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.