Heat Treat Oven
- Firebrick Box
- Metal Case
- Electrical Wiring
- Electronics Box and Final Assembly
- PID Controller Configuration
- First Heat Treatment
- Update 11/19/2014 -- Still Working
I had an aluminum project box lying around that happened to be the same size of a flashing rectangle. I built a little tower to keep it up off the main box for heat reasons. An AC fan and SSR heat sink go on top to keep things cool.
In the back is a power switch, fuse holder, thermocouple ingress, power cord, and air vents. The rear square of firebrick is lightly cemented in so it can be removed if necessary.
I repurposed a rolling kitchen cart to be able to wheel this out of the garage when I need to use it.
On the front is (left to right) power LED, relay closed LED, the PID controller, and a manual relay bypass switch. The bypass switch is so I can screw with the PID controller without the coils going hot. Note that the orange LED is hard to see -- probably should have used a different color. The red isn't super bright but it's good enough.
This is a Mypin TA4 PID controller. The manual is very brief so if you don't know what a PID controller does (I didn't) nothing will make sense. All the settings are in PID terminology. The Wikipedia article on PID controllers is very useful to learn the terminology.
The first thing I had to do was calibrate the thermocouple. It was reading around 45 F too high. I determined this by measuring ice cubes and boiling water. You compensate for this with the PVF setting.
The TA4 supports an autotuning feature. It isn't clear from the manual but from what I've gathered this is a common feature of a PID controller where the PID constants are chosen by the controller following a characterization of your system. Basically you wire everything up, tell it to autotune, and it turns on the heat for a short time to figure out how the system responds. The alternative to this is to use manual mode to use your chosen settings.
Important note about a Mypin PID controller bug: at 1275 F the PID controller seemed to exhibit a bug of some sort. It would display 1273, 1274, 1275, 0000, cut off the relay, drop down in temperature to around 1273, and repeat over and over. On a whim I switched it to Celsius mode and it had no such issue getting up to hardening temperature.
First victim for this device is a paring knife I made out of 1/16" 1095. It's my first knife and I didn't put more time into it than I had to. I mainly wanted something I could test with. Heated to 1475 F (800 C), kept it there for 10 minutes, and quenched it in ~130 F canola oil.
Lots of decarb! I quenched it in ~130 F canola oil. I'm 90% sure the warpage of the handle occured during the quench. The handle had some large indentations I accidentially ground into it, this could have something to do with it. I was able to straighten out most of it.
And that's the finished product. The hardening and tempering (at 450 F -- ~ 60 RC, in theory) both seemed to work fine. The knife was tougher to sharpen than my stainless kitchen knives but took an edge and chops things just fine.
Oven has held up for 2 hardening cycles and several tempering cycles without issue. It takes about 70 minutes to get to 1500 F and about that long to cool down to a tempering temperature.