Hardening is a way of making the knife steel harder. By first heating the knife steel to between 1050 and 1090°C (1922 and 1994°F) and then quickly cooling (quenching) it, the knife steel will become much harder, but also more brittle.

To reduce the brittleness, the material is tempered, usually by heating it to 175–350°C (347–662°F) for 2 hours, which results in a hardness of 53–63 HRC and a good balance between sharpness retention, grindability and toughness.

Tempering should be carried out within a reasonable time after hardening, preferably within an hour or so. It is of vital importance that the blade should be allowed to cool to room temperature before tempering is started. The transformation to martensite will otherwise be interrupted and the hardening results may be impaired.

A higher tempering temperature will yield a somewhat softer material with higher toughness, whereas a lower tempering temperature will produce a harder and somewhat more brittle material, as shown by the figure below.

A camping knife or a survival knife, for example, may be tempered at 350°C (662°F) so that it will be able to withstand rough handling without breaking. On the other hand, if the knife is expected to keep a sharp edge, it can instead be tempered at 175°C (347°F) for maximum hardness.

Tempering temperatures below 175°C (347°F) should be used only in exceptional cases, when extreme demands are made on high hardness, since very low tempering temperatures will result in a very brittle material. Similarly, tempering temperatures above 350°C (662°F) should be avoided, since this could give rise to brittleness and reduced corrosion resistance. Note that if the tempered blade is exposed to temperatures above the tempering temperature (e.g. during grinding), the properties of the knife will be impaired.

Correctly performed hardening will result in a good balance between hardness, toughness and corrosion resistance of the finished knife blade.

Hardening / Tempering your blade.

Hardening is a way of making the knife steel harder. By first heating the knife steel to between 1050 and 1090°C (1922 and 1994°F) and then quickly cooling (quenching) it, the knife steel will become much harder, but also more brittle.

NOTE: When you buy steel be sure you have a standard designation. ATS-34 is a popular knifemakers steel and the knife sites should have specs on it. ATS-34 is a stainless steel and most stainlesses are "precipitation" or age hardening. This means they are held at a certain temperature for a length of time and then cooled slowly. Quenching these steels leaves them soft.

How do you heat it up (torch with rosebud, a friends gas forge, an electric jewelers oven)?

It is hard to evenly heat a knife with a torch. Torches are satisfactory for some small parts but thin objects like knives need to be heated as evenly as possible or warping becomes a problem. Preheat your oven so that the part doesn't have to spend a long time heating. Long heat times near more oxidation. The most common error is overheating prior to quenching.

If you have inert gases in your shop (helium, argon, CO2), you could pipe a little in to that jewelers furnace and cut down on oxidation. Another good method is to use stainless steel foil. The part is sealed in a stainless foil bag, heated, then the bag is ripped open and the part quenched. Its expensive by the roll ($100 US) but you may be able to talk a local machine shop or heat treater into selling you a few feet. You can also purge the package with inert gas to reduce oxidation further.

What temperature to heat it to and for how long?

The "transformation" temperature is 1400° to 1600° F. for most steels. That's a red but not orange heat. The higher the carbon content the lower the temperature. Steels becomes nonmagnetic just as they enter the transformation range. This allows you to test the part with a magnet. (Note: Heat can demagnetize the magnet, and it will melt the ones made from powdered metal in a plastic matrix (like "refrigerator magnets"). Time depends on the material thickness. Obviously, heavy sections take longer to heat. For most steels you do not need to "soak" any longer than it takes to get to temperature. A few steels require a brief "soak" period. Check the specifications on the steel.

NOTE: Steels are quenched on what is known as a "rising heat". This means you DO NOT want to heat beyond the hardening temperature and then let it cool before quenching.

What type of oil to quench it in?

Many types of oil have been used and all work. Mineral oil, vegetable oil, motor oil. If you need a small amount of mineral oil, "baby" oil is the same with a little perfume added. I don't like to use motor oil because of all the additives (some toxic). Be sure you have enough. The oil heats up faster than water due to its lower density. If you overheat the oil you can end up with a fire. For one quench something between 2 quarts and a gallon should do it. Be sure to use a metal container. Have a fire extinguisher and (full) sand bucket on hand. And always wear safety glasses!

NOTE: Different steels require different quenchants. In order of severity of quench, brine, water, oil and even air depending on the steel. Quenchants should be room temperature or a little better. Common tool steels such as W-1, O-1, and A-2 tell you their quenchant by their prefix. W for water, O for oil, A for air. Again this can vary depending on the mass of the part.

Do you immediately temper it in a kitchen oven, or can it wait a while?

Tempering is best done immediately after the quench. After quenching the part will have a lot of internal stress. Tempering relieves some of that stress so the sooner the better. A kitchen oven works but tempering is often at the high limit of the oven. I generally turn off my forge and use the residual heat for tempering. The trick is determining the temperature. Some alloys require different handling but I cannot be specific without knowing what you are working with.

Tempering temperatures vary from 350° to 850°F depending on the steel and how it is to be supplied. One way to measure these temperatures is with "temperature crayons" Tempil Division of Big Three Industries make a line of them. The way the crayons work is they are made of different kinds of waxes or plastics that melt at different temperatures. As the part heats you test it occasionally with the crayon. When it makes a slightly wet streak, you are there. You should have a range of "Tempil" sticks so that you know if you have over heated or if you are almost there.

One of the best ways to temper thin sections is to heat a relatively heavy block or plate of steel up to the tempering temperature and then set the part on the plate and watch the temper colors "run". You can heat the plate on your stove top. To see the temper colors you must be ready to quickly polish the part after quenching so that you have some clean bright surface to watch change color. Judging temperatures by temper color is an art that takes practice. BUT, if the heat sink is the right temperature mostly what you are looking for is an even color indicating that the part is evenly tempered. After tempering the whole blade you can take a propane torch and temper the tang further. It should be fairly soft to prevent breakage. Some knife makers also "draw"' the temper more on the back of the knife so that the edge is hard (and brittle) while the back is softer (and less brittle). These are some of the things that make a fine hand made knife a work of craftsmanship that a factory can not match. However, the factory also has very good temperature and atmospheric controls for their heat treating.