Heat Treat oven window?

[JTknives]

While I was building my heat treat oven I had wondered if there was a way to have a viewing window. From my research it seamed like fused Quartz glass would be perfect because it will handle any temp we would use. The idea faded from my mind as time went by but today it popped back up. So I did some research and come to find out paragon and evenheat both offer Windows on there kilns. There window appears to be a single pane of Quartz.

But my thought was to do a double pane set up to isolate the heat better assuming that would help. I ran the numbers and it appears that air is quite a good insulator. To give you an idea how good an air gap insulates here it is compared to a K23 fire brick. Air has a thermal conductive of .0825w/mk @ 2000°. A K23 fire brick is .24w/mk @ 2000°. So as you can see air is much better insulator the the brick but the trick is to keep it from circulating and keep it still. I have some 1/4" thick round fused Quartz Windows that are 3.5" diamater that I think would be perfect for the job. My thought is to put one on each side of the fire brick and inset them flush to the brick. Then use high temp furnace cement to hold them in place. This would give an air gap that's very close to the same thickness as the brick which when comparing thermal conductivitys is a must.

So everything looks promising but there is one little catch that I'm wondering about. The window might be a good insulator but it does not insulate any radiated heat. Kinda the same thing as you feeling the Suns warmth standing inside your house. I wonder if that could create a problem in keeping the ovens temp even.

What do you think, it must have some merit if it's an option on kilns but I have never seen it on s heat treating oven per say.

 

[Drew Riley]

Maybe make a second door with a window but keep the original just in case?

I'd probably coat the inside of the brick holes with some kind of refractory cement as well to "clog" the pores. If you're really feeling salty, you could make an insulated window cover. Maybe a hinged lid with some ceramic wool in the inside. You could even inset the outer window 1/2" or so. You don't really need continuous viewing, but it would be nice to sneak a peak without having to open the door.

 

[nickandersonart]

I'd also love the ability to see inside the oven as things are coming to temp. Maybe I'll have to try out something similar to this. Good idea on the quartz glass

 

[Lieblad]

Aside from overwhelming curiousity to see, I think its kind of silly.

Just insert the workpieces & rely on your instrumentation displaying temperature etc.

 

[JTknives]

Only problem with that is the pid does not tell you the temp of the blade. It only tells you the temp of the tip of the thermal coupple. Now given enough time all things inside the oven even out in temp. But it takes time for a cold blade to come up to temp and that's why I check on it. Once it's at temp I start the clock.

 

[Stacy E. Apelt - Bladesmith]

Do what is done on big kilns and boilers with a viewing port - make an insulated cover that flips over it when not viewing inside. A flap with an inch of kaowool glued on it should work fine. Lift it up to peek, let it flip down to cover the port.

I have several 12X10" sheets of heavy special glass that were used in some sort of government viewing windows in NASA research (its great to have scientist friends). They are gold anodized on one side and very dark quartz glass. They were viewing ports to protect the eyes from intense IR and UV. For all I know, they were test windows for the space shuttle. I plan on making a hinged arm with one that can be swung in place to view the interior of the forge for visitors with much less danger to their eyes. Sort of like a large welding view shield.

 

[DevinT]

I agree that this is a good idea. You will learn a lot by watching the behavior of things heating up. Definitely would improve heat treating in the home shop.

Hoss

 

[Ken H>]

Only problem with that is the pid does not tell you the temp of the blade. It only tells you the temp of the tip of the thermal coupple. Now given enough time all things inside the oven even out in temp. But it takes time for a cold blade to come up to temp and that's why I check on it. Once it's at temp I start the clock.

You're right, PID only shows temp at tip of TC. That's why I put a couple of firebrick on bottom of oven which puts my blades (mounted edge up) about an inch below tip of TC. These firebrick require longer for oven to heat up, but once at temp it does seem to be more stable.

Ken H>

 

[Mahoney]

If you can find gold half-mirrored quartz glass, that would do the trick. Gold reflects IR. The "see-through" oven I worked with long ago at JPL was made from a gold half-mirrored Quartz tube. The closest I've seen available commercially are gold plated welders lenses, but I'm pretty sure those are just glass not quartz. It might work with the quartz glass on the inside of the oven and the welder's lens on the outside, gold surface towards the heat. If you go this route, be sure what you are getting, the last couple pair of 50mm lenses I bought turned out to be gold mirrored polycarbonate, fine for the cup goggles, but won't work as an oven window.