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Heat treating with a gas forge - some questions


Jonas Liebel

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Heya folks,

I am currently building a propane tank gas forge like the one in the picture below. I am also planning to increase my knive production rate to be able to make a little money with my stuff, but I do not yet want to invest several thousands of dollars into a heat treating oven.

Now i was asking myself the following: Is it possible (and reasonable) to use my soon-built forge to harden several blades at once? Can I control the heat in there by sticking in a thermocouple sensor and adjusting my burner? 

I am mainly going to work with 1095, and Im going to stick in a lot of clay-covered blades for hamons.

So, for example, would there be any problem with the following scenario:

I put 5 clay-covered 1095 blades into the forge (using one of those blade-racks), heat the forge up to 800°C (which is, as far as i know, the temperature to harden 1095 at) and then try to let the blades soak at that temperature for 10 minutes by regulating the heat via the burner. 

I hope someone can give any advice on that topic :)

Cheers, Jonas

IMG_20170921_172720.jpg

Edited by Jonas Liebel
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You're likely to get a hot spot where the burner comes in and differential hardening/overheating. You definitely can heat treat with it (I have been for years). The best thing to do is to get some steel pipe and put that through the forge and put the knives inside this. This evens out the heat and reduces the impact of hot spots.

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A propane forge inevitably has hot spots. Alan recommended me to use a caped steel pipe inside the forge to put the knives in. cap it one end and i screw your thermocouple inside. i doubt you could fit more than 3 knives inside it though and you would need a large quench tub because the temp of the oil will rise too quicky otherwise. I am certain some veterans will jump in and be of greater help :P

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Your experience may be different once you get up and running, but I think you'll have trouble with what you are thinking.

I use a forge made for a 20lb propane bottle, although I only use one burner.  When heat treating I usually have the burner turned down about as low as I can get it to run stably, and the temperature in the forge still tends to be over 1500F.  The temperature is also not very uniform.  For these reasons, I don't ever let blades just sit in the forge while I am heat treating.

Edit:well, you both beat me to the punch by a couple of minutes :)

Edited by Brian Dougherty
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-Brian

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Thanks everyone for your information.^_^

I see, hot spots are going to ruin my initial idea. About the blade-inside-a-tube-thingy , would I dunk the entire thing into my quenchant? I'm also kind of worried about the tube removing my precious clay outline :wacko: 

Once the forge is heated up, how long does it approximately take to heat up a medium size blade (moving it around inside the forge to prevent hot spots)? Of course this varies from forge to forge, but a rough guess is better than nothing.

 

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9 minutes ago, Jonas Liebel said:

Thanks everyone for your information.^_^

I see, hot spots are going to ruin my initial idea. About the blade-inside-a-tube-thingy , would I dunk the entire thing into my quenchant? I'm also kind of worried about the tube removing my precious clay outline :wacko: 

Once the forge is heated up, how long does it approximately take to heat up a medium size blade (moving it around inside the forge to prevent hot spots)? Of course this varies from forge to forge, but a rough guess is better than nothing.

 

You would remove the knife from the tube to quench. The pipe just helps get a more even heat. The time it takes to heat a blade will vary by the size of your knife also. It shouldn't take too long though, and you should be able to crank out a good number of blades pretty fast heating one at a time no problem. But it really helps to pre-heat the forge at a low temp. so that you aren't at risk of over heating. You may find a good pressure to work at that is best for HT. I keep mine at 6 psi.

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And the pipe is not a small tube, it's at least 5cm diameter.  You can flatten it a little to make it wider, but the important thing is to have one end closed.  You also want the pipe and forge up to heat before you add the blades.  Depending on the size of your quench tank, your oil will get too hot after five to ten blades in a row if you take three to five minutes per blade.  My tank holds around four liters and I can do five or six tomahawks (edge heated only) before I have to let the oil cool down a little.  But then I start with the oil hot already, as everyone should with 1095.

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Thank you Alan and thank you Brian for your warning, it might save me from some moment of stupidness.

I have an idea about several 5x2 cm steel tubes stacked together to get several blades hot at the same time, would that be a possible thing to do? Or does it only work with pipes? 

(Excuse the quick sketch, I hope you get the point)

IMG_20170922_094549.jpg

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You can use rectangular tubing with no problem.  However, any tube only helps with the hot spots, it does not eliminate them.

I think you are getting the cart in front of the horse on this one.  Heat treat a few blades once the gas forge is up and running and see how it goes.  I think you will decide that trying to do 6 at once is not going to yield optimal results.

 

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-Brian

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I agree with Brian.  The advantage of a tube with one end closed is twofold: it heats more evenly and you can throw a little charcoal or wood in the end to eliminate scale and decarburization.  If the tube is open on both ends it's just a chimney and will not help at all.  BUT:  while you can indeed totally eliminate oxidation and decarb, you can't fight grain growth over time.  That's why people doing multiples are usually dipping a rack of blades into a molten salt pot.  Simultaneous even heating and atmospheric protection, and simultaneous quenching.  You can do it with gas or coal, it's just risky.

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6 hours ago, Brian Dougherty said:

 

I think you are getting the cart in front of the horse on this one.  Heat treat a few blades once the gas forge is up and running and see how it goes.

Yes, I think i should stick to exactly that. :D  Thank you for your insights guys, they are very valuable to a newbie like me.

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I have my gas fow aimed at the floor away from where I position the blades for HT and when the forge is at heat I turn down the air flow so there is no direct flame, so no hot spots. I can see the steel go into the right heat treat colour as the black shadow dances and leaves back through the tang.

Von Gruff

http://www.vongruffknives.com/

The ability to do comes with doing.

 

 

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Using a muffle pipe tends to reduce the temperature variation. It is a useful method for getting a more even temperature from an existing forge, given that most "normal" forges are not built with Heat-Treating in mind.

However, it is also possible to design/build a forge and burner combination to give precise temperature control with minimal temperature variation, specifically for HT. 

There are a number of HT-dedicated drum forges around, to a design usually credited to Don Fogg and originally intended for HT of swords. These use a single, relatively small, burner to heat a 55-gallon drum, lined with 1" of Kaowool. Temperature measurement is usually by thermocouple and handheld readout, with manual burner adjustment to get to the desired temperature.

Because the temperature is so low, the gas usage is much lower than would be the case with a similar-sized "normal" forge and temperature stability is very close to that achievable with an electric HT oven. Build cost is low.

Alternatively, a small pilot burner can be used with the temperature controlled by using a thermocouple and PID controller to switch a solenoid valve on the main burner feed. It's a good modification for the geeky types who enjoy playing with control stuff, or for the frequent user who will recover the additional build time involved by not needing to make the manual adjustments each time the forge is fired up.

The Don Fogg design is technically very elegant and is just about optimized: other than the PID control, every "improvement" I have heard suggested would actually be detrimental to its performance (IMO/E). The only major downside to the design is the size and it does not seem readily scalable downwards to make a knives-only forge. 

A search for "Don Fogg heat treat drum" should bring up a good few hits. 

I spent some time trying to make something smaller that would work for knives-only. I think I probably did ok-ish. There are 2 or 3 guys who have used my HT forges to make knives to sell while they gather the money to upgrade to an electric HT oven. There's a youtube video one of my test-pilots made at: 

The target temperature of 816 degC is 1500 degF.

The clever bit of this setup is the commercially-made gas mixer (so nothing I can claim any credit for), which allows very fine control of the air:fuel mixture and therefore the temperature. The mixer used in this case is an Amal 354/12BLV

http://amalcarb.co.uk/downloadfiles/amal/amal_gas_injectors.pdf

These are available from the manufacturer http://amalcarb.co.uk/amal-gas-injectors/butane-injectors.html

In a "normal" forge, the flame retention cup is not usually needed, but I do use one in the HT forge. That way I can light the burner then insert it into the forge, eliminating the possibility of filling a big chamber with a gas/air mixture and then igniting it.

The forge is a length of 10" thinwall pipe with 1" of ceramic fibre blanket inside and the ends cut from 1" ceramic fibre board and lightly hand-pressed in (to allow the ends to blow out if I am careless enough to fill the chamber with the aforementioned gas/air mixture and ignite it). 

The Don Fogg design has the burner at the bottom and the workpiece/exhaust port at the top. I just couldn't get the temperature distribution even with this arrangement. Once I tried the burner at the top and the workpiece/exhaust port at the bottom, things got a whole lot better.

 

 

Edited by timgunn
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Brilliant, Tim!  Not surprising from you, but brilliant. ;)  I know a guy who did scale down the drum concept for knives, but not as far or as elegantly as you did.  He used a 20-gallon drum and a tiny venturi burner with thermocouple and hand control.  He used it to sell enough knives he could buy an electric heat treat furnace, then gave it away when the electric arrived.  Last time I talked to him he said he wished he had kept the gas one.  The electric one was giving him fits with oxidizing and decarb, two things the gas did not do.  Then again he likes doing long soaks on 52100.  

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Thank you Alan.

The rich mixture tends to help minimize scaling and I think it probably helps to minimize decarb too: Carbon soot from the incomplete Propane combustion gets deposited on the steel surface. It's difficult to envisage a mechanism that would burn Carbon out of the steel without taking the soot, though it doesn't necessarily mean there isn't one.

Over here, O1 is a doddle to get hold of in beginner bladesmith quantities, but most other blade steels are not. Like 52100, O1 does best with a long soak so I wanted to come up with a setup that would let a beginner get decent results with a reasonably low entry cost. The same burner can also be used in a small conventional forge, where it can even (comfortably) achieve welding temperatures. This helps to keep the total startup cost down.  

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I'm going to play Devil's advocate here- you're a man, not an assembly line. The risks you take in trying to harden several blades at once in a forge are not worth the time savings imho. Now, for drawing the temper, you can throw a couple of dozen blades in an oven at once.

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Thank you very much Tim for your detailed explanation of this great idea - but it's nothing for me, as I want to use my forge also for - well - forging, not only to heat treat.

Also, @Al Massey, you are right. I've dropped my idea of wanting to throw a bunch of them in there at the same time.

 

But I still have a question about heat regulation: Should i rather check the forge's heat with a thermocouple and try to aim for the right temperature inside needed for quenching, or should I rather check the heat of the blade itself by taking it out of the forge and measuring it's temperature with a hand-held-temperature-measuring-device-thing? (i might add that I do not know the name in english ^^ )

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Thermocouple in the forge.  When the blade is the same color as the inside of the forge it's ready to quench.  The handheld infrared thermometers don't work well at heat-treating temperatures, things hot enough to glow confuse them.

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Alternatively, you could go old-school with a magnet.

 

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