Fat Carbon Atoms in Skinny Austenite Interstitials?

[Lee Cordochorea]

Hi.

 

I'm an ameture blacksmith and have been studying metallurgy for the past couple decades in an attempt to make better tools next year than the ones I made last year.

 

There's one thing keeps eluding me... I'm hoping someone here can point me in the right direction or adjust my world-view.

 

According to all of the books, carbon atoms are supposed to be able to fit into the interstitial space in the middle of an austenite crystal cell. This is an FCC cell, so the biggest interstitial should be equal to 0.414 times the atom diameter.

 

I find iron diameter numbers ranging from 248 pm to 280 pm. This would give interstitials ranging from a bit less than 103 pm to just under 116 pm.

 

But the carbon atom sizes I'm finding numbers for are 134 pm to 154 pm in diameter.

 

How do they fit in there?

 

And, for that matter, how do they squeze in there after the carbides dissolve anyway?

[louis wu]

If you have have not already found the answers for yourself I will supply some.

 

Interstitial C in both FCC and BCC steel does produce local lattice strains and distortions. Too much carbon will produce a very brittle and essentially undesirable steel.

I calculate the interstitial hole to be ~104 pm diameter in FCC iron.

 

Interstitials and other lattice defects such as vacancies, edge and screw dislocations are not fixed in place permanently. Your blacksmithing is influencing, moving, concentrating, sometimes creating or eliminating these defects.