Is there something special about iron that makes it the only metal you
can substantially improve by carburizing? I know many of the other
common metals are very chemically different from iron, but it seems so
strange to me it would be the only metal.
I know this question is probably absurdly clueless, but I couldn't
find the answer anywhere online.
Anyone know the answer, or where to read up on it?
Actually, IMHO it a question that answers why this disscusion group
exists - and remains fundametally yet to be answered.
The word "improve" I think you really mean the ability to adjust the
strength of the metal iron - low carbon to high carbon can vary the
strength of iron in terms of tensile strength from about 20 ksi to
over 300 ksi- several authors have stated this as the greatest dynamic
range (or bandwidth) of strength of a single alloy system - quite
unique - a known consequent of the quantum theory of solids - but
still unexplain from first principles of quantum physics.
Looking at it chemically utilizing a quick look at the periodic table
- the only other carbide formers - titanium, chromium, vandium,
zirconmium, niobium, some of the rare earths and tungsten (a few other
I may have passed up) are too rare or difficult to process. Atomic
shells of aluminum, copper, zinc, magnesium, beryllium, lead, bismuth
and other metals don't combine with carbon as intermetallics.
The same question can be asked about iron/nitrogen system (nitridng
and the more recent high nitrogen steels).
Historically it is more amazing to me that humans have not yet found
another alloy system (excluding the stainless steels) with such a
bandwidth of strength.
Ed
The fundamentals of steel being the iron atom size relative to the carbon
atom, the interstitial space avalable to fit the smaller carbon atom, in
both iron's body centered cubic and face centered cubic lattice
configurations, and possibility of carbon being available in the form of
carbon monoxide and carbon dioxide and the ability to form Fe3C or Fe2C:Fe,
and the relative abundance of these elements all contribute to steel's
position as a material that can have it's properties "improved by
carburizing". There are other atoms near in size to iron in the periodic
table that may be of the correct atomic size for interstitial spaces but the
lattice structure may be different, no similar compound to Fe3C exists or
the material is very expensive.
Mark
Iron is the only one that changes crystal pattern in such a way to
make room for carbon to fit into the newly formed "open spaces" when
it's heated and then when the iron is cooled will trap(!) the little
suckers.
And it isn't just carbon that fits into those spaces, so does boron,
phosphorus and nitrogen. Case hardening compounds usually have all
those elements avaiable BTW. :)
Iron when heated to a certain temperature switiches from...
"body centered cubic" to the "face centered cubic" crystal pattern.
When iron is hot enough to switch to FCC it glows orange hot in the
dark and you can feel the heat on your face. ;) (face centerd cubic)
Both crystal patterns, BCC and FCC have the same density tho. :)
The atoms rearrange very slightly but the "open spaces" are
distributed differently, see? ...that's all an atom the size
of carbon (et.al.) need to get sucked into the open spaces.
Ok, got that? :)
Next, picture a cube made from tinker toys where all the sticks are
the same length. It "gives" when pressed from odd angles. Take one
stick out and put in the next longer stick. The cube is now under
tension (since you hammered it together?;) and doesn't give so easy.
That's symbolizes the distortion created by a trapped C, B, P or N
when the iron returns to room temperature and its usual BCC
configuration.
Got that one too? ;)
But a cool one anyway. :)
formatting link
"Metallurgy Theory and Practice" by Dell K. Allen
Alvin in AZ
ps- I'm the dumbest guy on s.e.m and so, best qualified
to answer the clueless questions. :)
pps- BBC iron is magnetic and FCC iron ain't, so, the iron
tells you when it's "hot enough".
=2E..
This question is very interesting and although some of the answers
already
have been given I would like to clearify a few things.
It seems clear the question is about strength, why carbon can increase
the
strength of iron but not any other metal.
The main reason for this is that iron has two different crystalline
forms, BCC
(bodycentered cubic) at low temperature and FCC (face centered cubic)
above 911 =B0C. Although FCC is actually more closepacked than BCC
it has larger interstitial places where the carbon can dissolve and
the
solubility in FCC is about 2 mass%, in BCC it is less than 0.02 mass%.
Iron with more than 2 mass% carbon is not called steel but is used as
"cast iron"
because its low melting point and although it is very hard is its also
very brittle.
Carburizing steel is usually made above 911 =B0C to obtain a high carbon
content
in the FCC phase (also known as autenite). For plain carbon steels
the
maximum hardness one can achive is for a little less than 0.8 mass%
carbon.
The strength of the steel depend on the cooling from the high
temperature.
That is the secret of the blacksmiths in the old times. If the iron
is cooled
slowly it will not be particularly hard because the austenite
transforms to
BCC (also known as ferrite) and another phase called cementite with
the
stoichiometry Fe3C. But if the hot austenite is quenched in water (or
with
more modern techniques in oil) the rapid cooling prevents the normal
transformation and a metastable phase called martensite is formed.
This martensite is very hard because the carbon atoms trapped inside
the crystal lattice prevents the deformation. In order to have some
ductility the quenched steel is usually "tempered" at 300-400 =B0C for
a short time which reduces the hardness but decreases the risk that
the sword,
or whatever the blacksmith is making, breaks when you hit a stone for
example. If one tempers the steel too long one obtains ferrite and
cementite
as with slow cooling and a rather soft steel.
When the steel is at high temperure it is soft and can be easily
formed and
the cycle heating+queching+tempering can be repeated as many times
as is needed to form the steel to the desired product and hardness.
Incidentally ferrite is only ferromagnetic to 770=B0C so the
dissaperance
of magnetic properties is not a criteria of the ferrite to austenite
transition.
Other alloying additions like Cr to obtain stainless steels usually
decreases
the strength and makes the processing more difficult as other phases
may also appear.
Bosse
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