Hardening and Tempering Knife Blades
Each steel alloy has a different temperature at which it must be heated for ideal hardening. The goal of hardening is to produce a homogeneous structure within the steel so there isn't a large collection of ultra-hard crystals and softer areas. When steel is heated to the ideal temperature (for that alloy), it transforms into a product called "austenite." When austenite is rapidly cooled, it forms "martensite." Martensite is the hardest transformation product of steel.
Click here for a primer and further explanation of heat treating knife steel.
The cooling process, which often involves a liquid, is just as important as the temperature to which the steel is heated. It's called "quenching," and the liquid used is most often some type of oil. However, a few steels require a water quench, and fewer still are hardened by air cooling alone.
If we just hardened the steel into martensite, it would be too hard to be serviceable. It would take a good edge, but it would shatter. A second heating process known as "tempering" is required. In tempering, the steel is heated to a much lower temperature, and it softens and becomes more ductile (pliable).
Once the blade has reached its maximum hardness, it's very brittle and full of stress. Tempering gives it a correct balance of hardness and toughness while relieving much of the stress. Tempering also requires lower heat for longer periods of time, again depending on the alloy and the mass of the steel in the blade. Some blades can also be selectively hardened and tempered - and require terrific control and a master knifesmith.
There are too many technicalities of hardening and tempering steel to cover them adequately in this small space. If you're interested in this subject, you might want to get a knifemaking book. There are many to choose from, so I won't make any specific recommendations.
Copyright 2005-2006 by Cheaper Than Dirt. Reprinted with permission
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