Maximum required furnace temperature

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Mstrick96

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Mar 6, 2018
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Well, I built a heat treat furnace and during testing managed to burn out a heating coil. I was at about 2100F. My heating rate was about 5 minutes to get to 1000F and about 25 minutes to get to 1500F.

Unfortunately, I tried to see how high I could go and burned the heating element out.

I know this is a higher temperature than I need, but I was stress-testing the furnace to see how high it could go. I want to design for more capacity than I will need. My goal was to get above 2000F which I achieved. I know that is at the limits of the type k thermocouple and (obviously) of the heating element.

My question now is what is the maximum temperature I should design for to heat treat steel for knife making? Is 1600F or 1700F enough? What is a useful upper design limit?
 
If you want to heat treat stainless steel, you need higher temperatures. I use mine at around 1,950F for stainless. I think there are steels that use higher temperatures.

What type of wire are you using for the coil?
 
For a do all oven it would need to be able to hit 2300° if you wanted to be able to do anything. Very rarely do you need temps over 2k but I have gotten steels that need 2100-2300 to process. The steel I’m currently experimenting with T1 has a recommended temp around 2300°.

Because you built your oven did you make sure your surface density of your coil was correct For the gauge selected. If you give me all you numbers I will run it through my spread sheet I put together. Need wire ga and the voltage and amps you want to run. If you burning out coils you went way to small of wire for the amps you are pushing. The elaments can only handle so much temp and that’s where surface loading comes into play. Also how are your elaments supported. You have to derate the coil loading if the coils are in grooves vs free hanging.
 
Coils are in grooves, pinned in place. 7.2mm FeCrAl wire. 220VAC up to 3000Watts. I am on a 20 AMP dedicated 220VAC circuit.

What heating coil material would I need to go higher in temperature? Also do you have a good source?
 
Are you sure it is 7.2mm? That would be a really thick wire with very little resistance and require a very long coil. I think I used about 1.3mm Kanthal.

There is a handbook for Kanthal online that gives max. surface loads for different element configurations. In a groove, it cannot handle as much surface load like JT says. I do not have the link for the handbook handy at the moment, but have it saved on a different computer and can dig it up tomorrow.
 
As above: 7.2mm would be incredibly thick wire for an oven. Is that the coil OD?

I bought some ebay coils that were very thin wire, coiled to a coil OD that was something of that order (I'd guess at 0.5mm wire diameter coiled round a 6-mm rod). Never tried them in an oven because they were obviously* too thin to last.

A search for "Kanthal handbook .pdf" will find it.

I have built a few HT ovens. The first few used 16ga (1.29mm) Kanthal A1 elements. These seemed ok for occasional hobby use, but there were a couple of element failures with heavier use by guys who make money from knifemaking. Later ones used 1.6mm Kanthal A1 and these seem to be holding up better. The later ovens were tested to 1300 degC, 2372 degF, which is the upper range limit for the type N thermocouples I use.

Previously, I'd always assumed 2150 degF to be the highest temperature ever likely to be needed, at least by anyone using an HT oven homebuilt by me. It's the top end of the range for S90V, which was about the most exotic steel I could imagine anyone too cheap to buy a "real" HT oven using, back when I built my first oven. Now I tend to think in terms of 1250 degC, 2282 degF as the maximum temperature anyone using one of my HT ovens might need. That's the top end of the range for M35 and I'm not aware of anything that needs hotter. Having built a few and learned from the experience, I also now feel that my HT ovens are probably not giving much, if anything, away to the "real" ones in terms of performance.

*to me
 
7.2mm is the OD of the coil before stretching it. I stretched it out to fit my groove pattern.

I’m going to find the Kanthal handbook. Sounds helpful.
 
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Ok I did an eBay search and found your coils. Thy are indeed .284” in diameter with a wire diameter of .047. I plugged all the numbers into my spread sheet and this is what I got.

Photo%20Oct%2007%2C%209%2050%2037%20AM.jpg


Here is a screen shot of what I found on your element
Photo%20Oct%2007%2C%209%2033%2008%20AM.jpg


And here is a graph showing max surface loading.
Photo%20Oct%2007%2C%209%2043%2058%20AM.jpg


So looking at the data you are WAY over driving it. So let’s say 1200C max temp on the graph. That’s a MAX surface load of 2.5w/cm^2. My calculator calculates your surface loading at 5.264w/cm^3. If the elements are in grooves thy have to be derated by 20%. So that 2.5w/cm^2 is actually now a max of 2w/cm^2.

my guess is those coils are for things like dryers and heaters and not designed for kilns. I can help you calculate the right coil to have made for your oven I just need max coil length you can use and your desired amp draw.
 
Great! Thanks! I will study this information. I think you are correct that the coils I used are not for kilns. Also, they have the same chemical formula as Kanthal, but claim only a little over 2100F tolerance.

I have more material coming....

I ordered some Nichrome wire
TEMCo Nichrome 60 series wire 18 Gauge 50 Ft Resistance AWG ga
My idea is to use this to pin the heating element in place.

Then as an experiment and also I ordered more of the original element plus a 15' roll of
(Alien 0.3x0.8+32GA)

we will see how they compare.

Jarod, I'll get you the information you need to help me calculate what I need a little later today.

Also, is your spreadsheet something you wrote? Is it possible to get a copy?
 
OK Jarod. Here are the details of what I am trying to do.

The attached photos show the furnace configuration and also one of the two heating elements.

There are two heating elements mounted in grooves int the sidewalls of the furnace. They are wired in series across 220VAC on a dedicated 20Amp circuit. Each straight section of the element is about 18 inches. I'm not sure of the length of the resistance wire since they come coiled and are then stretched to fit.

The internal volume of the furnace is about 18x7x8 inches. As you can see, I designed it so that it is easy to take the top off for repairs.

I had pinned the elements into the firebrick with short pieces of the heating element. They did not hold very well. The elements still expanded and popped out of their grooves. I've ordered some 18 Gauge Nichrome wire to use as staples for the new elements. Will that work? Is there a better way to attach the elements?

(I haven't had this much fun since I retired!!!)

Mike
 
Jarod, I took the top off of my furnace to remove the elements. I have two elements in series with each element about 6 feet long so roughly 12 feet of grooves cut for the coils. The furnace is on a dedicated 20 amp 220vac circuit. I'd like to keep the current draw at 15 amps or below though.
I appreciate your help and the education!

It's been several years since I put this furnace together so I don't have my notes for how I calculated the lengths I needed. I've only had it up as high as 2000F a couple of times and that seems to be its limit!

I downloaded the Kanthal Handbook. Looks really good.

 
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