Blacksmithing
and Cutlery
by Gérard HEUTTE
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Steel : Elementary knowledge
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Here are some basic informations to be known to approach the part devoted to the metallurgy.
Iron
Iron is a current element. The Earth's
crust comprises a strong percentage of it.
In practice, pure iron is very rare: The industrial
processes to pass from ore to metal always
leave some impurities.
Main physical properties:
> Atomic number: 26
> Density: approximately 7.87
> Melting point: 1538°C
> High thermal conductivity
> High electric conductivity
Iron has an essential property: Crystalline
polymorphism i.e. that its crystalline structure changes
according to the temperature:
T<A3=912°C : Iron is known as
Alpha-Iron (noted a
-Iron). It
is ferromagnetic (it reacts to a magnet) until 770°C and paramagnetic
beyond that. Its crystalline structure is "Body Centered Cubic":
The Iron atoms are laid out on the tops of a cube with an atom
at the center of the cube.
A3<T<A4=1394°C : Iron is known
as Gamma-Iron (noted g
-Iron).
It is paramagnetic (it does not react to a magnet). Its
crystalline structure is "Face Centered Cubic": The Iron atoms
are laid out on the tops of a cube with an atom at the center of
each face of the cube.
A4<T<1538°C : Iron is known as
Delta-Iron (noted d-Iron). It
is ferromagnetic. Its crystalline structure becomes again
"Body Centered Cubic".
"Pure" iron is not used in cutlery, at least for the edges.
Steel
The presence of Carbon in solution inside iron can
modify in an important way its behavior. Then this is a Binary system Fe-C.
We can distinguish:
> Iron : The
presence of a weak carbon rate (<0.05%) does not modify significantly the iron properties.
> Mild steel :
A limited proportion of Carbon (between 0.05% and 0.35%)
reinforces the mechanical characteristics of iron (impact strength).
The mild steel cannot be hardened.
> Steel : Between
0.35% and 2% of Carbon, steel acquires an essential property.
It can be hardened. By heating it to "red" then
by cooling it quickly, steel acquires a very great hardness.
> Cast iron :
Beyond 2% Carbon, it is cast iron. The cast
iron is generally breakable. It is not used in cutlery.
In cutlery, useful steels generally have between 0.5 and 1.6% of
Carbon.
The page on steel microstructure
explains in detail the combinations of Iron and
Carbon within steel.
The phase diagram
Fe-C
gives the crystalline structure of steels according to their
composition (i.e. percentage of carbon) and the temperature.
Alloy elements
In order to modify certain properties of steel,
some alloy elements can be added. The most current alloy
elements are: Chromium, Manganese, Molybden, Nickel,
Silicium, Tungsten, Vanadium. The proportions can go from
some tenth of percent until 20%!
Industrial steels are standardized. The designation of a
steel makes possible to know its composition. See the page
about
standardized designation of steels.
Certain "polluting" elements (sulphur, phosphorus) are also
present. Their proportion must be limited.
The addition of an alloy element adds a dimension (i.e. an
axis) with the diagram of phase. This diagram quickly becomes
complex and difficult to represent. For the ternary diagrams,
you will often see a series of diagram by fixing for each one one or
more alloy elements.
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