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Heat Treat Oven Build + Kanthal Wire Calculator


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So, I guess this is why it is best to let the professionals handle the calculations. For some reason the ones I came up with were slightly off. Also, they said the SA value for this application should be in the 4.5-9.5 range for faster element heat up times. We are only talking a few minutes here, but it matters. However, lower SA values do increase element longevity. So, I decided to go with their recommendation and with the APM wire. I for sure could use A-1 and the lifetime would be less, but apparently not as much as the Kanthal rep stated. Who really knows. For anyone interested, here is the final calculations from the folks at Hyndman, (resistancewire.com).

 

 

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Edited by Brandon Bearden
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Most of this has been over my head, and if it works I will pin the heck out of it, but I have a question:  What steels are you wanting to austenitize in this thing?  I ask because some stainless alloys need to get to or slightly above the application temperature listed above.  As long as you're sticking with carbon steels and low-alloy stainless steels it should be fine, but there are a few exotics out there that need to soak at 2100+ F.  

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I want to have the ability to heat treat any steel. Just finishing reading Knife Engineering and 2150 is about the max of any knife steel. This furnace will operate safely up to 2200. It should be rare, if ever to go that high. I think 2100 is about the max needed for 99% of steels. I could be wrong though. 

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23 minutes ago, Brandon Bearden said:

I think 2100 is about the max needed for 99% of steels.

 

It is, I just wanted to make sure you didn't have a specific exotic super-stainless in mind.  My Evenheat will do 2200, but they warn against it.  Apparently it significantly shortens the life of the elements to go much over 2100, but I also don't think they used the fancy resistance wire you have.  K-type thermocouples don't last long at that heat either, even though they are rated to go to 2400.  They'll last forever at more reasonable temps up to 2000F. 

 

Speaking of which, have you thought about adding an argon purge option?  Electric ovens are an extremely oxidizing environment, which is what necessitates the use of anti-scale goo or stainless foil packages.  A slow flow of inert gas through the kiln can prevent that.  It can also mess with the elements until they have a good oxide layer built up, but for your purposes of making this hot rod of a furnace it seems like a possible add-on.  Especially if you use shielding gas for welding anyway and keep it on hand.

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I do have argon in the shop and I have been thinking about this late at night while I can't sleep :blink:

 

I was thinking of two things, one, using a propane torch to burn off the oxygen or using argon to fill the chamber. 

 

The issue isn't getting that done at the beginning, the problem is when treating more than one knife. Say you have 6 knifes in there, which is kind of the point of building this thing so large... so you open the top to grab the first one, I would assume there would be an in rush of fresh air as the hot air escapes. Knowing how fast scale forms, I don't think I could open it, get something out, close it, purge it before the oxygen had time to do its thing. Also, I can't do anything but quench the steel right away too... unless it is a steel that has a few seconds to wait for quench. 

 

I have not researched it though. I don't know if argon sinks or rises, or how fast it would dissipate. I just Googled the first bit, argon sinks. Same for the burn off of the oxygen, I have seen than done in some youtube video but it wasn't for heat treating steel. I think it was a pottery or glass kiln. 

 

Any thoughts on this would be more than welcome. 

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On 10/14/2022 at 10:07 AM, Joël Mercier said:

Let's say at an average aus temp like 1475°f, how far off would copper wires give you false readings? Are we talking 1-2° or 10-20°?

Joel,

 

Sorry, can't really answer that.  I think it may have to do with a number of factors, like type of compensation in your controller, length of wire, temperature of the room...  I've always just taken the easy route and used the proper thermocouple wire the manufacturer recommends.

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So, I took the model quite a bit further today. I haven't incorporated all of the components or the frame just yet. But all the major layers are there including the ceramic bars that will support the baffle/shelf. I didn't put the bars in that will support the work pieces yet either. 

 

My question is this... hexagon or octagon. I cannot decide which I like better. There are two major differences besides the shape. #1) The octagon has 27% more volume, though this is not usable volume, just air. The actually rectangle hole in the baffle is 5" x 10" on both models. #2) Materials. The octagon, due to the increased surface area and volume, requires more materials to build. Also, due to the shape, there that many more angles to cut for the framing and structure in general. Basically, this will cost more and be more difficult to build.

 

Here is a side by side rendering. Let me know your thoughts!!!

 

 

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Octagon version...

octo side view.png

 

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Hexagon Version...

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hex top x-ray.png

 

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hex inside looking up.png

 

hex xray side.png

 

Edited by Brandon Bearden
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Meh, I decided to go with the hexagon shape even though I like the looks of the octagon better. I added some more components, thought out the lid hinge, lid switch, handles and base. I just need to decide where to put the thermocouple. I think most put it outside the control box and run the wires into the box and then to the controller. I was originally thinking to allow the thermocouple to enter from within the control box. It would be going through 4" of wool and 2" of CFB. I am just thinking everything out and then I will break this CAD into a cut sheet and component drawings. Then, it is just a matter of hacking, cutting, beating and banging everything into the predetermined shapes. 

 

Any feedback would be great.

 

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I've been away for a few days, and am just catching up on your progress.  

 

WRT the temp sensor location:  You could poke through the back wall of the electronics enclosure, but that would make me nervous.  I'd prefer to have an unbroken layer of insulation there.

-Brian

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I thought about that. I am going to put the thermocouple outside the control box, but allow the resistance wire to enter because otherwise, I have to build another box to protect them from being touched accidentally. Here are some renders of my final drawing. If there are any comments on the design, I would be happy to hear them. From here, I am going to part everything out and build the templates to begin building. Though, that will have to wait a couple of weeks due to work.

 

There is the path the wire will take laid out flat, a view of the components all wired up, a view of how the thing will look put together and a couple of size perspective shots.

cfb kanthal path.png

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kanthal path top view.png

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So, I printed out the layout sheets and began cutting the CFB on the table saw with full PPE, dust extractor and dust filter running. With this setup, there was no visible dust at all. I know it is there of course. Afterwards, leave the dust filter running and open the garage doors. 

Complex Wall.pdf Inner base and lid.pdf Simple Wall.pdf

 

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Edited by Brandon Bearden
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Two more pics. I went a little further and figured out a stellar way to cut the grooves. Japanese pull saw and a chisel. Fast and easy. I think I will be able to put all of the grooves in within five hours. It dry fit in the hexagon shape nicely, due to my uneven floor and the fact it took two hands to get it to seam up, I could not get a good pic.

 

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I have had good success cutting grooves in soft brick using a drill press (this may also work for ceramic board).  You can set the table at an angle and use a stop to keep the groove straight.  You can either make up a custom cutter, or just use an old drill bit.  If you grind it with a radius on the bottom you won't get those sharp corners that may be crack initiation points for your insulation.  In any case, please use a respirator while cutting this stuff.

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I see you're in a dust suit, which is good.

 

I had another thought, which is rarely good:  when Dan mentioned brick, it occurred to me that commercial kilns use brick not because it wears better, but because it acts as thermal mass to even out fluctuations once it's up to heat.

 

I know you designed this to heat fast and to cycle fast. Do you think the board will provide thermal ballast, as it were, to save a little wear on the relays?  It's good you're using ssrs, mechanical ones would sound like a band of castanets!

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I think that "commercial" kilns use brick because it is cheap. However, I would say "commercial" in this sense means consumer commercial. For local shops and such. They cannot afford CFB. The thermal coefficient of CFB is much lower. I think it will just work better. I noticed in my research that industrial ovens use CFB. As for the thermal ballast idea, yes, that is a much more intelligent way of saying what my intuition told me.

 

Time will tell. Perhaps I will be tearing this all apart in two years and building it with brick. Good point though.

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3 hours ago, Dan Hertzson said:

I have had good success cutting grooves in soft brick using a drill press (this may also work for ceramic board).  You can set the table at an angle and use a stop to keep the groove straight.  You can either make up a custom cutter, or just use an old drill bit.  If you grind it with a radius on the bottom you won't get those sharp corners that may be crack initiation points for your insulation.  In any case, please use a respirator while cutting this stuff.

 

I like this idea, the only thing is that I don't plan to use any staples unless absolutely necessary. curved profiles would necessitate that unless there was a flat bottom and that would require cutting a grove and following with a properly sized bit. Also, the bit would need to be something like a hole hog to reach the depths. Unless of course it was like a 3/4 hold and you fed the wire through, that would be impossible I think with this design. Also, I have designed it to be replaceable, and I wouldn't be able to do that after it is assembled with 3/4 depth holes.

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If you've got a way to deal with the dust, and it sounds like you have, don't faff about with pullsaws and stuff: use a router. I've built several HT ovens, mostly using IFB. The first one or two I used a saw to cut each side of the grooves, broke out the middle bit and filed in the bottom of the grooves with a length of bent studding (allthread).  That got really old, really quickly. For number three, I used a router and have done so on every one since. Where I've used CF board, it's been for doors and roofs, with no grooves in the board, but I don't have good enough dust control to feel comfortable routing the fibre board.

 

I have to say that my experience has been that you really do need the staples. I keep the staple spacing to under 3", using a 10mm or 3/8" OD coil of Kanthal A1, initially in 16AWG and later 1.6mm (about 14 AWG). When I've tried to get away with wider spacing, things have not gone well.

 

I am fairly certain that 12 kW is going to be waaay too much for realistic control. I'd suggest 2 x 6 kW coils in parallel to give the option of running them in series at 3 kW total if you find it's the case (I don't expect my opinion to make any difference to your using the 12 kW initial setup). I'd keep the cycle time as short as possible and use SSD(s) for switching. I use a 2-second output cycle time. 

 

Your thermocouple looks like a 6mm/1/4" Mineral Insulated assembly. I'm guessing it has an insulated junction? With 12 kW of heating, I'm inclined to think the thermocouple response will be too slow. The control thermocouple needs to be the fastest-responding thing in there. If it responds slowly and damps out the temperature fluctuations, you'll get the temperature at the edge of the blade (lots of surface area, very thin, so minimal thermal mass) going up and down with the power on/power off cycles of the elements, with the damping effect of the slow-responding thermocouple only allowing the controller to see the average temperature over the last ten or twenty seconds. I usually use a 3mm or 1/8" Mineral Insulated type N thermocouple (developed to be an "improved type K", with much less tendency to drift) with grounded junction to get a fast enough response (and I "only" use 3 kW of elements for a 23" or 28" HT oven). Sheath material is either the proprietary "Super Omegaclad XL", when the budget allows, or type 310 stainless when I can piggyback onto an order through work from a local supplier. The first run to high temperature gets a black Oxide layer on the sheath (to get it to normal operating condition) and the second run (from cold) is the tuning run.

 

I should note that I build my HT ovens to handle both Austenitizing and tempering temperatures and I am very careful to build/tune for minimum overshoot at tempering temperatures. If you are only looking at using yours for Austenitizing temperatures, this will be much less of an issue. 

 

 

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So, I think the pull saw will actually just be easier and better in the end. I enjoy hand work. Dust control for a saw is one thing, for a router is another. I don't have a good router table with good dust collection. I have a cheap one. 

 

I will highly consider the staples. What happens when you don't use them? It just snakes out?

 

You may very well be correct regarding the wattage. I hope not, but we will see. I can always go down in size, at a cost.

 

I switched to a different thermocouple with an exposed junction and Omega Clad a few days back. :) - Let me know if I still messed this decision up. It will take a few weeks for this to arrive.

 

I am putting a lot of faith in the controller I have chosen to be able to handle the control. I do want to be able to heat treat and austenitizing with the different programs.

 

Thank you so much for the comments. I really do appreciate them.

 

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Mainly, they snake out.

 

I have had one migrate lengthwise though: I'd missed with a couple of the staples and probably had about 6-7" between the ones that I'd got right (I blame age, poor close-up vision and a lousy viewing angle, but mostly age). The coils moved and bunched up much closer towards one end (staple) than the other. I was on 16AWG elements and a fairly narrow groove: probably one of the first couple I built with the filed-in grooves. I don't know what the mechanism was that made them bunch up, but I'm guessing that inconsistent groove width played a part, and that the movement occurred during heat/cool cycles at/between uses, rather than during a HT. They didn't close up enough to short out coils, but there was obviously more heating where they were close together and, given that I was trying for absolutely even heating, it was enough to register that I didn't want to do that again.

 

I don't think you'll have any particular problem with an exposed junction: Evenheat et al have been using Exposed junction type K thermocouples for a long time AFAIK. Type K drift doesn't seem to get considered much. I am acutely aware of it because my day-job involves processes with long periods at high temperatures. We used to control 24/7 at 1200 degC (2192 degF) with type S thermocouples back in the 1990s, but reduced to 1000 degC (2012 degF) for reduced NOx emissions by the early 2000s. We did some work to compare Types K, N  and S at 1000 degC. Type N were MUCH better than Type K, in that they agreed closely with the type S for months, where the type K were starting to show drift within days. The type S, being Platinum-based, were about ten times the price of the Nickel-based type K or N. We settled on type N. 

 

HT ovens are a different process, I have to say, but I can't ignore my experience at work and I use Type N for HT ovens. As I understand things, Type N was developed for use in MI assemblies, along with the Nicrobell sheath material. I was wrong in my earlier post: I get Type N thermocouples with Nicrobell sheaths from my local supplier. I get type K with the 310 Stainless sheath for use in forges (1300 degC seems to be a good welding temperature for bladesmithing beginners and is the upper limit for type N. Type K will go up to 1372 degC and is therefore a bit more useful for tuning forges).    

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Sorry, I have been traveling for work the past two weeks. That is good to know. Interesting to read about the thermocouples. I do have a type C on hand, but it requires a reducing or inert atmosphere. I will try to get my hands a type N and compare the K over time. 


@timgunn

 

One thing that is vexing the heck out of me in my design is how to support the "shelf". I was thinking of some sort of ceramic type rods beneath, seen in some of the cads, but sourcing those has proven very difficult and pricy. I haven't worked with CFB before, but I am wondering if I dado the shelf into the walls, will it hold on it's own accord, without any help? What about just a rod running from the floor to the shelf on either side? I know the pottery kiln shelfs utilize these principles. 

Edited by Brandon Bearden
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Finally got back to this project after a few weeks out of town for work. I have all the grooves in, and cut to height. Now I need to figure out how to cut the bevel on the top. I was going to use my track saw, but would need to build a jig to cut it from the top, pita. I will think on it. Here is a quick pic. It actually only ended up taking 2 hours to do all that. I used a fine saw, (oscillating tool), to cut the last grooves that connect everything.

 

grooves.png

Edited by Brandon Bearden
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