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jk
 
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Default OK what is the diferance between carbide and powdered metal ?

"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
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.


Extremely high-alloy HSS, such as CPM Rex 121, actually get some wear
resistance from the precipitated carbides. However, the main one there is
chromium carbide, not iron carbide.

Metals, metalloids, and "metallics" are defined loosely by their properties.
For example, silicon metal is just considered a metal by metallurgists. But
it has only some of the properties that we associate with metals. Add some
carbon, and it's silicon carbide. Add oxygen, and it's sand.


jk