One of the best tools for knife making is a file. They are easy to work with and are typically made from 1095 high-carbon steel, which makes them ideal for heat-treating.
If you want to make a knife from a file, you need to know how to soften it before you can use it. This will help you make an excellent, dependable blade.
Annealing
In the world of knife making, it can be a challenge to find file steel that is able to hold an edge. It is often cold and hard and will be difficult to shape into a knife.
One of the most common ways to soften a file to make it easier to shape is by heat-treating it. This alters the crystal structure of the metal in such a way that it makes it more malleable and increases its ductility.
Another benefit of annealing is that it removes impurities that can cause the blade to break under extreme circumstances. This also improves edge retention.
There are many different heat treating methods to anneal metals and alloys. The process involves heating a piece of metal above its recrystallization temperature, holding it for a certain amount of time, and then cooling to room temperature.
Heat Treating
Heat treating is the most important part of making a knife or any other cutting tool. If done incorrectly, it can lead to a blade that will bend like a pretzel, shatter when dropped or otherwise fail under stress.
Despite the importance of heat treating, it is also a difficult process to learn. This is because the process involves a large number of variables (soak temperature, soak time, ramp-time, quench rate, surrounding conditions, etc.) and is largely a science.
One of the most basic ways to heat treat a file is to put it in a 400 degree oven for two cycles of one hour each. This will cause the steel to change color from a straw or yellow brown to a light purple. This will tell you that you have hit the proper temperature and have softened the steel enough to make it hard.
Differential Tempering
In differential tempering (also called graded tempering, selective tempering or local tempering), steel that has been uniformly quenched and hardened is heated in specific areas to soften those areas. This is most commonly used in blacksmithing for making cutting tools, but has been applied to knives and swords as well.
Differential tempering creates a blade that has a harder edge for superior edge retention, but its spine and cheek are softer and more flexible. This is particularly beneficial for camp and hunting knives, as it allows a blade to take some abuse without the spine becoming brittle and breaking.
The smith packs the spine with a special clay during tempering that acts as an insulation allowing it to cool slower than the edge. This slows the cooling of the back and body of the blade, reducing its brittleness and creating more malleable iron. The edge, however, is more quickly cooled, producing a hard microstructure called martensite.
Torching
When you want to soften a file to make a knife, one of the most common methods is to torch it. This method can be a little tricky and requires some skill, but it is effective and can save you a lot of time.
To start the process, bury the file in a pile of coals. This will heat the steel and gradually bring it to its “austenitizing temperature.” Carbon steel loses its magnetic properties around 1425 degrees F.
Once it reaches this temperature, keep heating it. You should see the steel turn red-hot and non-magnetic after a few minutes.
After a few minutes, take the file out of the fire and check its color. If it’s evenly red-hot and non-magnetic, you have reached the annealing temperature. If it’s not, you’ll need to heat it up again.