Heat Treated, Non-Heat Treated Alloys
Date: July 2006
What are the differences between heat-treatable and
non heat-treatable alloys?
Your question is a very broad one, and can mean a couple of
things. If you are asking the most general question, the
answer is that heat treatable alloys are those that can have
their properties (usually their hardness) affected by some
form of exposure to heat. Non heat treatable alloys are those
that heat is not (or cannot be) used to increase hardness.
If you were asking what is the mechanism that makes an alloy
heat treatable, compared to one that is not, then there are
many reasons depending on the alloy. Perhaps some examples
are most useful.
High carbon steel (that is, steel with at least 1% carbon
content) is heat treatable. When you heat it to red heat, the
carbon and iron form localized crystals of "Martinsite" (iron
carbide). If you rapidly cool the steel such as by plunging
it in water, the Martinsite crystals do not have time to
break down, and remain in the matrix of iron. Martinsite is
very hard, and thus the resulting steel is hard. If the red
hot steel is allowed to cool slowly, the Martinsite crystals
slowly revert back to iron and carbon, and the steel is said
to be "annealed"; that is, very soft. Various steels are the
primary alloys that are heat treatable in this way.
Essentially all pure metals, and most other alloys do not
respond to heat treating. Aluminum alloys, for example cannot
be hardened by this method. Aluminum alloys are often able to
be hardened by another non-heat-related process called
"precipitation hardening". Copper is not able to be heat
treated either, but like many metals and alloys, it can be
"work hardened" when it is stretched, bent, or hammered,
which causes large copper crystals to break up into many
smaller ones that interlock and make the metal stronger.
However, it can be annealed by heating it to a read heat and
cooling it. This causes the many small crystals in the metal
to regrow into larger ones again.
There are many other examples, but hopefully this comes close
to answering your question.
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Update: June 2012