Let’s see if I can write this without getting real technical. It’d be a good thing because I don’t have a PhD in metallurgy or physics and I don’t fully understand a lot of what’s happening in heat treating tool steel.
Like water, tool steel exists in several states. Water can be vapor, water or ice. Tool steel is similar.
In its soft state tool steel is made up of ferrite and cementite, iron molecules and iron/carbon molecules. When heated to critical temperature, around 1450° F, the steel changes states or “phases” and becomes a different crystalline structure called austenite. If allowed to cool slowly, it returns to its ferrite and cementite phase. If cooled quickly, it changes again and becomes martensite. Martensite is the hard and stable state or phase we’re looking for.
I started out using methods I’d read about for determining when the phase change to austenite occurred. First I tried a magnet and found I really needed three hands to check the steel for loss of magnetic property. By the time I fumbled around with the magnet, the steel had cooled somewhat. I taught myself to work by color but I knew I was always guessing and color changed with ambient light conditions. I knew I needed something more dependable after teaching a workshop where we ended up heat treating on a loading dock after dusk and under mercury vapor lights. None of the plane irons ended up as hard as they should have been and we had to heat treat twice.
I started reading everything I could find on heat treating. Modern sources didn’t have anything to offer because they all dealt with using a furnace. We have a computer controlled heat treating furnace but using it is an all-day process. Hardening a batch small molding plane irons can be done with a torch can be done in less than an hour.
The old texts never mentioned judging color or using magnets. They were cryptic in their description but all mentioned some visual indication of a change in the steel. They said things like, “when the steel opens,” “when the steel sweats,” or “when the flux rises.” Finally, in a 1938 Machenery’s Handbook I found a description of “rising flux” in the section on heat treating high speed steel. “Oh, I’ve seen that,” I thought immediately. I set about being able to get the steel to show this with dependable repeatability.
When you harden small tools, the first thing you need is a good heat source. I use a Goss propane torch with a BP-5 head to get a flame pattern that’s about 2 ½” in diameter. Here’s the torch:
Stuff on the Internet often suggests using a MAPP torch. Don’t do that!! A MAPP torch has way to small a flame pattern. The flame pattern is a pencil point and the flame temperature is only about 100° hotter than propane. If you insist on using hand-held tank/torch units, use two propane torches with flame spreaders attached.
If you use a MAPP torch you’ll most likely overheat areas on the surface. If you do, you’ll end up with pitted steel; at the least, you’ll warp the steel.
We use O-1 steel and quench in peanut oil. Here’s the set-up in our shop but the vent hood isn’t visible in the photo. If you don’t have a vent hood, do this outdoors.
There are some important things that need to happen for this to be dependable. First the steel has to be clean. It it’s old steel it need to be lapped to make sure no rust or pitting remains. Secondly you want to preheat the steel. In the video, when the iron is moved to the quench, you can briefly see an iron behind the working area preheating.
A brief description of what you see in the video: When the steel makes the phase change there’s a volumetric change. It’s like water expanding as it freezes but tool steel shrinks slightly in the phase change to austenite. In the austenitic state the carbon has combined with the iron to form a crystalline matrix but the carbon is basically in solution and is free to flow through the steel. At this temperature the steel is above the flash point of carbon and the carbon works its way free to burn off. Each carbon atom that leaves the austenitic crystalline matrix leaves behind iron that’s no longer part of the matrix and is no longer in its shrunken state. It rises to the surface and forms small pools or puddles. A link to the video:
After hardening the steel is too hard and brittle for good use. You then need to temper the steel. I bake it in an oven at 350°F for 45 minutes to an hour.
It’s important to know when the phase change happens. The finest grain steels are created by heating only to the critical point for phase change and not kept at critical temperature for longer than necessary to get a uniform heat soak and complete phase change.
This is the most accurate method I’ve been able to find. Steel loses its magnetic properties below the actual critical temperature. Critical temperature is determined by the actual carbon content of the steel and specification tolerances for O-1 steel allow for a 15% variance in carbon content. When I use our furnace I have to assume carbon content is on the high end and end up heating to a higher temperature than I likely have to.
We heat treat tools that already have the bevel formed. In the video you can see the iron being moved in and out of the hottest part of the flame to keep from overheating the thin steel at the cutting edge.
The small pools of pure iron left on the steel are soft and about a molecule thick. They’re easily removed on honing stones.
You’ll also notice some honeycomb ceramic elements under the steel. I salvaged these a number of years ago from a propane barbecue grill put out for trash pickup. These are really helpful for helping heat the back of the tool as its face is heated. I didn’t pay any attention to the make of that grill. Here’s a photo, maybe someone will recognize it and know the make of the grill: