Now that we have the steel cut into the shape of our blade, we will proceed to the next step: Heat-Treating.

Simply put, heat-treating is the process used to strengthen the knife blade by changing its chemical properties and is usually done by heating and cooling the steel. The whole process is very technical, as a critical mistake could break the knife blade and you will need to restart the project.

Step 4: Heat Treating

Hardening

The first part of this step is called hardening, where the steel is heated to a critical temperature, then quenched to bring the temperature down. To heat the blade, we would need to set up either a gas forge or a coal forge. For smaller knives, a torch can also be used.

A hardening bath would also be needed. Different materials can be used for quenching, like oil or water. For beginners, especially those using 01 Steel, oil is recommended. Any oil can be used, ranging from motor oil to vegetable oil. Just make sure that the quenching material will be enough to cover the whole steel.

To start the hardening process, heat up the forge, and then place the steel in the forge. Make sure that the blade is by its spine during heating, as this will prevent the steel from overheating and melting. Once the steel turns bright orange, take it out of the fire and test it against a magnet, as properly-heated steel will not be magnetic. After checking, leave it out to start air cooling. This will also be the time to check if the blade is straight, as once the steel is hardened, the shape can’t be changed. Repeat the process two more times. This relieves the stress in the steel, which is also known as annealing.

On the fourth heating, instead of air cooling, submerge the steel to your quench bath for cooling. If using oil, be aware that flames may appear, so have proper protection. It is also recommended that the quenching be done on a well-ventilated area, as once the steel is taken out, it will produce some smoke. The steel after quenching will be very brittle, so be careful with the blade.

Tempering

Once the blade has been hardened, it is time to temper the steel, as untempered steel is as useful as a brick. Different steels have different temperatures for tempering and would yield a different tempering color. The safest is to aim for a brown to purple hue, which appears at 425 degrees Fahrenheit (or 218 degrees Celcius).

To begin the second part of the process, preheat your oven to 425°F (or 218°C), and then place the cooled steel on the middle rack of the oven. Leave the blade there for at least an hour. Once the hour is finished, the tempering process is now complete.

Now that the heat treating process is complete, the knife is ready for some cutting. But before we do that, we need to complete the knife project by finishing it, placing the handles and guards. Next time, we will finish our knife project.

Another shout-out to Basta from Instructables and WikiHow, as their guide has helped us make this.