In search of an improved HT apparatus

[Lukas MG]

What I'm not getting is this: How will you temper in a burner forge set up? Getting a blade up to quenching temp is the easier part (a trench in the ground works for that), having a way of heating it precisely to 180-220°C or so is the hard part. I'm not saying it can't be done in a propane forge but it's not what I'd call "controlled".

 

A large electric kiln is what I want to have. All in one. Blew my budget on the belt sander though so I guess for long blades that don't fit into my kitchen oven I will have to continue using a heat treating service.

Edited December 10, 2015 by Lukas MG

[Alan Longmire]

You have a separate tempering oven, either electric or built like a gas bbq grill. No insulation, tiny long ribbon burner. I plan on using rest of my steel drums to make this, using a pair of gas grill burners on either end, with the whole thing horizontal. Plus I can use it to roast a whole pig!

[Wes Detrick]

Depends on the stainless and the the temperature you are working, but tentatively yes.

 

What kills stainless in heat cycling is loss of the Oxide layer. When it gets hot, an oxide layer forms. When it cools, the mismatch between the expansion coefficients of the Oxide layer and the underlying Stainless Steel causes the Oxide to spall off. After a number of cycles, depending on thickness, there's nothing left. Different stainless steels will hang onto the Oxide layer through different temperatures. 316 keeps the Oxide layer intact through around 850 degC/1560 degF, so is good enough for most Carbon steel HT.

 

310 stainless has higher (25%) Chromium for a stronger Oxide layer and higher (20%) Nickel, which brings the thermal expansion coefficient closer to that of the Oxide, and will keep the Oxide layer intact through cycles up to about 1100 degC/2000 degF.

 

Finding 310 is usually difficult, so Nichrome or Kanthal are often used because they can be found in small quantities on ebay.

 

316 in conjunction with rigidizer works quite well for blanket retention: by the time the stainless wire fails, the shell is rigid.

 

Thanks Tim!

[Lukas MG]

Thanks Alan.

 

This thread has made me decide to do my own heat treating of larger sword blades. So far, the single burner drum furnace seems like the most doable version (also on a tight budget here and absolutely NO knowledge regarding electronics. I'll most likely electrocute myself trying to build an electric oven. Gas seems much safer. Right... ).

Anyway... the only thing I'm worried about is the whole "heat rises" thing (as Colin also mentioned). If you have any suggestions how to deal with that, let me/us know, please!

@Sam and Jesus, you seem to have experience with that set up, did you experience any trouble in that sense and if so, how did you deal with it?

@Alan, you seem to have thought on it as well... muffle pipe in the middle makes sense concerning indirect heating and avoiding hot spots from the burner but it doesn't really change the physical phenomenon that heat rises, does it? What I also don't quite get is your plan for intake/exhaust...?

[Alan Longmire]

Downdraft kiln technology. The only exit for the exhaust is at the bottom next to the burner, guided up by a suitable chimney of course. I have not built it yet, but it has worked for even firing of pottery for a few hundred years or so, and I suspect it will work for this application.

 

Heat rises, but is forced to exit from the bottom, thus making an even distribution. Set up a swirl and let physics and thermodynamics take over. That's the idea, anyway, and a potter friend of mine thinks it ought to work. The proof should be forthcoming by next summer.

[Sam Salvati]

it is a positive pressure system, the even ness and heat can be controlled by enlarging or closing the opening at the top which the blade goes through once the blade is in. I do this with a firebrick.

[Lukas MG]

Alan, something like this? http://f.tqn.com/y/pottery/1/8/N/0/-/-/downdraft.jpg

 

Obviously this is a pottery kiln so you wouldn't have the stack in the middle but your pipe instead. I assume you will have a slot in the top to hang in the blade?

 

Sam, I going to have to look up positive pressure system... I can't wrap my head around why enlarging/closing the top opening would get rid of the rising heat.

[Alan Longmire]

Very similar to that, with a pipe down the center for the sword to hang in. The blade is heated by radiant heat only since it is not in the chamber itself. A little charcoal in the pipe to provide a reducing atmosphere and I think it will work. Otherwise I'll just use a slit in the wool insulation.

 

What Sam means by positive pressure is that yes, heat rises, but more hot gases are being forced in under pressure all the time so the entire gas content of the kiln is forcibly exchanged whether it wants to go or not. Opening and closing the damper allows you to find the balance point where the heat is even from top to bottom.

[Sam Salvati]

I feel I shouldnt say this, but i look for the dragon's breath at the opening the burner shoots into and tune accordingly. my burner is NOT in the chamber at all, but outside it. I used a 2x4 to size both openings. I should mention i run my drum furnace vertical as I dont do many katana, and even when I did some, i still ran it vertical to no ill effect.

 

I've not tried the muffle pipe yet, but have had little to no scaling issues, the tuneability of a quality burner like the trex makes this possible i think. It works so well in fact with the Trex and 1" of wool with the ridgidizer, i am skeptical to think it could work better.

 

as to tempering, a hot oil bath is cheap and easy, and used to great effect for odd shaped or long blades. a pipe or box of steel any dimension you need and a couple gallons of cheap canola.

[Alan Longmire]

Sam: so you're running a vertical drum with top exhaust? I may be overthinking my design...

[Sam Salvati]

Yes, the burner enters into the drum down low, and points directly at a short stack of soft (reflective) firebricks to disperse the heat and keep a hot spot from happening on the tip. the exhaust port is directly across from that cut into the lid.

 

Yes I think you are

[Sam Salvati]

 

Here it is

[Alan Longmire]

Thank you sir! That saves a bit of extra work and cash for sure.

[Lukas MG]

Sam, could you maybe shot a pic of the interior of the drum? I'd like to see that reflective set-up and the location of the exhaust.

Edited December 13, 2015 by Lukas MG

[Lukas MG]

To get back on this, here's an excellent WIP of a single burner drum furnace for swords: http://www.bladeforums.com/forums/showthread.php/862158-Drum-forge-WIP

 

There's one thing I'm not sure about: The angle of the burner. I'm thinking it shouldn't be at a right angle but instead be oriented upwards. Any comment on this?

[Justin Mercier]

I'm picking up a pair of out of spec 100lb propane cylinders, to turn one into a slightly smaller volume 55 gallon drum forge and the other into an electric kiln just for tempering. A standard 100lb tank is ~48 inches tall, though that includes the nozle protector, but that should still give me around 43 inches of usable height inside. I think the horizontal furnaces we used at NESM were made from 100lb propane tanks.

 

As to the angle of the burner, heat rises, angle it so that you get a swirling motion in the drum and you'll eliminate hot and cold spots better, it doesn't need to angle up

 

Edited December 18, 2015 by Justin Mercier

[Lukas MG]

Thanks. How would I have to angle it to get a swirling motion?

Edited December 18, 2015 by Lukas MG

[Alan Longmire]

Aim it to one side, and the blast will naturally swirl upwards. Not necessarily a full tangent angle, but off to one side a fair amount.

 

Thanks for that thread!

[Lukas MG]

So like the one at the left?

 

[Alan Longmire]

Yes.

[Lukas MG]

Thanks.

 

Btw, here's another example, this time of a horizontal set up. Not sword length though making it longer would be no problem. The text is in German but the pics are telling enough.

 

http://www.messerforum.net/showthread.php?68396-Gas-H%E4rteofen-Weichgl%FChen-etc-

(better pics further down)

 

The maker/user claims he can reach a very even temperature throughout. Key apparently is a very slow heating up with little gas pressure. With 0,5bar it takes around 40min to reach 820°C and keeping that pressure, the oven will keep the same temperature over hours.

 

I have seen a version that is basically two chambered. The burner comes in at the front and heats up a channel of firebricks that spans almost the entire length of the drum, only at the end it stops so that the flame can rise into the top chamber and return to the exhaust port, where the blades are put in and placed onto the fire bricks that make up the roof of the top chamber. I really like that idea...

 

I'm really not sure what set up I should go with, horizontal or vertical. Plenty of people use horizontal kilns (evenheat for example, in fact, there seems to be no vertical kiln to buy out there) and make long sword blades with those... it can't be THAT hard to extract a hot blade out of a horizontal set up and plunge them into the oil without having it bend on you (or more so that it does anyway). And the prospect of horizontal set-ups having less hassle with even heating is very appealing...

Edited December 18, 2015 by Lukas MG

[Sam Salvati]

No need to swirl the flame, this just keeps getting more complicated then it needs to be.

[Jesus Hernandez]

Exactly what Sam just said.

[Collin Miller]

Would 3/4" kaowool be sufficient for a single burner horizontal set up? For some reason all of the 1" stuff I find is crazy expensive, and the 3/4" is less than half. So would 3/4" just be slightly less efficient and need replaced a little sooner, or is there an absolute reason why someone would need to use 1" thick?

Thanks for making this thread awesome, guys! Hopefully it'll help a few of us out and save a couple of us a lot of money.

Edit to add: What kind of thermocouple is recommended for such a furnace? I assume you'd want at least two or three sensors, to make sure the heat is even?

Edited December 24, 2015 by Collin Miller

[timgunn]

I've not tried 3/4" kaowool (or even seen it). If it's the high-density stuff (128 kg/m3, 8 lb/cu ft) I think it "should" work, but will give you a bit less headroom for getting the rest right than the thicker stuff. I'd rather expect the lower densities to give marginal insulation at 3/4", though if you get everything else pretty much perfect, it should still work OK.

 

I'd recommend a long type K mineral insulated thermocouple with a 1/4" sheath in type 310 stainless, Inconel or Super Omegaclad XL. That way you can put the tip anywhere you want it and can move it about to profile the temperature distribution. Once you are happy with the distribution, you can put the probe in a representative position and use it for the adjustments. I find 6mm diameter (1/4") is stiff enough to use horizontally at 1000mm (40") long at HT temperatures. Vertically, there should be no problem at all except for keeping the handle and the hand holding it cool. getting a 6' transition junction thermocouple and bending it is probably the best solution.