On Thursday, November 15, 2012 10:50:15 AM UTC, Jon Fairbairn wrote:
"Dave Baker" writes:
Unlike ferrous metals which harden with a fast quench and
anneal with a slow cool, copper also anneals with a
fast quench.

How does that work? As I understand it, annealing happens while
the metal is hot: the domains grow. A fast quench (of metals
that go this way) causes rapid contraction which puts
unconformities into the domains as they cool, so hardening
(outer) parts. I suppose in copper this doesn’t happen as much
for some reason?

Oh god. [/me tries to recall Metallurgy from when we were both
at Cambridge].

In both copper and ferrous metals, heating a work-hardened piece
allows the dislocations to move around, untangle themselves, and
dissipate. This removes the work hardening.

When you heat ferrous metals, they undergo a phase change to a
different crystalline form (which happens to be hard). If you
then cool them fast, they don't have time to change crystal
layout, so they stay hard. If you cool them slowly, they /do/
have time to change crystal form, so end up soft.

Copper doesn't undergo this phase change, so can be annealed
quickly.