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Diagrams with Time factor
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.