Annealing To Soften Metal

What Makes Metal Hard?>

There are many different kinds of metals and metal alloys (mixtures of metal). Each alloy is designed for a particular set of desirable properties, such as "resist the heat of reentering the atmosphere", or "resist the pressure of thousands of feet under the ocean", or combinations of properties, like "resist corrosion from salt water and withstand pressure".

But the physical properties of an alloy do not result only from the mixture of metals that are in it. It also results from certain treatment of the metal, like heat treating.

By carefully heating and cooling a piece of metal, it can be made harder or softer. If we were doing this commercially, and the properties of the alloy were very important, the protocol to get exactly the performance we want could be quite complicated: heat to this temperature, hold for this amount of time, cool X degrees per minute until it reaches this other temperature, hold it for this time, suddenly drop it Y degrees...

Annealing

We aren't aiming for precision here. Our goal is simply to make a chunk of metal as soft as we can, so we can do something like drill a hole in it.

The steps are pretty simple:

Heat the metal until it is glowing orange-hot, using a propane torch.
Hold the metal at this heat for a little while.
Let the metal cool slowly back down to room temperature.

Getting the metal hot enough is important. If you don't get the metal up to its "Curie temperature", this procedure won't work. The Curie temperature is different for every different alloy. The simplest way to see if you have gotten the metal hot enough is to test it with a magnet. When ferrous metal (that is normally attracted to a magnet) stops being attracted to a magnet, it has reached the Curie temperature.

If you don't have a magnet handy, it is often sufficient to just get the metal orange-hot. If you complete the procedure and the metal is still to hard, you can heat it up and try again.

Cooling the metal slowly enough is also very important. If the metal cools too rapidly, it will stay hard, or even get harder than it started out! Start by slowly moving the propane torch back, a little at a time. Do this whole procedure in a sheltered area, where puffs of wind won't suddenly cool the metal.

[photo] When you first apply the propane torch, you will see some smoke, flame and burning stuff. This is paint, coatings, and other stuff. Don't breathe the smoke, but don't get excited by any bright orange sparkles that you see.

[photo] Look for a dull orange glow, like that to the left side of this picture.