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Heat Treating
Heat Treating is normally thought of as the process of strengthening steel but it can also be used to soften or anneal it. After forging at the anvil or the factory, steel must be annealed or it is difficult to machine, or in our case, grind and drill. Annealing (again, in our world) is described as the process of heating a blade or billet to critical temperature (non-magnetic) and cooling it VERY slowly in order to soften it enough to work easily. The temperatures we are talking about here are different for each type of steel. When using high carbons and Damascus the average hardening temp is between 1475-1530 degrees. For stainless steel the temperatures get upwards of 1900. That is why it's best to have a computer controlled furnace so you know your temperatures are exact.
To harden by quenching, a blade must be heated into critical temp or the austenite “crystal phase” and then quickly cooled. Quickly cooled means to take it below the hardening curve in less than a second. The cooling or “quenching” may be done in several ways depending upon the type of steel or alloy you're working with. Some require forced air ; some oil ; and some water or brine . When the blade is cooled very quickly, a portion of austenite will transform to martensite , a hard crystalline structure. Un-tempered martensite is very strong but it is too brittle and the knife will most likely fail in use.
The way to remedy this problem is called tempering . Tempering is done at a much lower temperature than was the hardening – usually in the 350-600 degree range, again depending on the steel or alloy.
An easy way to wrap your mind around this is to talk about coffee. Coffee? Stay with me here…
People like their coffee many different ways – just ask “Star…uh…Fivebucks”. However, one thing is common and that is the coffee must have flavor. People who put creamer in their coffee are in fact “tempering” it, or making it less strong. Well then, why don't they just make weaker coffee in the first place? Well, it wouldn't have the proper flavor, now would it? So in essence, we're taking the coffee or in our case, steel, to its strongest point and then backing it down a notch.
Differential hardening is common in high end swords and blades, especially those made the old fashioned way in Japan. When using high carbon steel, I do mine with a clay backing just like the old Japanese blade smiths. The clay prevents the steel underneath from cooling as fast as the naked steel along the edge. This provides for a harder edge while leaving the back or spine softer to handle impact and twisting a bit better. A nice byproduct of this process is the Hamon line or hardening line. This is a visible line or series of lines on the blade where the hard steel along the edge meets the softer steel of the back. Hamon lines can be decorative if the maker etches the blade in a mild acid and polishes it out for more definition. It is curious to think about the radical activity that occurs in a split second that causes a Hamon line.
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