February 5, 2018 by SteelWolfKnives
Tool Spotlight – Temper Oven
Tempering is taking a hardened metal and reducing the brittleness while increasing flexibility. When balanced correctly, it changes the metal from incredibly hard and brittle to fairly hard and not-going-to-shatter-when-you-accidentally-drop-it. It is important that the metal stays hard or the edge would dull incredibly quickly. This is why we do the hardening and quenching step. But if we did not temper it the knife would shatter if we dropped or chopped with the knife. This brittleness is the same reason that although obsidian is sharper than almost every hand-made blade, it cannot be used as a knife. It is brittle enough to come off in layers or shatter when used to cut anything other than soft materials like leather.
So how does a temper oven help us?
A temper oven is a box made of insular material to retain the heat, with coils that heat up to a specified temperature. These coils allow us to control he exact temperature of the temper (or heat treat!). The more variables that we can control, the more consistently we can make the knives.
The second advantage is the ability to use an envelope for the process. A metal envelope is used to keep oxygen out of the process, which reduces the amount of cleanup needed after heat treating and tempering. Normally we cannot use the envelope because we have to see the color of the steel in order to tell when it has reached critical temperature. With exact control of the temperature through the temper oven, we can use the envelope to reduce oxidation.
The problem with a temper oven…
There are two problems with a temper oven for beginner knife makers. The first is price. While a can of propane and a torch or a handheld burner are not very exact for controlling temperature, they are very cheap. The temper oven is very exact and consistent so you don’t have to judge by eye, but that comes at a price.
The second problem is the size. With a blowtorch and propane, you can temper any reasonable size, from a small knife to a sword. With a tempering oven, the space is limited to the depth of the oven. This is often perfectly fine for a knife, all the way up to chopping/camping knives. I wouldn’t try an 18 inch knife or a sword in the oven, but for most fixed blades the size shouldn’t be an issue.