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Quick and dirty muffle furnace forge modification

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I always talk about using a pipe with one end welded shut as a sort of muffle furnace to do my small knife heat treating, but I never had pictures until now. The advantages are using this kind of semi-muffle are large, especially in a solid-fuel forge or a venturi gas forge that isn't very adjustable for atmosphere.


Step 1. Get a piece of plain steel pipe or tubing (no galvanized or plated pipe, please!) a little longer than the heatable length of the forge, and big enough in diameter/cross section to fit the kind of blade you want to harden. I had some 2" schedule 40 black iron pipe, and I flattened it a bit in the vise to make it more of an oval section so I could fit blades up to 2.5 inches wide into it. Since I am a holdout and still use a coal forge, the heatable length of the pipe is limited to the long axis of my firepot, which is about ten inches/ 25 cm. Weld or thread a cap on one end of the pipe. You now have a muffle that allows you to see the blade at all times (which means you can watch for decalescence in a solid fuel forge!) and at the same time allows you to control the furnace atmosphere. If you just heat in the pipe, you're running slightly oxidizing, but much less so than you are in a venturi forge. You will not get much scale at all forming in the pipe except for the last bit towards the opening. If you want a reducing atmosphere, which means no scale at all, just push some charcoal into the pipe. Bingo, no more oxygen at all.


pipe 1.jpg


The above shows the pipe in the fire.


Step two: heat pipe slowly and evenly until the inside is slightly hotter than the target HT temp. Judge by eye if you can, or by thermocouple if you have one. I do not have a decent pyrometer, so I just watch for a medium orange. Note in the following pictures the colors are off, as always happens when shooting incandescent steel.


pipe 2.jpg


In the fire looking straight in, coming up to heat. You can see the end cap is a little past the back of the firepot, allowing me to have a cooler spot to keep the tip from overheating while the rest of the blade is coming up to heat.



Step three: Turn off the lights, insert blade, watch for decalescence, quench immediately. You do not need to know the exact temperature for simple steels if you can watch the phase change take place. Once you see it happen your steel is ready to harden.



pipe 3.jpg


Blade in place, coming up to heat.


As I saide above, the big advantage of this technique is that you can actually see the blade in the forge plus you can totally prevent scale from forming. I think that's pretty useful, myself. B)

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Looks like it should work well. Most of my time years ago when I was using coal wasn't doing blades so I've not been around this but it should be a good reducing atmosphere. Any idea of how long your pipe should last?



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Alan, thanks so much for sharing the pictures with us. I have a question I'm hoping you, or anyone at all, can help with. I would like to do this exact same thing......use a pipe in my homemade paint can/MAPP forge. Obviously, my set up causes a hot spot in the middle of the forge that I would like to do something about, and a pipe muffling the flame is sounding good to me right about now. Of course....eventually......a proper kiln/oven with temp/time control...is in the plans. I tried using a piece of scrap steel to shield the flame from direct contact to the blade, similar to the pipe idea, but not the same. The problem I was having is that it seemed to be taking way too long to get to 1500, and using too much fuel. I had to scrap my scrap, and do it the way I've been doing it.....moving the blade back and forth in the flame for an even heat. Well......even"ish" anyway. My guess is I'm just going to have to keep doing what i'm doing, with what I have. Waiting for the blade to come up to critical inside of a pipe, inside of my paint can forge, is taking too long. Actually, it's not the time I'm concerned about, it's the cost of a tank of MAPP. Thanks for your ideas and thoughts.

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Gary, since I'm just letting the forge idle the pipe barely scales on the outside. Five years on this one so far...


Stuart, you need a bigger forge. MAPP/paint cans just are not big enough not to have a hot spot no matter what you do.

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  • 3 weeks later...

Yep! It'll work on any forge with a big enough hot spot, including induction if your coils are big enough. The most important thing is to have one end sealed. Otherwise it just acts like a chimney and oxidizes the heck out of stuff while not getting it hot enough.

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This is the technique you told me about in my early days and boy did it help with my knives..I totally rate this. I used the coal in the tube and got a great weld on a chainsaw chain knife.

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  • 2 weeks later...

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