Metallurgy of Steel for Bladesmiths- Dr. Verhoeven

[Mark Reich]

Carbide volume fraction is a way to measure how much carbide is in a steel.  So if you have 100 cubic inches of steel and a volume fraction of 5%, then 5 cubic inches is carbide all mixed up in little tiny bits.

These carbides are the same as those in hardened steel.  This is where the phase diagrams come in handy.  In a simple steel like 1095, the carbide is Fe3C, iron carbide, or cementite, all are the same.  It is present at room temperature in annealed 1095 steel, at about 15% volume fraction.  When heated for hardening, it will typically have 2% to 5% volume fraction.  These are the same carbides as at room temperature, and when quenched they are still the same.  The phase diagram can be used to calculate volume fraction.  Carbides can be bigger or smaller, it depends on temperatures used.

From the little I think I may know, I assume the reason for the drop in carbide percentage between annealed 1095 and hardened 1095 is because at austenitizing temp the carbon atoms leave the big clumps of carbides, and are free to roam about and create hardened steel upon quenching... right?

I presented this before reading the posts that came after this post by Mr. Me2.

I really don't have much comprehension of such studious posts as your's, Mr Jan. I'm sorry, you're just way beyond what bladesmiths need to understand.

I only know that martensite has a larger volume than the unhardened version, which may be pearlite and/or cementite with retained austenite...

My simplified explanation to that statement would be that at room temp, unhardened phase, the Fe atoms and C atoms mainly separate, with most of the C atoms clumping together in carbides.

After full austenization and Quenching, the C atoms have been free to move around amidst the Fe atoms, then, when quenched, the C atoms are "frozen" in place, and lock the Fe atoms in place, with the resulting phase becoming martensite.

If that's basically accurate, it might be understandable that atoms mixed and... (I don't know a term for) "frozen" or "locked" in place, upon quenching, would create more volume than when in a natural relaxed phase.