Stephen, of Windsor, CA asks:

I'm just starting out with blacksmithing and blade making. I just got an anvil and soon a coal forge. In the meantime I am making a dagger out of ATS 34. It is about 1 1/4 inch wide, 5/32 thick and about 6" of blade. I have it ground and semi polished. It has a 1/3 length shank and I'll gas weld a stainless bolt on it to secure the handle. The hilt is sterling silver and will be soldered on.

My question is about heat treating - hardening and tempering.

I can answer most of your questions but specs on the alloy stumped me. "ATS-34" is not a standard steel designation. It is a trade name identification. All the ATS steels I found in "Woldmans Engineering Alloys, 6th Ed", were manufactured by Deutsche Edelstahlwerke, G.m.b.H. The steels listed did not show any relevance between the number and alloy. They are just stock numbers. You need to find out more about it from where ever you obtained the steel. Give me any standard steel number (SAE, AISC, UNS,) and I will gladly look it up. Otherwise I can give you some testing pointers at another time.

Stephen wrote back: The steel is made by Hitatchi and was purchased from Sheffield's Knife Makers supply. The spec sheet they supplied was in Japanese!

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 warpage becomes a problem. The forge (coal, gas or oil) or oven would work, I've never used electric but I expect it is easier to control (if it gets hot enough). 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.

I've never done it but 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.

If you need a good affordable general reference on steel properties, the ASM Metals Reference Book is an excellent source of information.

Copyright © 1998 by Jock Dempsey, DEMPSEY'S FORGE