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The Craft Steel & Heat
Treating Performance
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have been written about heat treating elsewhere, and I’m not going to
try to give lessons on it here. The information is here so you can further
appreciate the details that go into every knife that leaves my shop.
Probably the one factor that makes steel the material the world rides on is its ability to be heat treated. The ability to work a material while it’s relatively soft, and then by heating and cooling at just the right rates and temperatures, that material becomes hard and durable. Imagine how much simpler it would be if you could do the same thing with a diamond! Though many other metals are heat treated to give them certain properties, I know of no other metal that experiences the dramatic transformation that steel does when heat treated. The following is a basic description of how steel is heat treated. I do all my heat treating in a gas oven that I built. It is the best oven (if I do say so myself) that I've ever seen used. It heats very evenly, and it has a digital controller that gives a temperature reading that allows me to heat treat a knife to within +/- 2 degrees F. The steel is heated in the oven to its critical temperature. That temperature is different for every alloy, but it’s basically the point at which the carbon in the steel is in a liquid state within the solid piece of steel. The steel is then cooled at a rapid rate, either in water, oil or for some steels, even air. This is where the magic happens. The molecular structure of the steel transforms into a completely different matrix than it was originally, and in the case of knife blade steels, is nearly as hard and brittle as glass at this point. That really doesn’t do for a knife, so a tempering cycle will follow. You will often see the word “tempered” used where the word “heat treated” should be used. Heat treating is the entire process, while tempering is just one stage of the process. Tempering is when the steel is heated again but to a lower temperature than before and allowed to soak. This reduces stresses in the steel and makes it a little softer, but at the same time, tougher (resistant to breaking). Most of my knives will get a soft back draw when the hardening and tempering cycles are finished. The spine is drawn a little softer still until it has a hardness similar to a spring. It is still not soft, but is extraordinarily tough and stiff. One unique aspect to bladesmithing is that the entire forging process is really part of the heat treating. To achieve a superior cutting tool, The temperatures during forging need to be closely controlled, as they will have a great deal to do with the finished performance of the knife. The explanation above is obviously an extremely simplified explanation of the whole heat treating process. The longer I forge and test my knives, the more I learn about the subtle nuances of the entire process, and the more I can tailor each individual blade to meet the challenges of a particular task. |
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