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Profiles and Bevels

There are any number of shapes and sizes of knives and a challenge for you is to go and design your own knife that suits your needs- a custom hand made knife. Using such a knife is a joy!
The number of kinds of bevels on the other hand is a more limited thing, though there is still a choice to be made.
The options are split into two. The first is single or double bevel and then the second is hollow ground (concave), or flat ground not to mention the combinations of these.
In this case we are going for a flat, single bevel.
Digressing slightly, hatchets that are sharpened to a fine edge some times have a convex (single) bevel.

If you want a template to work from then try this. Though I have found that it will only print to the right size if I use Photoshop (the program I edited it in), so you might have to play around with the image to get it to print at 230mm. (The size that most, if not all, R.M. knives are).

To get the bevel as near perfectly flat and at the angle we want (23 degrees is said to be a good compromise) I devised a jig, please see pictures below.

Hardening and Tempering

Now we come to the fun bit!
This is an interesting process and one that people either know very little about or nothing at all, but think it to be a difficult process.
It is in fact fairly straight forwards within the confines of our needs.
Basically we need to heat the blade up to 800 degrees C and then cool it down rapidly in order to harden it and then heat it up again to about 150 degrees C and let it cool in order to temper it.
When the metal is heated to 800 degrees C the carbon in the steel turns into carbide.
It’s interesting to note, as well as extremely helpful, that at this temperature magnetic attraction disappears!
The steel is also glowing red at 800 degrees C and gets progressively brighter the hotter it becomes.
So by using a magnet (carefully!) we can easily tell when the steel has reached the critical temperature.
We take it a little higher than this in order to make sure we have converted as much carbon as possible into iron carbide.
If we were then to let it cool down naturally the iron carbide would turn back into carbon.
So we ‘quench’ the steel, cooling it rapidly in cold oil and in doing so capture the iron carbide.
This hardening process changes the crystalline structure in a way that makes it harder, which is good for obtaining a sharp edge but it also makes it brittle, like glass, in fact if you were to drop the piece onto a hard surface at this point it may well break.
So we need to temper the steel in order to get a balance of hardness and durability.
Reversing the hardening process very slightly does this.
Using a gas blowtorch we reheat the steel to a little above 150 degrees C.
Again nature comes to our aide and gives us a visual guide to the temperature that the steel has reached.
As the steel gets up to 150 degrees it starts to turn yellow, then a darker yellow referred to as straw, then brown, blue and then nearly black.
This is a crude indicator, but one that serves our purpose well enough.
The hardness at these colours is as follows:

Pale yellow-hardness 750-800 Vickers- still quite brittle, but very hard.
Pale straw-hardness 720-770 Vickers- still low resistance to fracturing.
Dark straw-hardness 700-750 Vickers- suitable for a scriber.
Purple blue-hardness 650-700 Vickers- suitable for chisels and punches.
Dark blue-hardness 640-690 Vickers- suitable for springs.
Once the straw colour is reached on the cutting edge of the blade we can leave the steel to cool naturally or if we have been a bit heavy handed with the blowtorch and the colours are flowing fast we can quench the blade in water to halt the process.

In figure 3a, you can see the knife in the forge being heated up to the temperature needed for hardening (800 degrees C)

In figure 3,b the knife is being cleaned in order that we can see the colours running in the tempering process.

You may find this website very interesting if you want to know more about metallurgy. This is a great site that explains it in terms you can understand.