[Bill Harsey]

Here's a red-neck engineering attempt at tying up some of the loose ends about how tool steels harden.

We've already used the term Austenite or Austenitizing here without much of an explanation. If we get some understanding of this word, it will help us know what happens when heat treating tool steels for use as edged tools.

Austenite is the word used to describe the solid solution of Iron and carbon, when steel is heated above it's critical temperature (something like 1450 Degrees F for simple water or oil hardening tool steels, much higher like 1950-2150 F for air hardening steels).

When the steel is cooled quick enough this turns the carbon and other alloys which can form carbides into hard Carbides and that's called "Martensitic Transformation". All during this the steel remains the exact same chemical composition but it's a different shaped atomic structure because of where the carbon and Iron atoms freeze into place depending upon rate of cooling. Cementite (Iron Carbide) is produced in here somewhere too.


If the steel is cooled slowly, this hard carbide formation doesn't happen.

We control the rate of cooling from the austenitizing temperature to get a particular range of physical characteristics, like hardness, toughness and edge holding in the tool steel being made into knives.

Usually this "rate of cooling" is exactly as fast as we can get away with while not cracking the work piece. The thicker the work or more complex the shape, the more careful we have to be about cooling too fast.

The alloys and percentages of these alloys will determine the physical characteristics of the steel and how it is heat treated, that is quenched in water, oil or air OR a combination of oil/air interrupted quench etc.