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Step 6Heat-treat the Blade
Heat-treating steel's a two step operation. First, you heat it up, and cool it quickly. Second, you heat it up slowly, as slowly as possible, and watch for the colours. you're going from hard and brittle to soft and springy. Yellow is the brittle one, Blue is the soft one. (a blue temper is still quite hard, in good steels)
All hardened blades need tempering, but some more than others. Throwing knives need to be very tough, because they smash into things with great force. If it cracks, it's an embarassment to the person using it, and to the person who made it.
So, warm up the steel with a blowtorch, or in your forge, it doesn't matter, as long as it gets hot enough. There is a point at which the steel's internal structure changes from the soft crystals called Ferrite into Austensite. This is the point at which the steel is Quenched. (luckily, as the steel changes its crystal form, it stops being magnetic, that makes things a lot easier, doesn't it?) Quenching the steel cools it rapidly and changes the Austensite into Martensite. By quenching the steel, we are imparting tremendous stress to its crystals, this is what makes hardened steel hard. But it's brittle, too, so what are we to do?
Temper it. Change some of the Martensite into Troosite.
To temper the hardened steel, you need to warm it up again, but not as much this time. I suggest that you either do it slowly with a blowtorch, or, if you're using a coal forge, that you heat up a big lump of steel ot a dull red heat and let the knife draw heat out of the lump and into itself. You're looking for an all-over blue temper on this one. Don't let it get too hot, otherwise that lovely blue colour will lose its brilliance and the knife will lose its toughness. A quick note: Steels with more carbon in them need tempering slightly more for the same effect. Experience is the only real teacher here.
All hardened blades need tempering, but some more than others. Throwing knives need to be very tough, because they smash into things with great force. If it cracks, it's an embarassment to the person using it, and to the person who made it.
So, warm up the steel with a blowtorch, or in your forge, it doesn't matter, as long as it gets hot enough. There is a point at which the steel's internal structure changes from the soft crystals called Ferrite into Austensite. This is the point at which the steel is Quenched. (luckily, as the steel changes its crystal form, it stops being magnetic, that makes things a lot easier, doesn't it?) Quenching the steel cools it rapidly and changes the Austensite into Martensite. By quenching the steel, we are imparting tremendous stress to its crystals, this is what makes hardened steel hard. But it's brittle, too, so what are we to do?
Temper it. Change some of the Martensite into Troosite.
To temper the hardened steel, you need to warm it up again, but not as much this time. I suggest that you either do it slowly with a blowtorch, or, if you're using a coal forge, that you heat up a big lump of steel ot a dull red heat and let the knife draw heat out of the lump and into itself. You're looking for an all-over blue temper on this one. Don't let it get too hot, otherwise that lovely blue colour will lose its brilliance and the knife will lose its toughness. A quick note: Steels with more carbon in them need tempering slightly more for the same effect. Experience is the only real teacher here.
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