"jk" wrote in message
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"Ed Huntress" wrote:

"jk" wrote in message

Neither one, Bob. It's a metal-matrix composite. Think about
reinforced
epoxy. 'Same thing.
And so essentially is steel, which fairly uniformly is considered a
metal.

The carbon in steel comes in three forms, IIRC, but the one that makes it
steel is not a simple mixture or a compound. It is an incorporation into
the
molecular structure of iron -- the crystal structure -- that stresses the
iron crystals. Most hardening mechanisms involve such stress.

Some carbon combines with the iron to form iron carbide. Except for
cementite structures (a phase of steel), this has little to do with
steel's
properties. And there can be free carbon in very high-carbon steel,
somewhat
like the free carbon in cast iron.

Most of the phases in "steel" are carbides

I believe that the phases austenite, ferrite, and martensite are solid
solutions, not carbides. As I mentioned, cementite is an iron carbide that
can have larger effects on steel's properties, generally as the pearlite
laminate you describe below.

The carbides in steel (and there can be others besides iron carbide,
depending on the alloy) are not what give steel its strength or hardness.
Except for some pretty exotic alloys, the primary strengthening and
hardening mechanism is the molecular-level stress induced in the conversion
of austenite to martensite.

AS from the definition of cementite, from
http://metals.about.com/library/bldef-Cementite.htm
"Definition: An iron carbide (Fe3C) constituent of steel. It is hard,
brittle and crystalline. Steel which has cooled slowly from a high
temperature contains ferrite and pearlite in relative proportions
varying with the chemical composition of the steel. Pearlite is a
lamellar structure of ferrite and cementite."


Pearllite
Definition: A lamellar constituent of steel consisting of alternate
layers of ferrite (alpha-iron) and cementite (iron Carbide Fe3C) and
is formed on cooling austenite at 723oC. This produces a tough
structure and is responsible for the mechanical properties of
unhardened steel.

Ferrite
Definition: The solid solution of carbon in body-centered cubic iron,
a constituent of carbon steels.


But composite structures, by convention, are ones in which the matrix and
the bound material produce a composite material that shares or combines
properties of the two (or more) materials in a significant way. Free
carbon
in steel does not. Neither does the carbide.

Are you saying it is insignificant? or that it does not combine the
properites? To my mind it does both [Signficicant enough that we can
harden (some) steels] and the properties are not the same as either
elemental iron, low carbon steels, or iron carbide. Just because you
can change the location, size,shape of the composite material by heat
treatment, does not make it any less composite.

Let's add "macro" or something like that to distinguish the common
understanding of "composite" from molecular combinations (solid solutions,
in this case) like the steel phase called martensite. Martensite is a
combination of iron and carbon, but it wouldn't ordinarily be called a
composite. Carbon fibers distributed in a matrix of iron or steel would be a
composite. And pearlite is neither very hard nor very strong. It's present
in annealed steels, not in quench-hardened ones.

And, in terms of hardness or strength, carbides don't add much to the
properties of steel until you get up to the very high-alloy materials,
particularly the extreme high-end, powder-metallurgy high-speed steels, such
as CPM Rex 121 and comparable numbers from Fette and one or two others. In
those, there is enough free carbide in the steel matrix (vanadium and
chromium carbides, mostly) to improve the wear resistance of the steel, as
well as its apparent hardness. But it isn't the composite properties that
produce the steel's basic hardness or strength. It's a combination of
martensite conversion and either solid-solution hardening or precipitation
hardening (I forget which). These are pretty exotic steels.

--
Ed Huntress