Before heat treatment nearly all work on the knife must be completed. Heat treating hardens the steel being treated to a point where a lot of processes can no longer be done to the steel. Here is an example of the idea of hardening. Before heat treatment a knife can be filed by an ordinary metal file as the previous section showed but after a knife is heat treated, it is so hard that a file cannot cut into the knife at all. By hardening the knife it is not just the knife that gets harder but everything one would want to do to it gets harder as well, polishing, drilling, grinding, you name it.
As the picture below shows I am drilling a series of holes through the handle of the knife. These holes need to be drilled before heat treatment. It is possible to drill into the handle after the heat treatment but only with special carbide drill bits so I make sure to drill all the holes needed beforehand. On this particular knife I drill a lot of holes into the handle. All of these holes have a purpose for one reason or another. For example, some of the smaller holes I will drill will be used for pins for either the bolster or the handle itself. Other holes are drilled just to lighten the handle of the knife and allow for a better balance in the hand. Another purpose for many of the holes is to increase the bond strength of the handle scales when the knife is assembled. This particular principle will be covered in a later section.
Heat treating…. Ok. Here we go.
There are many different types of steel out there and each type has a different method for heat treatment. In this project the knife is made out of O-1 tool steel and in fact it is one of the easier and simplest steels to heat treat. Many stainless steels that I use take a much different process to heat treat thus requiring me to send them to a specialist. For example ATS 34 stainless steel must be heated to a high enough temperature where scale (a carbon buildup) forms all over the surface from a reaction with oxygen. It is thus preferred to heat ATS in either a vacuum furnace or in a bath of hot salts. ATS also prefers to take a bath in liquid nitrogen or something else that will make it drop to at least -100°F. The best and easiest way to handle these steels is to send them to an expert like Paul Bos. For relatively simple steels like O-1 my electric kiln, kitchen oven and bucket of vegetable oil work perfectly fine.
The first important thing to know when heat treating a steel is its hardening temperature. Many steels, especially the common tool steels, have a well established temperature range for hardening. O-1 happens to have a hardening temperature of 1450 – 1500 degrees Fahrenheit. To begin the process, I preheat my kiln to 1200 degrees Fahrenheit.
Ok, now that the kiln has reached 1200 I place the knife into the kiln. In the picture to the right I use a long pair of pliers to place the knife in the kiln. I place the knife in blade first and stand it up so that the edge of the blade faces up. I place the knife into the center of the oven to help ensure even heating by keeping both sides an equal distance from the heating coils. The door is closed and I wait.
Aahhh…. Just about the right temperature. I like to heat the knife to somewhere around 1475 degrees. Once the knife has hit this temperature I keep it sitting at that temperature for approximately five minutes to ensure the temperature is uniform throughout the knife. The thicker the knife the longer I will let it soak at the target temperature. If I were heating a 3/8 inch thick blade for example I would let it sit in the kiln at 1475 for 15 minutes or more. Since most blades are much thinner, usually no more than ¼ inch I do not need to leave it all that long for the piece to fully reach the correct temperature.
Now comes the fun part, the quench. Different steels use different quenching mediums just like they have different hardening temperatures. Regardless, all quenching is done for the same purpose, to quickly cool steel from its critical temperature to some temperature much lower thus freezing the steel into a much harder state. O-1 steel uses oil and in fact the O even stands for oil. Some other steels like W-2 use water and many stainless steels use air as a quenching medium. Water tends to be the quickest quenching medium taking only a few seconds to cool from critical temperature to 100 degrees or less. Some steels must be cooled much slower or they will warp or crack from the quenching shock. These steels use oil if they must cool just a bit slower, 20 – 30 seconds, or just room temperature air which can take several minutes. Here I am quenching the O-1 blade in a bucket of vegetable oil. That is flame coming out of the bucket. The one drawback to oil is it tends to catch fire when quenching. The big key with quenching in oil is to move the knife around as much as possible as it is quenching. The reason behind this is if the knife was just placed in the oil without other movement a vapor barrier would form between the steel and the oil from the superheated steel burning the oil. This vapor does bubble up as smoke but it also causes the steel to have less contact with cool oil and thus cool much slower. By agitating or moving the knife around in the oil the vapor barrier effect is minimized thus cooling much faster.
Virtually all steels require some kind of tempering after the quench. Except in a few stainless steels the steel is in its hardest possible state right after the quench. Now hard is good in a blade but there is such a thing as too hard. The harder a steel is the more brittle it tends to be. Brittle steel can break very easily as shown to the left with this piece of 3/8 inch thick O-1 steel that had been hardened but not tempered. The goal with tempering a knife is to draw the hardness down a bit to eliminate the brittleness and toughen the blade. A properly tempered blade is very hard yet can still be bent like a spring without breaking. Tempering is simply a reheating for a certain amount of time. Some steels require multiple temper cycles while other, simpler steels require only one. The only trick to tempering is to figure out the temperature needed to achieve the desired hardness. For O-1 steel I prefer to temper at 500 Degrees Fahrenheit in my kitchen oven for one hour. This tempering cycle will bring the knife blade to approximately 59 Rockwell C hardness rating. With an hour up I remove the knife from the oven and let it cool to room temperature.
Now that the knife has been heat treated it is very important not to allow the blade to heat up too much while finishing it. My rule of thumb is that I must be able to hold the knife with my bare hands. If the knife gets too hot to touch with bare hands I use a bucket of water to cool it off. On some post heat treat operations such as buffing and finish grinding I will dunk the blade in water quite often to ensure the knife does not overheat.
Ok, it is time to finish the blade off. After heat treat at the very least buffing is required to clean up the coloring and minor scaling from heat treatment. Occasionally I will go further and do a finish grind on the knife with a 400 grit or finer belt running at slow speed. For this particular project I do some minor buffing as shown to the right. Here I am using an 8” tightly sewn cotton buffing wheel with black rouge. This particular setup is very aggressive and can take a 400 grit scratch pattern off of a blade in no time but it can also heat up a blade very fast so water must be near. Since I am going to finish this project off with a satin finish I will not go further than this wheel for buffing. I am simply using this wheel to clean up the blade from the heat treatment and prep it for the next step.
The final finish on this blade is accomplished with a scotchbrite belt. This belt seems to be made of nearly the same material as the scotchbrite kitchen sponges and it leaves a nice satin finish on a knife blade. I set the scotchbrite belt onto my grinder running at low speed and simply run each side of the blade across the belt several times and viola…..the blade is finished.
Please continue to the next section on making the handle on this project using the links below. You can also go back to the knife making process page and skip to another section of this project.