Boiled in Oil

Finished the blade. I used a “V” bevel so I can mark pulling or pushing. Didn’t feel like setting up a sharpening jig (to rough in the bevel before heat treating) so I did it by eye. I probably should cobbled together a jig; 15 – 20 degrees doesn’t look right on that small a blade so I kept grinding the bevel too steep. BTW, I got the 15 degrees from Toshio Odate’s Japanese Woodworking Tools. One to heat treatment. Here is my super sophisticated setup:
Heat source: oxy-acetylene cutting torch held in a vise because I don’t have three hands.
Eye protection: Shade 3 so I don’t go blind and can see the blade color.
Pliers to hold blade while it gets really hot (you can see the blade on the pliers if you look closely).
Glove so hold pliers when they get hot and to keep the hair on my hand and arm if the oil flairs (it won’t on this small blade but does on big plane blades). I have tested this.
Big can full of motor oil. Because I had it and it works.
Procedure: Use the pliers to hold the very end of the blade and wave it in the flame. You don’t want to over heat the blade, keep it a ways away from the torch tip. The pliers act as a heat sink so grip as little of the blade as possible. You want to keep the blade at the critical temp for just “long enough”, which I don’t know how to describe but isn’t very long (as in 10s of seconds for most things you can treat with a torch). Two things are important: the critical temp (1,450-1,500F for O-1) and keeping it there. The critical temp is the point where the steel changes from magnetic to non magnetic. I used to use a magnet to determine this point (be quick and you won’t fry anything) but I’ve done this enough I can judge it by eye – the steel just starts to glow orange/red and seems to become almost translucent. It is very obvious when a spot on the steel falls back down under the critical temp. Which you do *NOT* want (as it won’t harden). So, keep the *entire* chuck of steel *just* hot enough for *just* long enough. Overheating is bad as the steel will start to degrade. At the magic moment, quench; that is, dunk the steel in the oil and move it around (to maximize the cooling rate). If you don’t move it, the oil boils. If you move it too fast, you get cavitation (forms bubbles), both bad because you oil next the blade at all times. So, plunge the blade into the oil and slice (not stir) for 30 seconds or so (the blade cools *really* fast, all it really has to do is get below the critical temp really fast and freeze the new crystalline structure). It will look like this:
In case it isn’t obvious, quenching (in motor oil) really stinks. For a long time. Use a different oil (peanut?) if you want people to think you are testing that new fangled way of cooking a turkey.
The shiny area at the top of the blade didn’t harden – the pliers kept the steel below the critical temp. I ran this blade a bit hot and got more scale than I should have. I remove the scale with a wire wheel.
I verify that the steel is hard by lightly running a file over it, the file won’t bite, it will skate over the steel. At this stage, the blade is quite a bit harder than a file.
Tempering: The blade is now hard like glass; brittle. Tempering trades some hardness for toughness. I use my high tech GE computer controlled oven, which just happens to sit in the kitchen (and also bakes cookies). I use around 350 (or a little less) for a hardness of just over 60. “Soak” the steel at temp for while (I think the official rate is 1 hour per 1/4” thickness). The temp is important so *don’t* use the thermometer on the oven, hand held optical one are pretty cheap (I think I paid $70 for one rated to 700F). Actually, if you aren’t too picky about hardness, using the oven thermometer would probably work just fine; the range 300-450F will give a hardness between 58 & 64. One thing I think is a bad idea is using color to judge temper. I think it is certain to give variable hardness and the steel will probably be softer than anticipated.

After cleanup and sharpening:

Before you decide you are going to put Ron Hock out of business, this method of hardening doesn’t scale very well, treating a #7 blade or a big 1/4” thick blade is a *lot* of work due to warpage and steel damage to the initial edge (the steel at the edges is going to be softer because of being exposed to oxygen during heating).

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