Tempering steel is an important process that makes a knife hard and tough. It allows the steel to take a lot of abuse without being brittle and breaking.
To temper, the steel is heated to a specific temperature. Usually between 800 and 900 degrees Fahrenheit, held at that temperature for a certain amount of time, then quenched (rapidly cooled) in oil or water.
Heat Treating
Getting your blade to the right hardness is one of the most important parts of knife making. It determines whether your blade will hold up under abuse, shatter when dropped, or bend like a pretzel when hacking on an unusually rebellious piece of wood.
Heat treating steel is the process of heating and cooling a material at a controlled rate to alter its physical properties in a beneficial way. It is an essential part of manufacturing and is used in a variety of industries including aircraft, automobiles, hardware-such as saws and axes, computers, spacecraft, and military equipment.
During hardening, the metal is heated until it is a certain temperature and then cooled rapidly by insertion into a cooling medium such as brine, water, or oil. This causes a chemical reaction that makes the steel hard. The result is an increased hardness that relieves internal stresses, which can be useful in many applications.
Normalization
One of the most important heat treating operations, normalizing resets the steel’s crystalline structure. It relieves the stress built up during the forging process and helps to prevent edge chipping.
To normalize, simply heat your blade above its critical temperature, let it cool, then repeat this cycle several times. This will result in a finer grain structure and, as it continues to cycle, it will become more refined.
Ideally, the initial heat should be enough to initiate pearlite formation in the metal (see the above fractures) and subsequent heats can help refine this as well.
If the metal is heavily spheroidized, this process can be more complicated and a higher temperature may need to be applied. In general though, the final hardness of normalized steel is similar to the as-received condition. The only exception was 1084 which was only a couple Rc different with normalized and spheroidized conditions.
Quenching
The quenching process of hardening and tempering steel is a mechanical method that strengthens and improves the toughness and durability of metals. It involves heating the material to a specific temperature and quickly cooling it to the desired level of hardness.
It can be done with various types of media, but water is often the most popular. Because water has a high specific heat and is non-flammable, it can cool the steel quickly and easily.
However, it can also cause a layer of insulation to form on the surface of the metal. This can lead to cracking and uneven hardness, especially when the moisture content of the quenching fluid is too low (less than 0.1 percent).
For this reason, it is best to use commercial quenching oils that are specifically designed for this process. These include Parks AAA and Parks 50, which have special properties of being either slow or fast quenchants.
Tempering
Tempering is a process that reduces the hardness of a steel blade. It is most commonly performed on steel that has been quickly cooled, in a process called quenching.
It is also sometimes used to increase the toughness of a blade for better resistance to breaking. For example, a camping knife may be tempered at 350degC (662degF) to make it more resistant to breakage.
The tempering temperature depends on the purpose of the blade and should be chosen carefully. A higher tempering temperature will produce a slightly softer steel with increased toughness, while a lower tempering temperature will produce a harder and somewhat more brittle material.
Differential tempering is a common technique used in bladesmithing to produce a knife with a hard edge and a softer spine. This allows the blade to take abuse without the spine becoming brittle and breaking. It can be done by either insulating the spine with an insulating clay or by not quenching the spine during the heat treating process.