Electrical Heat Treatment Oven

I am making knifes as a hobby. For hardening my first knifes a used my old charcoal grill. It works, but was not that precisely and also not really comfortable.

So I desiced to build my own electrical heat threatment oven. And this process I will show you here:

• fire light bricks (number depends on oven size)
• kanthal wire
• PID controller
• K-type Thermocoupple
• solid state relay with heatsink
• high temperature refractrory cement
• ceramic connectror
• some electrical cable
• power switch
• some hand tools (drill, saw, grater)

• Optional: PC fan with 12V power supply
• Optional: wood for the platform

In my case, I get the old kiln from my aunt, witch gives me most of the parts for my oven.

I get the old kiln from my aunt. So I decided to use as much as possible from the kiln. To do that, I have to disassemble everything.

I want to use the old kanthal wire. To remove it, you have to be very carefull because it breaks very easily. By heating up the wire with a torch you can make it softer, so it will bend and not brake. By heating up the wire I remove one layer after the other.

At the end. I have enough bricks to build the oven and at least one kanthal wire which is long enough.

At first, I need a big base plate. Therefor I used parts of the old base plate. I marked the saw lines and used and old bucksaw. You can use a saw which is specially made for cutting these bricks. But I don't have one and also I didn't want to buy one. The old bucksaw works fine, but after all that cutting, the saw is really done.

My base plate is around 20x10 inches, but you can scale it however you want. Important point is here, larger oven needs much more time for heating up, so you need more power or more time.

After the base plate is finished, I cut all off the bricks to size. First cut the ends off to get rectaangular bricks, then cut to the right length. With high temperature refractrory cement I filled up the gaps in the base plate to get a smooth surface and I also use it to stick the bricks toghether.

As next step I put the kanthal wire in the slots. Again be carefull to not break it. I drilled two holes in the brick on the backside to put the wire throught. There will be the connection to the control unit by using the ceramic connector.

My oven sits on a wooden plate. I also build a frame out of wood that holds everything together. If you are able to work with metal, you can also use angle iron or something like that. I have no welding machine, so wood was easier. I put two more layers of thinner bricks between the inner bricks and the wooden frame because I am not sure how hot the bricks become on the outer side. So to be safe I made the walls thicker. These layers and also the bricks on top where only stacked without using cement. You can fix them with cement, but I think it is not necessary. By looking on the picture you can see some gaps between the stones. I think and hope this does not effect the function of the oven. The gaps were a result of the free-hand cutting of the bricks. By using new bricks or spend some more time on cutting really straight you can avoid the gaps. But in my case, I can't cut off more material to get 90° cuts, because if I do that the bricks become to small.

After the oven is build up, we have to focus on the electrical part.

IMPORTANT: The oven is running with 230 V (I live in Europe). Working with mains voltage is really dangerous. Make sure you know what you are doing, or ask someone who knows it.

I made a small case out of 15 mm plywood. My front panel has cutouts for the PID controller and the main switch. The back panel and one side panel have cutouts for two pc fans for cooling.

In the picture you can see a small 12V power supply for the fans, the solid state relay (SSR) and the PID controller. Two cables run out of the box. One to the K-type thermocouple and one to the kanthal wire.

For cooling, the SSR is mounted to a heatsink. The two fans blow air into the box and the top panel has cutouts for air out

As small explanation: On the PID display you can set a temperature. The PID controlls the SSR and by doing that, it controlls the power that goes into the kanthal wire. With the connection to the thermocouple the PID knows the current temperature inside the oven which is needed to control the turn on and turn off times of the SSR.

In the picture you can see the stand made out of 2x2 inch. Its not the most prettiest work, but it is stable enough to hold the oven. I am not sure how heavy the oven is, but I think around 50 kg, so it really needs a stable platform.

At the moment on the intermediate level lays the door for the oven. Later this should be the palce for the quench tank, and some tools you need for heat threatment.

The door is also made out of the fire light bricks, 4 inches thick, with a frame out of wood. For the first test a take some bricks and stack them infront of the oven opening. Problem is, after a while they get really hot and that makes it difficult to remove them. So I put all bricks togheter and with a handle you can remove it without burning your hands.

After putting all componets together, I have to test the oven.

At first, I set it to just 80°C and put a meat thermometer in the oven to compare the displayed temperature to the real one. Result looks good, so the oven can be heated up in steps, 300°C, 400°C. Works just fine. At the end I set it up to 850°C. This will be the most used temperatur because you need this temperature for hardening carbon steel. It works really fine, the oven reaches 850°C after 15-20 mins. Of course you need to hold the temperature for some minutes for full heating up the bricks. But I think for my use, around 15 min is ok.