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HOW DO I HARDEN STEEL:
HOW DO I HARDEN/TEMPER METAL?

Heat Treating is a *HUGE* subject, and depends on the metal, and the intended use. Most of the time, this question is asked regarding steel, so we'll give a brief description of that.

HOW DO I HARDEN STEEL:
HOW DO I HARDEN/TEMPER METAL?

 

Heat Treating is a *HUGE* subject, and depends on the metal, and the intended use. Most of the time, this question is asked regarding steel, so we'll give a brief description of that.

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 formally occupied by an iron atom. This form of red-hot steel is called austenite.

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 use.

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 be relieved and is called termpering. The amount of termpering 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 degrees F. The 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

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 machinists. 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/hoby use. As its name implies, it is the type of steel used for drills, and is available in round form. 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 raped 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. It has been reported, by way of example, that you can make springs out of hacksaw blades by annealing, bending, hardening, then tempering by heating to a "metallic blue" and quenching in oil. I suspect lots of experimenting may be in order before you get things just right. Remember the steel must be clean (no paint, etc.) to see the colors.

Quenching in oil may be a fire hazard. Take the precautions, such as removing flammable materials from the area, wear the proper clothing, and have an extinguisher handy. Even quenching in water presents the risk of scalding from steam or splattered water.

As some people pointed out, not only are there's a gerbillion alloys, but zillions of treatments to choose from, and this is just for steels. Other metals, like brass, can be hardened by "working" the metal, by bending, hammering, peening, etc. Brass is usually annealed with a quench, which is the opposite of steel. It's best to carefully study you particular project first, especially if it's something that is valuable.

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Last modified: October 02, 2001