Blacksmithing
and Cutlery
by Gérard HEUTTE
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Carbon diffusion
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General
The diffusion of Carbon in iron is the source of
many phenomenon. Steel comprises Iron and Carbon.
The relative size of Carbon and Iron atoms makes possible
that the Carbon atoms can slip between the gap of the crystal
matrix of Iron atoms. Under certain conditions, these
Carbon atoms can move in the crystal matrix of Iron: This is the
diffusion.
As nature dislikes vacuum, the Carbon atoms will
migrate from the richest zones towards the poorest zones, to tend
towards a balance (homogeneous distribution of the Carbon atoms in
Iron).
Influence of temperature
Atambient temperature, the phenomena of
diffusion is practically non-existent! But with an increase in
temperature, it will accelerate!
Indeed, when the matter is heated, the movements of the atoms
around their position of balance increase. This agitation allows
an easier circulation of the "small" atoms in the interstices.
The speed of diffusion of Carbon in Iron remains low.
It is generaly evaluated in millimetres per hour.
Opposite, a curve showing the rate of for of Carbon in Iron
according to temperature. The figure speaks for itself!
This speed can vary in steels according to the composition.
Practical consequence: Decarburization
When steel is heated in an environment lacking in
Carbon, it tends to lose Carbon. This is
Decarburization.
This phenomena occurs when steel is heated in the oxidizing zone
of the forge, strongly lacking in Carbon. A
strong and prolonged heating amplifies this effect!
In practice, take care of:
> Not heating in oxidizing zone.
> Not heating too much.
> Not heating for a too long time and unnecessarily.
> Removing (after hardening) a fine layer of steel at the
level of the edge.
Practical consequence: Carburation
Conversely, if steel (or Iron) is heated in a environment
in excess of Carbon, carbon will be absorbed and will diffuse in
steel. It is the principle of Cementation.
As shown on the curve above this process is slow. This
explains the durations for cementations (a few hours).
The temperature will influence:
> The speed of transfer of Carbon.
> The Carbon rate in Austenite and thus after cooling
the final Carbon rate in the cemented steel.
See opposite the limit of solubility of Carbon in Iron. Thus if
you want to obtain steel with 1.55% of carbon, it will be necessary
to make a cementation at 1000°C during... a certain time taking into
account the speed of diffusion of 0.8mm/hour!
The passage of a horn on the surface of red steel to bring it
Carbon is thus an useless operation since Carbon will penetrate
only on some hundredths of millimetre, which will be removed
while polishing and sharpening!
Migration of the other elements
The alloy elements other than Carbon are not
concerned with the diffusion. Their size does not allow their
migration, even hot!
As example, a Molybden atom will need 1.6 years to move
1mm in steel at 1040°C and 2600 years in steel with 820°C.
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