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Knife making by the stock removal method Part 4 - Hardening and Tempering the Blade

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This is part 4 of the knife making tutorial. In this part we harden and temper the blade. . If you came directly to this page you can start the tutorial right here: How to Make a Knife Part 1

 

Hardening and Tempering the Blade

This is the part that most people find is the biggest obstacle but in reality it is quite easy and will only take you an hour. A lot of the other work like the filing and rasping will take you a few hours of work but this is a snap. You just need to know a few things to get it right.

First an Overview

When you are making a knife from raw steel like this there are two things you have to do to the blade. You have to harden it first and you have to temper it second. Hardening it brings it up to a very hard state, but in this state the blade is too brittle. It can easily break. It won't endure the demands of knife use. So you temper it which is a softening of it. Yes, but when you temper it you only temper it somewhat and some areas you want softer than others.

So, you harden the entire blade, with no worries about the tang. And then you selectively temper part of the blade.

The following picture and graphic will give you a good understanding of what we are trying to achieve with tempering. Although the picture isn't the greatest, you will still get a good sense of the tempering.

The back of the blade (non cutting edge) is blue. And the cutting egde of the blade is wheat colored. This is what you are shooting for when you temper. How this is done by heating with a torch or placing near the fire so the back edge is closest. The blue shows that part has been heated to around 600 degrees F. And the wheat part has been heated to about 450 degrees F.

What this accomplishes is a softening of the back edge which gives us some bend and resiliencey while maintaining a hard edge which will keep the sharpness of the blade.

So, Either use a torch or crank up the forge!

Hardening the Blade

Put two things in the fire. Put your knife and a poker or some long piece of steel. This long piece of steel is so you can bring up the temperature of your quenching oil.

Quenching Oil - This is a bucket of oil that you use to quench the knife blade. It can be new or used motor oil or some kind of cooking or vegetable oil. Just be safe, there will be some flaming and you should always have the right kind of fire extinguisher on hand.

Once your poker or piece of steel is red hot you stir that into the oil to warm it up.

Test with a magnet



Now get your blade up to red orange hot and monitor it along the way. It needs to get to about 1450 -1500 degrees F in order to harden. It can be difficult to judge this by eye but there is an excellent way to know when the temperature is right. You test it with a magnet. As the temperature goes up it will become less and less magnetically attractive. And once it gets to the right temperature a magnet will no longer stick to it at all.

Quench the blade

Once it becomes demagnetized quench it in your oil and stir it around. Don't hold it in the same place. Stir it around nicely! If you hold it in the same place it will form a layer of superheated oil all around the blade which will impede the hardening and locking.

The Hardened Blade

There we go. Our hardened blade.

Testing the Harness

You can test the hardness with a file. Lightly run a file over it. On the blade section it will have a glassy and smooth feel and sound. Compare this to how it sounds on the tang section that hasn't been hardened. There will be a difference.

If they sound the same you should repeat the hardening process. Bring it back up to temp and quench it again!

Now let's Temper that blade

Clean the Blade

Now clean the blade with sandpaper or emory paper. Get most of the scale off. This is because we will need to observe the changing color in the blade as we temper it.

heating the blade to temper

Now place the knife near the fire like this. See how the back is facing toward the heat? This will give us our hotter section on the back where we want the blade softer.

You can move this knife around as you are doing this to direct the heat. Monitor the color changes and when it gets to our blue at the back and our wheat at the edge you can go ahead and dip it in your oil to lock it in.

 

Tempering the blade

This blade is almost tempered. Just a little more and a little bit of moving it around to get it right. And then you can go ahead and quench it! Your hardening and tempering is done. Now we can go ahead and finish up the making of this knife.

NextLet's Continue on with the KnifeMaking Tutorial and finish making the knife

 

 

 

 

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