– Heat Treating…the Soul of the Blade –
The first step in the heat treating process was done with annealing the blade after it was forged. This caused the Austenite to form and made the steel workable. Now, the steel has to be hardened if it is going to take and hold a good edge. To achieve this, the Austenite must be converted to Martensite. For this, the blade must be heated to critical or non-magnetic temperature, and then rapidly drop that temperature from about 1450 degrees F to 400 degrees F or less in 6 to 8 seconds. This shock on the steel causes the formation of the hard, needle-like structure of Martensite to form. At this point, the blade is at its absolute hardest, though it is also very brittle.
A simple, yet highly effective way to harden the blade is to heat the edge of the blade to critical temperature using an oxyacetylene torch, then quench in oil that has been heated to the appropriate temperature. A trained eye gauge the temperature of the steel by its color. By not allowing the entire blade to reach critical temperature, the edge area alone forms the Martensite while leaving the spine to its softer, Austenite structure.
Most common forging steels are oil hardening steels. This means that the quenching medium used is oil based. Every smith has their preferred medium. While there are many excellent oils out there that are specifically quenching oils, mediums such as basic peanut oil or transmission fluid are just as effective as well. There are some steels that are water hardening and some that are air hardening, but these are less commonly seen. Some stainless steels benefit from a sub-zero quench to attain maximum strength and durability. This consists of quenching the blade in acetone and dry ice after the standard oil quench.
After the quench, the stress in the steel must be relieved through tempering. First, the oil residue must be cleaned off by hand sanding the blade with 120 grit paper. This effectively removes the oil and all the 60 grit marks. When the blade is a nice, shinny silver, it is placed into the oven at 375 to 425 degrees (depending on the type of steel) for about 2 hours. This brings the blade to a light straw color. It is very important to make sure that you have a good, accurate oven for this. If you are using a home oven, use a high quality thermometer to ensure when it’s set at a certain temperature, that it really ‘is’ that temperature inside. This process of slowly bringing the temperature of the blade up to a controlled level relieves the stress in the steel without removing much hardness. This is achieved by the formation of hard Cementite particles which are held in a tough matrix of Martensite.
– After the quench –
Once the blade has achieved a light straw color, the spine should then be drawn to further soften it and achieve greater flexibility. This is achieved by placing the edge of the blade in a pan of water and heating the spine until the colors run down to about a half inch from the edge. This ensures that the edge retains its hardness and only the spine is softened up. This process should be repeated 2 – 4 times depending on the method of quenching and weather or not you want more or less flexibility in the spine.
After achieving a proper heat treating, the blade needs to be cleaned up and have its finish refined.
– Sanding & Finishing the Blade –
I typically leave my blades sanded to 400 grit as it leaves a nice satin finish. I’ve never liked the idea of putting a mirror finish on a blade as I feel the only purpose for that is to highlight scuffs and scratches. I create all of my blades with the intent of being used, whether they are or not.
After the final sanding of the blade, I do the last tough-ups prior to putting on the handle. If the blade is a hidden tang construction, I file the shoulders and make sure that they are flat and even so they will meet up seamlessly with the guard. On full tang blades, I make sure the tang is completely flat on both sides so the scales will set without any gaps. The pin holes or cord wrap holes on full tang blades are drilled prior to heat treating to ensure ease of drilling.
– Ready to begin the handle –