Heat Treating
Rolled Alloys, my former employer, grew up supplying the heat treat industry, and a significant chunk of their sales still ends up in heat treat furnaces. I have spent a lot of time in heat treat shops, looking mostly at the furnace parts & fixtures, for which my employer sold the metal, but also the parts being hardened. Or softened.
“Heat Treating” in the broad sense is a process of modifying the properties of a piece of metal by heating it to some temperature, and then cooling it down again at a certain rate. It might be cooled rapidly, as by quenching in water or oil, or it might be cooled very slowly.
Heat treating of steel has been done since ancient times—even the Greek poet Homer uses heat treat analogies in the Odyssey. It might not be a bad idea for you to read what Homer has to say about Ulysses’ escape from the Cyclops. Also, the contest that Ulysses won when he finally returned home, unrecognized (except by his dog). In spite of your high school English teacher, Homer does have a couple of points to make about metals, along with plenty of blood’n guts to make it interesting.
To harden a piece of plain carbon steel one heats it until it glows red, and then cools it very quickly by quenching it in water. If you try that with a 20d nail, nothing will happen, it will be just as soft and bendable as it was when you got it at the hardware store. The hardness won’t even be measurable by Rockwell C, it will be somewhere below C20, where this particular hardness scale really doesn’t work. Probably about Rockwell B80, like a soft piece of 304 stainless, or, of course, a nail.
That nail is still soft, because for steel to harden, not only must it be heat treated, but that steel must contain a certain amount of carbon. Nails have maybe 1/10% carbon in them, and that is not enough to permit them to harden very much at all. At least 0.2% carbon is needed.
Some examples: Gears and axles commonly have about 0.4% carbon in them. That is, roughly 6 ounces of black carbon, or soot, dissolved in 99 pounds 10 ounces of the element iron. A metal file, such as the Nicholson file in your local hardware, is made of steel with about 1% carbon (99 pounds of iron, alloyed with 1 pound of carbon dissolved in that iron).
Carbon, carbon, carbon—any time steel is being heat treated, there is some amount of carbon involved, in the furnace atmosphere as well as in the steel. Including when no one bothers to tell you about it!
When Nicholson buys that steel, called AISI 1095, it is in the annealed condition, which is reasonably soft. They blank out the shape of a file, and then have a machine cut those tiny little teeth in the file blank. Then it is heated in a molten lead, or bismuth, bath to about 1440°F (782°C), which is a nice red color, and quenched straight down into cool salt water. When it comes out of the water it is very hard—hard as a file, you might say. That is 65 on the Rockwell C scale With those hard file teeth one can cut and shape soft steel parts.
Quenching—do quench straight into the water, or oil. If the part goes in straight, it may come out straight. If you belly-flop it, it will surely bend. Salt water, about 10% table salt dissolved in water, is better than plain tap water. Quenches harder with, amazingly, less distortion, less chance of cracking.
If you were to make a nail out of that same steel, 1095, and harden it the same way it would break the first time you hit it with a hammer. Likewise, if you drop your new file on concrete you may find yourself picking up two pieces. The metal is very hard, which is good for a file, but it is not tough, not ductile.
To make that hardened piece of steel a little tougher, we reheat it just a few hundred degrees. That reheat, or “temper”, might be anywhere from 350°F (180°C) (the temperature Mom baked her pies) up to 1100°F (~600°C) or so, hotter than the self-cleaning oven. The old word for temper was “draw”, and you will still hear heat treaters use this term, such as when referring to the “draw furnace”. The higher the tempering temperature, the softer the metal. We really don’t want the metal to be softer, but we have to accept it being softer if we want it tough enough to be useful for anything other than a file. One may also make truck springs of AIAI 1095, but in that case they are tempered at about 700—800°F (370—430°C), or until the hardness drops to about Rockwell C 40 to 45. That is a good “spring temper”.
We have a choice. Either the steel can be hard as a file, and brittle, or it can be tough, but soft. One can’t have it both ways. Except . . .
There is a means by which you really can have it both ways. Take a pinion gear, for example, or the square shank on an air impact wrench. Both need to be very hard so they don’t wear out. But both may take something of a beating, so they must be tough enough not to snap.
Say you made that gear, or wrench part, of some soft, tough, low carbon steel. If there were a way to put a “case”, or surface layer, of high carbon steel on it, then the surface would be hard, so it wouldn’t wear out. But underneath that thin, hard case the metal would be soft and tough, so it wouldn’t break.
The way this was done, a couple thousand years ago, or fifty years ago, was to first make the gear out of low carbon, soft steel. Then put it into a box packed with real wood charcoal, broken up to about the size of peas. Cover the box, and put the whole thing in a furnace at a good red heat. At that temperature the carbon from the charcoal will slowly diffuse into (or soak into; yes I know it isn’t a liquid) the low carbon iron. After about four hours there will be a high carbon “case” or surface layer 1/32” deep all over the part. If you turn that box over & dump the gear into a water quench, it will come out hard as a file on the surface. But the inside will still be soft, tough low carbon steel. In modern practice you might then temper it about 350°F (180°C) just to make it a little tougher, without much loss in hardness (for gears one does not temper, as the as-quenched residual compressive stresses in the case are helpful).
This process is most correctly called “carburizing”, though it is also called “case-hardening”. Today it is done in a furnace with a protective atmosphere containing, say 2—4% methane (or other hydrocarbon, maybe propane. or propylene) as the source of carbon.
James Kelly, Michigan January 18, 2008 revised March 6, 2009
