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Copper Properties and Heat
Name: Vinh
Status: student
Grade: 6-8
Location: N/A
Question:
What happens to coppers properties when heated?
Replies:
Hi Vinh,
When copper is heated until it glows dull red, then cooled (either
slowly or by plunging it in water), it becomes very soft and easy to
bend or to "work" by other means. This condition is called being
"annealed".
Copper cannot be hardened by any method of heat treating. The only way
copper can be hardened is by so-called "work hardening". This is
accomplished by (for example) rolling thicker sheets of copper and
squeezing them into thinner sheets. You can easily demonstrate this
yourself by taking a strip of copper and heating it red hot, then
cooling it in order to anneal it. Now try to bend the strip sharply 90
degrees or so. It bends very easily. Once bent, try to unbend it
straight again. You will notice that the bent area tends to stay bent,
and another area nearby gets bent back instead. What has happened is
that the first bend has "work hardened" the copper in that area, and
it resists bending back again; so the copper bends back elsewhere, in
a nearby area that has not been previously bent.
Why does this happen? When copper is heated close to dull-red-hot
temperature, the many small crystals of metal grow into each other and
form fewer, larger, crystals. This is the soft, or annealed condition.
Now, when you bend or stretch the annealed copper, the large crystals
of copper are microscopically fractured into many irregularly shaped
smaller ones. These small crystals do not easily slip past one another
when the metal is stretched or bent, and so they act to strengthen the
copper.
The above process of annealing then work hardening again is completely
reversible. When sheet copper is made by repeatedly passing a thick
piece of copper through successively closer and closer sets of
rollers, the copper is heated to anneal it several times during this
process, otherwise the copper would get so hard that cracks may
develop.
Regards,
Bob Wilson
A lot of things can happen, and it depends on how much you heat it. It can
melt, and then vaporize if you heat it extremely (although you would need
special equipment). In 'normal' temperature ranges, like those possible with
a butane torch, it simply expands slightly. Its thermal conductivity also
drops slightly.
Hope this helps,
Burr Zimmerman
Vinh,
If the copper is heated in the presence of oxygen, the heat speeds up oxidation
and copper(II) oxide (black in color) is formed. Copper(II) oxide has an entirely
different set of properties from metallic copper.
On the other hand, if the copper already has a good layer of copper carbonate
(notable for its green color - like in the Statue of Liberty), further
corrosion/oxidation may be prevented and heating will not have as strong an
effect.
If the copper is heated in such a way as no oxidation can take place (say in an
inert environment or in a vacuum), then it would go through the transformations
you would expect from any heated solid/metal: softening, melting, sublimation,
etc.)
Greg (Roberto Gregorius)
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Update: June 2012
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