Hardening a Blade

Dec 06 2014

Following up from my last post on making the nicker iron

I got prepared for hardening the steel. I bought a Bernzomatic TS8000 Map Pro torch. These were highly recommended, and burn hotter than propane torches. Got a can and a quart of peanut oil to quench in, and with some bricks, built a little oven. If I were heating up something larger, I’d have to get a little more intricate to let the heat build up, and use bricks designed for the purpose. This was mainly just to prevent me from setting my bench on fire.

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I don’t have any photos of the heating itself, as I only have two hands. I will say that it went disappointingly fast. With such a small piece of metal, and a powerful torch, it was cherry red in under a minute. I kept a magnet nearby to test it with. When you hit the right temperature for hardening O1 steel, it loses its attraction to magnets. Fascinating. And it worked. After the color was right and it passed the magnet test, I put it back in the flame to revert any temperature loss during the test, and then plunged it into the oil.

Again, the oil quench was sadly uneventful as well. No hissing or flames like you see on the knife forging videos. I mean, this iron is the size of a large nail, so not very dramatic at all. But honestly, that’s probably great for a first project of this type.

Here’s the blade after the quench and cleaning up the oil.

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Next step, I sanded it down to remove all the carbon from the heat treatment.
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When you harden steel like this, it gets very hard. And very brittle. Too hard for practical use. So you have to temper it by bringing it up to a lower temperature for a while. I think this is the more finicky step. Overheat it at this point, and it will be too soft and won’t hold an edge. Don’t heat it enough and it will be too hard and fragile and tough to sharpen.

The step is easily done in a toaster oven set somewhere between 300-400F depending on what you read. I think these toaster ovens have an accuracy range of plus-or-minus 75 degrees anyway, so this is best done by other indicators. As the steel heats up at this point, it will go through various color changes.
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You’re looking for a “light straw” color. Indeed, it turned a light straw, then a dark straw, and before I could react, a bronze/gold color, and then the tip started going purple and blue. You might just be able to see the blue in this next photo.
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Once I got it out of the oven and cooled down, the whole first inch was a beautiful (and useless) purple and blue. At that point, you’ve ruined any hardness you just created in the first step. Luckily, all is not lost. You just go back and do step one again.

The next time around, I was much more conservative on step two, and watched it like a hawk. You should just be able to see in the next couple of photos that it has a very light brownish tint (light straw, as we say in the metal working biz). I feel like I could have maybe gone a little bit further here, but didn’t want to risk going into the blue area again. I’m not sure how many times you can run through this process before something starts to break down.
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Once again, clean up the surface with sandpaper and then over to the diamond sharpening plates for final honing and shaping. In this phase, I got a pretty good feel for how I had done. The steel was definitely hard. A lot harder than when I was shaping it with the file yesterday. But it wasn’t so hard that I couldn’t sharpen and shape it the the diamond plates. So, I think I did pretty good.
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Here it is in its new home.
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To use it, you pull the plane backwards one or two strokes, to pre-cut the fibers. Then start planing.
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In the above two photos, you can see some so-so results. I think I need to adjust the shape of the nicker a bit, give it more of a cutting edge along the sides, as it’s ripping the fibers in some spots, rather than slicing them. And in general, I need some more practice and experience using and adjusting this particular type of plane. But all in all, this whole project has been a blast. Learned some new skills that open up all kinds of possible new projects.

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