We know that “tempering” is an important stage in axe making. It must be important for one brand to be called “True Temper,” companies like Emerson & Stevens to emboss the initials of the temperer right on the axe, and many companies to highlight tempering in their ads and labels.
Tempering is only one step in heat treating steel though.
Steel is an alloy of iron and carbon. Iron alone is too soft to be of much use to the tool maker. “High carbon” steel is considered steel with between .5% to 1% carbon content. For example, 1055 steel has a carbon content of .55%. What makes high carbon steel so alluring to the edge tool maker is that through heat treating you can make the edge harder and more durable. In other words, less sharpening and more chopping!
The chemistry is really quite cool. Hardening is possible due to two characteristics of the iron and carbon found in steel: (i) Iron can exist in different crystalline structures, and (ii) carbon atoms are 1/30 the size of iron atoms. I know I lost you at “crystalline,” but wait it gets good . . .
At room temperature, high carbon steel consists of ferrite crystals which have iron atoms at each corner of a cube and one in the middle. By heating the steel up to around 1500 F the carbon becomes dissolved in the crystalline structure of the ferrite creating a new structure referred to as “austenite.” Another way to think of it is that austenite is a solution of iron and carbon. If allowed to cool slowly back to room temperature the carbon atoms come out of solution and the steel returns to its original state.
But what if we cool the heated steel rapidly? By taking the hot steel and quenching it rapidly in water, brine or oil, the carbon does not have time to escape the iron crystal lattice or otherwise come out of solution. A new structure known as “martensite” is formed. This structure is hard because the carbon atoms are more tightly packed in with their iron brethren making the steel harder. But in this form the hardened steel is brittle. To solve the brittleness of the steel at this point it is tempered.
Tempering is simply raising the temperature of the hardened steel to reduce the stress in the steel caused by heat treating. This reduces the hardness a little, but reduces the brittleness a lot. Most steels need to be tempered at about 450°F for maximum usable hardness but every steel is slightly different.
Most axes are heat treated at the bit end only. On many older axes you can see a clear demarcation between the steel that has been hardened at the bit and the softer steel making up the remainder of the axe head. This is called the “hamon line.”
Of course there is much more to the chemistry (and craftsmanship) involved in heat treating tool steel. Apologies to all the chemists, blacksmiths and metallurgists out there for this oversimplification of the heat treating process!
More information on heat treating (and great diagrams on the chemistry behind it) can be found here: