Blacksmithing
and Cutlery
by Gérard HEUTTE
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Diagrams with Time factor
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The binary diagram Fe-C gives many information,
but its main gap is to not integrate the impact of time!
The metallurgists use in complement other diagrams. It
is not absolutely necessary to know them perfectly to make good
knives, but their comprehension makes possible to better understand
the important principles.
Time
The transformations of structure of steel depend
on the temperature and time. Indeed, the changes of structure
are not immediate. Moreover, the variations in temperature
spend time to penetrate the matter.
If you wish a part with a homogeneous temperature, it should
be maintained at the temperature desired during a "certain time"...
For work of cutlery, count approximately a minute per
millimetre of blade thickness.
Austenitization
For the majority of the heat treatments, it is
necessary to bring the blade at the austenitic state. The
diagram shown opposite (for a 35CD4 steel), is the diagram
of austenitization. You will find information on the
appearance of austenite according to the temperature and time as well
as the modifications of the grain of steel according to temperature
and time.
This kind of diagram is seldom available for a given steel.
But this example makes possible to understand the
process of formation of Austenite and the impact of a prolonged
maintenance at various temperatures.
The target is to go above the red line (full and homogeneous austenitization).
Simultaneously grain growth must be limited by avoiding too long temperature
stage.
In short: Not too much heat and not too long time!
TTT diagram
The metallurgists use a
complementary diagram: The TTT
(Time-Temperature-Transformation) diagram, also called isothermal diagram of
transformation. It makes possible to determine the changes in
the structures of steel, at constant temperature. It is thus of
a moderated utility, since the means necessary to this kind of
transformations are specific (salt baths), seldom available and
expensive!
One generally prefers to consult the CCT diagram, closer to the
usual conditions. Of course, there is a TTT diagram for each
steel.
The shape and the position of the "Nose" of the curve give an
indication of the time to cool steel during hardening.
The example below gives TTT curve of the XC80:
It is seen that time "Tn" is lower than the second!
CCT diagram
When the time factor must be taken into account, the CCT (Continuous
Cooling Transformations) is most useful!
It is relatively representative of the usual conditions of
cooling. The various zones of the CCT diagram define the
transformations of Austenite. There also, on the left of the
diagram a "nose" still appears. It will be necessary that the cooling curve
passes on the left from this "nose" for a good
hardening.
The presence of Chromium in steel strongly shifts the whole of
the zones towards the right, leaving more time to carry out cooling
(air hardening).
Here an example of CCT diagram:
There, the various zones are seen. The orange curves give
various coolings. Curve A gives greatest hardness and
corresponds to an optimized hardening. The following curves from
B to F provide a less and less hard final result.
Synthesis
It is not necessary to know perfectly all these diagrams
to work a steel correctly! These diagrams are a help to
understand phenomenon. They are really useful only for
the metallurgists.
The steel manufacturers provide the synthesis of all of
information while giving for each steel, the data necessary to its
work: Temperature of forging, annealing, hardening, medium
of quenching advised...
This information is thus really essential for a work of quality!
Effects on the way of working
From these concepts of metallurgy, you can
extract some principles about working method:
> Never "ill-treat" steel by heating like a brute!
It is necessary to give time for the transformations to be
done slowly. The defects and later deformations thus are
avoided.
> Do not heat too much, to avoid grain growth.
> Strictly respect the ranges of recommended temperatures.
> Make stages at the crucial temperatures to homogenize correctly.
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