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How to Temper a Carbon Steel Knife

Posted by James Mc on March 10, 2012 at 5:40 PM

How to Temper Your Blade.

All your grinding is finished and now what? If you’ve been working the steel in its annealed state (soft) it will need to be hardened. If you’ve been grinding away on a file it will need to be made softer or it will break.

From the annealed state you will need to have an adequate source of heat. Gas, coal, coke and charcoal forges will all work. Even a grill with a hair drier is capable of getting hot enough to reach the proper temperature to harden simple carbon steels. For smaller blades a torch will work just fine, although you need to be very careful as the 5200F flame will quickly destroy your work.

Before you start get a few things ready. You’ll need a quenchant. Old motor oil will work, transmission fluid or quenching oil. Never quench your blades in water, there is nothing as discouraging as having a blade you spent hours working on crack or break when it hits the water. I use a variation of Wayne Goddard’s goop, 1/3 wax; 1/3 hydraulic oil; 1/3 vegetable oil. Just melt the wax and mix in the other oils. This mix doesn’t smoke much and really doesn’t flame up unless the metal is overheated.

Take a moment to practice your movement from the heat source to your quench. It needs to be quick and sure. If you are using motor oil heat up a thick piece of mild steel and drop it in. Oil quenches better when it’s warm, don’t ask me why. If you are using the goop, heat up a piece of metal and melt a grove into it.

Go ahead and preheat the oven. Depending on the steel you are using you will typically be somewhere in the 375 to 425 range. Get your fire going or light your torch. When the fire has reach temperature begin running the blade edge down in the heart of the fire. I use a pair of large vice grips for this.

You’ll also need a magnet and a toaster oven. The lady of the house doesn’t like it when an oily blade stinks up the house so if you can do it out in a toaster oven. Also have an old file handy.

Start heating your metal, do it slowly. As the temperature starts rising the steel will begin to change colors. When the metal starts glowing pull it out and touch it with a magnet. The magnet will not stick when the steel reaches the proper temperature for quenching. Keep heating a little more and repeat the magnet test. Be careful not to overheat the steel, this will weaken your blade and may cause failure. When the steel doesn’t stick to the magnet put it back in and heat it up just a little more and quickly quench it. Daggers will go point down; single edged knives can go edge first. Do not let the blade lean, this can cause uneven cooling and result in a warped blade.

When the steel has cooled to around 130f take it out and test it with the file. Run the file down the edge. If the file cuts the blade then something went wrong and you will have to repeat the process. If it is properly hardened clean a section of the blade as to read the tempering colors. Don’t do this with a grinder, use a belt sander or flap wheel. The hard wheels can break the newly hardened steel.

Put the blade in the oven. Let it cook for an hour and take it out. Set the hot blade on a piece of wood to cool. Do not cool it off in water. At this point inspect the color on the section of the blade. It should be a straw yellow color. Temper larger blades to a darker, almost brown color. Remember, the objective of the temper is to make the blade hard enough to hold an edge but not so hard that it is brittle. It’s always a good idea to repeat the tempering process.

If you’ve done everything right you are now ready to clean the blade up polish it, and put the handle on it.

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