Hardening

Tools Utilized

• Coal or Gas forge
• Deep Baking Pan
• Peanut Oil
• Locking Pliers
• Magnet
• Optional Oxy Torch

Have you ever heard someone brag about how sharp their knife is? Or how long it will hold that sharp edge? Well, the ability of a blade to become very sharp and hold an edge is primarily based on the level of hardness the metal is capable of achieving. The carbon within the 5160 steel we use is what gives it hardenability; however, the normalization process also has the effect of annealing, or softening, the metal.

Ok. It's not soft relative to things like wood or feathers, but in a fully annealed state the metal is much softer than it is capable of becoming. And soft metal doesn't make for a sharp or resilient edge...

That being said, it is possible to have too much of a good thing. When metal becomes too hard, it is brittle and can shatter easily. For that reason, we are going to harden just the edge of the blade while leaving the rest of the metal in a softer state. This will leave us with a blade that will hold a razors edge, but at the same time have the flexibility to bend without breaking.

We learned two ways to accomplish this in class. First, you can check out the absurd yet highly amusing saga of Mike Williams hardening and tempering the blade in one step with an oxy-acetalene torch! This method is highly unreliable and you would likely never do this with a blade you just spent a lot of time forging and gringing... But it's amazing to watch anyway!

• Part 1 - Mike Hardens with Oxy Torch
• Part 2 - Mike Grinds the blade
• Part 3 - Mike Sharpens on Grinder
• Part 4 - Sharpen on Whetstone
• Part 5 - Mike chops and cuts again
• Part 6 - Passes Bending Test
• Part 7 - Finally Broken

Now that we've had our fun, let's get back to work. The following steps will get you a hardened hunk of metal:

1. Fill a metal baking pan with enough peanut oil to be able to fully submerge the blade while holding it straight up and down (meaning edge down, spine up.)
2. The oil needs to be brought up to about 140 degrees. This can be accomplished by heating a piece of sacrificial metal up and quenching it in the oil until a baking termometer reads 140.
3. Next, carefully and slowly heat the blade to non-magnetic. We do this in the same manner as when we normalized the blade, taking care not to get any particular area too hot.
4. Once the blade is uniformly heated to the non-magnetic state, it must be removed from the forge and edge quenched as quickly as possible.
1. Since the edge of the blade is curved, begin by dunking the tip into the quench and then rotate the length of the edge downwards to quench it.
2. Almost any oil will work for this proceedure, but Mike recommended Peanut oil because it has a high flash point and because the smell won't get you kicked out of the house. :-)
5. While holding the edge in the oil, the blade is moved rapidly forward and backward to help speed the cooling.
1. Note: Do NOT move the blade from side to side as the uneven effect of coolking more rapidly on one side than the other will likely result in a warped blade.
6. After holding the edge in the quenching oil until the rest of the blade cools to black, the entire blade is dipped into the quench.

Seth once again demos the process for us. (Video: 10.1 MB)

More information on Annealing from the 1924 edition of Machinery's Handbook

Next Step

Beginning - Definitions - Forging - Normalizing - Grinding - Hardening - Tempering - Summary