[JoshEarl]

Tempering
This is where tempering comes in. By heating the blade again, you allow the martensite to soften slightly and you relieve some of the internal stresses. You’re sacrificing some hardness to gain resilience. This is a good tradeoff for a razor, because 65 HRC is virtually impossible to hone anyway.

After the blade has cooled and I’ve wiped off any excess oil, I take it over to my grinder and do a couple of passes on the contact wheel, cleaning the forge scale off the surface and leaving the hollows bright and clean.

It’s a good idea to test the hardness of the steel before you go on to tempering. The easiest—and least accurate—way to do this is to try to file the blade with a good file. Files are hardened to around 65 Rockwell C, and by observing how the file interacts with the steel, you can gauge whether it hardened or not. A file won’t dig into a hardened blade—it will slide off the surface as if the blade were glass. The sound it makes is a good indication, too. A hardened blade makes a high-pitched, brittle sound. (The file may take off some scale, which is soft, but once it gets down to clean metal is should barely even scratch it.)

My tempering oven right now is a small toaster oven. These things vary in accuracy, so I got an oven thermometer to confirm the temperature. You could also use a kitchen oven.

To temper the blade, you heat it to a prescribed temperature and hold it there for two or three hours. The tempering range of steel starts around 350 degrees F and goes up to 600 or 700 degrees. Around 350 F, the martensite starts to transform into tempered martensite, which is slightly softer and much tougher than untempered martensite. The higher the tempering temperature, the softer the steel will get.

For razors, I usually temper at 400 to 425 degrees F for two hours. I’ve experimented with tempering blades at 350 F, and the steel is usually too hard to hone easily. On one O1 blade that I made and tempered at 350 F I spent more than five hours trying to get it honed. I eventually gave up and gave it another tempering cycle at a higher heat, and after that it honed right up.

Antique razors seem to be between 58 and 62 Rockwell C, and several of the modern custom makers shoot for a hardness of 62 to 64. I’ve actually made a couple of blades that were pretty soft, maybe 52 to 54, and they still shaved well. The actual number doesn’t matter much, in my opinion, unless you’re looking to produce and sell blades and want to be able to make specific hardness claims. There is a wide range of “acceptable” hardness levels—the blade will still shave nicely, although edge holding and ease of honing will vary.

As the blade tempers in the oven, the bright, shiny parts that I cleaned up will start to discolor again. This is oxidation forming on the surface of the steel. These tempering colors are a good indication of what’s going on inside the steel. The first color to appear is a faint bronze tint. The bronze darkens to brownish, then it starts to go iridescent reddish purple, followed by blue, then finally gray. For blades, you want to temper in the medium bronze range. The exact colors vary by the type of steel, so you’ll need to experiment. I usually go by the temperature, and the tempering colors just provide a nice secondary reference.

After the temper, the blade is ready for finish grinding. It’s important to keep the steel from getting too hot—anything over 300 or 350 degrees will start to soften the blade again. By letting the blade get too hot, you’re doing further tempering. That’s why the blade will go blue if you let it get too hot. Those are temper colors. The best way to prevent overheating the steel is to grind without gloves on and dip the blade in a bucket of water every couple of passes. You will cook your skin long before the steel gets hot enough to affect the hardness.