms and SiX carbon atoms, located at the center of each "face" of the cube.
I'm assuming this is somehow inaccurate. If this were actually what was going on, then in fcc state, there would be x number of carbon atoms associated with each set of four iron atoms, but in a bcc state, 5/6 of them would have somehow disassociated themselves. And if that's the case, where did they go and what are they up to?
The only way I can justify my understanding (as above) is to theorize that in an fcc state, each carbon atom is actually located at the center of SIX "faces." In other words, there would be six cubes of iron atoms sharing each and every face-centered carbon atom. However this also seems unlikely, since it would seem to imply that the whole resulting polycrystal would be dramatically more compact.
So...basically, I'm just pretty muddled, and I'm stabbing in the microscopic dark. Anyone care to sort me out?
The question: If the ratio of iron:carbon in bcc = 8:1 and the ratio in fcc = 8:6 (4:3), what happens to the missing 5 carbon atoms in bcc?
Thanks,
Zack
Zack Jonas
Journeyman Smith
to my understanding the excess carbon is bound up as carbides and/or as pockets of graphite with in a matrix of ferrite.
MP
Sorry Zack, close but not quite. You have the shapes and numbers correct but the elements wrong. All of the atoms in the unit cell will be iron; the carbon will be squeezed into the spaces between them. Carbon is an interstitial alloy, i.e. it fits in the spaces between the host (solvent) atoms. If the carbon was in the center position of the body or the faces, it would be a substitutional alloy, like chrome or moly.
To find your carbon you look between the iron atoms. Remember that the bulk of steel is merely a mixture, a solid solution of iron and carbon. Without the bonding of a compound the carbon is free to move interstitially all it wants. Couple this with the fact that iron is allotropic and that is why steel can have such drastically different properties in the same material depending upon the phase. FCC has many more spaces between the atoms and thus can hold many times more carbon interstitially. When carbon comes out of solution it then has concentrations great enough to begin forming compounds like Fe3C.
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