Recommendation? DIY Heat Treat Ovens - Specific Design Questions

[Cushing H.]

Thanks hubert. Looks like i should go with 1.5” brick on the sides and back cut to 5.5” height, giving chamber dimensions of 6” wide by 5.5” high. Does that sound like it would be functional in your experience? Then maybe 4” of kaowool top sides and back, ending with maybe .5” of board on top of the wool. That is 5.5” of insulation, which might be overkill. Maybe just one layer of kaowool?? (Then again the extra insulation will help with the 15A limitation...) You think the wool will compress too much on the bottom - so just go with brick there? Ill have to think about the door - maybe just wool with a frame to hold it in place...

i have to get my hands on a welder to make that frame...

 

[Hubert S.]

I think the chamber dimensions would work nicely. You should get enough power on 110V. I would use the full size brick and cut off the corners. The bricks can be cut very easily with a hand saw, but it will dull the blade. I used a rusty old PVC saw that I never use for anything else.

I am not sure about the wool on the bottom. I only have board on the bottom. It is 2" thick and strong.

My door is hinged at the bottom and it's just two bricks sitting in a frame. I have two tabs on the frame with pivot holes and a 5/8" rod welded to the door. The rod is bent into an L to make a handle. There is a weight at the end of the handle, just a piece of 2" round bar.

Aside from the larger chamber, the other thing I would do differently is the coil. I went through the side near the controller and have a loop that goes to the back, then other side and back again. I uncoiled it at the back thinking it would get too hot there otherwise. It was a pain to install. I am not sure what I will do on the next one. Probably an easy to remove back panel and a coil on each side. If they need to be in series, I would do that on the outside. That would make installation and eventual replacement a lot easier. I am watching JT's thread to see how the pros do it, though.

 

[Cushing H.]

Before i forget, i would very much like to see your coil winding jig!

For the coils, i was thinking of trying JT’s approach of putting them around ceramic rods - several suspended sideways across the top. That way replacement is easy - just lift off the top bricks and you have full access (but it does decrease the chamber height - though i guess you could carve channels in the top bricks to mitigate that?). The problem is you then have two access points to externalize the wires... but that is ok - this is not a commercial product! (I dont really like the idea of running a wire back inside the chamber - that seems like a short just waiting to happen... along those lines - cant forget to ground the frame!).

id really like to shoot for a side opening door with wide swing to get that hot door out of the way of hands...

 

[Hubert S.]

Here is a photo of the front of the jig (the outfeed side). You can see two concentric holes, the one on the infeed side is the size of the mandrel, 5mm in this case. The one on the outfeed side is the mandrel size plus twice the wire diameter. The front piece is baltic birch, the back a piece of 2x4. You screw them together first, drill the small hole through both pieces, then take apart and enlarge the outfeed piece.

The second picture shows a small channel I cut into the plywood piece for feeding in the wire. This channel goes all the way to the hole.

To use this, you drill a small hole through the end of the mandrel that the wire fits through. Clamp the jig in a vise and feed the wire through the hole in the side and pull out a few inches. Insert the mandrel and feed the wire through the little hole in the end. Chuck the end of the mandrel in a drill and slowly start winding. Pull back on the drill lightly. After the first few windings are established, you can go quite fast, but once the coil gets longer, you have to slow down. I used an aluminum rod as the mandrel, and the weight of the coil made the end bounce around a lot. It helps to have a second person stabilize the end of the rod.

Let me know if you have any questions.

 

[Cushing H.]

Here is a photo of the front of the jig (

Now that is a really cool idea. Thank you!

I would use the full size brick and cut off the corners.

Re. this point: I am thinking I would rather use the thinner brick and have the sides (and top) uniform everywhere to lay the wool flat against.

ON the bottom, I agree that just brick and board is best - make the board thicker for support .

BTW, the website for the retail source for firebrick near me is this (https://ssfbs.com/). they have good data for thermal conductivity and other properties for firebrick, fabric (their version of wool), and board. From those you can do the heat flow calculations to verify outer temperature and wattage requirements for the elements. (if not, at least for the blankets "wool" they supply ready-made "heat flow calculations" - for example with an inner temperature of 2300F and 6 inches of insulation, the estimated outer temperature is 310F. this is just for the blanket - but for ballpark results you could apply that to the sum thickness of all your materials. (which makes me think that 4 inches of wool on top of the firebrick would be a REALLY good idea......)

 

[Hubert S.]

Now that is a really cool idea. Thank you!

I can't take credit for it, I saw this type of jig somewhere a few years ago, but I cannot remember where.

Re. this point: I am thinking I would rather use the thinner brick and have the sides (and top) uniform everywhere to lay the wool flat against.

The bricks are a bit fragile. If you put your coils in the side walls, I would be concerned about the bricks fracturing, depending on the depth of your grooves.

BTW, the website for the retail source for firebrick near me is this (https://ssfbs.com/). they have good data for thermal conductivity and other properties for firebrick, fabric (their version of wool), and board. From those you can do the heat flow calculations to verify outer temperature and wattage requirements for the elements. (if not, at least for the blankets "wool" they supply ready-made "heat flow calculations" - for example with an inner temperature of 2300F and 6 inches of insulation, the estimated outer temperature is 310F. this is just for the blanket - but for ballpark results you could apply that to the sum thickness of all your materials. (which makes me think that 4 inches of wool on top of the firebrick would be a REALLY good idea......)

I am a little surprised by the example, but 2,300F is a lot more than the 1,975F I had mine up. Maybe I have not had it on long enough, but I can touch the outside of the oven without burning my fingers, and I don't have anywhere near 6 inches of insulation.

 

[Mike Read]

One thing I would recommend is to have the top of the frame/box removable and the bricks not cemented in place. It is very useful to be able to lift the lid off and lift out the roof bricks for access to the elements.

 

[Cushing H.]

I am a little surprised by the example, but 2,300F is a lot more than the 1,975F I had mine up.

I dont have the experience to judge one way or another .... but the calculations made a certain amount of sense to me based on JT's commenting that he used an IR sensor to shoot the outside of his bricks and came up with about 250F (as I recall). In either case, calculations like this are based on some pretty variable mechanisms - the hardest one to know being how quickly air around the outside carries heat away. I would view any calculations like this as extremely tentative ... and much more trust measurements (like your fingers or JT's IR thermometer....

 

[Cushing H.]

One thing I would recommend is to have the top of the frame/box removable and the bricks not cemented in place. It is very useful to be able to lift the lid off and lift out the roof bricks for access to the elements.

Oh definitely. I was thinking that to get to the elements to service them, just lift off the top insulating materials and you have full access to the inside of chamber....

 

[Natlek]

Ok ... but my question is: done your way, everything inside of the outer box “floats” ... no cement, everything is held in place by friction (and a tight fit)????

Yes ...floats is right word .
What you want is to trap heat inside chamber, right ?So, I would overlap bricks like on drawing, half inch would be enough...they are so easy to cut so I don t see any reason to not do that .And that will add rigidity to structure if you don t want to use cement.About bottom ,I hope you will not HT train wheel inside Bricks are extremely light so i don think that there should be problem to kaowool to support that weight .Weight would be evenly distributed all over kaowool................For insurance if you want you can cut square pieces from one brick and insert them as support for bricks ..you will easy cut kaowool for them to pass..............I would use 1.5 inch bricks for all side if you mount HT element on top side ...

 

[Cushing H.]

Thanks Natlek - what you say makes sense. I am thinking 4 inches of kaowool - do you think that is too much (I would just assume the outside not get too hot)? I will likely go with 2" bricks in the ceiling (or maybe 2 1/2) because I will will likely notch them out to allow for the heating elements to be recessed....

 

[Natlek]

Well , maybe it is to much....................this is my small tube furnace after half hour on 1200 Celsius .Isolation is exactly 4 inch ceramic blanket .....128kgr / m3. Surface is cold , it heat up after probably one hour I turn it off, it need more then one hour for temperature to drop below 500 celsius

 

[JTknives]

They crack because they have not place to expand ....tighten up with angle iron Kaowool would give them place to move little , think about that .
This is how I would build one if I ever make one .Outside is sheet metal ...............
Maybe two layer kaowool on bottom and on top is better ..........

I’m sorry but I don’t agree with this statement. Bricks will crack no mater what and compression helps keep them from collapsing. If your spanning more then one brick wide you have to put them under compression or support them some other way. The bricks for the rear of my forge are allowed to shrink and expand as thy want and thy are all cracked. The bricks in my oven have a few cracks but are super solid. I personally do not see any practical reason to do multiple layers of anything besides brick. My current oven usually runs for 3-8hrs at a time and the outside usually sits in the 250 on the body. That’s with the temp for aebl and other high temp stainless. There is not a single place on the oven That I can’t touch. Only place I wouldn’t touch is the glass window in the door. But around it you can touch for a few seconds with out burning your self. And this is with only a 3” brick wall

 

[JTknives]

Thanks Natlek - what you say makes sense. I am thinking 4 inches of kaowool - do you think that is too much (I would just assume the outside not get too hot)? I will likely go with 2" bricks in the ceiling (or maybe 2 1/2) because I will will likely notch them out to allow for the heating elements to be recessed....

I would not go with thin bricks. Your just inviting problems with bricks cracking and collapsing. Also I would not notch the roof bricks for the coils. There are precise formulas to calculate the surface loading you can have on a coil. As soon as you put a coil in a groove or enclose it you have to derate the coil surface loading. You do have to derate the coil if it’s going to be supported by a rod but if you also tuck It in a groove you will have to derate it even more. It has to do with the max surface temp the wire will achieve. The more refractory around it the higher the wire surface temp will be. Also if you use a standard SSR with on off control you have to derate. If you proportionally control the coils you do t have to derate as it’s not on off pulses. If you need or want help let me know.

Yes ...floats is right word .
What you want is to trap heat inside chamber, right ?So, I would overlap bricks like on drawing, half inch would be enough...they are so easy to cut so I don t see any reason to not do that .And that will add rigidity to structure if you don t want to use cement.About bottom ,I hope you will not HT train wheel inside Bricks are extremely light so i don think that there should be problem to kaowool to support that weight .Weight would be evenly distributed all over kaowool................For insurance if you want you can cut square pieces from one brick and insert them as support for bricks ..you will easy cut kaowool for them to pass..............I would use 1.5 inch bricks for all side if you mount HT element on top side ...

overlapping is all well and good but really all it’s going to be good for is securing bricks in place. The heat is moving through the bricks with conductivity through the material. It’s not running around looking for cracks it can squeeze through like a mouse. I don’t know what your experience with heat treat ovens are but I generally get the feeling that you are just making stuff up some times. I apologize if I am out of line and I’m not trying to offend you. But you have to remember people here will take what you say as fact if thy are new and inexperienced. I did not go to school to do this or just search google. I have real world experience as a heat treater and have heat treated thousands of blades. I’m not trying to toot my own horn I’m just trying to give the guys a bit of what I have learned by actually doing it and building my own ovens. So no hard feelings, We are all friends here.

 

[JTknives]

Oh and here is a quick video showing the temp of my oven after running 3-5hrs at 1950°

 

[Cushing H.]

Jt - thank you. I was hoping you would enter in here. Could you please supply more information (or places to go look) regarding derating of coils in contact with slots or rods? On one hand it makes sense, but on the other hand, resistance of the wire (and thus power output) is pretty solidly defined ...and that energy surely just bounces around (via radiation) the inside of the chamber? I am sure I am missing something as I do not doubt you- please clarify?

 

[Natlek]

I’m sorry but I don’t agree with this statement. Bricks will crack no mater what and compression helps keep them from collapsing. If your spanning more then one brick wide you have to put them under compression or support them some other way. The bricks for the rear of my forge are allowed to shrink and expand as thy want and thy are all cracked. The bricks in my oven have a few cracks but are super solid. I personally do not see any practical reason to do multiple layers of anything besides brick. My current oven usually runs for 3-8hrs at a time and the outside usually sits in the 250 on the body. That’s with the temp for aebl and other high temp stainless. There is not a single place on the oven That I can’t touch. Only place I wouldn’t touch is the glass window in the door. But around it you can touch for a few seconds with out burning your self. And this is with only a 3” brick wall

If bricks crack ,something is wrong with that oven .Why they don t crack in my friend oven ? That oven run non-stop from Monday to Friday ? What you and me think is irrelevant my friend.What think and do engineers who construct professional HT furnace matter .And they use additional insulation .....and I understand WHY .Do you understand that to ?
Actually ,most manufacturer don t use bricks anymore , ceramic fiber board is most used in building as hot face ........and I understand WHY !
What would change in your HT oven if temperature outside is 80 F instead of 250 ?

 

[Natlek]

overlapping is all well and good but really all it’s going to be good for is securing bricks in place. The heat is moving through the bricks with conductivity through the material. It’s not running around looking for cracks it can squeeze through like a mouse. I don’t know what your experience with heat treat ovens are but I generally get the feeling that you are just making stuff up some times. I apologize if I am out of line and I’m not trying to offend you. But you have to remember people here will take what you say as fact if thy are new and inexperienced. I did not go to school to do this or just search google. I have real world experience as a heat treater and have heat treated thousands of blades. I’m not trying to toot my own horn I’m just trying to give the guys a bit of what I have learned by actually doing it and building my own ovens. So no hard feelings, We are all friends here.

I never take post from other members as offend , don t worry about that...........
The heat is moving trough the bricks with conductivity through the material ............Right ! But there is other mouse ....small one and that is HOT air inside .How you will stop it to go out from oven? WHY temp .drastically drop when you open door on your HT oven ? If hot air goes out ,cold one will replace it ..THAT s one reason why every decent HT oven have some kind of solution for sealing door side . Overlapping bricks,cement , help to trap that air inside ...At the end your lovely bricks work in same way ....You have high temperature resistant material in which are trapped billions of bubbles with air and exactly that AIR is your REAL insulation in oven you build . (W·m−1·K−1) of air is 0.026 .
BTW , I m mechanical engineer who chose to work with automobiles ...with everything that have combustion engine

 

[Natlek]

I don’t know what your experience with heat treat ovens are but I generally get the feeling that you are just making stuff up some times.

What is that I'm making up ? Some example from my previous post ?
Maybe my advice that additional insulation is useful ??

 

[Natlek]

I’m sorry but I don’t agree with this statement. Bricks will crack no mater what and compression helps keep them from collapsing. If your spanning more then one brick wide you have to put them under compression or support them some other way. The bricks for the rear of my forge are allowed to shrink and expand as thy want and thy are all cracked.

It s gas forge , right ?Maybe they can t handle thermal shock when you fire it up ?