Heat Treating......

I was at a knife store yesterday and the subject of heat treating came up. I have searched the net but the more I read the more I get confused. Is there anyone who can explain this in near plain english?

I read that temering and heat treating are different things but they look like very much the same.

As a side note my wife actually got me knife for Christmas! So I guess she is ok with the subject now.

Scuba Doo
This is a page at crucible that should help.

[This message has been edited by Edward R Schott (edited 01 January 1999).]
"Heat Treating" means raising the steel to a very high specific temp and then lowering it at *usually* a fast pace. It all depends on the steel but the idea is to get *maximum* hardness.

Too hard for actual use; "tempering" means to soften it up to a specific usuable hardness/toughness level. "Differencial temper" is where you make the spine softer and the edge harder to get edgeholding and toughness.

Jim March
Heat treating is frequently used to describe the whole process of taking an annealed, or soft piece of steel, and turning it into something that will hold an edge.
It involves heating the steel, and like Jim said, cooling it. Steel like a car spring just has to be brought up to the temperature where it changes and can be hardened. You can actually find that temperature by testing a blade with a magnet. One the magnet wont' stick any more, its at the right temp. You then cool it quickly by 'quenching' or putting the blade into oil or water or synthetic quenching fluid. It turns hard as it cools.
Stainless steel is different in that it has to be brought to a certain temperature, held at that temperature for a fairly specific amount of time to 'soak' in that heat. Then it is air cooled. The simplest method is to take it out of the furnace and let it cool. Some shops are set up to take those blades and them cool them further in something like liquid nitrogen for several hours.

Depending on the steel, and its use, it MAY be too hard to use......too brittle and it may chip or shatter. To make the steel usable for a blade you 'temper' it. You do that by re-heating the blade again, but to a much lower temp than for the heat treating step. Most steels will have a range of hardness they can become depending on the temperature. Put a car spring in an oven at 400 degrees for an hour or so, and you get something like Rc 57-59, which is hard enough to hold an edge, but tough enough not to chip or shatter with use. Stainless steels may have to be tempered at a similar temperature, or up to 800-900 degrees. Depends on the steel and final hardness you want.

That's about it. Some guys use more than one tempering cycle. I have two cycles and a cryo treatment done on 440C and D-2. Some that forge will even heat the blade for hardening more than once, though I don't quite understand the benefit to that, but for some it seems to work.
Basic idea is to heat treat steel to get it hard....if too hard, temper it softer.

hope that helps
Here is some more info David,

Iron will, at common temperatures, organize itself into an atomic structure that is called "body centered cubic." This consists of overlapping cubes with an atom at each corner, and one more in the center of the cube. But above roughly 1400 degrees F there is a change in structure to "face centered cubic" and the central atoms migrate to the faces of the cubes. This latter form is not magnetic.

Steel is basically iron with some carbon mixed in, though modern alloys have various other metals and substances as well. When steel is heated to the critical temperature (about 1400 degrees F), the iron will change to face centered, and the carbon atoms will migrate into the central position formerly occupied by an iron atom. This form of red-hot steel is called austentite. Since it is not magnetic, a magnet may be used to determine when the critical temperature has been reached (though the magnetism may be lost before the transition, so this is only approximate). Complete migration of the carbon atoms may take a minute or two.

If you let this cool slowly, the iron atoms migrate back into the cube and force the carbon back out, resulting in soft steel called pearlite. If the sample was formerly hard, this softening process is called annealing.

If you cool (quench) the sample suddenly by immersing it in oil or water, the carbon atoms are trapped, and the result is a very hard, brittle steel. Too brittle for most uses. The structure is now a body centered tetragonal form called martensite.

So, the next step is to heat it back up, to between 200 and 800 degrees F or so, depending on the desired end hardness. This allows some of the hardness to relieved and is called tempering. The amount of tempering that is desirable depends on the final use. Cutting tools are very hard, knife blades less so because they must flex under use rather than break. Tempering is a trade-off between hardness and flexibility.

Accurately measuring the tempering temperature is important. A nice, expensive thermostatically-controlled oven is great. Or, some special compounds can be applied that melt or change color at the right temp, such as Tempilstik and Tempilaq. If the steel is clean to start with, then you may notice that it goes through certain color changes as it heats up, with understandably vague descriptions such as "light straw" indicating about 440 degrees F, and purple=520. These colors are not incandescence colors, but are viewed in normal room light. The colors are due to types of surface oxidation that are temperature dependent.

When quenching, it is often very important to avoid stirring a part because this will cool one side much more quickly than the other, and might cause warping. For knife blades, as an example, move it strictly up and down during the quench.

Case hardening is a bit trickier, and involves heating the object in some sort of agent that promotes hardening at the surface. Liquid cyanide works well but should be out of the question for the home machinist. Luckily there are substitutes available from suppliers, one being called Kasenit, for example. Note that hardness is often measured using a "Rockwell C" scale, with 63 being very hard and 35 being fairly soft.

A type of steel called "drill rod" is especially useful for home/hobby use. As its name implies, it is the type of steel used for drills, and is available is round or square form (square drills?). Drill rod is also very useful around the shop because it is usually made to very accurate dimensions. Some types of drill rod are formulated for hardening via heating then quenching in oil, while others are quenched in water. The difference is that water will cool more quickly because it's a good conductor (though it may also form a steam "jacket" that moderates this effect), while oil will cool more slowly. Since rapid cooling may warp a part, this could make a difference in the final product.

There is also an "air hardening" steel, though it seems to be quite a bit more expensive than other steels.

Most of this is from info I gathered around the web so hopefully it answered your question and then some.

Best Regards,
Mike Turber
BladeForums Site Owner and Administrator
Do it! Do it right! Do it right NOW!

The latest issue of the American Rifleman has a good article on cryo treatment of rifle bbls. and the effect on accuracy. It is well worth reading. Walt
Wow......what a great thread.
Thanks to those who provided the information.....I have saved it with my knife FAQs.

Brian W E
ICQ #21525343