[JoshEarl]

Hardening
After the steel has cooled to black for the third time, it’s time to harden it. Normalizing usually takes about 15 minutes, so my quenching oil has reached my target temperature of 130 F. The forge is holding a steady temperature of about 1450 F. (I usually have to keep tweaking it, actually. I turn the gas up and down a bit, and move the torch closer to and farther away from the fire hole on the side. But I can hold it within 20 degrees if I baby sit it.)

I pop the blade in the pipe one more time and allow it to come slowly up to temperature. Heating slowly is desirable because it allows the interior of the steel to heat up, not just the outside.

I use my thermocouple to judge the hardening temperature, but there are a couple of other clues that can be useful. Iron looses its magnetism at 1414 F, and on 1084 steel this is close to the correct hardening temperature. The magnet approach is widely recommended for beginners, but it won’t let you get the maximum performance out of the steel in many cases. The reason is that most steels require some “soaking time” at the optimal hardening temperature. The maximum amount of carbon that can be dissolved into iron is right around .8 percent. With steels that have more than or less than .8 percent carbon, you need to apply more heat and more time to get all of the carbon into solution. For example, 5160 requires some soaking because the amount of carbon, which is .6 percent, is on the low side for a good blade. You need to get all of that carbon into solution if you want the steel to reach full hardness. In a steel like O1, which has around 1 percent carbon plus some alloying elements like chromium, the soak is required to allow the carbon to free itself from clusters of carbon and chromium atoms.

The steel we’re considering, 1084, has just about the perfect amount of carbon in it. So it requires the lowest heats and shortest soak times of any steel. I recommend beginners start with 1080 (more readily available), as it will give you good results with primitive equipment.

Another clue you can use to find the proper hardening temperature is decalescense. If you watch carefully as the blade nears the correct temperature, you can see the steel transforming to austenite. Black shadows flicker and pulse rapidly over the surface of the steel. This takes some practice and a dark environment to see, but it’s amazing to watch. When the flickering stops the transformation to austenite is complete.

As the blade comes up to temperature, I mentally rehearse the quenching process to make sure that I’m not going to drop the blade or trip over something while I’m transferring it to the quenching tank. After it reaches 1450 F, I allow the blade to soak for about three or four minutes. This is longer than is probably necessary, but it doesn’t hurt anything.

The biggest pitfall you want to avoid during the hardening heat is overheating the steel. As the steel overshoots the hardening temperature, the austenite grains start to merge, creating bigger grains. Bigger grains make the steel more brittle—bad bad bad. Overheating by 100 degrees is enough to have an effect.

Once the soak is complete, it’s time for the quench. This is the fun part. I grip the tang firmly with a pair of tongs, and in one smooth motion, I pull the blade out of the forge and immerse it, point first, into the oil. With my preheated Parks #50, there’s very little fuss—a few puffs of smoke, some bubbling on the surface of the oil. It’s important to agitate the blade in the oil, which accelerates the cooling. I’ve tried both agitating up and down and back and forth, and neither causes any warping. Agitation is has been recommended by industrial heat-treaters for decades, and those guys know their steel.

What happens during the quench? The goal of the quench is to cool the steel fast enough to transform all of the austenite into a structure called martensite. If steel cools slowly, the dissolved carbon will collect together, leaving pockets of plain iron. But if it cools quickly, the carbon gets trapped in the iron, forming martensite. Martensite is the hardest, most brittle form of steel.

The quench, therefore, needs to cool the steel quickly and evenly, but not too quickly. (If you use a quenchant that’s too fast for a particular type of steel, it’ll stress the steel and cause it to crack.) You’ll hear bladesmiths talking about missing the “nose.” This is a reference to a type of graph called a isothermal transformation diagram, which shows the different structures steel forms as it cools. To simplify, you need to get the steel down below a specific temperature quickly, and then the cooling speed becomes less critical. On 1084, for example, the steel needs to go from 1450 F to around 800 F in just under 1 second. After that, you have as much as 30 to 60 seconds to get it down to room temperature.

That’s the reason that I preheat the oil; warm oil is thinner and moves more freely, pulling the heat away from the blade more quickly.

If you do all these steps correctly, the chances of the blade cracking or warping are minimal. My personal opinion is that cracked blades are usually a result of using poor quality steel, failing to normalize, or overheating during the hardening heat.

Once the blade has cooled enough that I can touch it with my bare hands, I take it out of the oil and set it on the newspaper and allow it to cool to room temperature. At this point the blade should be 100 percent martensite, and it is loaded with internal stresses caused by the rapid cooling. One good visual clue is to look at the surface of the steel. As the blade heats up in the forge, black scale forms on the steel. If you get the hardening temperatures and quench just right, the scale gets blasted off the blade, leaving an otherworldly, ghostly gray-white finish on the blade.

The blade is probably between 65 and 67 on the Rockwell C hardness scale—crazy hard. If you dropped it on concrete, it would shatter like glass.