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I Forge Iron

5160 heat treat question

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So I've decided to build another blade. Let's see this would be number 4. Anyway, after some searching in the archives I've found that several of the "guru's" here recommend a differential treatment. Here's my question / scenario. When I'm making a chisel lets say, I was taught to "shadow" the tool up and down a little bit so as not to create a sharp transition from the quenched to unquenched parts of the tool. Do you do this same thing with the blade if you're going with an edge quench?

Another question / scenario I've been thinking about. Since the edge of the blade is not straight. Do you worry about having a hardened portion that's not parallel with the edge? I doubt I'm describing my issue here, so bear with me. I can see where you'd have a very narrow hardened portion near the ricasso, then a wide hardened portion near the heel, then a very narrow hardened portion at the tip. Is that Ok? Or is there something I'm missing in the equation. I found a post where someone recommended welding plates across the ends of a piece of 3" angle iron to form the edge quench container and this is an issue I'm seeing with edge quenching a blade this way.

Any other recommendations?

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The reason you were told to treat chisels as you described is because their use involves impact, and by lengthening out the transition zone, the shock of impact is better disbursed, and the tool less likely to break.

Knives are a different animal, intended for a different purpose. Although a knife blade is expected to endure some impact, its not nearly to the degree a chisel would encounter.

Usually the transition zone on a knife blade is very narrow. The soft material which remains in the upper 2/3-1/2 of the blade acts somewhat like a shock absorber, helping to make the edge somewhat more durable at the same hardness, versus an fully hardened blade.

I use what I call a "limiter plate" in my quench tank. My tank is 36" long X 5" wide X 6" deep. The limiter plate is a 3/8" thick piece of plate aluminum, chocked full of 3/8" hole, with a 1/4-20 threaded hole at each corner. Each of these holes has a 5" carriage bolt thread through, with the heads facing the bottom of the tank. All I have to do to adjust the depth of the quench (how much of the edge gets hardened) is to turn the bolts....raising or lower the limiter plate level in the quench tank.

When quenching, I tend to leave blades overly thick (the edge is approx the thickness of a nickel) to help retain the quenching heat. I also use an oxy/act torch with a #3 tip and a "soft" flame. I try only to heat that area of the blade that I want to quench/harden. Once the proper heat is achieved, the tip goes in first, for a slow count to 7. Then I rock the blade back so that the rest of the edge/bottom of the ricasso goes in, and again, a slow count to 7. Keep doing this until all the fumes stop coming off the oil, then drag the blade off the limiter plate, and allow it to cool down IN THE OIL, until you can handle it bare handed (usually about 5-10 mins). DO NOT TRY TO SPEED UP THE COOLING BY DUNKING THE BLADE IN WATER. Once the blade is cool enough to handle, get it into a pre-heated tempering oven for a minimum of 2 hours. Everything I do get triple tempered, meaning 3, two hour cycles, allowing the blade to cool to room temp between tempering cycles. Spectrographs of test materials have shown that at least two tempering cycles are required to get rid of most of the retained austinite...the third is just for insurance.

Thanks for posting that link to the JS article! I'm glade to see it's getting some use!

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