Hardening with mercury?

I read this over at practicalmachinist.

Sounds like a good application for a Cole drill. It isn't a good idea, and unsafe as hell, but somtimes in an emergency a friend of a friend's uncle once removed and twice arested would heat a carbon steel twist drill to the critical temperature and quench in mercury. That thing would go through some pretty hard stuff! None of us would ever resort to using mercury but once upon a time it was a common method for this sort of thing. The Rivett 608 lathe spindle was hardened in mercury acording to the sales brochure.
I've never heard of this before. Is there something to this?
Wes
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In theory, no. Once the quenching rate is fast enough, quenching faster doesn't make the resulting part harder. The more highly alloyed steels get full depth hardness at more moderate cooling rates. A very high cooling rate in a highly conductive fluid might make some difference to hardness depth in a large part in straight carbon steel. I don't think there are any alloys that <i>decrease</i> the hardenability of steel, though there are some that don't do anything to increase it. No doubt I will be corrected if I have that wrong.
This is leaving aside consideration of metallic glasses, where cooling rates from the initial pour (ie during steel <i>making</i>, not during heat treatment) must be incredibly steep (on the order of a million degrees/second, as I dimly recall).
Adam Smith Midland, Ontario, Canada
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Pre-carbide, this was the ONLY way to get a drill that would crack through the case-hardened surfaces of old military rifles for scope mounting. I've seen it in some early 20th century gunsmithing books. Only works with plain carbon steels, which most drills these days aren't, even the "carbon steel" ones have other elements added or dragged in. Mercury vapor doesn't add anything to anybody's life expectancy, since carbide, cobalt and diamond drills are readily available this should be relegated to the bin for historical machining techniques-and left there! I've not done it myself, nor do I wish to. Wonder how many of the Rivett heat treat drudges made it to old age?
Stan
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