Ferrite: Pure iron, body centered cubic crystal at ordinary
temperatures.
Cementite: Fe3C, iron carbide. Where the carbon goes in steel.
Austenite: The solid solution of carbon in iron. In low alloy steels it
can normally only exist at elevated temperatures. Face centered cubic
crystal, allowing lots of room for carbon atoms. The solubility of carbon
is austenite is about 2 %.
Pearlite: The lammelar structure that results from slow cooling from a
high temperature. It consists of alternating plates of ferrite and
cementite. The spacing of the plates, or relative "fineness" is
determined by the temperature at which it was formed, as is the hardness.
Fine pearlite can be as hard as HRC 48-50, while coarse pearlite formed
by an anneal is usually only HRC 20-25 (numbers given refer to 1075-1095).
Martensite: The usual condition that is referred to as "hardened" steel.
It is the result of rapid cooling of austenite, trapping the carbon atoms
inside the face centered cubic structure, causing them to be greatly
distorted into a tetragonal shape that is highly stressed, and thus quite
hard. Normally tempered to achieve a comprimise between hardness and
"toughness".
Hardening: Heating the workpiece to a temperature above A1 ("critical")
to make austenite, then cooling rapidly in some kind of quenchant to
produce martensite.
Tempering: Re-heating the as quenched martensite to a lower temperature
to relieve some of the stress. This lowers the hardness some, but greatly
increases the impact resistance, or "toughness". Also precipitates small
"temper carbides" out of the martensite as a function of the stress
relief.
Annealing: Softening the material so that it can be easily
fabricated/machined. There are a number of methods and structures that
can be described as annealed. Classic "full anneal" requires that the
workpiece be heated to austenitic condition, then cooled very slowly
under near equilibrium conditions to produce a structure of coarse
pearlite. This is the normal condition for hot rolled bar. Tool steels
are usually shipped from the mills in a condition called "sphereodize
annealed". This consists of a ferrite matrix, with all the carbon as
spheroidal carbides of relatively large size scattered evenly throughout
the ferrite matrix. This is the most easily machinable condition for
hypereutectoid (above .8%C) steels.
This might be a good time to talk about the effects of alloying elements
in steel and what they do to it. Bear in mind that I am talking about LOW
alloy steels, that is, no individual element at more than 2% of the steel
(with the balance Fe).
Mn=Manganese. manganese is present in all modern steels to some degree.
The primary reason for this is to tie up the sulphur that may be present.
Sulphur in steel not containing manganese will end up in the austenite
grain boundaries as a compound FeS or iron sulfide. Iron sulfide is
liquid at temperatures that are in the forging range, and if present,
lubricates the austenite grains at their boundaries and makes the steel
come apart in a situation referred to as "hot short". If the Mn is above
about 1%, and increasing with the percentage, the hardenability of the
steel is dramatically improved. Mn is generally only considered an
"alloying element" if it is above 1%. The classic example is O-2. O-2 is
essentially 1090, but with 1.6% Mn. This renders it oil hardening, in
rather thick sections, by delaying the decomposition of the austenite as
the temperature drops, and gives more time to get the piece cooled and
still not form pearlite.
Cr=chromium. Chromium is a useful alloying element in low alloy steels
not because of any "stainless" quality, but because it improves the
hardenability of the steel quite a bit. It is also a carbide forming
element, which can help keep the austenite grain size small by "pinning"
the grain boundaries of the austenite. This is only true until the
time/temperature combination to dissolve all of the carbides is reached,
then the austenite grains are free to grow.
Ni=nickel nickel is useful as an alloying element because it increases
the strength of the ferrite phase by entering into solid solution in the
ferrite. It is not a carbide forming element, and in fact will reject
carbon and render it graphite if it is present in large enough amounts in
steel or cast iron with high carbon contents. It does help in creating
some of the very high strength alloys, but none of these are over .7%C,
and the only one I know of that is that high is L-6 tool steel, with most
of the others being at or under .4%C.
Mo=molybdenum. A strong carbide former, it is also very helpful in
increasing the hardenability of low alloy steels. In higher amounts, it
has many of the same effects as tungsten, imparting hot hardness to the
steel, and is the major alloying element in the "M" series of high speed
steels (M-1, M-2, M42, etc.)
V=vanadium. vanadium is usually only present in quite small amounts in
low alloy steels, typically under .25%. At these levels, it serves mostly
as a carbide former to inhibit grain growth in the austenite when the
steel is heated. Vanadium carbides are VERY persistent and difficult to
dissolve into the austenite solution, which is why it is so effective at
keeping the grain size small.
In my mind, as a bladesmith, the ideal knife steel would be something
about 1.1%C, .25V, and about .4Mn, and that is all. This would be a
shallow hardening steel, quite similar to W-2, which is something that I
have never, ever, seen anywhere in a form that could be positively
identified as such by the people that actually made the steel. A
close
second would be the steel we were discussing that led to this, the
"carbon V" which is also quite similar, with the addition of a little Cr,
which would make it deeper hardening, not a bad thing to have. 52100 is
about as close to this as we can get with readily available
alloys, so far as I know. It is very good steel, but one really should
have temperature controls to do the heat treating on it, it is picky
about being over-heated, it does not like it, not even a little bit.
Terry Primos has set up a
useful page listing the properties of common steels check it out.