Heat Treating In a Coal or Gas Forge, One Approach

Last Revised: October 22, 2023

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The Question: (How to heat treat steel in a gas or coal forge)

Recently a guy who lives on the on the banks of the Suwannee River in Mayo, Florida emailed me to tell me that he had just built a "Hans Peot" gas forge and asked me how I control temperatures for heat treating. He has recently started making knife blades and has sent them out somewhere for heat treating, with disastrous results, according to him.

Here's my reply.

(By the way, I'm not a knife maker)
You'll see that I talk about both gas and coal (coke) forges:
"Gene, I live close to the MPLS/St.Paul MN area. There is a company there called "Minnesota Clay" that sells a 2500 degree pyrometer for right around $100. It uses a 14 gauge C/A (Chromal/Alumel thermocouple that is protected by ceramic beads. It will withstand quite a bit of use in that gas forge.
When I have had to do some fairly accurate heat treating in the past (like gun parts and machine tool parts), I have used a piece of water pipe large enough to hold the workpiece in my COAL FORGE as follows: I build a good fire in my coal forge. Once I have a good bed of coke formed, I place the pipe down into the coke and cover it with lots of fresh coke. I place my thermocouple into the tube along with the workpiece and cover the ends with more coke, leaving a little peep hole.
I then use the blower to gently bring the heat up and to keep it at the desired temp. You'd be surprised at how evenly you can hold the heat this way----for example, 1450 + or - 20 degrees. I can also use the peephole to see that the workpiece and the wall of the tube are at the same heat; the part almost disappears when it is at the same temperature as the pipe wall.

I am sure that the gas forge will work well, too. Just keep balancing the air with the gas so you keep a neutral flame. With something as low mass as a knife blade, I'd use the pipe idea and maybe even add some mass inside the pipe along with the blade to help keep the temp even."
Of course it's up to you to be ready to snatch the part out of the pipe and get it to the next stage (quench or whatever) in good time.

The benefit of this process is that you can SOAK the parts as long as needed to thoroughly and evenly heat them.

The Problem: (Can't get a hole drilled in a small cast iron frying pan)

A guy brought me a small (4" square) cast iron frying pan that he wanted to drill a hole in.
He is going to add a clock mechanism to it and needs a well centered 5/16" hole. He couldn't get a nice sharp drill bit to cut at all.
I double checked by reducing my drill press speed to 150 rpms and used my favorite sulphurized cutting oil, but no luck AT ALL!

After googling around a bit, my thought that the part had been cooled too quickly when cast was confirmed. I don't know much about cast iron, but I think the term is "chilling".
So I thought I'd treat it like a piece of tool steel that needed to be annealed. But from what I had read, getting all that carbon out of CAST IRON's iron grains takes a long time.
So, I built a good fire in the coal forge and did some other needed work while the forge heated up. Then, when everything was good and hot, I made sure I had a good bed of coke and then laid the frying pan on the fire.
After covering the pan with 2 or 3 inches of coke, I warmed it slowly until I could see it glowing.
Then I stuck a thermocouple in there at about 1PM. I added a 6 inch layer of fiberglass insulation to that and blew slowly until I got to about 1500 F, then walked away.
5 hours later, the temp was about 1250 F After 2 more hours, I still had about 900 degrees. I left it overnight and had 200 F in the morning.

Then the drill bit cut readily into the pan. Just like you'd expect cast iron to work.