How to Make a Hunting Knife

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Tempering

The second step in heat-treating is designed to soften a piece of steel very slowly until it reaches a precise hardness, in this case, the point at which the brittleness is removed from the blade you're working on while leaving the steel hard enough to hold an edge and otherwise serve as a knife.

It should be kept in mind, too, that some knives should be tempered differently than others, depending on the "sharpenability" desired and the kind of use the tool is designed for. A long vegetable knife, for example, is a lot of work to sharpen and is subjected to much less strenuous usage than, say, a survival knife, and, in my opinion, should be tempered softer than the second implement.

When heated, steel turns from a silver gray to a light gold or straw color, then to darker and darker shades of bronze before becoming reddish, then a bright and very striking vermilion, a light blue or peacock, and, finally, the dark blue of soft steel. The metal becomes progressively softer through this color range, with the cutoff point between hardened and softened steel somewhere between the peacock blue and dark blue stages.

I've found the point of best compromise for most carbon saw-blade steel knives somewhere in the medium "straw" range, which produces a blade hard enough to take and hold an edge, but with enough flex to make it nearly impossible to break. For a particularly hard blade, I temper at the light straw color, while some makers of vegetable knives draw their steel to a much softer (vermilion) color.

For starters, you can't go too far wrong if you clean your hardened blade to an even shine with an old 80- or 120-grit belt and heat the entire piece of steel slowly and evenly in an oven to about 450 degrees. Watch the blade as it turns from silver gray to the color you desire, then take it from the oven and allow it to cool.

Annealing

The third, and final, part of the heat-treating process consists of carefully heating (thus softening) the handle portion (tang) and the back and center of your blade just down to its ground edge. Turn the specified areas a deep blue but do not allow the color to run into the cutting edge or its hardness will be lost and the knife must be rehardened.

I do this annealing with a small welding tip on my acetylene torch and I keep a bucket of water handy so I can dip the ground area of a blade quickly whenever I see the heat running too close to the edge. WARNING: Never douse the whole blade during this annealing process. If you do, the metal may regain its hardness while remaining blue, then crack if you later try to straighten it.

Finishing

If your knife will have a very fine (thin) edge, you should do the final grinding of the blade after the metal is tempered (because a very fine edge will warp during the heating process).

Grinding an edge after it is tempered, however, is also risky, since the heat generated by the final grind can remove some of the steel's hardness. If you do not have a water-cooled belt grinder, then, go slow and dip the blade in water after every one or two passes across the wheel. And use a new belt on your belt sander for this job because new abrasives grind with less heat than either old ones or grindstones. (I like a 40-grit belt for this process.)

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