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Introduction / New Heat Treating Oven Design


Florian F Fortner
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Hello Bladesmith's Forum,

 

This post is my first, so I'm going to introduce myself briefly: My name is Florian Fortner, i am based in Vienna, Austria and came to make my own weapons from long and unsatisfying use of reproduction rapiers. After handling and measuring many originals (we have a very nice armoury here in Vienna), I know exactly what to make, however there are always problems along the way... (for help on those i will ask in another thread!)

 

 

So here I want to present you the heat treating oven we built for hardening and tempering, after reading many posts and thinking hard.

 

The rapier blades we need are really long, around 53" including tang, and about 1" wide and 0.32" thick. I assumed that heat treatment only makes sense in a vertical oven to prevent warping. As we do the stock removal thing and don't have access to a forge, we built a vertical kiln around a quartz glass tube to save weight and skip the groove cutting, safety switches and some other disadvantages of pure brick ovens.

 

tube_wire.jpg

 

The glass tube holds up to temperatures of 2000°F and is also electrical insulation. The kanthal wire is wound directly around the tube and kept in place by a strip of ceramic fibre blanket. I have split the wire in two sections, so each section can be connected to a phase of our european 220V supply to distribute the load and have a total power of 6 kilowatt.

 

tube_insulation.jpg

 

On top of that is another layer of fibre blanket and a wrap of glass-fibre tape. This construction is covered by a stack of fireclay-tubes held together by an angle-steel encasing.

 

tube_schamott.jpg

 

The electronics (Auber controller, cutout switches, electronic relays) are placed in a metal box, detachable from the oven. Here i used a Neutrik Speakon connector for the 3 power lines and earth, and a XLR connector for the thermocouple.

 

electronics.jpg

 

After some first tests we found out, with the help of a 59" K-type thermocouple inserted from the top, that the heat distribution is not evenly across the entire length. So we had to add more insulation. The first choice of aluminium foil covered stone-wool directly wrapped around the oven did not work out, because it doesn't hold up to the temperatures listed in the datasheet and turns brown while smelling awfully burned.

 

So we built a solid case of a industrial sheet metal ventilation tube with matching lids and filled the whole thing with extruded vermiculite, which has excellent insulation capabilities and is dirt cheap as well.

 

The final oven looks like this (in the background you can see the oil filled gas bottle we use for quenching):

 

final_oven.jpg

 

Now the temperature distribution is quite constant to within 3" of the tube ends.

 

 

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Florian,

 

Welcome ! Thank you for the interesting post. When designing this furnace ( beautiful design by the way), you must have considered just wrapping the heating element around the tube on a lathe instead of wrapping a spiral, spiral....what made you decide to do it that way?

While measuring temperature differences in the tube , how large were those differences and how large a difference would you have ignored?

I am curious what your guess might be as to the temperature gradient that would get established if the tube were used vertically just by the convection in the closed tube?

Though I have no particular need for such a furnace I find your project fascinating, thank you.

Jan

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What a great HT furnace design!

 

Thanks for thinking originally and sharing your ideas. Had I seen this before purchasing my kiln, it would have saved me a lot of money!

 

Welcome to the forum!

 

Dave

-----------------------------------------------

"It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood, who strives valiantly; who errs and comes short again and again; because there is not effort without error and shortcomings; but who does actually strive to do the deed; who knows the great enthusiasm, the great devotion, who spends himself in a worthy cause, who at the best knows in the end the triumph of high achievement and who at the worst, if he fails, at least he fails while daring greatly." -- Theodore Roosevelt

http://stephensforge.com

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Thanks to all for the warm welcome!

 

I decided to wrap the spiral to get the parameters of the wire right. If I had wrapped the wire directly the length would force a real close wrap which would also be more difficult to hold in place. The spiral has a diameter of appr. 1/2" and stays put well.

 

Without the outer shell and the vermiculite, the temperature differences at the very top and bottom were around 350°F which is a lot. With the shell we get to 170°F which is acceptable because just 2" away from the tube end the temperature is almost comstant.

 

At the top of the tube is the tang with a thread which shouldn't get to hot or quenched anyway because of the danger of cracks (usually a rapier blade finally fails at the pommel thread when it doesn't break while sparring)

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