Gas forges and thermocouples

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Gas forges and thermocouples

Postby Simon Balle » Wed Sep 13, 2017 2:49 pm

Hello!

i am a stock removal guy by necessity (read apartment dweller)
and i have run into a bit of a question outside of my area of expertise.

my father is taking up forging again, and has decided that he wants to get a gas-forge for it.
the forge in question looks to my (admittedly untrained) eye to have all the bells and whistles one could want from a gas forge.
the only thing that raises any questions about it, for me. is that it comes with a thermocouple, it isn't PID controlled, but it does have a thermocouple for reading the temperatures inside of the forge.

2 burners 13x20x40 cm internal volume, its not a round gas forge.

my questions are thusly

1. how much should he be able to rely on the thermocouple for heat treating?

2. what kind of temperature gradients are to be expected from the square internals and top mounted burners?

3. i might in the future look to convert the current digital temperature reader, into an actual PID controlled device. what is some must have functionality for such a controller?

feel free to ask me to elaborate on anything, im not a native english speaker, so something important might have gotten lost in translation.
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Re: Gas forges and thermocouples

Postby Stefano Z. » Thu Sep 14, 2017 8:56 am

Hi,
is it a pre-made forge for sale? do you have any pictures?
The temperature gradient can be quite wide, anf for heat treatment it is advisable to create a muffle...just a piece of iron pipe placed inside the forge creating a sub-chamber into which placing your blade. The best would be sticking the TC probe inside this pipe, close to your blade.
The pipe will damp the intense flame heat, and redistribute its radiant heat toward your blade, more evenly. The pipe thickness should be such as to have a bit of inertia (3mm should be ok).
Probably during the heat treatment you'll be using only one of your burners, at its minimum output, more or less.
Definitely, a TC probe is an useful help, but i like a probe that i can move inside my forge, so i can check for eveness and finding solutions to minimixe gradients.
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Re: Gas forges and thermocouples

Postby Simon Balle » Thu Sep 14, 2017 1:33 pm

http://www.ebay.de/itm/Schmiedeofen-Gas ... 2930945043

this is the forge in question. the probe doesn't look to be able to move around. which is a large part of why i asked about the temperature gradients.
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Re: Gas forges and thermocouples

Postby scott.livesey » Tue Oct 03, 2017 10:42 am

i would get several thermocouples and displays and see what temperature is where the knife will sit. I set up my furnace so I had 4 thermocouples almost touching. when furnace controller said 800C, all other displays said 800C +/- 10C. then I spaced thermocouples so they would read from tip of blade to end of tang. the displays again showed a range of +/- 10C, more than good enough for O1 and such. i second the idea of a muffle as basic as a piece of pipe.
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Re: Gas forges and thermocouples

Postby R. 'Deker' Dekelbaum » Tue Oct 03, 2017 3:14 pm

I agree with all of the above stated notes on thermocouple placement and the use of a muffle (if you can get stainless pipe, it will last longer).

For my addition to the conversation I'll make a couple of comments on the forge design.

I've never much cared for gas forges with square chambers and burners firing straight down. In particular when working with knives, this makes the largest portion in the center of the forge an area that will more rapidly oxidize your steel. This design is also more prone to "hot spots" in the center of the forge and a generally less even heat within the forge chamber.

My preference for smaller forges is a round forge body/chamber with a burner set on a tangent to the forge body so that the flame will swirl around the outside of the forge chamber. In my experience this reduces the speed of oxidation of your steel, and more evenly distributes heat through the entire forge chamber.

If you're serious about building a PID controlled forge, atmospheric burners will make that a lot more difficult, if not impossible. A forced air burner would make that a more realistic possibility (and generally be much more fuel efficient than a venturi burner setup as well). In fact, most of the PID controlled forge designs I've seen have had a major shortcoming in that they only regulate fuel delivery and not the air delivery as well. This means that keeping a stable atmosphere in such a forge will be quite difficult and may cause a lot of oxidation of your steel.

The most efficient, most controllable gas forges I've seen all make use of a "ribbon burner". These are used in glassblowing furnaces and deliver excellent even heat, more fuel efficiency, and don't need to be pointed to "swirl" the flame in the forge chamber. Unfortunately, there aren't many people making forges for sale that use ribbon burners, so you'd have to build your own.

For a very interesting read on automated furnace control, take a look at https://docs.wixstatic.com/ugd/c870d3_e3b7fc88bda04a39bd7be8d4c9f7db16.pdf. It's an excellent writeup of furnace control systems by Charles Correll who makes some excellent burners and glass furnaces here in the US.

So, in summary. See if you can find a forced air forge with the burner set to swirl the flame inside the chamber, in my opinion that would be a better bet. You can always add a thermocouple probe easily if it' snot equipped. Here is an example photo of a forge set up this way that can be ordered with a forced air burner here in the US. I know it wouldn't make sense to order from here, but it will at least give you an idea of the layout I'm talking about.

Image
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Re: Gas forges and thermocouples

Postby scott.livesey » Wed Oct 04, 2017 11:44 am

another source for gas furnace pieces, these were originally for glass. http://www.joppaglass.com/index.html here are pictures and prices for "Ribbon" burners http://www.joppaglass.com/pop_off/pop_off_complete.pdf and http://www.joppaglass.com/price/priclist.html the site also has a good tutorial on building electric furnace/kilns
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