Heat Treatment Oven Build Process
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Posted 16th February 2025
Following yesterday's paint job, I thought it would be good to take the chamber back up to temperature just to check that the paint didn't suffer any ill effects. With the thermocouples sticking out into the air rather than only poking slightly out from the bricks, the temperature is much more consistent:
The top-left reading is from the back of the chamber; the mid-left one is from the front (the thermocouple in the door). It took longer for the temperature at the door to stabilise than that at the rear; I suspect that's a combination of the two simple factors: the geometry of the element slot (a bit more heating at the back than the front) and the fact I'd put a large lump of steel (roughly 12 mm × 100 mm × 100 mm) at the front of the chamber and that's quite a lot of thermal mass.
With the thermocouples measuring air temperature (rather than, perhaps, brick temperature), heating was much more rapid; it reached 700°C in about 15 minutes and got to 780°C (at the back of the chamber) in 20 minutes. That took 28 minutes on the previous test. I think it can be made a little bit faster again. The controller was hunting quite a bit: turning the power down before it had got close to the target and then having to put more power in when the temperature started dropping too early. I'm pretty sure that's a result of having run the auto-tune algorithm with the thermocouples semi-buried: the controller "thinks" there's more thermal mass and hence has to turn the power down earlier. Once it's fully cooled, I'll run the auto-tune again and that should help a bit.
While it was warming up, I took a few more photos, this time with the SLR. They're still not great; I think the dingy overcast day doesn't help and, let's be honest, neither does my lack of photographic talent!
Once it had been up to temperature for quarter of an hour or so, I opened the door and got a photo inside:
In that photo, you can see the big glowing lump of steel I put in the chamber for this test. It was just a random bit of structural steel I dug out of my odds and ends drawer, so I didn't try to quench it once it was up to temperature or anything like that: I just left it in the oven to cool along with everything else. The only reason I put it in there was that I wanted to prove to myself (not that I ever doubted it really) that I could use the chamber to heat big bits of steel. There is no way I would have been able to heat treat something that size with just a blowtorch but the oven makes it possible. I can't imagine ever needing to harden a piece of 12 mm × 100 mm × 100 mm steel, but I can see me wanting to make reasonable thickness (3–5 mm) plane blades in future and if the blades are wide then there's quite a bit of steel to heat up.
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